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BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
VARIABLE VERSUS CONSTANT AIR VOLUME
Sizing of central equipment is based on climate conditions
which indicate heating and cooling loads. In a CAV system,
typically outdoor air is supplied, cooled, distributed to the
various zones, and then reheated for the individual needs of
each space. The sizing of the central equipment is based on the
sum of the loads from all air terminal units served by the CAV
system. In Figure 2 for a single duct CAV system, the maximum
load from all air terminal units is 18.9 m
supply fan is sized for 18.9 m
return fan is sized for 16.5 m
The diversity of the heating and cooling loads in a VAV system
permits the use of smaller central equipment. If a building with
a VAV system has glass exposures on the east and west sides,
the solar load peaks on the two sides at different times. In
addition, due to building use, offices and conference rooms on
the east and west sides are never fully occupied at the same
time. The interior spaces do not use reheat and receive only the
amount of cooling required. Perimeter zones use reheat, but
only at minimum airflow. Therefore, the instantaneous load of
the VAV system central equipment is not the sum of the
OUTDOOR
RETURN
RELIEF (OR)
AIR FAN
EA
RA
INLET VANE
DAMPERS
OA
FILTER
FLOW
MEASURING
COOLING
STATION
COIL
OUTDOOR
AIR
3
2 m
/s
MINIMUM
T
T
T
T
Fig. 1. Single Duct Variable Air Volume System.
3
/s. Therefore, the
3
3
/s and, with 2.4 m
/s exhaust, the
3
/s.
FLOW MEASURING STATION
3
12.0 m
/s
MAXIMUM
REMOTE
EXHAUST FAN
SUPPLY
AIR FAN
3
0.7 m
/s
3
14.0 m
/s
MAXIMUM
75% DIVERSITY,
AIR TERMINAL UNIT
3
TOTAL19.0 m
/s
INTERIOR ZONES
T
STATIC
PRESSURE
T
TUBES
PERIMETER
ZONES
T
T
maximum loads from all air terminal units. Instead, the
maximum instantaneous load on the central equipment of a
VAV system is a percentage of the sum of all maximum
individual loads. This percentage can vary for different
buildings.
In Figure 1 for a single duct VAV system, the maximum air
terminal unit load is 18.9 m
means that the supply fan is sized for only 14.0 m
the return fan is sized for 12.0 m
coils, filters, and ducts can also be downsized.
VENTILATION
Care must be used to assure that the AHU ventilation design
complies with relevant codes and standards which are frequently
revised. Ventilation within a constant air volume system is often
a balancing task. During occupied, non-economizer periods of
operation, the mixing dampers are positioned to bring in the
required OA. The system balancing person determines the
specific minimum ventilation damper position.
268
ENGINEERING MANUAL OF AUTOMATIC CONTROL
EXFILTRATION
3
1.3 m
/s
VAV AIR
TERMINAL
UNITS
T
T
REHEAT
COILS
C4072
3
/s with a diversity of 75 percent. This
3
/s instead of 16.5 m
3
/s. Similarly
3
/s and the
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