Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
268
Fig. 1. Single Duct Variable Air Volume System.
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
3
/s. Therefore, the
supply fan is sized for 18.9 m 3 /s and, with 2.4 m 3 /s exhaust, the
return fan is sized for 16.5 m
3
/s.
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
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
3
/s with a diversity of 75 percent. This
means that the supply fan is sized for only 14.0 m 3 /s. Similarly
the return fan is sized for 12.0 m
3
/s instead of 16.5 m
3
/s and the
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.
RETURN
AIR FAN
OUTDOOR
RELIEF (OR)
EA
OA
RA
12.0 m
3
/s
MAXIMUM
COOLING
COIL
FILTER
OUTDOOR
AIR
2 m
3
/s
MINIMUM
14.0 m
3
/s
MAXIMUM
REMOTE
EXHAUST FAN
0.7 m
3
/s
T T
INTERIOR ZONES
T
T T T
T
T
REHEAT
COILS
PERIMETER
ZONES
C4072
EXFILTRATION
1.3 m
3
/s
VAV AIR
TERMINAL
UNITS
T
FLOW
MEASURING
STATION
75% DIVERSITY,
AIR TERMINAL UNIT
TOTAL19.0 m
3
/s
STATIC
PRESSURE
TUBES
INLET VANE
DAMPERS
FLOW MEASURING STATION
SUPPLY
AIR FAN
T