Constant Air Volume - Honeywell AUTOMATIC CONTROL Engineering Manual

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
are fully open, the outdoor air volume will be 5,000 cfm. To
increase outdoor air volume, it is necessary to modulate the return
air damper. This airflow control provides a slightly positive
building static pressure with respect to outdoor air in a properly
designed system.
As supply air volume is reduced, so is return air volume.
The return air fan is normally sized smaller than the supply
fan. Flow measuring stations are located in both the supply
and return ducts so that the return air fan can track the airflow
of the supply fan with a constant flow differential. Thus, as
airflow through the supply fan reduces, the control system
reduces the airflow of the return fan. This control system can
maintain either a fixed airflow difference or a percentage of
supply airflow difference. For more information on VAV
systems, see AIRFLOW CONTROL APPLICATIONS.
ENGINEERING MANUAL OF AUTOMATION CONTROL
OUTDOOR
RETURN
RELIEF (OR)
AIR FAN
EA
RA
INLET VANE
DAMPERS
OA
FILTER
FLOW
MEASURING
COOLING
STATION
COIL
OUTDOOR
AIR
5,000 CFM
MINIMUM
T T
T T
Fig. 1. Single Duct Variable Air Volume System.

CONSTANT AIR VOLUME

A Constant Air Volume (CAV) system controls space
temperature by altering the supply air temperature while
maintaining constant airflow. Since the airflow is constant, the
system design provides sufficient capacity to deliver supply air
to the space for design load conditions. In many systems, reheat
coils allow individual space control for each zone and provide
heating when required for perimeter zones with different
exposure. The CAV system shown in Figure 2 has the same
interior and perimeter zone load requirements as the VAV system
in Figure 1. The CAV system, however, does not use static
pressure sensors and flow measuring stations since the airflow
is constant.
In a CAV system, the supply and return fans are manually
set to meet the total airflow needs. The cooling coil discharge
temperature can be reset as a function of the zone having the
greatest cooling load. This improves operating efficiency. The
reheat coils on the constant volume boxes are controlled by
individual space thermostats to establish the final space
temperatures.
FLOW MEASURING STATION
25,000 CFM
MAXIMUM
REMOTE
EXHAUST FAN
SUPPLY
AIR FAN
2,000 CFM
30,000 CFM
MAXIMUM
75% DIVERSITY,
AIR TERMINAL UNIT
TOTAL 40,000 CFM
INTERIOR ZONES
T
STATIC
PRESSURE
T
TUBES
PERIMETER
ZONES
T T
268
EXFILTRATION
3,000 CFM
VAV AIR
TERMINAL
UNITS
T
T
REHEAT
COILS
C2662