08 Vent Demand

I nternational MSc Programme Sustainable Energy Engineering I nternational MSc Programme Sustainable Energy Engineering
SUSTAINABLE ENERGY UTILIZATION
Lecture:
- VENTILATION DEMAND
Assist. Prof. Igor BALEN
Air Handling
System
Room With
Defined
Requirements
Supply
Air
Outlet
Air
Purpose of an air-handling system
Basic concepts and terminology
VENTILATION includes the intentional introduction of air from the outside
into a building.
Basic concepts and terminology
VENTILATION AIR
is air used to provide acceptable indoor air quality (IAQ) and thermal comfort
(TC).
VENTILATION DEMANDS
- provide outside air (oxygen) for breathing of humans (or/and animals)
- control of indoor air contaminants
- covering of the building’s thermal loads (temperature and humidity control)
- setting of uniform conditions in the occupied zone
Basic concepts and terminology
AIR-HANDLING UNIT – conditions air for a building
100% OUTSIDE AIR
UNIT
- no recirculation of
return air through
the air-handling system.
- all the supply air is
treated outside air
(called makeup air)
- all return air is dischar-
ged directly to the out-
side as relief air
- air-handling unit that
provides 100% outside
air to offset air that is
exhausted is called a
makeup air unit (MAU).
- the pressure in ventilated spaces is, in most cases, equal to the
atmospheric pressure of the outside; the air-flow delivered to a space (SA)
equals the airflow brought back from a space (RA); such spaces are
described as neutral or balanced.
- in mechanical ventilation systems, SAF and RAF might be sized differently,
when necessary.
- in these cases, a building can be:
(1) pressurized (positive) relative to outdoors - V
SA
> V
RA
- certain amount of air is exfiltrated from the space through openings
and cracks
(2) depressurized (negative) relative to outdoors - V
SA
< V
RA
- certain amount of air is infiltrated to the space through openings
and cracks
- examples of pressurized spaces: clean rooms, operation theaters
- examples of depressurized spaces: kitchens, toilets, laboratories working
with toxic substances
Pressure in ventilated spaces
Pressure in ventilated spaces
- normally, slightly positive space pressure should be maintained if the
required level of IAQ and environmental control of the conditioned space is
higher than that of the surrounding area.
- controlled pressure difference between space air and the air in the
surrounding area (positive or negative) depends on the space requirenments
and usually is below 13 Pa - an excessive pressure differential may make
opening and closing doors difficult.
The space pressure difference between the conditioned space and the
surrounding area is affected by:
(a) stack effect,
(b) wind effect,
(c) operation of the air systems and exhaust systems, and
(d) the air tightness (amount of leakage area) of the building shell
Pressure in ventilated spaces
STACK EFFECT
- the phenomenon where the temperature difference between the cold
outdoor and warm indoor air columns causes a density difference between
these air columns that creates a pressure difference between the cold and
the warm air columns.
Pressure in ventilated spaces
STACK EFFECT
- neglecting vertical density gradients, the stack pressure difference for a
horizontal leak at any vertical location is given by:
) ( ) ( H H g p
NPL i o st
− − = Δ ρ ρ [Pa]
- for a high-rise building with two openings on the external wall, measured
from lower opening, can be calculated from:
) / ( ) / ( 1
2
2 1
0
o i
NPL
T T A A
H
H
+
= [m]
ρ
o
– outdoor air density [kg/m
3
] T
o
– outdoor air temperature [K]
ρ
i
– indoor air density [kg/m
3
] T
i
– indoor air temperature [K]
H
NPL
– height of neutral pressure level H
o
– vertical distance between
above reference plane [m] openings [m]
H – height above reference plane [m] A
1
– area of lower openings [m
2
]
A
2
– area of higher openings [m
2
]
Pressure in ventilated spaces
STACK EFFECT – AIRFLOW
- airflow caused by stack effect when inlet and outlet areas are equal:
i o i NPL D st
T T T H g A C V / ) ( 2 − Δ =
&
[m
3
/s]
C
D
– discharge coefficient for opening (usually 0.65)
A – free area of inlet openings [m
2
]
ΔH
NPL
– height from midpoint of lower opening to NPL [m]
- increasing the outlet area over inlet area (or
vice versa) increases airflow but not in
proportion to the added area.
- when openings are unequal, use the smaller
area in above equation and add the increase
as determined from the figure.
Pressure in ventilated spaces
WIND EFFECT
- pressure changes for airflow around obstacle:
Pressure in ventilated spaces
WIND EFFECT
- airflow around building:
Pressure in ventilated spaces
WIND EFFECT
- wind pressure relative to outdoor static pressure on the building surface:
2
2
w
C p
o p w
ρ = Δ
[Pa]
- approaching wind speed at the observed height is calculated from the
meteorological wind speed, corrected for height and terrain roughness:
n
met
w
met o
H
H
w a w
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
[m/s]
C
p
– wind surface pressure coefficient
w – approaching wind speed at winward side [m/s]
a
o
– correction factor for terrain roughness
w
met
– wind speed from a meteorological station [m/s]
velocity profile exponent
- for urban areas: a
o
=0.35, n=0.4
- for suburban areas: a
o
=0.6, n=0.28
- for airports: a
o
=1.0, n=0.15
Pressure in ventilated spaces
WIND EFFECT
- local wind surface pressure coefficient (C
p
×100) for tall building wall:
Pressure in ventilated spaces
WIND EFFECT
- surface averaged wind surface pressure coefficient C
s
:
For tall building wall For low-rise building wall
If the wind direction is normal to the windward surface of a high-rise building
with a depth-width (L/W) ratio of 4, the average C
p
value is about +0.60 on
the windward side, about -0.5 on the leeward side and on the flat roof, and
about -0.25 on the other two sides.
Pressure in ventilated spaces
WIND EFFECT
The influence of the wind effect on space and building pressure characteri-
stics is as follows:
1. Rooms on the windward side of the building are usually at a positive
pressure and on the leeward side at a negative pressure relative to the
corridor pressure. It is best to build clean spaces, such as conference rooms,
on the windward side of the prevailing wind, and laboratories with toxic gas
exhaust systems on the leeward side of the building.
2. Outdoor air intake should be located on the side with a positive surface
pressure coefficient C
p
in the prevailing wind. Exhaust outlets should be
located where C
p
is negative, preferably on the rooftop.
3. Sufficient total pressure must be provided by the supply fan to overcome
the negative pressure at the outdoor intake and by the exhaust fan to
overcome the positive pressure at the exhaust outlet. Alternatives should be
provided to allow outdoor air intake and outlet when the wind direction is
different from that of the prevailing wind.
Pressure in ventilated spaces
WIND EFFECT – AIRFLOW
- airflow caused by wind only:
Aw C V
w w
=
&
[m
3
/s]
C
w
– effectiveness of openings (C
w
is assumed to be 0.5 to 0.6 for
perpendicular winds and 0.25 to 0.35 for diagonal winds)
A – free area of inlet openings [m
2
]
w – wind speed [m/s]
COMBINED DRIVING FORCES
- wind pressure, stack pressure, and mechanical system acting together:
I w st
p p p p Δ + Δ + Δ = Δ [Pa]
ρ / 2 p A C V
D
Δ =
&
[m
3
/s]
pressure that acts to balance inflows and outflows
(including mechanical system airflows)
Ventilation requirements
OUTDOOR AIR REQUIREMENT PER PERSON
ASHRAE Standard 62 - the recommended minimum outside air ventilation
per person for breathing, for any type of space (non-smoking) is 8 L/s
(V
o,p
=30 m
3
/h) - satisfies the odor perceptions of 80% or more of visitors.
- outside air supply per person can be higher, depending on the type of
space occupancy (see recommendations in literature); it can be up to 100
m
3
/h for offices in modern high multi-floor buildings.
- for usual residential and commercial applications, outside air supply per
person is in range V
o,p
=30-60 m
3
/h (>50 m
3
/h satisfies 90% or more of
persons)
- for smoking sapaces, outside air supply should be increased for at least
+20 m
3
/h
- for N persons in a space, total minimum outdoor airflow rate is:
p o o
V N V
,
& &
= [m
3
/h]
Ventilation requirements
OUTDOOR AIR REQUIREMENT ACCORDING TO THE ALLOWABLE
CONCENTRATIONS OF AIR CONTAMINANTS
- pollutants effect health of the occupants
- pollutants include non-biological particles (synthetic vitreous fibers,
combustion products, nuisance dust, and others); bioaerosols; gases and
vapors that may be generated due to industrial processes (usually known
from the type of process), by building materials, furnishings, and equipment,
by occupants and their activities in a space, or brought in from the outdoors.
- different standards for industrial and nonindustrial indoor environments
- outside air supply rate, used to dilute the concentration of a specific indoor
air contaminant, can be calculated as:
o i
con
o
C C
m
V
−
=
&
&
[m
3
/h]
m
con
– total contaminant source strength [μg/h]
C
i
– steady-state indoor concentration [μg/m
3
]
C
o
– outdoor concentration [μg/m
3
]
Ventilation requirements
OUTDOOR AIR REQUIREMENT ACCORDING TO THE ALLOWABLE
CONCENTRATIONS OF AIR CONTAMINANTS
- indoor concentration of contaminants C
i
should meet the specified value
stated in standards (i.e. CO concetration in garages and tunnels)
- concentration of contaminants is usually expressed in the following units:
ppm – parts of contaminant by volume per million parts of air by volume
μg/m
3
– micrograms of contaminant per cubic meter of air
ppm=(24.45/M)(1000 μg/m
3
); M – relative molecular mass of contaminant
- example – CO
2
concetration in indoor environments
steady-state indoor concentration 1000 ppm, normal outdoor concentration
350 ppm, volume production of CO
2
by one person 18 L/h
7 . 7
00035 . 0 001 . 0
3600 / 18
,
=
−
=
−
=
o i
con
p o
C C
V
V
&
&
L/s per person
Ventilation requirements
HUMAN RESPONSE TO CARBON-MONOXIDE (CO)
Ventilation requirements
OUTDOOR AIR REQUIREMENT ACCORDING TO THE ALLOWABLE
CONCENTRATIONS OF AIR CONTAMINANTS
- with balanced ventilation system, for a single space, change of the
contaminant concentration in time can be calculated from:
θ
θ
θ θ
θ θ
θ , , 0 ,
,
,
) ( ) 1 (
s
ACH
s i
ACH con
i
C e C C e
V ACH
V
C + − + −
⋅
=
⋅ −
=
⋅ −
&
C
i,θ
– contaminant concentation in space at perfect mix [m
3
/ m
3
]
V
con,θ
– total contaminant source strength [m
3
/h]
ACH – air changes per hour [1/h]
V – space volume [m
3
]
θ – time [h]
C
i,θ=0
– start contaminant concentation in space [m
3
/ m
3
]
C
s,θ
– contaminant concentation in supply air [m
3
/ m
3
]
Ventilation requirements
OUTDOOR AIR REQUIREMENT ACCORDING TO ACH
- number of air changes per hour (ACH) represents a ratio of the outside
airflow entering a space in one hour to the space volume (internal)
- therefore, the outdoor airflow rate is :
V ACH V
o
⋅ =
&
[m
3
/h]
- ACH depends on the space volume, shape, type of occupancy...
- ACH criteria for ventilation requirement is used when contamination
sources are not specified; it is also used as a control for the supply airflow
calculations by other methods.
- for usual residential and commercial spaces, ACH is in range 4-8 h
-1
.
- for different types of spaces, recommended ACH can be found in the
tables given in literature.
Ventilation requirements
SUPPLY AIR REQUIREMENT ACCORDING TO CALCULATED
COOLING/HEATING LOAD
- volume airflow rate of ventilation system, to maintain a required
temperature, can be calculated from cooling and/or heating load results
supply airflow from sensible cooling load: supply airflow from sensible heating load:
AC p
COOL s
AC
t c
q
V
Δ
=
ρ
,
&
[m
3
/s]
H p
HEAT s
H
t c
q
V
Δ
=
ρ
,
&
[m
3
/s]
- if the ventilation system operates with constant airflow during the whole
year, calculation using cooling load usually gives higher airflow because the
temperature difference between supply air and room air is much lower:
COOLING SEASON: Δt
AC
=3-8(10)ºC
HEATING SEASON: Δt
H
=10-25ºC
Ventilation requirements
SUPPLY AIR REQUIREMENT ACCORDING TO DEHUMIDIFICATION
DEMAND
- volume airflow rate of ventilation system, to maintain a required humidity by
decreasing humidity ratio x, can be calculated from latent load results:
AC
l
AC
x r
q
V
Δ
=
0
ρ
&
[m
3
/s] or
- used in cases of high latent loads in a space (i.e. ventilation of swimming
pools)
AC
w
AC
x
m
V
Δ
=
ρ
&
&
[m
3
/s]
Ventilation demand
ROOM AIR MOVEMENT – ENTRAINMENT FLOW
- also known as conventional mixing flow
- conditioned air is normally discharged from air outlets at velocities much
greater than those acceptable in the occupied zone; diffuser jets mix with the
ambient room air by entrainment, which reduces the air velocity and
equalizes the air temperature
- creates relatively uniform air velocity, temperature, humidity, and air quality
conditions in the occupied zone.
Ventilation demand
ROOM AIR MOVEMENT – DISPLACEMENT FLOW
- the movement of air within a space in a piston-type motion
- air is supplied from air outlets at low air velocities
- outlets are located at or near the floor level, and the supply air is introduced
directly to the occupied zone
- to function properly, stable vertically stratified temperature field is essential
- desirable for removing pollutants generated within a space
Ventilation demand
OCCUPIED (COMFORT) ZONE
Distance from
internal walls
Distance from
external walls
Window
Outside of this zone it is not necessary to
maintain thermal comfort parameters