Cooling Towers
Content
c
Cooling
towers
à stract
The o ective of this experiment was to explore various operating conditions of a Hilton Bench
Top Cooling Tower H892 and to analyse its performance associated at each condition.
the influence of varying cooling load, water flow rate and air flow rate on the performance was
investigated. The experiment involved keeping two of the three varia les constant while varying
the third during each run y isolating the influence of each mvaria le in operation. Data , such
as the water temperature and air pschrometric properties at various points, was collected and
manipulated into parameters useful in analyzing the operation of the cooling tower. The most
important parameter is the efficiency. The steady flow equations ( energy and mass alances)
were employed inorder to investigate the forms of water lost from the system and to provide an
insight on the amount of energy transferred etween phases under different conditions.
It was determined that the maximum efficiency was achieved at-----------------------------------------------------------------------at a cooling load of ----------------for the case where ----------is kept
constant and the----------------is varied.
It was therefore concluded that optimum cooling is achieved when--------------------
Ta le of contents
à stract
Ta le of contents
1.c Introduction
2.c Theory
Cooling tower fundamentals
Cooling tower terminology
Mass alance analysis
Energy alance analysis
Psychrometry
3.c Experimental
àpparatus
Cooling tower preparation
Experimental procedure
Precautions
4.c Results
àpproach temperature
Temperature range
Efficiency
Mass alance
Energy alance
5.c Discussion
6.c Conclusion
7.c References
8.c àppendices
Introduction
à cooling tower is a reection device, which emits waste heat to the atmosphere through the
cooling of a water stream to a lower temperature. The type of heat reection in a cooling tower is
termed ³ evaporative¶ in that it allows a small portion of water to e cooled to evaporate into
moving air stream to provide significant cooling to the rest of that water stream. Cooling occurs
as a result of direct heat transfer y conduction and convenction etween two phases. The heat
from the water stream is transferred to the air stream raising the air temperature and its relative
humidity and this is discharged to the atmosphere.
Humidification processes are frequently carried out to control the cooling and the recovery of
water y contacting it with low humidity air. It is therefore common practice for chemical
engineers to intergrate cooling towers into plant design to remove all unwanted heat from
process streams. the cooled water produced y the cooling tower may then e distri uted
throughout the whole plant for various cooling purposes using heat exchangers. The conservation
of water, for oth economic and environmental reasons is thus promoted.
the Hilton Bench tower uses the counter- current evaporating method ( also known as wet
cooling method). This method is widely used when cooling heated water in industries. When the
heated water comes into contactwith air, a large heat of water allows a high heat transfer from
the water to the air with little water evaporating into the air , hence the water is conserved. àn
advantage of this is that heated water can e cooled down to the inlet wet- ul temperature of the
air.
The purpose of conducting the experiment was to investigate the influence of varying the
cooling load, water flow rate and air flow rate on the performance of the cooling tower and to
determine all end state properties of the air and water from charts and ta les. Experiments were
performed at different cooling loads of 0.5kW, 1.0kW and 1.5kW at constant air and water flow
rates. àlso the influence of varying the flow rate on the performance of the tower was analysed
y perfoming experiments at constant cooling loads and at orifice differential of-------. The
ssame approach was taken when analyzing the tower performance at differe air flow rates,
constant water flow rate of 40g/s and cooling load.
Theory
Cooling towers fall into two main su -divisions: natural draft and mechanical draft. ( coulson
et.al . (1999))
Mechanical draft cooling towers are much more widely used and these towers utilize large fans
to force air through circulated water. The water falls downward over fill surfaces which help
increase the contact time etween the water and the air. This helps maximize heat transfer
etween the two.conversely, the natural draught cooling tower uses uoyancy through a tall
chimney. Warm , moist air naturallyrises due to the density difference to that of dry, cooler
outside air.warm moist air is less dense than drier air at the same pressure.the moist air uoyancy
produces a current of air through the tower, thus the natural draught cooling tower does not
require the aid of any mechanical devices to induce tha air flow.
One of the important mechanical draught cooling towers is the forced draught cooling tower.
This tower follows a simple operation, atmospheric is forced vertically through the tower
through the use of a fan, while the water flows down through gravity.this produces
countercurrent flow in the cooling tower. Included in the coling tower structure is packing
which serves the purpose of increasing the surface area of contact etween the air and water
allowing more efficient cooling.
The mechanicaldrught cooling tower produces greater coolingthan the natural draught cooling
tower and occupies less space. Mechanical draught cooling towers are generally used for
achiving higher cooling or for capacity considerations, however they are utilized due to
economic considerations( such as costs) when compared to natural draught cooling tower.
c
c
c
c
c
c
c
temperature rangec c c¢cc
This is the difference etween the temperature of the water entering and that leaving the cooling
tower.
Ôc
cc cÔc
¢
ccÔc
¢c
c
Changes in water and air flow-rates adveserly affect the temperature range. àccording to Hill
and Stanford, 1967, y increasing the air flow rate in the cooling tower, an increase in the
temperature range is expected since a higher flow-rate of air increases the contact time of water
to less himidfied air which increases the mass transfer driving force.
Increasing the water flowrate has the inverse effect, lowering the temperature range and
evaporative cooling y increasing the exposure of water to a more humidified air. Therefore it
can e seen that as temperature range increases so does the performance of the cooling tower.
cc
The difference etween the temperature of the cold water leaving the tower and the wet- ul
temperature of the air is known as the approach.
Ôc
cc cÔc
¢c
ccÔc
cc
c
The approach fixes the operating temperature of the tower and is a most important parameter in
determining oth tower size and costc
The smaller the approach temperature, the higher the performance of the cooling tower under the
operating conditions. For optimum efficiency of the cooling tower, the approach temperature
would ideally e zero since it is not possi le for the air to coolthe water any further than the wet
ul temperatureof the incoming air. However, in practice it is impossi le to achieve a zero
approach temperature as inorder to achieve this, the tower should e infinitelt tall. Cooling
towers are thus not designed to operate with an approach temperature elow 2.80C and it is
generally accepted that an approach temperature of 5.50Cis attaina le. ( Mcca e et al, 2005).
cc
cc
cc
Wet-Bul Temperature - The lowest temperature that water theoretically can reach y
evaporation. Wet-Bul Temperature is an extremely important parameter in tower selection and
design and should e measured y a psychrometer.
Temperature can e measured with two thermometers simultaneously - one with a wet ul and
one with a dry ul - in order to determine the relative humidity of the surrounding air. This
method was used historically y meteorologists, ut we will use it extensively in this experiment
to measure the water concentration in the air. If the air is saturated with water vapor (100%
relative humidity), then the wet ul and dry ul temperatures e equal.
If the relative humidity is less than 100%, then the wet ul temperature will e lower due to
evaporation of water from the surrounding wrap.
To o tain an accurate wet ul temperature, it is important to make sure that the wet ul
reaches steady state under the conditions of air flow and relative humidity.
The relative humidity is determined y looking at wet- ul and dry- ul temperatures on a
psycrometric chart.
c
c
cc c
c
c
The water entrained in the air flow as fine droplets and discharged to the atmosphere is defined
as drift ( Mckelvey and Brooke, 1959). Drift loss does not include water lost y evaporationc
c
c
c ccc
ccc
c
c
thus, the mass of water evaporated can e determined y taking the difference etween the mass
flow rate of water in the air entering the cooling tower and the mass flowrate of water in the air
leaving the tower.
Using psychometry the following equation is o tained
ccccccc
c cc cc
c
c
c
cc
c
!c
the psychometric chart is used to determine the a solute humidity and the specific volume at the
respective dry ul and wet ul temperatures of the inlet and outlet air.
Heat Load
The amount of heat to e removed from the circulating water through the tower. Heat load is
equal to water circulation rate (gpm) times the cooling range times 500 and is expressed in
BTU/hr. Heat load is also an important parameter in determining tower size and cost.
c
Pumping Head c
The pressure required to pump the water from the tower asin, through the entire system and
return to the top of the tower.
Make-Up
The amount of water required to replace normal losses caused y leedoff, drift, and
evaporation. It is the amount of water required to compensate for the loss of water due to
evaporation and losses due to drift during the normal operation of the cooling tower. This can e
further expressed y the equation given elow
c cc ccc ccc
c
c ccc
cc c
cc ¢c
c !c
c cc
cc c
cc
c
c !c
c
ccc
cc c
ccc
c
c c
c
c !c
c
Bleed Off (Blowdown)
The circulating water in the tower which is discharged to waste to help keep the dissolved solids
concentrating in the water elow a maximum allowa le limit. às a result of evaporation,
dissolved solids concentration will continually increase unless reduced y leed off.
Efficiency of the tower
This is defined as the ratio etweenthe range and the ideal range(i.e the difference etween the
cooling water inlet temperature and the am ient wet ul temperature).
The cooling tower efficiency can e expressed as
"
c
where
"ccc
c c
Ti is the inlet temperature of water to the tower (oC , oF )
To is the outlet temperature of water from the tower (oC , oF )
Tw is the wet ul temperature of air (oC , oF )
4
Heat transfer is occurring primarily in the load tank, where the water is rought up to the feed
temperature. à small amount of heat is also lost to the surroundings y
radiation/conduction/convection. Work is done on the water y the pump.
Energy is transferred along with mass loss, ecause dry air enters, and humid air leaves.
l = the rate of heating added to the system
K = rate of work done y the pump on the water
H
= rate of enthalpy loss in exiting vapor
H
= rate of enthalpy gain due to entering air and entering water from make-up tank
Now let's set up the energy equation for the portion of the
process illustrated at the right:
l + K = í
at steady state
[work done on system] = [energy loss due to enthalpy change]
defining some mass flow rates and enthalpies:
e the rate of (liquid) water addition from the "make-up" tank
the mass flow rate of air
s the mass flow rate of steam
the specific enthalpy of (dry) air
the specific enthalpy of steam
su script entering the cham er
su script leaving the cham er
The energy alance equation is now written:
lc cKcc
c
c
c
c
steam/air leaving
steam/air entering
water entering
from make-up tank
c
ccc
c
cc
Conservation of mass gives us the following simple equations at steady state:
àir in = air out
( m
)( m
)
Water in = water out
(m
) (
)
This equation simply says that the mass flow rate of water entering the system
( ecause the entering air is humid) plus the mass flow rate of water entering from
the make-up tank equals the mass flow rate of water leaving the system as
vapor/steam.
The ratio of water vapor to air can e determined if the humidity is known.
Cooling
towers
à stract
The o ective of this experiment was to explore various operating conditions of a Hilton Bench
Top Cooling Tower H892 and to analyse its performance associated at each condition.
the influence of varying cooling load, water flow rate and air flow rate on the performance was
investigated. The experiment involved keeping two of the three varia les constant while varying
the third during each run y isolating the influence of each mvaria le in operation. Data , such
as the water temperature and air pschrometric properties at various points, was collected and
manipulated into parameters useful in analyzing the operation of the cooling tower. The most
important parameter is the efficiency. The steady flow equations ( energy and mass alances)
were employed inorder to investigate the forms of water lost from the system and to provide an
insight on the amount of energy transferred etween phases under different conditions.
It was determined that the maximum efficiency was achieved at-----------------------------------------------------------------------at a cooling load of ----------------for the case where ----------is kept
constant and the----------------is varied.
It was therefore concluded that optimum cooling is achieved when--------------------
Ta le of contents
à stract
Ta le of contents
1.c Introduction
2.c Theory
Cooling tower fundamentals
Cooling tower terminology
Mass alance analysis
Energy alance analysis
Psychrometry
3.c Experimental
àpparatus
Cooling tower preparation
Experimental procedure
Precautions
4.c Results
àpproach temperature
Temperature range
Efficiency
Mass alance
Energy alance
5.c Discussion
6.c Conclusion
7.c References
8.c àppendices
Introduction
à cooling tower is a reection device, which emits waste heat to the atmosphere through the
cooling of a water stream to a lower temperature. The type of heat reection in a cooling tower is
termed ³ evaporative¶ in that it allows a small portion of water to e cooled to evaporate into
moving air stream to provide significant cooling to the rest of that water stream. Cooling occurs
as a result of direct heat transfer y conduction and convenction etween two phases. The heat
from the water stream is transferred to the air stream raising the air temperature and its relative
humidity and this is discharged to the atmosphere.
Humidification processes are frequently carried out to control the cooling and the recovery of
water y contacting it with low humidity air. It is therefore common practice for chemical
engineers to intergrate cooling towers into plant design to remove all unwanted heat from
process streams. the cooled water produced y the cooling tower may then e distri uted
throughout the whole plant for various cooling purposes using heat exchangers. The conservation
of water, for oth economic and environmental reasons is thus promoted.
the Hilton Bench tower uses the counter- current evaporating method ( also known as wet
cooling method). This method is widely used when cooling heated water in industries. When the
heated water comes into contactwith air, a large heat of water allows a high heat transfer from
the water to the air with little water evaporating into the air , hence the water is conserved. àn
advantage of this is that heated water can e cooled down to the inlet wet- ul temperature of the
air.
The purpose of conducting the experiment was to investigate the influence of varying the
cooling load, water flow rate and air flow rate on the performance of the cooling tower and to
determine all end state properties of the air and water from charts and ta les. Experiments were
performed at different cooling loads of 0.5kW, 1.0kW and 1.5kW at constant air and water flow
rates. àlso the influence of varying the flow rate on the performance of the tower was analysed
y perfoming experiments at constant cooling loads and at orifice differential of-------. The
ssame approach was taken when analyzing the tower performance at differe air flow rates,
constant water flow rate of 40g/s and cooling load.
Theory
Cooling towers fall into two main su -divisions: natural draft and mechanical draft. ( coulson
et.al . (1999))
Mechanical draft cooling towers are much more widely used and these towers utilize large fans
to force air through circulated water. The water falls downward over fill surfaces which help
increase the contact time etween the water and the air. This helps maximize heat transfer
etween the two.conversely, the natural draught cooling tower uses uoyancy through a tall
chimney. Warm , moist air naturallyrises due to the density difference to that of dry, cooler
outside air.warm moist air is less dense than drier air at the same pressure.the moist air uoyancy
produces a current of air through the tower, thus the natural draught cooling tower does not
require the aid of any mechanical devices to induce tha air flow.
One of the important mechanical draught cooling towers is the forced draught cooling tower.
This tower follows a simple operation, atmospheric is forced vertically through the tower
through the use of a fan, while the water flows down through gravity.this produces
countercurrent flow in the cooling tower. Included in the coling tower structure is packing
which serves the purpose of increasing the surface area of contact etween the air and water
allowing more efficient cooling.
The mechanicaldrught cooling tower produces greater coolingthan the natural draught cooling
tower and occupies less space. Mechanical draught cooling towers are generally used for
achiving higher cooling or for capacity considerations, however they are utilized due to
economic considerations( such as costs) when compared to natural draught cooling tower.
c
c
c
c
c
c
c
temperature rangec c c¢cc
This is the difference etween the temperature of the water entering and that leaving the cooling
tower.
Ôc
cc cÔc
¢
ccÔc
¢c
c
Changes in water and air flow-rates adveserly affect the temperature range. àccording to Hill
and Stanford, 1967, y increasing the air flow rate in the cooling tower, an increase in the
temperature range is expected since a higher flow-rate of air increases the contact time of water
to less himidfied air which increases the mass transfer driving force.
Increasing the water flowrate has the inverse effect, lowering the temperature range and
evaporative cooling y increasing the exposure of water to a more humidified air. Therefore it
can e seen that as temperature range increases so does the performance of the cooling tower.
cc
The difference etween the temperature of the cold water leaving the tower and the wet- ul
temperature of the air is known as the approach.
Ôc
cc cÔc
¢c
ccÔc
cc
c
The approach fixes the operating temperature of the tower and is a most important parameter in
determining oth tower size and costc
The smaller the approach temperature, the higher the performance of the cooling tower under the
operating conditions. For optimum efficiency of the cooling tower, the approach temperature
would ideally e zero since it is not possi le for the air to coolthe water any further than the wet
ul temperatureof the incoming air. However, in practice it is impossi le to achieve a zero
approach temperature as inorder to achieve this, the tower should e infinitelt tall. Cooling
towers are thus not designed to operate with an approach temperature elow 2.80C and it is
generally accepted that an approach temperature of 5.50Cis attaina le. ( Mcca e et al, 2005).
cc
cc
cc
Wet-Bul Temperature - The lowest temperature that water theoretically can reach y
evaporation. Wet-Bul Temperature is an extremely important parameter in tower selection and
design and should e measured y a psychrometer.
Temperature can e measured with two thermometers simultaneously - one with a wet ul and
one with a dry ul - in order to determine the relative humidity of the surrounding air. This
method was used historically y meteorologists, ut we will use it extensively in this experiment
to measure the water concentration in the air. If the air is saturated with water vapor (100%
relative humidity), then the wet ul and dry ul temperatures e equal.
If the relative humidity is less than 100%, then the wet ul temperature will e lower due to
evaporation of water from the surrounding wrap.
To o tain an accurate wet ul temperature, it is important to make sure that the wet ul
reaches steady state under the conditions of air flow and relative humidity.
The relative humidity is determined y looking at wet- ul and dry- ul temperatures on a
psycrometric chart.
c
c
cc c
c
c
The water entrained in the air flow as fine droplets and discharged to the atmosphere is defined
as drift ( Mckelvey and Brooke, 1959). Drift loss does not include water lost y evaporationc
c
c
c ccc
ccc
c
c
thus, the mass of water evaporated can e determined y taking the difference etween the mass
flow rate of water in the air entering the cooling tower and the mass flowrate of water in the air
leaving the tower.
Using psychometry the following equation is o tained
ccccccc
c cc cc
c
c
c
cc
c
!c
the psychometric chart is used to determine the a solute humidity and the specific volume at the
respective dry ul and wet ul temperatures of the inlet and outlet air.
Heat Load
The amount of heat to e removed from the circulating water through the tower. Heat load is
equal to water circulation rate (gpm) times the cooling range times 500 and is expressed in
BTU/hr. Heat load is also an important parameter in determining tower size and cost.
c
Pumping Head c
The pressure required to pump the water from the tower asin, through the entire system and
return to the top of the tower.
Make-Up
The amount of water required to replace normal losses caused y leedoff, drift, and
evaporation. It is the amount of water required to compensate for the loss of water due to
evaporation and losses due to drift during the normal operation of the cooling tower. This can e
further expressed y the equation given elow
c cc ccc ccc
c
c ccc
cc c
cc ¢c
c !c
c cc
cc c
cc
c
c !c
c
ccc
cc c
ccc
c
c c
c
c !c
c
Bleed Off (Blowdown)
The circulating water in the tower which is discharged to waste to help keep the dissolved solids
concentrating in the water elow a maximum allowa le limit. às a result of evaporation,
dissolved solids concentration will continually increase unless reduced y leed off.
Efficiency of the tower
This is defined as the ratio etweenthe range and the ideal range(i.e the difference etween the
cooling water inlet temperature and the am ient wet ul temperature).
The cooling tower efficiency can e expressed as
"
c
where
"ccc
c c
Ti is the inlet temperature of water to the tower (oC , oF )
To is the outlet temperature of water from the tower (oC , oF )
Tw is the wet ul temperature of air (oC , oF )
4
Heat transfer is occurring primarily in the load tank, where the water is rought up to the feed
temperature. à small amount of heat is also lost to the surroundings y
radiation/conduction/convection. Work is done on the water y the pump.
Energy is transferred along with mass loss, ecause dry air enters, and humid air leaves.
l = the rate of heating added to the system
K = rate of work done y the pump on the water
H
= rate of enthalpy loss in exiting vapor
H
= rate of enthalpy gain due to entering air and entering water from make-up tank
Now let's set up the energy equation for the portion of the
process illustrated at the right:
l + K = í
at steady state
[work done on system] = [energy loss due to enthalpy change]
defining some mass flow rates and enthalpies:
e the rate of (liquid) water addition from the "make-up" tank
the mass flow rate of air
s the mass flow rate of steam
the specific enthalpy of (dry) air
the specific enthalpy of steam
su script entering the cham er
su script leaving the cham er
The energy alance equation is now written:
lc cKcc
c
c
c
c
steam/air leaving
steam/air entering
water entering
from make-up tank
c
ccc
c
cc
Conservation of mass gives us the following simple equations at steady state:
àir in = air out
( m
)( m
)
Water in = water out
(m
) (
)
This equation simply says that the mass flow rate of water entering the system
( ecause the entering air is humid) plus the mass flow rate of water entering from
the make-up tank equals the mass flow rate of water leaving the system as
vapor/steam.
The ratio of water vapor to air can e determined if the humidity is known.
