Condensers and Cooling Towers

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COMMERCI L HV C EQUIPMENT

ondensers and ooling Towers

T ec hnic al

evel opm ent P r ogr am

 

Technical Development Programs (TDP) are modules o technical training traini ng on HV AC theory, system design design,, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HV HVAC AC equipment in commercial applications. applications. Although TDP topics have been developed as stand-alone modules modules,, there are logical group ings o topics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth o this offering allows for customization into a complete HV HVAC AC curriculum - from a complete HVAC design course at an introductory-level or to an advanced level design course course.. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.

Introduction to HVAC Psychrometries Load Estimating

Controls Applications

This TDP module discusses the most common heat rejection equipment: condensers and cooling towers. Heat rejection is a process that is an integral part o the air conditioning cycle. cycle. The heat is rejected to the environment using air or water as the medium. In order to properly ap ply system concepts to a design design,, HV HVAC AC designers must be aware o the different heat rejection methods. Also presented is the concept o total heat o rejection rejection,, it s deriva derivation tion,, and how it ap plies to the process o air conditioning, as well s the controls that are used to regulate each type o heat rejection unit.

© 2005 Carrier Corporation. All rights reserved . The information in this manual is offered as a general guide for the use o industry and consulting engineers in designing systems. Judgment is required for application o this information to specific installations and design applications. Carrier is not responsible for any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design . The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmitted in n y form or by any means electronic or mechanical for any purpose purpose   without the express written permission of Carrier Corporation.

Printed

in

Syracuse NY

CARRIER CORPORATION Carrier Parkway Syracuse   NY 13221 U.S.A.

 

 abl e of

ontents

Introdu ction .............. Introdu .............................. ........................ ........ . .. ...  ... ............. ............................ .................................... .............. ......................... ..................... ......... 1 Conden Conde n ser Tota Totall Heat of ejection ................... ...................... .. .. .  ... .  . ..   ..... .. .  .  .  .  . .... ..... ...... 2 Heat Rejection Factors .. .. .. .. .. ......... .  ..... .. ....   .......   .... ....   .   . .. ..   .. .. .  .. .. .  . ..   . .   .. 3 Condensers ......   .  .  .. ..   ..  .. .. .. .  ...............   ............................... ................................... ..  .   .  .   .. .. .  . .  .. .. .  .. .. 4  

 

 

 

Water-Coo ledversus Condensers .. .. . ........ ... .............. .. .  .......... ........... ...... ........ ......... ............ ......................... ....................... 5 Once-Thru Recirculating ........................................ ........................................ .. ........ .  .. .. .  .......... .  .. .. .. 5 Water Require Requirement ment Calc Calcu u lation for Reci Rec i rculating Systems ...... .....   ...... .... .......... .....   ....   6 ARI Conditions . ..... .... .. .. .. .. .. ................... ....................................... ........................................ ...................   .  .. .... .. .. ..   .. .  . .. .. ..   . .. 7 Water Consumption and Make Makeup up Q u antity ............. .......................... ............... ..   .........   ....   .............................. ..............................8 8 Con Co n structio n and Types o f Water-Coo led Condensers .........   . ..   ............................... ............................................. 8 Fouli Fo uli ng Factors ................ .. .... ..   .. .. .. ........... ............................. ..............................  .  ......   . ..... . .... ......   .... 13 T u bi ng Mate1ials .............   . .  .....   ..... ...  .  ..... .... .. ...... .. ...... .  . .  ...... ..... .......................... .............................. .... 15 Effects of Antifr Antifreeze eeze .... .... .  ............. .  . ........ .......................... .  . ....  .  .  .. .. ..... .......... ...... 15 Condenser Pass Arran Arrangeme geme nts .................................................... ...................................................... .. .. ..... .... .. .  .. ..... .. .. .. .. .. . ..... 16 Selection Inp Inpu u ts ......... ....... ............. ..   ..  .. .... ..... .... .. .  . ......... .. .  .................................... ..................................... .......... 17 Air-Cooled Condensers ....................... .. .... .. .. ..   . .. .... .. .. .. .. ..   . ..........   .... ..... ......   .............   .... . .... 17 A i r-Coo led Condenser versus Air-Coo led Condensing Unit.. ..... .   .............   ...................... 18 Subcooling Circuit ...... .......... .. .. ....   .. ...... .. ................................... ............................................ .... ...... .. ....... ...... ..  ... 19 P lacemen t. ............... ............... .......... . ........................... ............................. ....... ............... ... ........... ........ ..   .  .  . ....................... 20 Sellection .......................... Se ............................ ..... ... .  ........... ...... ........................ ................................ ......... .. ..   .. . .. ....  .. .. .  .... ..   . .. ...... 2 1 Evaporative Co Con n de den nsers .. .. . ... .. . ..............................   ..   ...   . .  ..... .. ..  . .. ...... .. .  .  .  .. ...... . 22 Evaporative Condenser Se lection Parameters ........................................ ... ....   ..   ......... . ..... 24 Condenser Econo mics ......... ......... .. .. . ... .............................. ............................... ....... ......... ...... ...... .... .. ..... .. .. .. .  .. ....... .. .  .. .  25 Coo li ng Towers .............................. ................................  .. ..  .. ............................................................. ............................................................... ...   .. .... .. .. .... .. 27 Basic Terms ................................... ...................................... .. .. .. ...  .. . .... ..   ..   ....   ......   .................. ................................... ..................... ... 28 Entering Wet Bulb Temper ature ...... ......   ....   .   .  .... ....... . .  .. .. .  .. ..... .. ..... .. .. .. .. .. .  .  ................ 28 Approach.......................... Approach ............................. .. .. ................ .. ..............................  . ......... ......... ......   ..   .............. ..   ... 28 Range .. . .  .. .. .. .... .. .. .. .  ............. ........ ......... ........... . ..... ............ ......   ........... . ............ ......... . ...... . .......... 29 Totall Heat of ejection .......... . ..... .. .......................................  ............................ ....... ............ 30 Tota Drift Wi W i ndage) ...... ............... .........   .. .  .. ..... .. .  ............. ........   .... .... .  ..... .  .....   ..... ......... 30 Evaporration ...................... .. .... .... ....   . .  . .. .. .  .. .. .  .. .. .....   . . ...... . ....... ......... . ......................... 3 1 Evapo B low-down B leed) .................... .  .. .. .......... .  . ............................ ............................ ...... . ............................... .................................   31 Makeup ......................................... ................................................ .......   .. .. .  . .  .. .. .. ..... . .... ....... ...................... ............... . ....... . .................. 32 Cooling Tower Psyc Ps ychrometric hrometric Plot. ........... ...... .  .   ......   . .. .. .  ..   . .. .. ...  . ............ . ........ 32 Types o f Cooling Tower Towerss .... .... .. .. .. ....   . .. ..... .. .... ......... ........ ............. .... ....   . .  . .......... ................   . ... 33 Natural Draft Atmospheric) .. . ....   . .  ..................................................... ....................................................... ........ ......... ...... .. .... .. 33 Mechanical Draft ................. ....   . .. ................................ .................................   .... ......... ....... ....... ........................ .......................... . 34 C losed-Circuit Cooling Towers Fluid Coolers) .. ... .. . .  ....   .  .  ............... .... .... . ...... ........... . 36 App li cation o f Coo ling Tower s ......... ................... ................ ......   ......   ..... .....   .....   ..... .... ....... .............. .... 37 P lace acem m en t ........................... .. .   . ............... .............................. ........................... ............   . ..........................................  . ... 3 7 Effects of Reduced Coo li ng Tower Water Tempe ratur raturee ...... ................ .....   . .........................   .. .  38 Hydronic Free Coo li ng ................... ....................................... .................................... .................. ............ . ...................   ............... 39 Cooling Tower Relief Profil Profi les ............................. ...................................................... ............................... ...... .   ..............   ..... . ......... . 40 Cooling Tower Differen Differences: ces: Electric versus Absorption Chillers Chillers .   .......... .... ......   .   41 Cooling Tower Selection ......... .................. ........... ...  .. .  .. .. .  .. .. .. .. .  .. .. .. .. .. .. .... . ................... .................. 43 Water Treatment ..   .   .   .   .   . .. .. .. .  . ........ . .. .. ........... .. ... .... . ............ .  .. .. .  .. .. ............. .. .. ....... ... ........... 44

 

Control S y s t ~ m s ......... Control .............  ...   ........... .......... ...  ........ ...... .  ..... ...... 46 .....................................  .....   .   ...  .  ..  ..... ....   ..  .... 4 7 W a t ~ r C o n d ~ n s ~ r s ................................... Air Cookd C o n d ~ n s ~ r s ..................................... ......................................  .....  ......... .  .....  .....  .....  .................. 4 7 Contro l ...... .  ........ .  ........... ...... .....  ....  ...... .............. ...  ...  .   .  .  ...  48 R e f r i g ~ r a n t Side Control .  ..   ..... .....   ....  ..  ......   ..  ..........   . 48 s i d ~ Control. ....... .. .   ........... ... . .   .  . ....... .. .  p o r a t i ~ n s ~ r s .  .... .... .....   ......   ..... ..... ............   ............  ...  ..............  ....  ........... 50 Cooling T o w ~ r s ................... ....................................... ..................................... .......................  ..  .......  ........... ............  ..  .....   ..  ...... 5 1 thee Coo Coolin ling g Tower .... ...... ........... ...  .  ...  .....  .....  ..... .  ......... .................. 51 W a t ~ r Bypass o th Airflow Control Contro l on Cooling T o w ~ r s ..  .  ..................... .......................................... ............................................ ...................... 52 Winter Op era eration tion o Cooling Towers ...  ..  ......   ........ ...  ............. ....... ........ ........... 53 Summary ........................................................... ................................................................... .  ........   .................  ........  ....  ..... . ..  . .... 54 Work S ~ s s i o n ......  ...... ............... ........ ....... ........ ............... .  ................................ ....................................... .  ............ 55 ....................................................................... ...................................... .....................................  ...  ......  ........... ...  ..  ...  ....... ....... .... 57 ................................................. ....... ......  ......  ....... .  .........   ..  ..  .......   ...  ...... 57 ........................................ R e f ~ r ~ n c ~ s Work S ~ s s i o n Answers .......... .   .  .... ..... ..  ....... .  ..  ..... .  .....  .... ......... .....  ......  ...  .... 58 C o n d ~ n s ~ r

and Cooling

T o w ~ r

 

CONDENSERS AND COOLING TOWERS

Introduction Cond Co ndeense serrs and cooling towers are th thee mo most st common kinds

of

h eat rej ec ecttion e quipm quipmee nt.

There are thr three ee t ypes o f condensers: water-cooled  water-cooled   air-cooled air-cooled   and ev evaaporativ e Water-cooled and air - cooled condensers use a airWater Cooled sen se ns ibl ible-o e-onl nly y cooling process to rej ec ectt heat. Evaporativ Eva porativee condensers use both sensible and lat latee nt heat principless to rej ect h eat. principle Coo ling Coolin g towers are simi lar to ev evaapor porat ativ ivee condensers beca ecau use th theey also utili ze la la ten entt cooling through th thee process of evap evaporation oration . We will di sc scu uss thr three ee kind kindss o f cooling towers in thi thiss TDP: natural   m ec natural  echanic hanicaa l  and close clo sed-circuit. d-circuit. We will di sc scu uss total h eat o f rejection rejection   it itss de ri va tion   and how it applies to tion thee proc th process ess of air condition ing Application Ap plicationss for condensers and cooling tow ers   as well as th ers thee controls that may ma y be use sed d to maintain proper r  fri rigera gerant nt and water te mp mpera eratur tures es will also b e

Evaporative Figure 1 Three Types o f C ondensers P ho hottos · Wate aterr  c oo oolled: C o urte sy o f S ta nd nda a rd R efr efrig iger era a tion ; Evapo Eva po rative ative:: Court ourtesy esy of Baltim or oree A irco il Co C o mp any

Cooling towers are heat rejecters.. They do not rejecters condense refrigerant so they are not consi cons idered condensers .

covered.

Figure 2 C oolin ooling g Towers Pho hottos re r eprod produc uced ed wit w ith h permi perm i ssion of Bal Baltti mor moree

Commercial HVAC Equipment

Compa any i rco il Comp

Turn t o t he Ex pert S

1

 

CONDENSERS AND COOLING TOWERS

ejection

ondenser Total Heat o

The heat to be rejected by b y the condenser in condensing the refrige refri gerant rant is equal to the sum of the refrigeration effect RE) of the evaporator plus the heat equiva equivallent of the work o f the com pression. RE + Compressor w o r k

THR Total Tota l Heat Rejectio Rejection) n)

Heat rejection in the condenser ma y be illustrated on the P-H pres sure ure-- enthalpy) diagram. A pressure enthalpy diagram is used because condensing take takess place at constant pressure,, or nearly pressure nearl y constant pressure when blended refrigerants are used , line F-G). This diagram ma y a lso be used to show the pressure ri se o f the condensing medium as it absorbs heat from the refrig refrigerant erant curved line) . The THR

of

the condenser

is

is

Tota l Heat Rejection= RE + Work of Compression) Compression ) or E- H THR

UJ

:: >

/) /)

UJ

::

a.

de de of

ENTHALPY finedrefrigeration b y line E -Heffect , whichlinethe sumand the A-B) the heat of compression line C-D). Figure s the ratio between compressor dis Cond ense r Tota Totall Heat o f Rejection shown on p-h diagram) diagram) charge and suction pressures increase, increase , the refrigeration effect decrea decreases ses and the heat of compression increases. This is because the work done by b y the compressor has mcreased.

These are the equations to calculate the THR In cases where the brake horsepower bhp)

THR

= RE +

bhp

* 2545)

In cases where the compressor kW is known:

= RE +

3414 Btuh

is

kW

* 3414)

equivalent

to

units

oft he

of

Btuh:

compressor s)

is

known :

If you know the compressor bhp or kW: kW :

2545 is a constant; constant ; it is the Btuh equivalent o f one bhp . Brake horse horse power is the application rating for the compressor.

THR

in

1. Total Heat Rejection = RE + (bhp or

*

2545)

2 . Total Heat Rejection = RE + kW * 3414) 2545 is the Btuh equivalent of one bhp 'v'

__..__

3414 is the Btuh equivalent of one kW

If you don don  t know the compressor energy consumption consumption:: 3. Total Heat Rejection

one

=

RE

*

(Heat Rejection Factor)

What is the heat rejection factor?

kW. Figure 4

Total H ea eatt o f R eject ejection ion Formulas

Commercial HVAC Equipment

Turn to the Expert S

2

 

CONDENSERS

ND COOLING TOWERS

THR reflect reflect s the work done by th thee co mpr mpresso essorr as well as the eva evapor porator. ator. THR can be ex expr presse essed d in Btuh equal ual to 1000 Btuh. tonss, or MB tuh. One MBtuh i s eq ton Where refrigera efrigerant nt is use d to coo cooll the motor tor,, such as in a herme ti c-t -ty ype co mpr mpresso essorr des esiign, added hea eatt the heat from the mot motor or losses) al al so beco ecom mes part o f th thee THR in thee con th cond denser ser..

Kilowatts

Heat Rejection Factors H ea eatt rejection factor is a multipli multipliee r applied to th thee coolcoo ling capa apaccit ity y to find the co nd ndee nse r total hea heatt of rej ec ection tion .

_ h_e_n_a_c_h_i_ll_er

The amount of hea eatt added to the th e coolin coo ling g ca capa pacc it ity y to arrive arri ve at th thee THR THR for an y give ven n application is a fun funcction o f thee co mpr th mpresso essorr efficienc fficiency y and th thee condenser coo coolin ling g method air air,, wa watte r, or eva evapor poraati tive) ve) cool coo led. As an exa exampl mplee, co mpr mpress essors ors use sed d in HVAC H VAC equipm eq uipmeent t ypic picaa ll y ha have ve a of

full load hea heatt rejection factor in the the ran range ge 1. 15 to 1. 25. Water-cooled screw and ce ntrifu ntrifuga gall co compr mpress essors ors are very ffi cie cient nt,, so th they ey t end to ha have ve hea eatt rej ec ection tion factors betwee etween n 1. 15 and 1. 18 . Co mpr mpresso essorrs use d in air-cooled appli pplica cation tionss t ypi pica call y ha have ve heat rej ec ection tion factors closer to 1. 25 . Thi hiss effi fficciency is a function thee satur th s aturaated condensing t emp mpera eratur turee, which is lowe owerr fo fo r wat watee r-cool r-cooleed chill hillee r co compr mpresso essorrs.

of

Us ing Usin g a valu va luee of 1.1 7 as an exa exampl mplee for a water-cooled chill hilleer, for every ton 1 2,000 Btuh) r  fri ge geration ration effec effectt , the load on th thee wate waterr-cooled co cond ndeenser would be: 12,000

1.17 = 14 ,040 Btuh hea 1. eatt re r ej ec ection tion for eac each h ton

A hea eatt rej ec ection tion factor of esult ultss in 15, 000 Btuh 1. 25 res hea eatt rej ec ection tion per per ton of coo cool l 1. 25 = 15,000) . ing. 12 ing. 12,,000 Co nse sequ queentl ntly y, 15,000 Btuh pe r coolin coo ling g ton was use d for man many y ye ars as repr prese esentati ntative ve o f all chill hillers. ers. For mod modee m water coolled chillers coo chillers,, howeve oweverr, thi thiss valu va longerr acc accurat uratee du duee lu e is no longe to effic efficiien cy impro improveme vement ntss .

of

coolin coo ling g ca cap pac acit ity y

A multiplier that is used to quickly find he condenser total heat of rejecti reject ion

Typical Water-Cool Water-Coo led Condenser Applications= 1. 1 . 15 to 1 .18 Typical AirAir-Cooled Condenser Appli cati ons= 1 25

Cooling Tons

Cooling Tons

Example: 100-ton water-cooled chiller has a condenser total tota l heat of rejection of

1 17

1

tons

=117 tons

Figure 5 Typical Typ ical H eat Rejection

Commercial HVAC Equipment

ac tor torss

Tu rn to t h e E.xp   rtS

3

 

CONDENSERS AND COOLING TOWERS

Condensers Cond ondeense serrs remo move ve h eat from the th e r efri frige geration ration sys systt em . Lik Likee th thee ev apor aporaator tor,, th thee cond ondee nse serr i s a h eat trans tran sf er dev eviice. Heat He at from the th e hi hig g h-t h-teemp mpee ratur e, hi g h-pr h-press essur uree refri ge gerra nt va vapor por is is tran transs f err rree d to a hea h eatt-aab sorbin orbing g m e dium air or w at er) th that at passes over or throu throug g h th e co nde den nse serr . Co ndee nse Cond serrs do th three ree thin things: gs: des esup uper erh h ea eatt th e refr efriige rant gas , cond ondeense th thee h ot re fri ge ger r a nt gas into a liquid liquid,, and sub ubccool the th e liquid re r efri ge gerra nt

I ir-Cooled Condenser

J

• Condensers remove heat from the refrigeration system • Condensers are one of the four basic refrigeration cycle components • Their ma in functi funct ion is to condense the hot refrigerant gas into a liquid Figure 6 Conde onden n ser Definiti Definition

Cond ondeen se serrs ar aree on onee of th thee four bas ba si c r efri frige gerration compon omponee nt ntss . Th e oth otheer three thr ee a re th thee evapo ev apo rator , c ompr ompress essor or,, a nd m et erin ring g dev eviice. Th Th e m et e rin ring g dev eviice shown in F igur uree 7 is i s a th thee nn nno o stati taticc exp ex p a nsion va vallve.

Refrigeration

ycle

Thermostatic Expansion Valve

l

-

G

Evaporator Refrigeration Effect)

® Compressor Work o Compression)

1

2 = 3 Total Tota l Hea Heatt of Rejection) Rejection)

Figure 7 Con de den n se serrs reje rej ect

um

to

the

e heat f rom the evaporator nd

e co c ompr essor ssor  

Commercial HVAC Equipment

ExpertS:

4

 

CONDENSERS AND COOLING TOWERS

Water Cooled Condensers Water-cooled condensers emplo y water as th thee condensing medium . Most water-cooled con dens e r sys system temss rec ecirculat irculatee th thee wat wateer throug throu gh the condenser th theen out to a cooling tow towee r, which then rejects th thee hea eatt to th thee atmosphere.

Once Thru versus Recirculating Syst ems empl Syst employi oyin ng water-cooled cond en sers ma may y be classified as once-thru or waste waste   water systt em s or rec sys ecir ircculatin ulating g water sys systt ems. In the th e pa passt , th there ere were man many y water-cooled co cond ndeen se r applications that utilize utiliz ed wate water suppli upplieed from city water mainss or from natural so urc main urces es such as ri ve verr s, lak lakes es,, or wells. These did not rec recirculat irculatee th thee water. The condenser water in these these syst system s pa passe ssed d throug throu gh the th e condenser onl y once and was wasted to a sewer or returned to th thee so ur ce. Thi s res ult ulteed in unnecessary unn ecessary water cos costt s and th thee rmal po pollution llution.. Today,, thi Today thiss application is not u se d nearl y as often as a recirculating systtem . sys With th thee ever incr increeasin ing g quantity quantit y o in insstallation tallationss, th thee de  mandss on wa mand watt er di stribution and treatment sys systt ems beca ecam me unr unrea ea so nabl nablee and virtu irtuaall y all municipalities municipaliti es now ha have ve ordi nanc nces es controlling the use o city water for co cond ndeen sin ing g purpo purposes. ses. These ordinanc ordinances es t ypica ll y require a water conservation dev evic icee , such as cooling ower r,oso may ma y be arecirculat ec irculateetowe d thro thr ughwater thee co th con n dense r and u se d repeatedly.

Once-Thru Chiller with Condenser Source o water river)

Optional Valve Water to waste or source

Pump • Much less common due to environmental concerns • Water

s

sent to waste

• Large consumption

o

or

returned back to source

water

• Source example: river , lake, well

Figure Once Thru Water  Cooled Condenser Sy Sy st steem

Commercial HVAC Equipment

Turn Tu rn to th e ExpertS:

5

 

CONDENSERS AND COOLING TOWERS

Water

Requi rement Calculation for Recirculating Systems Systems

In order to explain so me concepts involving recirculating water-cooled condenser systems, we should now discuss some basic information on cooling towers since they are almost always part o f the water-cooled condenser system. A separate section

of

Water-Cooled

this TDP tower is dedicate dedicat ed to cooling towers s where they th ey will be covered in detail.

Condenser

r

3 gpm/ton

When a water-cooled condenser uses recircu lating water from a cooling tower tower,, the te tem perature of the water leaving the tower on a design   da design day y is typically typicall y 85 o F in much o f North America. This is becau se much o f North America

• Water is reused F ig_u_r_e_9

has a de sign wb (wet bulb) temperature of

Typical Recirculating Water Cooled Condense r System

Condenser Water Pump

Cooling Tower

• The water-cooled condenser is typically part

of

a water-cooled chiller

• A cooling tower rejects the condenser heat to the atmosphere • Flow rates and temperatures are industry standards for North America • Piping and pumps circulate water

78 o F Cooling towers are often sized for a 7 ° F approach (difference in leaving tower water and entering wb) . A 7 ° F approach result resultss in an efficient tower selection at a reaso nable first cost.

we use 14 ,040 Btuh as our total heat of rejection (1 (12 2 ,000 * 1.17) for a typical water-cooled condenser pe r one ton (1 (12 2 ,000 Btuh) refrigeration effect, effect, we can solve for gpm and it will reflect the gpm per one ton of cooling for a recirculating water-cooled condenser system. If

Capacity or load (Btuh) = 50 500 0 * gpm *ris e The constant 500

= 60

minutes per h o u r 8 33 pounds per gallon

In this example example,, there is a 2.6° F approach. Approach, Approach , as it pertains to water-cooled condensers, condensers , is the dif ference between water leav eaving ing the condenser and the condensing tem perature of the refrigerant. t is not the same approach as described above for cooling towers This approach is rep resentative of a high quality quality shell and tube-type condenser as used on larger water-cooled chiller chillers. s.

of

water at 60° F

• Typical water-coo water-cooled led condensing temperature

97 .0° F

• Typical water lea leaving ving the condenser

94.4 o F

• Typical difference between water leaving the condenser and condensing temperature

2 .6 ° F

• Typical entering condenser water from tower

85 .0° F

• Water rise in the condenser

gpm/ton

=

14,,040 Btuh 14 9.4

* 8 .33 * 60

14 ,040 9 .4

9.4° 9.4 °F 14 ,040 1.17 * 12 14, 12,,000) is the THR for 12, 12 ,000 Btuh 1 ton) for typical water-cooled chillers

* 500

3 .0 gpm/ton

Figure 10

Recirculating Water Water   Cooled Cond Co ndenser enser Flow Rate Ca lcu culati lation on

Commercial HVAC Equipment

Tum to the ExpertS

6

 

CONDENSERS AND COOLING TOWERS

Solving for gpm gpm,, we arrive at three gpm per ton o f cooling for a recirculating (cooling tower) system. This is the ARI (Air Conditioning and Refrigeration Institute) standard gpm for a water cooled condenser cond enser on a chiller. On once-thru systems, systems , the gpm in circulation is typically typicall y less than with recirculating systems. This is because the entering condenser water temperature from the lake or river is lower than 85° F As an example, example , with 75° F entering condenser water temperature temperature,, the flow rate works out to be 1 45 gpm gpm//ton ton,, but mos mo st municipal codes still find this unacceptable water usage.

The 85 ° F temperature of the water exiting a cooling tower is a function o f the entering wet bulb temperature o f the air. This design design   wet bulb varies based on local climate. Cities like Houston in humid North American areas may may use 86° F o r even 87 ° F as their tower water tem perature for condenser selection. In some Asian cities, cities , due to even higher design wet bulb temperatures , as high as 90° F has been used as the tower water temperature entering the condenser. This is often referred to as ecwt (entering condenser water temperature). If in doubt as to yo ur local design wet bulb bulb,, consult

Good Tower

limates

with yo ur local cooling tower supplier. Wet bulb te tempera tures for various locations are also shown in th thee Carrier Load Estimating System Design Manual and in the AHSRAE Fundamentals Handbook. RI Conditions

The 3.0 gpm/ gpm /ton ju st derived is is a tradi tional condenser flow rate and is utilized by by ARI as the basis basi s for standardization for wa ter-cooled chillers.

• 3 gpm/ton

ARI incorporates chiller certification programs,, develops standards programs standards,, and certifies manufacturers ' software and chiller products within specified tolerances o f performance . Here are the ARI conditions for rating water cooled equipment:



• 3 0 gpm/ gpm /ton in th thee condenser water loop

in

condenser

• 0 .00025 fouling factor • 0.0001 fouling factor

in

in

condenser

cooler

° F ECWT Entering Enteri ng Condenser Water Temperature)

8

• 2.4 gpm/ton

in

the chilled-water loop 1 ooF rise)

• 44 ° F leaving chilled-water temp Figure ARI Co Conditions nditions fo r Water Water  Coo Cooled led Chill hille ers

• 0.00025 fouling factor in condenser • 0 000 1 fouling factor in evaporator • 8 °Fecwt • 2.4 gpm/ gpm /ton in the chilled water loop • 44° F leaving chilled water temperatur temperaturee • The units for fouling are are::

h

*

2

oF / Btu

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7

 

CONDENSERS AND COOLING TOWERS

Water Consumption and Makeup Quantity

Makeup wat wateer requirements for a rec ecir ircculatin ulating g syst system can al so vary du e to geog geograph raph y How ever for purposes purpo ses of makin making g a co compari mpari so n, we will approximat approximatee 1.5 3 gpm pm//ton = .04 5 gpm pm//ton o f th thee recircul recirculaate d fl ow rate mu musst be made made up . A once-thru water-cooled con denser u ses 1 45 gpm pm//ton ton,, approximately,, whil approximately whilee a cooling tow toweer, usin ing g th thee ev evaporati aporative prin princi cipl plee, u ses only onl y 0.045 gpm pm//ton . I t is apparent from thi thiss comparison that a coo cooling ling towee r reduc tow duces es water consumption as much as 97 as compared to con den se rs using water on a once-thru bassis ba That is why coo coolin ling g towers are use sed d in the th e vast maj orit rity y of open wa ter er-cool -cooleed condenser appli pplica cation tion s

Once-thru Condenser System

1.450 gpm/ton

Cooling Tower

0 .045 gpm/ton

Water

Savings

= ·450 -

0 ·045   100 1.450

= 96 .9

Lost by evaporation and other factors

Figure

12

Water Consumption Comparison: Co mparison: Once-t Once-th h ru versus Coo Cooli ling ng Tower

Construction and Types ofWater Cooled Condensers The majorit majority y of water-cooled condensers in u se tod today ay may be clas sifi ifieed as: • Tube-in-tube • Shell and coil • Shell and tube • Brazed-Plate typ typee

Figure

13

Types o f Water-Coo led Co nd ensers Photos : S hell and Tu be Courtesy o f Sta Photos: Standard ndard Refr Refriigerat geratiio n ; Shell She ll and Coi l  Tube -in -in-Tube -Tube and P lace Type: Co urt urtesy esy o f PI Heat Transfer

tfM

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8

 

CONDENSERS AND COOLING TOWERS

Tube in Tube

The tube-in-tube conden ser also called a coaxial con denser when wrapped in a circular circ ular fashion) consists o a tube-shaped condenser com posed o a s ri es o copper water tubes in inssid idee refrigerant tubes. The passages that the refrigerant flows through are small . These condensers tend to be used on packaged prod ucts in the small er tonnage ranges such as water so urc urcee hea eatt pumps. Tube-in-tube con densers are not mec echanicall hanically y cleanable because o their con figuration.

Used in small packaged products 5 tons or less Tube  i n tube condenser in small water cooled

Figure 4 Tube in Tube Condenser P hoto: Tub e  in Tube: Courtes Courtesyy of AP APII Hea t Tra Tr a nsfe sferr

Water Water   side must be kept clean and strained

Refrigerant in outer tube

/

Water outlet

/

Small passages Figure 5 Tube in Tube Cross Section

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9

 

CONDENSERS AND COOLING TOWERS

Shell and Coil

The shell and co il co cond ndeen se r co con n si st s of a cy cylindric lindricaal st ee eell she ll cont ntaa inin ining g on e o r mo re co il bundlees o finn bundl finneed wa w ate r tubing Th Thee coil is is continu continuo ou s so int intee rm ediate jo j oint intss ar aree eliminat liminateed . Con Con den se rs o thi thiss t ype are ava vail ilaabl blee for both horiz horizontal o r ve verti rtica call she ll ar- Available n vertical or horizontal range gem ment. configurations Continuous coil construction Th e cond ndeen se r wate wat er fl ow s into thee tub th tubes es and ho hot gas from fro m th thee com presso pr essorr fill s th thee sh ell. Cond en se d refri frige gerant rant dro drops t o th thee b ott tto om of th thee sh e ll wh whee re a liquid sump is is pro prov v id ed Thiss type o cond Thi ndee nse serr i s ge gen n e rall rally y limiteed to sys limit systt ems of o f about 2 0 to t on s or less . Clea eanin ning g th thee tub tubes es i s accom plisshed by pli b y ch ch emi miccal me m ean s

/

Figure 6 Shell She ll and Coi Coill Photo

n dense denserr

Court Cou rtesy esy o f PI Heal Trans ransfe fe r

Shell and Tube

The shell and tube tube co nd ndeen se r cons cons ist s of a cy cylindri lindrica call sh ell cont ntaa inin ining g a numbe numbe r o strai traig g ht tub es that are are supp uppo ort rteed b y tub tubee sh s h ee eett s at eac each h end o th thee sh ell   as w ell as int intee rm edi at e supports A wat wateerbo rbox x is att ttac ach hed t o both end Provides tubee shee tub eett s Th e w ate rb ox is th thee area at th thee end o th thee shell and tube tube con dense r that prov prov id es access t o th thee tubes. tub es. Th Thee fi eld piping piping co conn nnec ectt s to the the cond ndeen ser at the th e w at erb ox conn conneec ti ons The tio The wat wateerb ox may ma y ha hav ve a bo lt lteed removabl vablee pi piece ece call ed th e w a t e rbo rbox x cove overr or hea h ead d.

ost

Efficient Design

Used n larger equipment 50 tons and over)

Water

n

tubes

Water--side tubing is Water mechanically cleanable

Figure 7 She ll and Tube Conden Conde n ser Photo hoto   Cou Courrtesy of Standard Refriger Refrige rat atiion

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10

 

CONDENSERS AND COOLING TOWERS

Wat e r flows Wate flow s within the th e tub tubes es and re refri frige gerrant va vap p or fill s th thee spac pacee betw twee een n th thee she ll and the th e tubes. tub es. At A t the th e bott botto om of the th e sh s h e ll is a des desiig n to t o co ll ec ectt the th e co nd ndeense d re r e fri ge gerrant. A maj maj o r ad adva van n tage o thi thiss t ype o ndeen se r is th thaat th thee co nd tubes tub es may ma y be cl cle ean ane echaniec hanicall ally y e dby rm em ovin ov ing g thee w at e rbo th rbox x cover cove rs or h ea ead ds on th thee end . Cle leaa nin ning g b y me ch ani nica call m ea ean ns redu duces ces foulin uling g and efficc ien cy in c reases effi i d on e reg ul arl y .

I 3 pass unit shown

~

Condenser

Hot Gas from Compres Compressor sor

l:==::::;r:::====='-1  

Section Water

n

Subcooled Liquid to Evaporator

• Baffle separates bottom of condenser • Refrigerant gas condenses in top of condenser • Liquid drains into subcooler section below baffle • Coldest water wate r enters subcooler and liquid refrrigerant s subcooled below saturation ref

Figure 8 ross Section o f Typica l Shell an d Tube

on enser

Sh ell and tube tub e cond ndeense rs ar aree u se d on mos m ostt wa w at er chille chill ers ab ove appr a pprox oximat imatee ly 50 ton s Th They ey o  er a fl exi exibl blee , m a intain intainaa bl e des esiign that all ows f o r tube tub e cl eanin aning g and tube tub e repla ce m ent on sit itee . These t ypes of of co nd ndeense rs ar aree found o n t he lar larges gestt ce ntrifu ntrifuga gall and sc scrrew chill c hillee rs Marine wat Marine watee rb ox co nn nnee cti ctio o ns a re sh own in the th e fi g ur uree . Th ese all allow ow fo r ac ac cess t o th thee tub tubes es witho with out turbing g di sturbin thee th ld-insst a ll e d fi e ld-in nnec ecti tio on co nn p1pm g. Fo r m ore in o rma rma-tio ti on rega egardin rding g m ann e watee rb ox wat nnee ction ctionss, refe r t o co nn TDP DP-623 -623 , Watee r Wat Coolled Chill Coo hillee rs .

Marine Type Waterbox Connections Blank End

Figure 9 Large She ll and

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ondenser

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11  

CONDENSERS AND COOLING TOWERS

Brazed Plate Heat Exchange rs

Brazed-plate heat exc exchan hange ge rs ar aree use sed d as a s condensers on chill chilleers up to approximately approximatel y 60 ton s Often mec echanical hanical cl ea eaning ning is is re quireed in large quir largerr siz es so a shell and tub tubee t ype condenser is use d . Brazed-plate co nd ndeen se rs consist o • Smaller capacity design a se ri es o plat plates es bra ze d toge tog eth theer up to approxi approx i mately 60 tons) with every seco second nd plat platee turn turneed • Good efficiency for the cost 180 degrees. So me plat platee heat ex • Not mechanically cleanable chan hangers gers are mec echanicall hanically y fastened to ge geth thee r in insstea ead d o • Require clean, clean , strained waterflow braze br aze d Brazed-plate condensers re quiree cl ea quir ean n waterflow or else th they ey can be dama damage ged d or plug plugge ged d . They gen ge nera ll y requir quiree very fin finee strain ers and do not work well i the condenser water syste system m is very dirty dirt y . Since th they ey are su sce sceptibl ptiblee to fouling   th fouling they ey are bes estt applied with a close d-circuit condenser water systtem sys

• Also used as evaporators

Figure

2

Brazed Pl P late Heat Exchanger Condensers Photo: Courtes ourtesyy of AP APII Heat Transfer

Brazed-plate condensers are much small malleer than the th eir she ll and tub tubee counterpart is They ma may y be less th thaan one third the th e size o an equivalent she ll and tub tubee heat exchan exc hanger. ger.

Closed versus open circuit

Brazed-plate hea eatt exchangers are ex cellent for jobs job s requmn qumng g compact condensers.

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12

 

CONDENSERS AND COOLING TOWERS

Fouling Factors Fouling or scaling on the waterside o f condenser tubes is an important factor in water-cooled condenser selection. Fouling, Fouling , or scaling scaling,, is caused b y the building up o f mineral solids solids,, which precipitate out of the water water,, or b y entrained so solids lids,, such as silt, silt, which deposit on the tube surface. Typically,, water Typically cooled condensers are selected in the range betw betwee een n 3-12 feet p e r secon second d water velocit ve locity y in the tubes. tub es. At lower velocities velocities,, increased fouling is po ss ible as with low cooling to tow wer flow and with once-thru systems. This is be cause the scrubbing action of more turbulent flow IS diminished and sediments se diments can de posit more easily on thee tube walls. th

Fouling is the build-up of deposits on tube surfaces and depends on the quality quality of water i.e i.e.., dirty river, river , etc etc..) • Expressed as a number 0.00025 or 0.0005 or 0

• Minimal in evaporators - Closed piping circuit • Greater in condensers • ARI sets at 0 .00025)

- Basis of chiller ratings for condensers

• Lower water velocities result in higher fouling rates

Refrigerant

Figure 21 Fouling Scalin Sca ling g Resistance Resistance))

Incn:ased Incn:ase d fouling pot poteential mus mu st be considered i f th thee condenser water flow is reduced for ex t end ndeed periods period s o f time from traditional flows. An exa example mple o f this would be a low flow 2 gpm//ton condenser water sys gpm system tem operation. In these systems, the potential exists for greater foul ing than ARI standard three gpm /ton sys systtems . In low-flow systems, there is a higher rise so the water exiting th thee condenser is warmer. Heat also contributes to greater fouling . The rate

Fouling

o f tube

dds

fouling is is also a function

resistance

thee o f th

quality

o f condenser

water.

For cooling tower applications applications,, ARI Standard 550/ 550/ 590 for vapor-compression chillers utiliz utilizes es a fouling factor o f 0.00025 in the th e condenser as a ba bassis for chiller rat rat mgs. Des ign ignee rs should not arbitrarily as sum umee excessive fouling factors such as 0 .00 1, thinking the they y have a robust de sign b y doing so. Excess Excessive ive fouling utilized as a basis o f chiller selection ma may y result in ad ditional heat exc exchanger hanger area with a high highee r first cost.

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13  

C OND ONDE E NSERS A ND COO COOLI LIN N G TOW TO W ERS

e lection of a fouling factor provides for a certain amount of scale bui builldup dup,, which is then taken into account in the selection of the condenser. ftoo low a va vallue is selected selected,, frequent clean ing of the condenser tubes may be required .

Normally Normall y, a fouling factor is chosen based on ex perience for a gi g iven area operating hours hours,, water quali quality) ty) so that the chemical or mechanical cleaning o f tubes is

Fouling

required schedule not moremay thanbeonce a year. freq uent frequ cleaning practical andAis more dependent on the actual job conditions. On larger chillers, chillers , the control panell may contain a feature that per pane mits di disp lay of the difference in leav eavin ing g water temperatu temperatu re and refrig erant temperature approach or approach leaving temperature difference) in the condenser and cooler. This is va valluable because the operator can see if the temperature difference has incn:ased from the initial job commissioning , often a result o f increased or exces sive fo fou u ling. The approach is indicat ndicatiive of heat exchanger effi Cency.

RUNNING TEMP CONTROL LEAVING CHILLED WATER EVAPREF

CHWOUT

CHWIN

55 1

44 1

COWIN

40 7 CONDREF

CDWOUT

85 0

94 4

OIL PRESS

98 1

OIL TEMP

21 8

MTR MPS

132 9

93

An excessive difference could mean in creased fouling in the condenser 3° F Normal)

Figure Water Cooled Chiller Control Panel

This value will increase as tube fouling increases. f it increases to the point of exceeding the lift capabi li t ies of the compressor compressor,, operational problems may occur. In selecting a water-cooled condenser, condenser , a good rec ommendation for comfort cool coo ling appli app licatio cation n s is to u se the current ARI val va lues for fouling in cool coo ler and con denser.. As of this writi denser writ ing ng,, these va vallues are:

Regular maintenance treatment programs

Tum

to

the

ExpertS:

nd

water

0.0001 h

*

0.00025 h

2

*

* °F I Btu cooler fouling 2

factor

* °F I Btu condenser fouling

factor

Commercia l HVAC Equipment

14  

CONDENSERS AND COOLING TOWERS

Tubing Materials Wh en con Whe onssid ideerin ring g effi ciency cy,, th thee manufacctur manufa tureer ' s standard co copp pp er tub g is th thee bes t choi hoice ce in the th e cond c ondeense r. ing in Standard tubing tubin g for a ce ntrifu ntrifuga gall chill hillee r is shown s hown he he re and is oft ofteen fin fin  ne d or enhanc nhanceed  int inteern rnaall y and extte rnall ex rnally y to promote promote heat trans tran sfe r .

Internally and externally enhanced condenser tubing

Enhan nhanccin ing g impro improves ves th thee refri ge gerr ant coe co effi cient o f heat tra tran sfe r and the th e wateersid wat idee heat tra transfer . Figure

3

Larrge Water Coo led Condenser T   ing La

On large large r wat er cool ooleed centrifu ntrifug g al s and scr creew chill hilleers, th theere are oft fteen variou ariouss choic choices es for non-sstandard tubing nontubin g ba se d on applic appli cation re requir quireem ent ntss . On small maller er recipro rec iproca catin ting g and sc scroll roll chill hill ers, th these ese tubin tubing g choi hoices ces Application Tubing Materia Materiall Cost factor do not ty t ypic picaall y exi exi st. Fresh Water Copper 1.0 Glycols Copper 1.0 Corrosive ater Cupro nickel 1 .3 Special Process Stainless steel 2 3 Sea Water Titanium and Cupro nickel 3 4 Figure 4 Water Coo led Co   en ser T  ing C ost Factor Facto rs

Effects

o

ntifreeze

Antifreeze is som Antifreeze omeetim times es use d in th thee recir circcul atin ting g condeen se r loop in cond insst ead o f fr fres esh h wate wate r for purposes purpo ses of freeze fr eeze prot protec ection. tion. The The u se of antifr antifreeze eeze ver ve rsus fr fres esh h watee r will ffec wat ectt th thee cond condeen se r wat er pr press essur uree drop drop,, flow rate rate, and capac capac ity.

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15  

CONDENSERS AND COOLING TOWERS

Figure 25 shows the Figure th e effects o f using propy prop ylen lenee g lycol in the th e cond condeense r o f a t yp ypical ical wate water cooleed centrifugal chille cool chill e r . As th thee perc ent o f gl gly ycol incre increases ses   the effect on the th e effici fficieenc ncy y is shown hown.. The effici enc The ncy y is not af fectee d that much in thi s fect particular example. How Howeeve r  it iser-cool important toe r sto eleere el th wat wate ed chille chill rcte flthe ecte the ex exact act pe pe rc rceent of gl gly ycol col   if an y use d in the th e cond condeen se r. If thee percent changes th changes   a rese r esellec tion should be be don donee as th thee componeent compon ntss in the th e chill chillee r ma y be af fe ct cteed.

ia 0.5992 ¥

0.4

100

75

50

25

Full Load Figure 25 Effects o f G ly o l in in

Condenser Pass

e

o nd enser

rrangements

Pas ses ar Passes aree defin fineed as th thee numb numbeer o f tim times es th thee wat watee r trave traverrses th thee length o f th thee cond condeense serr prior to exiting . Wat Watee r-cool r-cooleed conde cond en se rs ar aree oft ofteen of fe red in one on e  two two   and three three -pa -pass ss arran arrange ge m ent ntss . Th Thee numb numbeer o f passes i s normall normally y relat lateed to maxi• Low Pressure Drop, Drop , mum allowable allowabl e tub tubee ve velo ity y lo cit One-Pass • }AREA= A Low Rise or maximum allowable allowabl e pres es-sur uree drop re requir quireement ntss . A watee r-cool wat r-cooleed conde cond ense serr with a Medium Pressure Drop , twotw o-pa pa ss arran arrange gem ment will be be Two-Pass Medium Rise moree effici mor fficieent than the th e sam amee condeense r with onecond one-pa pass. ss. A three thr ee-pa -pass ss arran arrange gem ment will • ~ ~ A R E A A3 ~ ~ be mor moree effici e fficieent that the th e tw two o Three-Pass _ High ~ ~ ~ ~ s ~ r o r o p : ....._ pa ss ve verrsion of th thee sam amee con den se r. How Howeeve r  th thee pr ess essur uree Figure 26 drop may ma y be too hig hi gh for the the ond on d en ser P ass   hig hi gher pass. pass.



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Commerc ial HVAC Equipment

16  

CONDENSERS AND COOLING TOWERS

Selection Inputs Wat er cool Water  coolee d con cond densers ar aree alm ost always se lected as part o the wate water cooled chill hilleer or package packaged d air con condition ditioner er Th Thee followi owin ng factors mu musst be tak takeen into account beca ecau u se th they ey affect th thee se lec ection tion o th thee unit: • Ent ntee rin ring g condenser water temp era eratur turee • Fo ulin uling g factor

1. Entering water temperature to condenser on design day 2 . Fouling factor 3

Pressure drop restrictions

4 . gpm 5. Total hea heatt of rejection Also affecting the condenser selection :

• Pr Press essur uree drop

Tubing design Glycol concentration Pass arrangement

• gpm • Total hea eatt

o

rej ec ection tion Figure 27

Selection Inpu Inputs ts for Water  Coo oolled Condenser

Air Cooled Condensers Air cool coo led co cond ndeensers ar aree th thee mo mosst common commonlly u se d condensers mod modeem HVAC syste system m s Aircoolled co coo cond nd en se rs are commonl commonl y appli e d on medium to large commercia commerciall jo job b s Res esid ideential split systtems are al sys also a large use r o air cooled equip• Simplicity due to packaged design ment. They ca can n be u se d in • No condenser water pump and piping multiples multipl es to form systems • Ease of maintenance reaching reachin g several seve ral th tho ousand • Simplified wintertime operation tons o in insstall tallee d capac capacit ity Condens ing Condensin g pr pressures essures and temp e ratur ratures es ar aree hi ghe r for air cool coo led than wate wate rcoolle d co coo cond nd ensers. Thi Thi s usuall y translates into a less effici fficieent refrigera efrigeration tion cycle for the the same same sized system.

Figure 28 Air Cool Coo led C on ensers

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17  

CONDENSERS AND COOLING TOWERS

H ere ar aree som somee ofth e reason onss airair-ccool ooleed co cond ndeen se rs ar are popular popular:: • Simplicit implicity y of in insstall tallaation due du e to a pac packa kage ged d des desiign • Cond ondeense r wate wate r piping piping and cond ondeense r wa wate r pump are are not re requir quireed • Chemi cal tr treeatm atmeent is i s not re requir quiree d be becau se th theere is no cond ondeen se r wate water loop • Ease o f maint mainteenance • Wint Wintee r ope op eration is is simplifi s implifieed sin ince ce th theere is no wate wate r inv in vol olve ved d so fr free ee ze-up conce conce rn s do not exi xisst Man y ye ar Many arss ag o, air ir--cool cooleed cond ondeen se rs we re limit limiteed primarily primaril y to small comm ommee rcial re refri frige gera ra tion syste system m s and room a ir condition onditioneers . N ow they they ar aree u se d far more more oft ofteen than wate water-cool cooleed cond ondeense serrs in the th e H VAC indu indusst ry ry.. The reliabilit The liability y of airair-ccool ooleed producct s for both res produ resid ideential and comm ommeercialial-ssize d proj ect ctss ha hass im prove d compar ompareed to pas pa st des esiign s . Even Eve n when the conde cond ense serr or conde cond ens ing in g unit is is remot motee from the th e evaporator evaporator as in a split sys systtem , compon componeent ntss ar aree prepr e-mat matcched so incompatibilities incompatibiliti es can be avo void ideed .

A ir-Cooled C ondenser v ersus A ir - Cooled Condensing Unit The term air ir-c -cool ooleed cond ondeen se r re refe rs to a hea heatt re r ej ect ctee r coil and fan) without an integ int egral ral com com  pr ess essor or sec section tion . A n air ir-c -co ooled conde cond ensin ing g unit re ref ers to the th e sam e cond ondeense r unit but with a compr ompress essor or sec section tion . The airir-ccool ooleed con con  dense serr has ha s hot g a s inl inleet and liquid line lin e outl outleet conn onnecectionss for fi e ld pipin tion piping g . Th Thee air-coo air -coolled cond ondeensin ing g unit

Air Cooled Condenser

ha s suction and liquid line has lin e co nn nnec ection tionss bec ecau ause th thee hot gas lin linee is fa faccto tory ry in in stall talleed bet\ vee n thee th compr ompress essor or and cond ondeense r coil. A irir-ccool ooleed cond ondeen se rs and cond ondeen sin ing g unit unitss ar aree easy to ins in stall , re quirin quiring g only onl y pow powee r, contr ontro ols, and refri frige gerant rant conn co nnec ection tionss . Mainteenan Maint nance ce is simpl implee and they do not have have to be b e win win

Compressors Figure

9

rlir rli r Coole d C on ond d ens ing vers versu us Unit r ir ir   Coo Coolle

Cond Con d ens nse er

te riz rizeed in th thee fall .

rW tl

1

the Experti

Commer cial HVAC

quipment

18  

CONDENSERS AND COOLING TO W ERS

Their primary disadvantage is that th they ey usually must operate at hi hig g h e r condensing t empera tures than water-cooled condensers or evaporative condensers to keep their physical siz izee reasonable. The following is a calculation showin howing g condensing temperature requirements for a t ypical air coolled condenser: coo condenser : • Inlet ambient) air temperature: 95

°F

• Air Rise:

15 ° F

• Leaving Difference:

15 ° F

9

°F 15°F 15°F

Leaving Difference

15°F 15° F

Refrigerant Condensing Temperature 125 125° °F

• Co Cond ndensi ensin ng Temperature: 125 ° F The hi higher gher cond co ndens ensin ing g tempera tures,, o course tures course,, increase compressor kW input and in increase crease operating costs. One mu must st consid consi der potentiall potentially y higher hi gher maintenance and water treat ment costs for water-cool ed water-coole conden co ndensers sers u sed with cooling towers towers ve rsu s the simpli ci t y o ver conden conde nsers.

Design Air Inlet Temperature Air Rise

air-cooled

Difference between condensing temperature and leaving air

125°F Condensing Temperature

Figure 3 rl.pproximate Design rl.ir Cool Coo le Co n ens in ing g Temperature

The cir circu culation lation o air over an air-cooled co conden ndenser ser is normally normall y provided in an upward draw-thru flow as previ ously shown. The co cond ndeenser surfac surfacee is usuall usually y o the copper tube and aluminum plate fin type as illu illustrated strated.. Fans for air cooled dut duty y, just as with cooling towers towers,, most often are ax ial type. Centrifuga Centrifugall fan condensers are avai availabl lablee especiall espec ially y i indoor placement and/ and/ or ductwork is required.

Subcooling

ircuit

The addition o a separate liquid subcooling circuit to an air-cooled condenser incr increases eases the compressor capacity approximately approximatel y 1/2 percent for eac each h one degree o liquid subcooling. Liquid subcoolin subcoo ling g increases the refrigeration effect, that is Btu, Btu , ab sorbed in the evaporator per pound o refrigerant. Liquid su subco bcoo olin ling g also helps to prevent the flashing o gas within the liquid line lin e . Flash gas is th thee fla flash shing ing o liquid refrigerant into a gas as a result o pressure change. When com pressor capac capacit ity y is mar marginal ginal,, liquid subcoo subcoolin ling g will frequentl y permit use o a sma ll e r comp co mprressor. Subcooling coils are genera ll y sized to provide from 1 to 2 0 degrees o subcoolin ubcooling g . This produces a 5 to 1 percent in increase crease in compressor-condenser capacity at a g iven condensing tem perature.

Commercial HVAC Equipment

Turn to th thee Expe rt S

19  

CONDENSERS AND COOLING TOWERS

The diagram show howss sch schematicall matically y th thee circuiting of an air-cooled condenser with int integ egral ral sub cooling circuit. L iquid from th thee condensing sec section tion is collected in th thee ret eturn urn hea head der t th theen passes pa sses into a separate circuit for subcool ing To obtain subcooling, subcooling , the systtem must be sys be charged with re th e sub-coolin ub-cooling g frigerant so that the circuit is is completely filled with r  frig rigee rant. Additional charge is ; ; ; ; ; ; ;:.;:;:; Saturated Liquid theen adde th add ed according to the ~ ~ ~ ~ ~ ~ Optimum h a r g e ~ ~ manufacturer's charging recom i q u i d : : = = = = = : ~ ~ @ ~ = ~ ~ mendations to fill the th e subcoolin ubcooling g Ensures proper operation of liquid Sight circuit. Air-cooled condenser ratings Air-cool with subcooling circuits are di vided into two categories , Optimum Char harge ge   and Mini mum Charge.  

metering device Adds 0.5 to total system capacity per degree of subcooling

Figure

Glass

3

Subcooling

ircuit

Optimum charge ratin ratings gs are for a sys systtem charged with refrig frigee rant to obtain th thee des ign num ber of deg egrrees of sub coo coolin ling g . In thi thiss case, case , gross hea eatt rej ec ection tion is is th thee sum o f des esup uper erh hea eatin ting g, cond co ndeensing nsing,, and sub coo coolin ling g . Liquid leaves at th thee saturat saturateed co cond ndeen sin ing g temp erat atur ure Minimum charge ratings are tho se obtained when th thee subcoolin subcooling g coil is not charged with liq uid and the th e subcoolin s ubcooling g circuit is u sed for condensing refrigerant. Gross hea eatt re r ej ec ection tion the th en equal qualss thee sum th s um of de-sup erh rhea eatin ting g and condensing of th thee r  fri ge gerant. rant. Th e liquid r  fri ge gerant rant le leav es at the th e saturat sa turateed conde cond ensin ing g t emp mpee rature Minimum charge ratin ratings gs will give hi hig gher va valu lues es o f hea eatt rej ec ection tion than op timum charge. This is beca ecaus usee th thee subc ubcoo oolin ling g circuit occupies condenser surfac urfacee . The hea eatt transfer tran sfer for co nd ensin g is ing much hi hig g her than for subcoolin g How However ever,, th thee co combin mbineed compressor-condenser rating will be be hig hi gher with optimum charge because of th thee incr increase eased d refrigeration effect pe r po pound of refrig e rant circulated.

lacement

Air-cooled condensers are available for eith itheer an in inssid idee or outside location . However However,, the vast majority majorit y are for outside application. In Inssid idee pla place ment often requir quires es a centrifugal fan to overcome cem the re sistanc tancee of th thee inl inleet and di disc scharg hargee ductwork. When in insstall talleed outside outside,, th they ey ma may y be locat locateed on th thee ground g round,, or on th thee roof. Roof location locationss are common for commercial applications. Again, Again , des esiign consideration mu musst be b e gi g ive ven n for hi g he r te t em peratur ratures es as soc sociat iateed with units unit s in insstall talleed on bla blacck roofs in direc directt sunli unlig ght. Th e ve rtical coil condensers should be ori orieent nteed so that the th e pr preva evailin iling g winds for the th e area area,, in summ ummeer, will te tend to he h elp the th e fan produce produce airllow . In addition addition,, field-fabricat ed and in insstall talleed wind baffles baffl es are recomm commeend ndeed for the th e di sc schar harge ge sid idee of th e condenser to reduc ducee th thee wind eff ffec ectt , esp espe cially durin during g cold weather coo coolin ling g operation operation.. The wind effect ma may y reduce th thee temp mpera eratur turee o f th thee coil co il in wint winteer, makin making g head pr press essur uree control difficult.

Thm to th thee Expe rt s

Commercial HV C Equipment Equipment

20  

CONDENSERS AND COOLING TOWERS

Mounting of an outdoor air-cooled condenser or condensing unit indoors is not recom mended. The unit nameplate nameplate ma may y indicate indicate,, "outdoor use only only"" and building inspectors inspectors can question the application. application . I f the area is Ground Mount Application large enough (such as an airplane hangar) there would be little concern about elevation of the temperature in the space from the should rejected However, However , equipment onlyheat. ap plied in its intended location and local inspectors have the final say.

Select placement

re

s

Figure

32

Placement Choices or Air Cooled Condensers

Selection Air-cooled condenser ratings are usually usuall y presented in terms ofBtuh or tons oftotal heat rejec tion or refrigeration effect versus temperatur temperaturee difference difference,, where where:: Temperature Difference (M) = con densing temperature temperature - entering outdoor air temperature .

rformance Inputs]

I

increases, increases thee heat rejection An th ca pacitAs pacity y increas increases es , proportionately. increase in condensing temperature re duces the compressor capacity and increases the power required . Typical input inputss required for computer selection software are: o

entering air temperature

o

total heat o f rej ec ection tion

0

1 1t

llu

disc di scharge harge

Other AJC

R

)

1 1

Untitled e

"f

£.ntAil T . . . ,

:t

Cand t o d e i i n t ~ l e d

Heal Reject Q..aeT

n

iubCool

I I I

Circ:WtA 100..01 100 30..01 30 15.01

if D isc Line Lo if Disc Line Size

r

n.

It

'flfl

nl

.

I I I

25 . o l

Chillet Options

Circ:Wt B 100.. 01 y...., 100

30.. 0 1 30 15 . 0 1 . , 15. .

~ . I

n. 25 .o l h

rsuctians .vicev,_ Cooler f

subcooling amount (typically 15 ° F estimated

1I I

UniiT11111l-:: UniiT11111l-

M

o

-I

line

I tandard

los s loss

(typically 2 oF Figure

33

Selection or Air Cooled Condenser

Commercial HVA HVAC C Equipment

:JI

tl

Turn

to th e

Expe rtS:

21  

CONDENSERS

ND COOLING TOWERS

An analysis o cost cost,, both first and operating, operating , will frequently show that a lar larger ger condenser condenser,, al though higher first cost, cost , can result in better overall economies for the buyer. This is the result o the larger condenser lowering the condensing temperature. However However,, the law o diminishing re turns will prevail. Most air-cooled condensers are selected as part o a sp lit system with selection software as shown in Figure 33 The balance capac capacit ity y between indoor unit and outdoor air-cooled condenser is automatically calculated

Evaporative

ondensers

Evaporative condensers combine the functions o water and air-cooled condensers int into o one design. The hot gas discharged from th thee compressor is circullated through coi circu coill tubes that are sprayed on the outside with water. The evaporative effect o the water on thee tube surface helps condense the th refrigerant gas inside. The net effect when the sprays are operating is to deliver higher system efficiency than a dry dry,, air-cooled condenser condenser.. Figure 4 Evaporative Condenser Pho to to   Courte ourtesy sy of Baltimore Ait  oil C ompan ompanyy

In a water-cooled system using a cooling tower tower,, all the water required for the condenser about 3 gpm/ gpm /ton) is pumped through the coo coolin ling g tower condenser circuit. In an evaporat evaporativ ivee con denser,, only enough water is circulated within the condenser casing to insure a constant wetting denser o the conden condensser coil tubes. The spray-pumping horsepower will be less than that required for a coolin coo ling g tower o the same capacity. However, However, the fan hp will be comparable for cooling towers and evaporative con densers o equal capacity capacity . The make up water requirements are also the same for an evaporative condenser or a cooling tower. Evaporative condensers are de signed for outdoor installation and are avail ava ilab le in horizontal and vertical vertical component arrangements. The sizes offered by manufacturers will vary, vary , but units are availabl availab le in the approxi mate range o 15 tons to over 2000

Tum to the Experts :

Figure 5 Evaporative Condenser with C ondenserless Chiller C ondenser P hoto hoto   Court ourteesy o Balt Baltiimore A ircoil C ompany

Commercial HVAC Equipment

22  

CONDENSERS

ton s o f tot totaa l ma y also a v ~ coole d tra transform e rs or

  c ~ s s

ND COOLING TOWERS

prim ary primary of is t o circ uit uitss in coil s to to cool coo l w fluidss . fluid install ed out ins utssid idee, du ctwo ctwork rk is is not normally normall y v a p o r a

vaporative condensers

platfo rm s

v ~

c o

a r ~

on ro roo o fs or on on

inte e r n to musst f wint mu given give

t i o n

~ ~ z ~

p

u suall y m o u n t pads at at

unit is  is r o b l

g ~ r a n t .

o il  on

q u i r

r a t i

ju st as with c oolin ling g

Evaporative Evaporati ve cond co ndeensers can can be dr draain ineed of wa watter and run as as a dry co il unit

f thaan 45 of th cap ca pac acit ity y is q u i r in wint intee r, it will be be e c ~ s s a t o se lec ectt unit on it itss dry coil ca cap pac acit ity y. Theen Th unit will li ke kelly be in u and co control ntrol of hea ead d pr press essur uree w ith a ir vo lum lumee damp ers or a VF D (Vari abl blee Driive) may be necess Dr ecessaa ry to redu ce unit capac capacity.

_T._'h_e_c 'ap,__ac_i....::ty

As a second second poss possibilit ibility y, should to incc ludin in luding g a remot e ind indoor oor sump sump or loca ocatin ting g unit w ithin a he at ateed a c ~ wh wheere r ~ e z during durin g off cycl es will not be b e a probl probleem. f th thee entir e ntiree unit is is insid ins idee, du ctw two ork is u suall y du ctw two ork should p rovid ed to to

o s ~

on both and di disc sch harge of of the th e unit. Da Damp mpeers in the the durin uring g of f cycl e to g rav aviity fl ow of outd utdoo oorr air. air . Evap orati rative ve cond ondeen se rs a re mor moree on a cost per-ton bas is than a co olin oling g The reaso eason n is i s th thee cost cos t of the coil in th thee evap evapora orati tive ve co nd enser. However owever,, thi thiss ex ex can be a and co nd ndeense r pump ca can n be l i m i n by use of an s e r .

Commercial HV C Equipment

Turn to the ExpertS

23  

CONDENSERS AND COOLING TOWERS

Evaporative Condenser Selection Parameters There are two acceptable practices for selecting an evaporative condenser: condenser: the evaporator ton method and the heat rejection method. A lthou lthough gh both are used and acceptable, the preferred is the heat rejection method. The principle reason is accuracy accuracy.. The evaporator ton method estimates the power required for an open reciprocating compressor and uses this as the basis for selection selection.. The heat rejection method uses the total heat o f rejection rejection.. Evaporator Ton Method: Method: • Select the type erant

of

'

I

refrig-

• Enter the proper evapora tortonnage • Enter the condensing temperature • Enter the outdoor design wet bulb temperature • Enter the saturated suc tion temperature

a

r

Options

I

Figure

-·--

1'

.:1 ]

6

Evaporator Ton M e thod o f Selection S creen Capture

Court Co urtesy esy of of B altimore A ir irccoil C ompan ompanyy

Heat Reject Rejection ion Method Method:: •

Se lect

the refrigerant

used • Enter the specific heat rejection capacit capacity y re quired • Enter the condensing temperature

Design Conditions _ ..

• Enter the outdoor design wet bulb temperature

C ~ T _ . . . .

Selection programs also have the ability to match chill ers that have independent refrigeration circuits due to multiple compressors with dedicated evaporative condens ers.

T

·

1

000 .00

.

_ . ...

.... .. .. ... rn:oo

-

Figure

:J

•f

.,

Sele< tlonRequ -

-

--· 1 ' r--:>

 

.

_

j r >3

-r

7

He  tRejech on Me th od of Se lection Scrree Sc een n C ap tu r e Cou Courrtesy of Ba Ballt imo morre A irco rcoil il Company

Thrn to

the

ExpertS.

Commercia l HVAC Equipment

24  

CONDENSERS AND COOLING TOW TO W ERS

Subcooling

oils in Evaporative

ondensers

Manufacturers of eva evaporativ porativee co nd ndense enserrs can pro prov vid idee subcoolin ubcooling g coils as options. This ge gen n e rall rally y is an exce ll ent and necessary reco ecomm mmeendation whe when matc matchin hing g an eva evaporati porative ve condenser with a packa package ged d condenserless chiller chiller.. Eac Each h degree of liquid sub-coolin ub-cooling g incr increeases th thee refri ge gera ra tion capacity o f a sys systt em by b y about 0 .5 perce ercent. nt. Also Also,, pa paccka kage ged d chillers may may require subcoolin ubcooling g in the condenser to assure that pure pure liquid refri gera gerant nt arrives at th thee chiller mete eterin ring g dev evic icee for proper prop er control. A liquidliquid-gas gas mixtur mixturee at the chill hiller er expan expanssion dev devic icee is not des desirabl irablee and is to be avoid avo ideed . I t should be be noted that so m e manufacturers rat ratee th theeir chillers and compressors with various

deg egrrees o f subcooling. If a co compr mpresso essorr is so rat rateed and a subcoolin ubcooling g coil is not use d with th thee evaporati eva porative condenser condenser,, dera eratin ting g and operational probl probleems could occur. If a subcoolin ubcooling g coil is use d; th thee compressor rating mu musst be b e corr correc ectted for the th e diff differe eren nce in the th e actual subcoolin ubcooling g available from th thee subcoolin ubcooling g coil at job conditions and th thee numb number er of deg egrrees o f sub ubcoo coolin ling g actually in  cluded in the th e compresso compressorr rating. Fan Performance Data Limited airflow dat dataa is provided by th thee evaporati evaporative ve

condenser manufactur manufactureer . Standard Standard hp motor s1zes are ba se d on zero ex extterna ernall stat atiic pr press essur ure. Whenever ductwork is is required required,, it is necessary to qualify qualif y th thee motor and fan selec selection tion in the th e standard unit. Only centrifugal fan eva evaporati porative ve cond con denser unit unitss should should be be considered for ducte ducted applications. The 100 pe rc ent air quantity quantity give ven n for eac each h umt 1s 1s base d on wet coil operation. If thi thiss cfm is exceed excee de d, mo is  turee carryover ma tur may y res esult. ult. The limitin limiting g cfin for dr dry y coil operation is depend ndeent on th thee fan performan rformance, ce, base d on motor hor se pow powee r and noi noise se leve l.

Condenser Economics hus far far,, we ha have ve di sc scu u sse ssed d water cooled condensers usin ing g natural water on a once-thru ba bassis, as well as recirculating water from a cooling tow toweer . We have also descri escrib bed eva evaporativ porativee and air-cooled condensers. et   s summari ze our di sc et  scu uss ion so far. Figur uree 38 show howss th thee effect o f th thee condensing me dium an d co cond ndeensin ing g m ethod on condensing temp mpee rature.

Commercial HVAC

quipment

Rise

(Of

Te m perature (0 F

Leav ing Difference (o F

Tempe r ature ( Of

80

20 20

95 100

5 5

100 105

85

10

95

5-10

100-105

Evaporr ative Evapo Cond 75-78 75-78°° F Wb

-

-

-

-

100 105

Air

95 105

15 15

110 120

15 15

125 135

Condensing Media Once Thru Water Coo li ng Towe r

75-78°° F wb 75-78

Inlet Te m peratu peraturre

(o f

75

Outlet

Figure 38 o ndensing Temperatur Temperaturee versus

o ndensing Media

Condensing

Turn to t h e ExpertS

25  

CON CO N  E N SERS

N

D COOLING TOW TO W ERS

In se lec ectin ting g a compr ompressor essor or condensi condensing ng unit unit,, th thee desig esign ner mu musst assume a t entati ntative ve condens in g or di sc schar harge ge t emp mpeeratur raturee in anticipation o f ba lan lanccin ing g th thee co compr mpresso essorr against th thee condenser. The tabl tablee shown s hown may may be use d to de det e rmin rminee t entati ntative ve condensing temp mpee rature s consistent with the condensing m edium to be be us ed t ma may y be not noteed that condensing t emp mpee ratur ratures es ran range ge from 105 ° F (which is a ty t ypical va vallue for all package packaged d wate wat e r-cool r-cooleed equipment) with 75° F onc oncee-thru wate wat e r to 130 13 0 ° F with 110° 110 ° F condens condenser er air. Figur uree 39 shows a seco second nd ta ble showin howing g th thee effec ectt o f disc di scharg hargee temp mpee ratur raturee on com presso pr essorr r  fri ge geration ration effec effectt and required kW input. As th thee condensing t emp era press essur uree tu re and corresponding pr increases incr eases,, it is apparent that the the refrigeration ca capacit pacity y i s dec ecrreas eas in g and thee th kW//ton kW of refrigeration ffec ectt (RE) is is in creasmg . From th thee tabl tablee, it is app ppare arent nt that the the condensing t emp mpeeratur raturee of th thee compressor has an impor tant influe influ enc ncee on compressor capacity and power power requir quireement ntss .

CAPAC ITY CONDENSING TEMP (°F) TONS

D D

kW INPUT

kWITON

kW//TON kW

100

52 86

100

38 2

72

100

105

52 15

98 6

4

77

107

110

51 41

97 0

42 7

83

115

120

49 84

94 0

47 9

96

133

130

48 10

91 0

53 6

1 15

159

Based on Sc Scrrew Co mpr mpresso esso r , 40 40'' F Suct ction ion R-134a

WATER COOLED

AIR-COOLED

Figure 9 ffec ectt o f

ondensing ondensin g Temp e ratur raturee

Rememb er that sav savin ings gs in water cos costt s lik likee chemical tr treat eatm ment and make makeup mig mi g ht offset th thee increase in creased d pow  r costs of air-cooled condensers. One should not ge gen ne rali ze abo about ut the relative m eri eritt s and costs of a give ven n condensing method as compared to another another.. There are too ma many variabl va riables es in vo vollved such such as outside des esiig n conditions conditions,, availability and quality quality o f water water,, and re re lati lative ve costs of pow poweer and water. Eac Each h situation shou houlld be anal nalyze yzed d on it itss own m erit ritss and th thee bes estt select selection ion sho should uld be be made co ns ist en entt with the th e circum stanc tances es.. Whi Whiccheve everr hea h eatt rej ect ection ion equipm quipmeent cho se n , lowe owerin ring g the co nd ndeen sin ing g t emp mpee ratur turee to the the unit   s optimum optimum,, g iv ives es th thee ma max x imum energy sav savin ings. gs.

Tum to the Experts.

Commercial HVAC Equipment

26  

CONDENSERS AND COOLING TOWERS

Cooling Tower Towerss In a cooling tower sy stem stem   the warm water leaves the water-cooled condenser and is pumped to the top of the tower. This water is then distri buted and broken up into droplets by one o f several methods so that a large surface area ma may y be brought in contact with outdoor air air..

Cooling towers are heat rejecters . They do not rejecters. condense refrigerant so they are not considered condensers.. condensers

Figure

4

C ooling Towers Tower s of Pho Ph o tos Cou Court rtesy esy

lti mo morre A ir irco coil il Compa Compan ny

The vapor pressure of the air is lower than that o f the water so a sma ll percentage o f the water is evaporated. The latent heat o f evaporation for this process is taken from the remaining water water   thereby cool in ing g it The cooled water co ll ects in a sump at the bottom of the tower where it is returned to the con denser to once again pick up the heat load.

From Water Cooled Condenser

Cooling Tower Figure

41

Bas ic C oolin ooling g Tower Operating C har ac acteristi teristiccs Illustrat Ill ustratiion   Cou Courrtesy of Ba Ballt imo morre A  l ·

Commercial HVAC

quipment

o il

Compan Compa ny

Turn to

th

ExpertS

27  

CONDENSERS

ND COOLING TOWERS

Basic Terms Entering Wet Bulb Temperatur Temperaturee Wet bulb te t emp mpee ratur raturee is th thee low lowes estt te t emp e ratur raturee that water can reach by by evaporation evaporation.. Design ent nteerin ring g wet bulb temperature ew ewbt bt)) is the mos ostt important parame param et er in tower tow er selection and should be det ermin rminee d for th thee spec ecific ific climate zone . For man many y areas in North A merica erica,, 78 ° F is comm co mmo on . Co n sult cooling towee r application data from tow manufacturers manufactur ers or ASHRAE for des esiign wet bulb va valu lues es

• Entering et Bulb Temperature is the lowest temperature temperature that water can theoretically reach by evaporation

Figure

4

Entering Wet u lb

Note

e mp mperat eratur ure e

Typically th thee 0 4 perc ent data is use d for des desiig n, which mea ean n s thi thiss va valu luee is exceed excee ded 0 4 perc ent o th thee hour hourss in a year. The perc enta ntages ges refe eferr to to th thee pe rc rceenta ntage ge o 8760 hours hour s in a typical yearr . Th yea Theerefor fore, e, 0.4 pe pe rc rceent me me an s about 35 hours pe per ye ar There is so me variation in eng in ee rin ring g practic practicee . So me enginee rs u se th thee 1 or 2 perc ent des esiign va valu luee, which is th thee ir pe p erso nal preefer pr fereenc ncee . When in doubt, doubt, co nsult with th thee local coo cooling ling tower tower suppli upplieer

pproach Approach is th e diff diffee renc ncee betwee etween n th thee water leav eavin ing g th thee tow tower er and th thee ent ntee rin ring g wet bulb temp mpeeratur raturee o th thee air. Establi Es tablisshm hmeent o th thee appro pproac ach h fixes fix es th thee operatin operating g temp mpee ratur raturee tant param parameet er in de dete rminin rmining g both towe towe r siz izee and cost.

o

thee tow th toweer and is an impor-

A 7° F approach is common in HVAC HVAC beca ecau u se man many y geographic geog raphic regions in North America have a 78° F ew ewbt bt des d esign ign and u se 85 ° F water leav eavin ing g th thee tow toweer

Commercial HV C Equipment

Turn to the ExpertS

28

 

CONDENSERS AND COOLING TOWERS

The clo closer ser th thee approach , the lar larger ger the coo coolin ling g tower and vice vice--versa. In fact fact,, as th thee approach approaches   zero approaches zero,, th thee tower cost and siz izee starts to approac approach h   infinit y. A 7° F approach in ·mo most st cases result resultss in a reaso easonabl nably y pric priceed tower tow er capable of pro prov v idin iding g th thee cooler condenser water re quireed for effici quir ficieent system operation. Larger approach pproaches es ma may y reduce th thee siz e and a nd cos costt o f th thee tow tower er,, but at a hi gher energy cost for the chiller resulting from the the warmer condenser water temp erature. Small Smaller er approaches for a fixe d wet bulb result in cooler condenser water, water , in tum increas increasing ing th thee efficienc fficiency y of th thee chiller chiller.. • Approach is the difference between the water leaving the tower and the entering wet bulb temperature of the air • A 7° F approach is common. in HVAC for systems with 78° 78 ° F entering wet bulb and 85° 85° F water leaving the tower

,-.,... ..}.~;:,.. =--:-:-,-

T I T I T I

r

85° 85 ° F - 78 ° F

= 7 o F) A A . p : - : p : : : r = - o a = c ~ h : t - - -  

Usually Usuall y, the ew ewbt bt will be les lesss than des design. ign. That means the the cooling tow towee r will be capable o f deli live ve ring cooler ecwt. The result is great gre ater er chiller effici fficieenc y.

Figure 43 Coolin Coo ling g Tower pproach

Range Cooling tower ran range ge is thee differ th differeenc ncee in t emp mpera era turee betw tur twee een n th thee water

• Range is the difference in temperature of water entering the tower and water leaving the tower

ent ntee rin ring g the tower and the the water leav eavin ing g the tower.

• An approximate 9.4 - 1ooF range is most common in HVAC applications

An approximate 9.4 to 10 ° F ran range ge is most com mon in HVAC 95 °F inl inleet minus 85 ° F outl utleet is a 10 ° F rang e).

Figure 44 Coolin Coo ling g Tower Range

Commercial HVAC HVAC Equipment

L :: :=: :

:; ;; ::;; J

_ _ lfl_...

Turn to th t h e ExpertS

29  

CONDENSERS AND COOLING TOW TO W ERS

Total

Heat

of Rejection

Total heat of rejection (THR) is the amount of heat to be removed from the circu circullatin ting g wa w a te r within the tower. This consists o f the peak cooling load o f the buil bui lding plu s the heat of th thee com om pressors (work of compression). Manufacturers   eqmpeq mpment selection programs for water-cooled equipm equ ipment ent will calculate the total heat of rejection for the application. This can be used to properl properly y size the tower.

• Total Heat of Rejection is the amount of heat to be removed from the circulat circulating ing w ater within the tower • It is equal to the refrigeration effect plus the work of compression • For water-cooled chillers THR = 1.15 to 1 18) • Cooling Tons

r " " ~ o o . . - ~ i

: :

: = = ~ = n

Figure 45 Total Heat o Rejection

Drift (Windage) Drift is water that is entrai entrained ned in the airflow and discharged to the atmosphere. Drift can vary widely based on tower location and prevailin prevailing g winds . It is approximately 0.001 to 0.002 percen perce nt o f the circulated condenser gpm/ton ton,, that gpm,, so , at 3 gpm/ gpm value is 0.00006 gpm/ton or • Drift is water that gets entrained in the airflow and discharged to 0.006 gallon for an hour the atmosphere full-load operational on a • Drift can vary widely and does not include water lost l ost by evaporation 100-ton cooling tower. • Drift is very small and can usually be neglected in most ca lculations for make up • Drift 0.001istoapproximately 0.002 of the tower gpm

Figure 46 Drift Windage Windage))

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CONDE N SERS AND COOLI NG TOWERS

vaporation

For each pound of water that a cooling tower evaporates, it removes approximately 1050 Btu from the water that remains. The exact value is dependent on water temperature and can be found in the thermodynamic properties o f water under the enthalpy hfg) heading. The more evaporation that takes place, place , the more heat that is removed. Lower entering wet bulb temperatures create a greater evaporative effect. Evaporation rate approximately 1 percent, percent, at 3 gpm//ton gpm ton,, that is 0. 0 1 * 3 gpm = 0.03 gpm gpm//ton.

• For each pound of water that a cooling tower evaporates, it removes about 1050 Btu from the water that remains • A lower entering wet bulb creates a greater evaporative effect • Evaporation rate equals approx im ately 1 percent of the towergpm

Figure 47 Evaporation

Blow-down Bleed) Water contains impurities. When water is evaporated, most o f these impurities are left be hind. I f nothing were done about it it,, the concentration o f impurities would build up rapidly. Blow down o f some of the water is continuous continuouslly required to limit this build up. The blow-down rate re quired is best determined by a water treatment specialist. They are prepared to make the necessary tests and recommen dations for the specific site conditions The blow-down rate de termines the water chemistry chemistry,, or cycles of concentration of the water. This can vary de pending on the makeup water quality,, the treatment program, quality program , and the materials of construc tion of the tower.

J=.

• Water contains impurities and when it is evaporated these impurities are left behind • If no action is taken, the concentration of impurities will build up rapidly

~

• The bleed rate is best determined by a water treatment specialist who is trained to perform the necessary tests and make recommendations

/

~

~

••••••• •• •

• Bleeding off some of the water is continuously required to limit this build up

J

_l



.utu: u tu: ff

_l

Bleed

Figure 48 Blow-down Bleed) Bleed)

Cycles of concentration COC) is a term used with blow-down and is defined as the ratio o f dissolved solids in the recirculating water to the concentration found in the entering make-up wa ter. The higher the COC the lower the blow-down or bleed rate. I f the COC valve is high high,, you have a low bleed rate.

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CONDENSERS AND COOLING TOWERS

Makeup Makeup is the amount of water required to replace normal losses caused by drift drift,, eva evaporation poration,, and blow-down. The

efficiency

of

a

is influenc cooling tower influence ed by all ofthe factors govern ing the rate at which water will evaporate into the air. With that in mind, mind , let et   s look at the various types of cooling towers on the mar ket. First let let   s see how a cooling tower process looks on the psychrometric chart.. chart

• Makeup is the amount of water required to replace normal losses caused by drift, evaporation, and blowblow- down down..

Figure

49

Makeup

Cooling Cool ing Tower Psychrometr ic Plot The cooling tower process can be plotted on the psychrometric chart art.. Let Let   s assum ass umee we have outside design conditions of 95° F dry bulb and 78 ° F wet bulb. For this examp examplle we will use 85° F ecwt and a range o f 10° F for the tower towe r water. The total heat gain of the air equalls the hea equa eatt given up by the water flow. flow. The tower airflow multiplied by the diff differe erence nce in enthalp y of air entering and leav ing the tow tower er will equ equal the water

Water Wat er Leaves Towe Towerr 85°F I

flow multiplied 500, , multi plied by thee M ofbythe500 th condenser water.



"

We can plot the entering air conditions of 95/78° F Notice the air undergo undergoes es sensible cool coo l ing and humidification as it exits the tower at sa saturat turateed severa severall degrees less than the water tem perature of9 ° F . In this exa exampl mplee, our ap proach is the th e traditional 7 o F disscussed earli er for climates di with a design wet bulb of78° F

.50 55

60 .65 .70

.75

80

.85 .90

95

Figure

50

ooling oo ling Tower Psychrometric Plot

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CONDENSERS AND COOLING TOWERS

Types of Cooling Towers Cooling towers are classified ac cording to the method o f air circulation.. circulation

Natural Draft Atmospheric)

Mechanical Draft o

o

Figure Types o

Natural Draft

Induced Draw-Thru Forced Blow-Thru

5

ooling Towers

Atmospheric)

When air circulates b y natural convection convection,, it is i s classifi classifieed as a natural draft or atmospheric tower . through the tower by tower. The capacity o f natural-draft tow ers varies with wind velocity velocity,, as does the drift loss. Outdoor location is re quired. Because o f the relatively relativel y slow air movement, movement, atmospheric towers are inherently large . Atmospheric towers are generally not the type used for standard comfort air conditioning systems because o f their large size and uncertain capac ity.. Therefore ity Therefore,, we will not devote any more time to natural-draft towers in this TDP .

Generally not used for comfort air conditioning applications

Air Inle Inlett .. . .. . Water Outlet

Figure 52 Natural Na tural Draft

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Mechanical

raft

When air circulation is pro prov vid ideed by a fan or bl ower ower,, th thee tower is call ed a mec ech hanic nicaal draft towee r Towers of thi tow thiss t ype are furth furthee r class class ifi ifieed as induc induceed draft or force forc ed draft. draft .

Induced-draft

With induc induceed-draft towe tow ers, th thee fan moves lar larger ger air quantities at higher high er velocities than natural draft ty t ype . This re duc duces es tow towee r footprint co compar mpareed to natural draft towe tow ers Water di disstribu tribu may y be accompli acco mplisshed by spra pray y ti on ma nozz no zzlles or by some t ype of g ravit ravity y base d perforat rforateed di disstribution bas basin in.. So m e manu fact actur ureers pr prov ovid idee spra pray y eliminator liminatorss at th thee air di disc schar harge ge to limit drift losses lo sses .

• Air exit velocity - Like a 5 mph wind - No recirculation - Fan in warm airstream

• Widely used - Crossflow or counterflow design

• Applications: - HVAC chillers) - Clean process

Cool Water Out

Air is drawn through the tower with a fan

Because th thee fan fanss are lo cat ateed in the th e Figure 5 moi st di mois disc schar harge ge air str trea eam m, th they ey should be be mad madee of corrosion-resistant Mechanical Draft - Induced Type Illu str  tion : Co urte rtesy sy of of B altim altimo ore A irc irco o il Co Company mpany matee rial mat rialss such such as as aluminum. Som omee atmospheric tow towers ers,, and almost all mec echanical-draft hanical-draft towe tow ers, contain fill fill,, a mate material that acts to increase increase hea eatt trans tran sfe r and ga gain in max ma ximum ex expo po sur uree of th thee wat watee r to the the airflow. In ye ars passt , fill was primaril pa primarily y mad madee of slatt lattee d lumber lumb er Curr urreent desig esign ns do not u se wood. The hea eatt trans tran s fer surfac urfacee referr rreed to as fill   or wetdeck   is t ypicall y PV wetdeck PVC C (pol poly y v i Fill helps the water gain n yl chlorid hloride). e). maximum exposure to the airflow

Steel redwood

nd ceramic

PVC is the most commonly used material •

Current designs for HVAC tend not to use wood for fill

Figure 54 Coolin Coo ling g To T o wer Fill F ill Pho to

Court Co urtesy esy of B alti altimo morre Air Airco coi/ i/ Co Compan mpanyy

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CONDENSERS AND COOLING TOWERS

Fill is typically available in a fi lm typ typee de sign. The fill causes the water wa ter to spread into a thin film and flow over a large vertical area. This desig esign n is significantly more efficient than the splash type used in the past. Mechanical-draft towers may may be classified as crossflow or counterflow counterflow.. This nomenclature referss to the heat transfer arrangement used to cool the water refer water.. In a crossflow-tower, crossflow-tower, which is most common,, the fill sheets hang vertically common in the tower. Fill heights can be from 2 feet long to over 2 feet. Ur passes through the fill horizontal to

rossjlow towers

w terflow



Fill is located in banks on two sides double inlet)

Figure Induced Induc ed Draft Crossjlow

Double Inlet

Photo: Courte Co urtesy sy o Ba Baltim ltimore ore A ircoil Company

In a crossflow tower tower,, warm water is distributed over the top o f the sheet and flows by by gravity down both sides o f each sheet. The cooling air enters the front face o f the fill and traverses across the sheet horizontall horizontally y at 9 ° to the waterflow, waterflow, exiting through a set o f drift eliminators. In a counterflow tower tower,, the warm water is distributed over both sides o f the fill sheets, which are typically 12 inches tall, tall , and arranged in layers up to six feet high in the tower. The en en tering air moves 18 degrees opposite o f the falling water in an upward di rection,, or counter to the falling rection water. The eliminators in a counter flow tower are mounted above the water distribution system system.. Figure 56 shows a counterflow cooling tow toweer with a blow-thru design, design , which is dis cussed in the next section.

t

Hot Water

Warm Air

ut

In

Figure

6

Forced Draft Counterflow   Tower Photo: Court ourtesy esy o Baltimore Airco Aircoil il Compa Compan ny

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CONDENSERS AND COOLING TOWERS

Forced Force d Draft Blow-Thru Forced-draft towers use a centrifuga centrifugall or axial fan to blow air through the fill . Today, over 8 percent o coo coolin ling g towers on HVAC applications use axial fans. Axial fans conserve energy be cause they require less hor horsepower sepower than centrifugal fans in cooling tower designs . While the axial fan is a less efficient type o fan than a centrifugal , its u se in low static draw-thru cooling cooling tower designs results in lower overall horsepower horsep ower than centrifugal fans. fans. Centrifugal fans in cooling tower design are applied in • Fan forces air the blow-thru configuration. through tower •

Uses centrifugal fans -

High horsepower

-

High static pressure



High entrance velocity



Small footprint



Counter-flow

Wet Deck Surface

- Air flows opposite to water

• Applications -

HVAC Chillers)

-

Clean process

Figure 57

Closed-Circuit Cooling Towers Fluid Coolers)

Forced orced--Draft Blow-Thru Illu stra ti o n

C ourt ourtesy esy o f Balti Baltim mor oree

A closed-circuit cooling tower is an evapo evaporativ rativee condenser except th that at in instea stead d inside the coil. A common Water application is in closed-loop Distribution water so urc urcee heat pump System systems.. The purpose is to systems maintain the water loop benveen a fixed minimum and maximum temperature by stagi stagin ng the spra spray y and fan. A water sensor inst instead ead o a refrigerant sensor se quences the fan and spray spra y stages. Clos losed-circ ed-circuit uit cooling towers benefit from the evaporativ evaporat ivee cooling spra spray y coill co coi conc ncept ept and resemble evaporative condensers clo se ly except for the physical design and circuit ing o the coil inside .

Cool Fluid

o

i rcoil Compa ompany ny

refrigerant,, water or glycol refrigerant

_ _ _

C losedosed-C C ircu ircuiit C ooling Towers Fluid C oolers oolers)) Co urt urtesy esy of Baltimor Baltimoree

circulated

Often used with systems and chillers where a closed condenser loop i s desirable

WSHP

Figure 58 Illu strat tratiio n

is

i rcoil C ompan ompanyy

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C

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36  

CONDENSERS AND COOLING TO W ERS

Closed-circuit cooling towers are also used with water-cooled chillers when a closed condenser water loop is being used. The coil and fan design result in a higher first cost than cool ing towers for the same tonnage. Closed-circuit cooling towers can often be justified based on the benefits they the y suppl y: less maintenance, maintenance , abilit ability y to run dry in winter winter,, less down time, time , and limited fouling if any) occurs on the outside o the tubes where it can be controlled by water treatment. Use o closed-circuit cooling tow ers results in less condenser and piping fouling than with an open cool ing tower.

pplication

of

Figure

59

C lo lose se   C ircuit C ooling Tower Fluid C ooler ooler)) P h oto : C ourt ourtesy esy of B alti altim mor oree Ail Ail·co ·coil il C om  ny

Cooling Towers

Placement When selecting the cooling tower location, location , sufficient clearance should be allowed for the free flow o air to the inlet o the tower and for its discharge from the tower. Obstructions will reduce airflow causing a reduction in capacity



The top o unit discharge must be level with or above any adjacent walls . Small amounts o recirculation can result in a decrease in actual heat rejection capacity

When sel se lecting the location, location , sufficient clearance sho sh o uld be allowed for the free flow o air to the inlet o the tower. tower . Insufficient clearance would necessitate a single inlet tower in this

jq

J

application.. application Obstructions will reduce airflow caus ing a reduction in capacity. causi capacity . •

A 2° F recircu lat atiion can equal up to a 19 reduction in capacity.

Figure

6

P laceme acement nt of of C oolin ooling g Towers

lr

;

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CONDENSERS AND COOLING TOW TO W ERS

Cooling tower loc locat ation ion should be such that the air di discha scharrge will not cause conden conde n sat sation ion on nea earb rb y sur faces or wetting beca ecau use of drift. Before the tower is po sition itioneed, consid cons ider er what iss ssu ues would arise if a plume (visible fog-like di disc scharg hargee) existed . Ge nerat eration ion of a plum plumee de pends on outside co condition nditionss, so is not prediictab pred ctablle . Note the direction of pr preva evailin iling g wind. The tower should be be locat locatee d away awa y from the source of exha exhau ust heat and contamination contamination . Locate cooling towers

PrevailingW

~ b

ind

J

< ( (

Avoid placement discharge couldwhere causeair condensation or wetting on nearby surfaces

( (

( ( ( •

Figure 61 oolin oo ling g Towe r Discharge

o n cern

Each cooling tow Each towee r should be be located and positioned to preve prevent nt the th e introduction of the warm di sc schar harge ge air and thee associated drift into nea th n ea rb y outdoor air intak intakes es and building buildin g op enin nings gs . This drift may ma y contain wat watee r treat m ent chemicals or biolo biolog gica call contaminants, including Legion Leg ioneell a . A lwa lways ys avoid situation ituationss that may ma y allow haz ardous mate rial rialss to ge gett into the th e ventilation sys systems tems of buildings buildin gs .

Effects of Reduced Cooling To T ower

ater Te Tem m perature

There is a limit on how low the temperature o f th thee condenser water ent ntee ring the th e water water-c -cool ooleed chiller can be without without head pressure controls bein ing g requir quireed. For water chill chillers ers,, an ent ntee rin ring g con densee r water temperature o f ap dens As a rule of thumb thumb,, proximately 55 to 60° F is typically water-cooled equipment the minimum minim um acceptab acceptablle at full con denser flow. Below that that,, the minimum minim um differential pressure be tween cool coo ler and conde conden nser ma may y not be maintained and and so m e form o f head pressu press ure contro controll is required.

efficiency is increased approximately approximate ly 2   fo forr every 1o F decrease in enteri entering ng condenser water temperature 85

8

75

70

65

60

Entering Condenser Water Temperature Temperature

ule o f Thumb

All points shown reflect a fully loaded, loaded , 500 500--ton centrifugal chiller

Figure

62

Effects o f Reduced

ooling Tower Water Temperature

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CONDENSERS

Head pressure control

ND COOLING TOWERS

Figur iguree 63 is an exa example mple of the effect on a typical screw chiller of reduc duceed ent ntee ring condenser water temperatures. The data is is for Based on a R-134a screw chiller wi th a leavi eavin ng chilled water t emp mpee ratur raturee of 45 ° F Condenser Entering Water Temp

While cooler condenser water in 80 .0 ce rtain creases chiller effi c ienc ncy y  85 .0 situations can exist where th thee tow toweer wa 90 .0 ter tem emp pera eratur turee will be too co ld fo r 95.0 chill hiller er operation. For in stance   after the system syste m has been off all ni nig ght ht   an earl y Figure 63 morning start-up o f a chiller may require Condenser Entering hea ead d p ress essur uree controls beca ecau use th thee water from th thee tow tower er is be low the th e minimum o f 55 to 60° F

Capacity Tons

Input

Ton

110 .8

71 .6

0 .65

106.3

76 .0

0 .71

101.6

80.4

0 .79

97 .1

86.5

0.89

kW

kW

Water Tempera Temperature ture ecwt ecwt)) Effect

A t ypi pica call way to pro prov vid idee hea ead d press pr essur uree control is to u se a cooling tow toweer bypass b ypass with a thr three ee way va vallve controlled dir direc ectl tly y by th thee chiller hea ead d press pressur ure Refe eferr to the th e control section o f thi thiss TDP for detai taills

Hydronic

ree Cooling

Hydronic free coo coolin ling g is often ofte n u se d in sys systtems th thaat do not incorpo incorp orate an airside free cooling cycl cy clee but have a coo coolin ling g tower. In fall and sprin pring g  th thee wet bulb temperature will be low lower er than the the

Heat Exchanger

summ ummeertim rtimee periods. The cooling these ese low lower er wet we t bulbs bulb s tower can use th to suppl upply y cold water to the th e buildin building g  allowing th thee chiller to remain off lin linee as lon long g as po ss ible

To and from

Cooling Tower

When return cond en ser water form th thee cooling tow toweer is sufficie fficientl ntly y cold co ld   it is di verte verted d throug throu gh a plate frame hea eatt exc exchan han ge r wh whee re it cools water in the th e chilled water loop loop   and all chill hillers ers in the the syst system are turn turneed off. Because condenser and chilled water st rea eam m s do not mi x   foulin fouling g o f th thee

Figure

chill hilleed water loo oop p is i s not a concern.

Hydronic Free Cooling Cycle

Building Return Water 6

P hoto hoto:: Courte ourtesy sy of API H e  tTransf Transfee r

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CONDE NSE SER R S AND COOLIN G TOWERS

Example: The operating leaving chilled water temperature for a system is 44 °F. A pl p late-fra ate-fram me hea eatt ex changer is used and provides an approach o f 2 ° F. For a certain certain set of ope operratin ting g co condition ndition s, th thee cooling tower is ab ablle to produce 42° F supp supplly water. Wi W ith the 2° F hea eatt exc exch hanger appro approac ach h, cooling tower water can be used to produce 44 ° F water in the ch chill ed water loo oop p . Th There fore , th thee plate frame supplly cooling to the bui build ldin g. A ll chill e rs ca can n be turned off. heat exchanger can be used to supp Heat exchanger approach defines the performan performan ce o f the pl plaate-fra e-fram me hea eatt exc exch hange gerr . Th The ap a p proach is the difference between the temperature o f su pp ly water from th the coo coolin ling g tower ent entee rin ring g the heat exchanger and the temperature o f water leaving the heat heat excha exchan nger ger.. Some packaged products, products , like vertical indoor units, units , can incorporate a h ydronic wa watte r-t r-to-a o-aiir economizer coil inside the unit to supp ly free coo li ng for that unit. In a strainer-cycle method of free cooling, cooling , tower water is strained, and then introduced directly into the chilled water loop to produce cooling. Because the open tower water is being mixed into the closed sys tem,, a high quality strainer (side-stream filter) is tem recommended at the tower.

s tr _ a _n_er__,cy -c_l_e _

_ _

 

The presence of an intermediate heat exchanger reduces the overall effectiveness o f th thee pl plaate frame method versus the strainer cycle . However However,, far more building operators li ke hav avin ing g no ad a d ditional water quality concerns since plate-frame heat exchangers do not mi m ix th t he open ope n loo oop p with the closed chilled water loop. ooling Tower Relief Profiles Relief ' pertains to how much the cooling tower delivers progressive ly co collder wat wate r as a function of reduced load on the chiller and reduced ewbt profile.

The term cooling tower ' 'turndown 'turndown   is also used interchangeably with relief to designate the same concept. In most regions of North America the relief profile might resemble the values in the ecwt column. An excep tion might be areas like Houston and Miami. At less than 100% o f load load,, the assumption is the outdoor conditions of dry bulb and wet bulb have fallen off. As a result, the cooling tower can pro duce cooler water in the fashion shown. The two right-hand columns reflect progressively more humid locations offering less relief. This is a direct result o f the wet bulb profile .

ECWT Humid Areas of

Chiller

ECWT ARI

Capacity

(o F)

North 1.0° America F/10%

ECWT ASIA 0.5° F/10%

100 00% % 90% 80 % 70% 60% 60 % 50 % 4 0% 30% 20 % 10%

85  0 81 0 77.0 77. 0 73  0 69. 0 65. 0 65.0 65 0 65 .0 65.0

85. 0 85.0 84. 4.0 0 83 0 82. 2.0 0 81.0 80.0 7 9  0 7 8  0 77.0 77. 0 76 .0

89 .6 89. 89.. 1 89 88..6 88 88.. 1 88 87.6 87.1 86.6 86 .1 85..6 85 85..1 85

T he te rm 't urn urnd down  is use d in te rc ha ng eab eablly with w ith re lief

Figure 65 ooling oo ling Tower

elief Profiles

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40  

CONDE NSERS AND COOLING TOWERS

third column r e f l ~ c t s a 1° Flowering o f the tower water temperature in chiller load. The fourth column is 0.5° 0 .5° F per 10 load reduction. so t

Column 4 r ~ f l e c t s h ~ d ~ s i g n entering

s o m ~ c o n d e

Asian climates where t h n s ~ r water to t h ~ chiller

~

d ~ s i g n

~ ~ d s

~ n t e r i n g

to

p ~ r

each 10

reduction

wet bulb is initiall y higher higher,,

89 6 ° F s ~ l ~ c t i n g

a

c ~ n t r i f u -

gal chiller s ~ l e c t i o n at full and part load for u s ~ with the cooling tower profile,, is a task normally profile normall y p r o v i d ~ d by t h ~ c h i l l ~ r manufacturer'' s represen manufacturer t a t i v ~ working with the th e design e n g i n ~ ~ r

C oolin ooling g T ow

r

Differenc Diff erences: es: Electric versus Absorption Chillers

Both ~ c t r i c and absorption c h i l l ~ r t y p ~ s r e q u i r ~ the cooling t o w ~ r to be s i z ~ d to h a n d l ~ t h ~ totall heat tota h eat o f rejection rejection.. As discussed previously, previously , the total h ~ a t o f e j ~ c t i o n is equal to the cooling capacity o f chill er plus internal heat g ~ n e r a t e d by t h ~ compressor in an ~ l ~ c t r i c motor-driven internal h ~ a t o f electric chillers is g e n e r a t ~ d pri marily by the compressor motor doing its work. Water c o o l ~ d e lectric c h i l l ~ r s utilize a multip lier of about 1.17 on the cooling load to represent total h ~ a t o f rejection rejection.. For example a 500-ton ~ l e c t r i c c h i l l ~ r typicall typically yr ~ q u i r e a cooling tower to be sized to handle 500 1 1 7 or about 585 tons total heat o f rejection rejection..

Total h eat of of rejection

Absorption chillers p r ~ s s o r but t h ~ y g e n e r amoun amo untt o f ~ a t than ~ l p ~ r coolling ton. coo ton . This r ~ j ~ c by t o w ~ r

h a v ~ a t ~ e c t r i c

h ~ a t

no com a greater c h i l l ~ r s

must be

Figure

Heatt Re Hea Rejj ection Factor

ARI gpm/ gpm/tt on

Electric

1.17

3.0 3. 0

Single-- Effect Single Absorption

2 .50

3 .6

Double-Effect Steam

1.80

4 .0

Double-Effect Direct--Fired Direct

1.80

4.5

66

Coo ling Tower Differenc Differencees   Electric vers versus us Absorption Ch Chill illeers

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CONDENSERS AND COOLING TOWERS

he large heat rejection factors

The absorption cycle has an ongoing reaction in the absorber sectio section. n. This exothermic exothermic   reaction gener atees heat and, at and , coupled with the heat input into the generator,, creates a large total heat of rejection re generator quirement.

Single-Effect Absorption Chiller

• Cooling tons

2 .5 (approx) equals total heat

• At ARI se selection conditions, conditions , 3 .6 gpm gpm//ton

is

of

rejection tons for tower sizing

t ypical for single-effect absorption.

• Indivi Individual dual job selections can vary. vary . Double-Effect Absorption Chiller Dire Direct ct fir ed or Steam)

• Cooling tons 1.80 (approx (approx..) equalss total heat of rejection equal tons for tower sizing • At ARI selection conditions conditions,, 4 to 4 .5 gpm/ gpm/ton is ty t ypical for double-effect absorption . • Individual job selections can vary. considering a new project with absorption chillers, chillers , the larger cooling tower flow rate, rate , size size,, and first cost must e factored into the analy anal ysis. f

When replacing absorption chillers

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CONDENSERS AND COOLING TOWERS

Cooling Tower Selection To select a cooling tower, tower, the following must be be dete etermin rmineed : Entering Wet Bu Bulb lb Temperature Temperatur e ewb ewbt) t) Typically this is the th e de dessign ign   wet • Entering wet bulb temperature bulb for yo ur exact location. In se lecting a tower, tower, we need to determine • Entering condense condenserr water temperature ecwt) the worse case condition under which the tower must function . Thus the • Leaving condens condenser er water temperature lcwt) wet bulb temperature of concern is not the mean coincident wet bulb bulb,, • Gallons per minute through the condenser which is an average type typ e value. When selecting a tower tower,, the 0.4 per Figure 7 cent wet bulb te te mp era erature ture from the Coo Coolin ling g Tower Selection Parameters ASHRAE Fundamentals book is typically typicall y us useed .

Entering Condenser Water Temperature ecwt) This is sometimes called the cold-water temp mpee rature exiting the tower. This va lu e is used in the selection of a water-cooled chiller and is usually usuall y 85° F for most of North America. However, However, there are several locations whe wh e re lower temperatures can be selected.

Leaving Condenser Water Temperature lcwt) This is sometimes referred to as hot water temperature entering the tower.

Gallons per Minute gpm) o he Condenser Gallons Usually Usuall y three gpm/ton is used . When considering a non-standard flow rate such as two gpm/ton,, as discussed earlier gpm/ton earlier,, consider the effects of increased fouling and the higher condenser

water temperature energy penalt penalty y on the chiller. The total heat o f rejection is normall normally y printed out on the water-cooled equipment selection program. f the selection program did not calculate total heat o f rejection rejection,, it is easy to do by hand. hand . f there is fresh water in the tower and yo u already know the th e range and gpm you desire desire,, (say 95 85 = 10° F range) yo you u can calculate the THR with the equation: Btuh = 500 *cond *c ondens enser er gpm * (M).

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CONDENSERS AND COOLING TOWERS

Today, with computerized Today, selection programs and CTI Certified ratings ratings,, cooling towers can easily be selected by simply providing the above informa tion. Site specific needs will help determine tower.

the

type

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B.A.r.. t;ooiiiiCJ TOW I

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Figure 68 Cooling Tower Selection Program Screen Cap ture: Courtesy Cour tesy o fBa ltim ore A ircoil Co Compan mpanyy

Water Treatment

1

A water treatment specialist is a wise investment. A specialist is trained and knowledgeable on creating treatment programs for cooling tower condenser systems, systems, evaporative condensers, condensers , and closed-circuit cooling towers . Problems a water treatment spe Cooling tower fill and tubes cialist can help prevent include: affected by Scale scale,, corrosion scale corrosion,, sludge forma Corrosion tion and microbiological Sludge contamination.

Contamination

Figure 69 Effects o f Scale, C orro orrossion ion,, and C ontamination Photos: Cou rt rtesy esy o fBa ltimor ltimoree A il·co ·coil il Co Com mpany

1

Information /tt:xt in this st:ct st:ction ion provided by ChemSt:arch

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NDE ND E N SERS

ND COOLI NG TOW TO W ERS

Scale Scalle is an acc Sca accumulation umulation of min minera erall depos eposit itss and acts as an ins in sulatin ulating g barri barriee r, reducing th thee systtem   s ability to trans sys tran sf er hea heatt , thus increas increasin ing g en e ne rgy costs to operate th thee syst sys t em Scale can also increase incr ease r  fri gerant rant hea head d pr press ess ure , which can cause se riou riouss mechanical damage damage to the th e compres ri ge sor,, th sor theere by adding replac placemen ementt ex exp pen se , downtim downtimee , and Scale build up inconveeni inconv nieenc e Hi Hig g h h ead press press ure i s caused by th thee build up of scale in the th e condenser tub tubes es,, and on th thee tube surfac urfaces es within eva evaporati porative ve condensers and clo close d-circcuit cooling se d-cir towee rs . Thi s causes compressors to work hard tow harder er

Corrosion Uniform and p1ttm g corrosion can occur in both the th e chi ll ed-wat d-watee r sys systtem and cooling p1ttmg towee r/ condensing sys tow systtem . Uniform corrosion is caused by low pH leve l evells indicatin indicating g an acidic condition co ndition,, which ge gen n e rall rally y thin the th e m etal throug throu g hout the th e systtem ; wh sys whee reas pittin pitting g is a localized cavity ca cau u se d by Corrosion loca ocall cell action associated with a prese presenc ncee o f oxygen bubbles bubbl es . A combination o f th thee oxygen leve evell, t emp mpeeratur raturee , and pH cause caus e localiz localizeed pitting t is important to maintain a prope prop e r pH leve leve l Eros Erosion ion corrosion is caused b y th thee fric tion o f th thee so solid lidss mo mov v in ing g throu throug gh th thee syst sys t em ; thi thiss can be minimizeed by maintainin minimiz maintaining g a sys systte m as cl ea ean n and as fr free ee o f su sp ended so solid lidss a s po poss ss ib le

Sludge Formatio Formation n Cooling tow toweer s e ith   r pu pussh (blow-thru) blow-thru),, or pull (induced-draft), (induced-draft) , air into the th e tow tower er in a cross flow or counterflow dir direc ection tion to the th e water dropl dropleet s A ir broug broug ht into the th e syst syst em will contain airborne particles particl es and debri s Note

Slud ludge ge th thaat fouls foul s th thee condenser tub tubes es is as se riou riouss as scalle The res sca esultin ulting g slud ludge ge will adhere or depos eposit it on con dens nseer tubes tub es , causing poor h ea eatt trans tran sf er and sub ubse queentl ntly y se qu hig h condenser h ea hig ead d press ure Slud ludge ge can ca n also plu plug g con den se r tubes or lin lines es,, imp impeedin ding g water flo flow w , caus causin ing g poor h ea eatt trans tran sfer er,, and providin providing g a growth env ironm ironmeent and food sour ce for bac ba cte ria ria..

Biological Growth Microbiological contamination (algae, (algae , ba bacte cte rial slim limee , and fun fung g i) i),, when circulated throu throug gh a cooling tow towee r/condenser sys systt em , can re duc ducee the effecti effec tive ven ness and effic efficiien cy o f th thee syste stem m . The specific aspects o f th thee probl probleem s ge gen nera eratt ed by these th ese microor microorga gani nissm s are outlined n ext.

Algae needs

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CONDENSERS AND COOLING TOWERS

The fo rm ati on of algae in a coo coolin ling g tower/co ower/cond nd en se r systt em occ sys occur urss because of th thee spec ecial ial env ir iro onm nmeent ntaal co connditio diti ons pr prev eval al ent in the th e syst syst em . Al Alg g ae a re chl oroph phy yll containin ontaining g micro microo orga gani ni sm s capable of multipl multiply yin ing g rap idly idl y and produc produ cin ing g large ma masses sses of plant mate mate ri al

  ontrolo f biological growth

Algae growth g rowth ca can n co comp mp ro mi se th thee wat watee r di stributi tributio on systtem by cl sys clogg oggin ing g th thee wat atee r di disstributi tributio on in the th e syst system and af fect the th e coolin coo ling g tower   s operatin rating g effici enc y . In additi dditio on, dea ead d algae can can co combin mbinee with airbo airb orn rnee debri briss and oth thee r co cont ntaamin minaant ntss to form slud ludge ge,, whi hicch ca can n foul thee cond th ndeense serr , cau caussin ing g maj or mec m echanical hanical brea breakd kdo own wnss or or furthee r re duc furth duceed effici fficieenc y Slud ludg ge can be be a foo d so ur urce ce fo r bacte bacteria g rowth and provide provid e a frie fri endl ndly y env ir iro onm nmeent fo r such org organi anissms as sul fate reducin ducing g bact bacteeri a . Slim limee depos it itss , are ca cau use d by the prese presen nce of ex exccess essiive am amo ount untss of bact ctee ri a in th thee co coo olin ling g tower/cond ndeense r sys systtem wate water suppl y Slim limee  formin rming g bac actte ria will adh ere to co cond ndeense serr tub es , cau sin ing g poor heat trans tran sfe r as se riou riouss as that crea eatted by scal calee . De D ead algae al gae and and slim limee in a sys systtem serr tubes tub es and a nd cl og th thee syst sys tem   s filt filteerin ring g sc scrree ns . Fin inaall y, excess excessiive can be b e com comee lodge ged d in co cond nd ense slim limee buildup can pro produc ducee a fo foul and di sag sagrree eeabl ablee od odo r . Fung i ar Fung aree non-chlor n-chloro oph phy yll or org gani anissm s th thaat li ve and grow in the th e dark a reas of a coo coolin ling g co n dens e r sys systtem . Fun F ung g i thrive thriv e in the th e dark a rea s of th thee syst system and us usuall ually y att ttac ack k woo wood d mate mate rial rialss, which us u sed to be be mor moree common in earli arlieer tow e r des esiigns, cau caussin ing g pr preematur turee fa ilur iluree of comp compo o nentss . nent (Info//text provide (Info provided by b y Ch Chem Se arch)

Condenser and Cooling Tower Control Systems For a re refri frige ge rati ratio on sys systtem to function prope prop erl y, th thee cond co ndeensin ing g pr press essur uree and a nd te temp mpee ratur raturee mu st be maintain maintaineed within ce rt rtaain limits . Thi Thiss is kn kno own as head pres pressur suree contr co ntro ol Abnormally Abnormall y

hig hi gh

condensing conde

temp mpee ratur ratures es cau se loss o f capacit capacity y, extra ex tra powe powe r co con nsumption umption,, and ove ove rloadin ading g of th thee compr compresso essorr motor and poss ossibl iblee pe rman rmaneent damage damage to the the compr co mpreessor and motor . Saf Safet y or limit control ontrolss no rm a ll lly y pr pro ote ct ag ain ainsst these th ese condition conditionss . Too low of a co c ond ndeensing pre pressur uree will cause caus e insuffi insufficcient press pressure ure for liquid fee feed d dev evic ices, es, which will starve thee evapor th vaporaator tor,, res esultin ulting g in loss lo ss of cap ca pac acit ity A hea ead d pr press essur uree contr ontro ol sys systtem maintain ma intainss syst system head press pressur uree at a pred pre dete rmin rmineed minimum le leve l

Why? • Maintain liquid subcooling and prevent liquid li ne flash gas • Provide sufficient pressu re dr d rop across TX TXV

How? 1. Water regulating v alve 2 . Flooded  Flooded  head pressure control (uncommon in comfort air-conditioning) 3. Condenser fan cycling (co (common mmon ) 4 . Variable condenser fan speed control (common) 5. Vane/damper contr control ol system

Figure 7 ondenser Head Pressure

o n tro troll

et hods

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CONDENSERS

ND COOLING TOWERS

Conse qu entl Con ntly y  co contr ntro o ls and co ntrol a lgori gorithm thmss can be use d to regulate condensing tempera turres. Th tu Th e met eth hod use d will vary dependin nding g on th thee temperature ran range ge o th thee condensing media dia   thee t y pe o condensing equipm th quipmeent use sed d   and th thee load va ri ati tio on o th thee syst system .

Water Cooled Condensers On onc ncee-thru water sys sys tem s  both the the ec ecwt wt and the the refrigeration loa oad d may vary widely Th e water fl ow rat e thro thr ough a water-cooled con den se r ma may y b e co ntroll ntrolleed aut uto omaticall y with a water regulating va vallve. Th The va vallve is in insst all e d in the the di sc schar harge ge watt er lin wa linee and operates in response to the the condensing pr essur e in th thee co nd ndeense r . Man Ma n y n ew chill hillee rs come equipp quippeed with a built-in hea ead d press essur uree co contr ntro o l fea f eatu turre that can control th thee va vallve t o maintain proper proper hea ead d pres es sur e

Once Thru Water Control Modulating Valve

condenser Water to waste or source Pump

Modulating valve throttles the to maintain minimum water temperature condensing Figure 71 Water Cooled C ondenser H ead Pressure Cont Contr ro l

Air Cooled Condensers Two gen gen e ral categories o hea ead d pr press essure co ntrol for air-cool ir-cooleed condensers mee eett th thee requir quiree m ent o maintainin maintaining g a minimum head pressure at th thee inl inleet to the th e liquid feed dev evic ice These tw two o catt ego ri es ar ca a re: 1. Refrigerant sid idee control 2 A ir sid idee control

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CONDENSERS AND COO LIN LING G TOW TO W ERS

Refrigerant Side Control

Refrigerant sid e control is accompli accomplisshed by re  ducing th thee amount o f active condensing surfac urfacee

Refrigerant side he ad pressure control

avai lablee in the availabl th e condenser by floodin flooding g th thee coil with the liquid refrigerant. This t ype o f control requires the use of a receiver and an excess charge o f re frigerant to ba back ck up into the th e coil co il There are several seve ral ways thi thiss ma may y be accomplished. On Onee method u uses ses a bypa pass ss valve to by b ypa pass ss hot gas into the liquid line line to res estrict trict the th e flow o f refrig e rant from the th e condenser and appl apply y di diss  charge pressure to a re Hot Gas restricts flow of ceptacle for th thee liquid \ l i q u i d refrigerant from condenser refrigerant called a re F looded ce1ver The hot gas works against head pr press essur uree to maintain mainta in a fixed minimum pressur press ure Thiss Thi

Condenser Bypass

f

Bypass Valve ___

system of head pressure controll can be use contro sed d at very lo w ambient co condinditions.

From Condenser

-

Suction Line

Figure 72 Air  Coo Air Coolle Condenser Head Pressur Pressuree Co Contr ntro ol

Airside Control Air irssid idee co control ntrol has has the advantag advantagee o f not requiring a rece eceiv iveer or an excess charge o f refri ge gerrant. A mea ean n s o f startin tarting g th thee compressor during winter winter op e ration is is usually usuall y requir quireed . This can be accomp li shed by bypa bypass ss in ing g th thee low pressure cut out on start up until the th e head pressure ha hass built up to maintain refrigerant refrigera nt flow through th thee liquid feed device .

Very common in comfort air conditioning air-coo led units 0

c:

Head Pressure Profile

tO

Fan 100

on signal

Co ntrol o f condensing pr press essur uree with airs airsid idee co contr ntro ol may be accomplished by th thee followin following g method s: Fan off signal

• Cy Cyclin cling g th thee fan fanss • Fan speed spee d con control trol • Cycling of th thee fan co combined mbined with fan spee eed d control

TIME

Figure 73 H ead Pressure Affecti Affectin ng Fan Fa n Cy Cyclin cling g

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CONDENSERS AND COOLING TOWERS

Fan cycling can be accomplished in response to variations in head pressure or to outdoor am bient temperatures . Fan speed control is accomplished with acces sory electronic speed controller that take a sig nal from the condenser pressure//temperature sen pressure sors. Fan speed control requires the controlled fan motor be capable of speed reduction reduction.. f not not   it must be replaced with a compatible motor in the field provided by the manufacturer .

Control Section of Multi Fan Air Cooled Condenser

Solid State Fan Speed Control

Fan Fa n Speed Controllers

Figure 74 Solid State Speed Control

On multi-fan units  units   usuall usually y one fan will be cycled on outdoor temperature temperature   another fan on pressure and the last fan speed controlled off of condenser coil temperature or pressure by by a con denser fan motor speed control.

Modulating damper control is not as common as the previous methods but merits a discussion . Damper con trol has been used in combination with fan cycling. The damper is mounted on the active fan section and modulates to reduce airflow to reduce the airflow when the other fans are off. Propeller fans have a characteristic opposite that of centrifugal fans fans:: increasing power input with increasing resistance. Therefore  Therefore   the motor must have adequate horsepower for operation with the dampers throttled. Mechanical Control

2 Fan Unit

4 Fan Unit

Figure Fig ure 75 rlir Cooled Conde Condenser nser Head Pressure Contro Controll

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CONDENSERS

Evaporative

ND COOLING TOWERS

ondensers

An evaporative condenser has characteristics much like a cooling tower. At light loads or low outdoor wet bulb conditions conditions,, condensing temp mpeeratures will drop to unacceptable levels unless suitable controls are used . In addition addition,, IR the condenser can experience icing f TOOUTSIDE) caused by lower-than-acceptable Discharge Dischar ge D amper spray water temp tempeeratur ratures. es. Evaporative condensers use sev Evaporative eral types of control that are staged to maintain the correct head pressure rangee These stages are: rang

IR FROM OUTSIDE)

• unit is started with dry coil

W TER PUMP

• spray water is initiated • low-speed fan airflow initiated

Figure 7

• high-speed airflow initiated

Due ed Evaporative

ondenser

Control o f an evaporative condenser ma may y also be accomplished by any or a combination thee following methods: th

of

• Cycling the fans • Modulating dampers • Variable frequenc y drives VFDs) Cycl ing the Cycling th e fan is a simple proc process. ess. Motor operation is controlled by a condensing-pressure controller. When the condensing pressure falls below a prescribed limit, limit , the fan is cycled off. The spray pump continues to run . Depending on ph phys ys ical arrangement and load characteristics , rapid cycling o f the fan can occur. The general rule calls for a maximum of six starts starts per pe r hour hour.. Modulating damp dampers ers on centrifugal fan units may may be installed in the discharge air connection of the unit. The dampers modulat modulatee airflow through the unit in response to condensing pressure. f thee unit is to run year-ro th year-round und on a wet-coil basis, basis , outdoor installations are not recommended be cause of the problems of control and protection from freezing. Decreasing airflow through the evaporativ eva porativee condenser will prevent freezing o f the recirculated water water during winter operation. The most precise means to control an eva evaporative porative condenser is with the use of a variable fre quency quenc y drive VFD) to control the airflow through the unit. The VFD controls the fan s) speed in response to condensing pr press essur ure As the condensing pressure drops drops,, the fan speed can be reduced to allow only the n intermedi te se sons minimum required airflow to maintain the predetermined condensing pressure . Installation Installationss that are to be b e controlled by VFDs require the use of an inverter duty dut y motor de signed signe d per NEMA standards, standards , which recognizes th thee increase incr eased d stresses plac placeed on motors by by a VFD.

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CONDENSERS AND COOLING TOWERS

Operating the unit dry was suggested earlier in our discussions on the application of evapora tive condensers condensers.. Such an arrangement permits the unit to be located outside and operated in freezing weather. Remember, Remember, condensing capacity capacity with a dry coil in freezing weather is onl only y about 45 percent o f wet-coil capacity. capacity . dry coil operation is satisfactory satisfactory,, it will probably probabl y be desirable to provide a means of con densing temperature control such as modulating dampers,, or cycling the fan motor to handle low dampers outdoor air temperatures below 35° F . f

Note

Cooling Towers The capacity capacity of a cooling tower is a function o f the entering wet bulb temperature . In cases where it is desirable to maintain condensing temperatures in the water-cooled condenser above a minimum limit, limit , several control methods have been used .

Water

Bypass of the Cooling Tower

This method is used to prevent nuisance tripping o f the chiller during morning start-up start-up.. At night,, the water temperature in the tower sump may have dropped below the minimum tempera night ture the chiller can handle . This is approximately 55 to 60° F for most chillers chillers.. At start-up, start-up , heated water exiting the condenser bypasses the tower and raises the tower loop temperature to acceptable chiller operating water temperatures . Care should be taken to locate the control valve so that the loop volume o f the bypass circuit isn isn   t f

too the, the bypass is may far yaway fromlarge. the chiller, chiller circuit ma take too long to heat up and the chiller can still trip off on low pressure at start up.. up

Condenser Water Pump

3 way diverting valve bypasses some of the water around the tower to wer to maintain a minimum water temperature 5 5 to 60° F

Figure 77 As shown in the diagram , a diverting valve is installed between the Wat Wateer Bypass Bypas s o Cooling Tower condenser inlet and discharge lines . Sometimes operating personnel like to control this valve manuall y Normally Normally,, the valve is con trolled automaticall automatically y from entering condenser water temperature, temperature , or more recently directly directl y from the reference head pressure control signal on the chiller.

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CONDENSERS

ND COOLING TOWERS

Fouling can increase

As the temperature o water leaving the tower decreases  decreases   the warm water from the condenser discharge is bypassed to warm the inlet water to a minimum level. This action reduces the amount o water to the tower and decreases its ability to cool water. water . Instead o constant water flow with a diverting valve ar rangement   another method utilizes a 2-way modulating valve. rangement This valve cou ld also be contro ll ed from chill er head pressure. Since there is a reduction in flow through the condenser water loop   the pump must ride the curve or be fitted with a VFD dur loop ing periods o lower flow requirement. That is because deadheading the pump is a concern. concern . Water bypass methods o head pressure control shou should ld be limited to morning start-up start-up.. For prolonged operation  operation   airflow control on the tower s a method o capacity control is more desirable.

Airflow Airflo w Control on Cooling Towers The primary method o control ling capacity on a cooling tower is to modulate the airflow through the tower in proportion to the load. ASH RAE 90.1 requires that all motors above 7.5 hp have the ability to be run at 2/3 speed or less to save energy. The methods to meet this requirement and maintain the desired leaving fluid temperature from the tower are s fol lows:

Figure

8

C ooling Tower Fan with Pony Motor Ph o to

Cycling the Fan Motor s) On and

C o u rte rtesy sy of Baltimor altimoree Ail·c oil C ompany

ff

This metho may be sufficient where close control o the leaving water temperature is not critical. The more cells or motors there are in the cooling tower installation  installation   the more stages o control are possible . Wear and tear on the machinery must be considered.

Two-Speed Motors

Two-speed motors provide an additional stage o control control   which can be important on one and nvo-cell tower installations. However However   the used o two-speed motors has declined because VFDs a re a more popular popul ar choice for approximatel approximately y the same cost.

Thm

to

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CONDENSERS AND COOLING TOWERS

Pony M otor otorss Pon y motors pro prov v id e an additional stage o f co control ntrol like like twotwo-sspee eed d motors motors , but have have th thee ad vantag va ntagee of motor re redundancy. Should one motor fai faill , th thee unit can be run on the the other motor until a repair is po poss ssibl ibl e. Standard singl inglee-spee eed d motors motors ar aree al so u se d , which are u suall ually y avai availabl lablee off thee-shelf ' at suppl th upply y hou houses ses (versus nvo -s pee eed d motors motors, which are ofte often n sp ec ecial ial or ord de r  . Tradition of

all pony y meoqual torrs in to aresize about thee sfan th izee motor iz thee. main th fanpo moto oris Howeve ever som so mnd e dmotor. esign esig n s utili utilize ze a p oy, ny pon motor to the thle/ 3main motor. At that pomot int it i.sHow s imply simpl y ar, seco second

Variable Frequency Drives

Ds)

VFD s adju VFD adjusst th thee motor spee eed d and thus thus th thee fan sp ee eed d . VF Ds pro prov v id e th thee clo closes sestt co ntrol o f th thee leav eavin ing g fluid te temp mpee ratur raturee o f all th thee method thodss . The cost of a VFD is similar s imilar to the th e tw two o -spee eed d motor. motor. The cost of a VFD is al so offset by th thee fact that VFDs VFD s eliminat e liminatee th thee nee eed d for se parat paratee motor start ers.

A ir Volum olumee Dampers on Centrifugal Fan Cooling Towers By addin adding g static to the th e di s charg chargee of th thee centrifugal fan fan,, actuator-controll actuator-controllee d dampe damp e rs reduc ducee th e fan horse the horse pow poweer, sav savin ing g en e rgy. How However ever,, thi thiss m ethod is is not as as en e nergy effi c ient as VFD con trol , and ma may y be pla plag g ued by b y actuator actuator and linka linkage ge probl probleem s in the th e fi eld .

Combination Methods Combination method thodss, such as as wh wheen a VFD is use sed d on one ce ll o f a n vo -cell in insstallation and th thee other cell is cycl ed on and off to mee eett th e load load,, are also u se d .

Water-cooled chillers

A ll of th these ese m ethod thodss can be controlle controlled by a t em pe ratur raturee se nso sorr in th thee leav eavin ing g fluid line line from th thee tower. tow er.

Winter

peration of Cooling Towers

Where it is n ecessary t o op erat ratee a coolin coo ling g tow toweer in winte winte r whe wh en freezing freezin g temp mpee ratur ratures es a re encount ounteered , pr prec ecau au tionss mu tion musst b e ta tak ken to preve prevent nt f reez eezin ing g o f water in expose exposed d piping piping and th thee tow toweer sump during during shutdown pe riod riodss .

For Winter Fre Freeze eze Protection Heater is immersed in cooling tower basin

Figure

9

Wint inteer Operation Operatio n - Tower H eat   g El E lem ent

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CONDENSERS

ND COOLING TOWERS

There are t\vo commonly use sed d met m ethod hodss to overcome th these ese freezing dan dangers: gers: l . A remot motee sump is i s locat located ed insid insidee in a hea eatt ed area area,, and th thee co nd ndeen se r water pip es are run and size d so that water drain drainss rapidly from th thee tower and does not re remain in the tower or the piping. 2.

An elec ectric tric,, or stea steam m or hot water hea eatt er i s locat locateed in the th e tow towee r ba sin or sump and oper ates whenever th thee sump water t emp mpera eratur turee falls be low 40° 40° F . Thi hiss is a very common commo n method of freeze protection for nonnal air-conditioning applications.

In all applications, applications , where freezing temp erat eratur ures es are encount encountere ered d, provi provission must be made for draining drainin g all ex expo posed sed lin lines es and equipm quipmeent during durin g ex exttend ndeed shutdown pe period riodss . Often Often,, th thee lin lines es are hea eatt trace traced which mea ean n s th they ey are wrapped with an electric heate eaterr ca cabl blee and heav il y in inssulat ulateed.

ummary In this thi s mo uk we ha have ve di sc scu usse ssed d the th e fun funcction of th thee condenser in the th e refrigeration cycle. We have ha ve de sc scrib ribeed the th e variou va riouss t ypes o f condensers available and the condensing media they th ey emplo employ y. We hav havee pr prese esent nteed application data for water-cooled co cond ndeen sers sers,, open and close d-cir d-circcuit cooling tow towers ers,, and eva evaporativ porativee and air air-coo -coolle d co nd ndeens nsers. ers. The vario variou u s t ypes of controls u se d for maintainin maintaining g condensing t emp e ratur raturee and hea ead d press pr essur uree have ha ve bee n rev eviiewed. Fac Facto torrs that influence th thee se lection o f prop propeer hea eatt rej ec ection tion equipm quipmeent are li sted here here in order o f importanc e: l . Av Availabilit ailability y of water 2 . Ene rgy costs 3.

Size and sco scop pe o f cooling plant re requir quireed

4.

Spac pacee requirements

5.

Quality and availability o f maint mainteenanc nancee staff

6 . Water tr trea eatm tment ent costs 7.

Length

of

operating seaso season n

Custom ustomeer pre pre fe renc ncee may change priorit priority y on a per-jo er-job b bas basi s As with any equipm quipmeent sel elec ection tion,, ca carreful review of key job parame param et ers and owner/occupant nee eed d s will guid uidee the des esiigner in se lec ectin ting g th thee prop propeer ty t ype and size o f condensing equipm quipmeent and control sc sch hem es to u se. 8.

Commercial HV C Equipment

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CONDENSERS

ork

ND COOLING TOWERS

ession

1

Name three ty t y pes o condensers u sed in the HVAC industr y

2

Name the two components that comprise total heat o rejection:

3

What is a ty t ypical heat rejection factor for air cooled equipment?

4 . List 3 factors that affect fouling rate on water cooled condensers.

5 . Sketch a t ypical air cool coo led refrigeration cycle and show the position o the condenser relati re lative ve to the other three components. Describe the function o the condenser. condenser .

6

What t ype

7

How does a crossflow tower differ from a co count unterflow erflow tower?

o

condenser is u sed on larger water cool coo l ed chill ch illers ers and why?

Commercial HV C Equipment

Turn to the ExpertS

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CONDENSERS

ND COOLING TOWERS

8

Name th thee four factors requir quireed for cooling t ower se lec ection. tion.

9

Defin finee cooling tow toweer re reli   fprofil profiles es and ex explain plain on what ty t ype o f chiller th they ey are ex extr treemely important.

10

What kind of tow toweer is is t ypicall pically y use d with a clos closeed loop water so sourc urcee hea eatt pump sys systt em ?

11

Wh y doe Why doess th thee cooling tow towee r size for an absorption chi ll e r diffe differ from a t ypical vapor compress pr ession ion chi ll er?

12 .

Defin finee th these ese four terms: Ent nteerin ring g Wet Bu lb Temperature:

Approach:: Approach

Range:: Range

To tal

13 .

Heat o f Rej ec ection tion :

Describe th thee method thodss o f hea ead d press essur uree control co ntrolss for eac each h ofth followin following g: Water Cool Coo led Condensers:

A ir Coo Coolle d Co nden se rs :

T

t

the

Commercial HV C Equipment

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CONDENSERS

ND COOLING TOWERS

Appendix References API Heat Transfer, Transfer, Buffalo Buffalo,, NY. http: http:// //www www .apiheattransfer.com apiheattransfer.com// ARI Standard 550/ 550/ 590 590,, Water Chilling Packages Using the Vapor Compression Guide. Guide.   http :// ://ww www w .ari. .ari .org org/s /std td// BAC Cooling Towe Tower Selection Program 6.02 with Product Information Information,, 1/ 2003. http :// ://www.baltaircoil www.baltaircoil .com com// BAC Evaporative Condenser Selection Program 7.1 with Product Information, Information , 6/ 2004 . http: / vv .baltaircoil baltaircoil..com com// Carrier Corp . S yracuse racuse,, NY , System Design Manual Part 1, Load Estimating. Cat. No. 510-304. http :// ://training training .carrier carrier..com com// Cooling Coolin g To wer Institute Institute,, Houston , TX . http: // //www.cti www.cti .org org// Standard Refrigeration Refrigeration,, Me Melr lrose ose Park Park,, IL. http :// ://www.stanref.com www.stanref.com / ChemSearch,, Cicero ChemSearch Cicero,, New York York.. http http:// ://w .vw .chemsearch.com chemsearch.com//

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urn to

the Experts

Carrier Corporation Technical Training 800 644 5544 www.training.carrier..com www.training.carrier

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