Energy Efficient Buildings
Sizing Air Handlers and Chillers
Air handlers, boilers and chillers must be sized to meet peak loads. The quantity of air
supplied by the supply air fan must be large enough to meet the greater of the peak
cooling or peak heating load. In most large multi-zone buildings, the peak cooling load is
greater than the peak heating load since all zones are subject to cooling loads, while
only exterior zones are subject to heating loads. This chapter discusses how to calculate
the volume flow rate of the supply air fan and the cooling capacity of the chiller required
to meet the peak cooling load.
Example
Consider the single-duct reheat system shown below. To begin, design conditions for
the indoor air, outdoor air, required ventilation and incident solar radiation should be
determined. Peak sensible and latent cooling loads for each zone are then calculated
based on these conditions. In the example below, only design loads for Zone 2 are
shown. However, in actual practice, design cooling loads for all zones served by the air
handler should be calculated and summed to determine the total sensible and latent
loads. In addition to these conditions, the temperature of the cooling water supplied to
the chiller and the temperature of the air leaving the cooling coil should be specified.
These conditions and peak sensible and cooling loads are shown in the figure below.
1
Return Air Fan
Qsen 1
Qsen 2
Zone 1
Zone 2
Qlat 1
Qlat 2
Tz1
Tz2
1'
Reheat Box 1
Filter
Supply Air Fan
Reheat Box 2
Cooling Coil
3
2
0
3-Way Valve
3-Way Valve
CW Supply
CW Return
HW Supply
HW Return
3-Way Valve
HW supply
HW Return
0: T0 = 90 F RH0 = 55% : h0 = 39.8 Btu/lba wo = 0.0167 lbw/lba p0 = 0.07 lba/ft3
1: T1 = 72 F RH1 = 50% : h1 = 26.5 Btu/lba
3: T3 = 58 F
Zone 2 : Qs = 42,000 Btu/hr, Ql = 18,000 Btu/hr, 25 people requiring 20 cfm outdoor air
/person
Chilled Water : TCWSupply = 55 F
1
The process lines on a psychrometric chart are shown below. The mixed air condition,
2, is a mix of the return air, 1, and outdoor air, 0. As the mixed air travels over the
cooling coil, some air is cooled all the way to saturation while some air bypasses the coil.
The ‘coil conditioning line’ is defined by state point 2 and the temperature of cooling
water entering the cooling coil. The average temperature of the air leaving the cooling
coil is 58 F. Thus, state point 3 lies on the coil conditioning line at the temperature of
the air leaving the coil. The space conditioning line between supply air, 3 and return air,
1, shows the sensible and latent heat added in Zone 2.
Coil
Condition
Line
2
3
0
1
Space
Condition
Line
58
72 75.1
90
The procedure to calculate the required air flow rate and cooling capacity follows from a
series of mass and energy balances on the various systems. In this example, the mass
and energy balances can be solved in five steps, by sequentially analyzing systems with
only one unknown. Use the following five step method to determine peak loads and the
required sizes of the air handling cooling coils and the chiller.
Step 1: Find mass flow rate of supply air m3
In most commercial buildings, the total building cooling load is greater than the heating
load due to internal heat gains from occupants and electrical equipment and because
interior spaces experience only cooling loads. Thus, the mass flow rate of air required to
meet the peak building cooling load is usually greater the mass flow rate of air required
to meet the peak heating load. When this is the case, air-handler fans are sized to
handle peak cooling rather than peak heating loads.
A sensible energy balance on the air and heat flows into/from zone 1 gives:
2
 m c T m c T 0
Q
s1
,3,1 p 3
,3,1 p 1

 3.1
m
Q s1
42,000  Btu 
1  lba  F 
1
 lb 




 12,245 




c p T1  T3 
1  hr  .245  Btu  72  58F
 hr 
The total mass flow rate of air through the supply air fan, m3, is the sum of the mass
flow rates of air required through each zone.
m3 = m3,1 + m3,2 + …
Step 2: Find mass flow rate of outdoor air, moa, and fraction outdoor air, foa
Next, find the fraction of supply air that must come from the outdoors in order to meet
occupant requirements for fresh air. The fraction outdoor air, foa, is:
foa = moa / m3
.
Outdoor air requirement: 20 cfm
person



ft 3
 lba 
 min 
 lb 

moa  25 persons  20 
 2,100  
  .07 3   60 

 ft 
 hr 
 hr 
 min  person 
foa  fraction outside air 
2,100
 17%
12,245
Step 3: Fix state point
on psychrometric chart
Next, find the state of air leaving the cooling coil from the set-point temperature, T3,
and an energy balance on the zone. A total energy balance on the zone gives:
 3h3  m
 3h1  0
Q s  Q l  m
Q  Q l
 Btu  42,000  18,000  Btu   hr 
 Btu 
h3  h1  s
 26.5  
 21.6 




3
m
12,245
 lba 
 hr   lba 
 lba 
(T3, h3) fix point
Step 4: Fix state point
on psychrometric chart
Next, find the state of mixed air entering the cooling coil from sensible and total energy
balances on the mixing box where outdoor and return air are mixed. A sensible energy
balance on the mixing box gives the temperature at point 2.
3
m ,0 c p T0  m ,1 c p T1 - m ,2 c p T2  0
 o To  m
 1 T1' 2,100  90  12,245  2,100  72
m
T2 

 75.1F
2
m
12,245
A total energy balance on the mixing box, gives the enthalpy at point 2.
 oho  m
 1 h1  m
 2h2  0
m
 h m
 1h1 2100  39.8   12,245  2100   26.5
m
 Btu 
h2  o o

 28.8 
2
m
12,245
 lb 
The specific volume of air at point 2 entering the cooling coil can now be determined
from the psychrometric chart.
 ft 3 
v 2 (from psychromet ric chart)  13.6 
 lb 
Step 5: Find total heat removed from air, Qc
Finally, the total heat removed from the air can be determined from a total energy
balance on the cooling coil.
 2h2  Q c  m
 3h 3  0
m
1 ton
lb
Btu 
 Btu 
 2 h2  h3   12,245   (28.8  21.6 
Q c  m
 7.3tons   88,164 
 hr 
 lb ) 
 Btu 
 hr 
12,000 
 hr 
A small fraction of the energy removed from the air leaves the cooling coil as cold
condensate; however in design calculations, the energy carried away by the condensate
is typically considered to be negligible compared to the energy removed by the cooling
coil. Thus, cooling coils are typically sized to handle the total cooling removed from the
air.
Final Specifications
The total cooling coil load, volume flow rate through the supply air fan, cooling coil bypass factor, and cooling coil sensible heat ratio can then be determined. The values for
this example are shown below. All of these values are typically required by cooling coil
vendors in order to specify the correct cooling coil. In an HVAC system with multiple air
handlers, the total peak cooling load on the chiller would be the sum of the peak loads
of the air handlers.
The total load on the cooling coil is:
Q c  88,164 Btu/hr / (12,000 Btu/ton  hr)  7.3tons 
4
The volume flow rate across the fan is:
 ft 3  1 hr 
lb
 2 v 2  12,245   13.6   
ν 2  m
  2,775cfm
 hr 
 lb  60  min 
The cooling coil by-pass factor is;
BF 
T3  Tcc
58  55

 15%
T2  Tcc 75.1  55
The sensible heat ratio across the coil is:
SHR (coil) 
Q s
Q tot,coil
 lb 
 Btu 
12,245   .245
 75.1  58 F
 3 c p T3  T2 
m
hr 
lb  F 




 58%
 3 h2  h3 
m
 Btu 
88,164 
 hr 
5