Variable Versus Constant Air Volume; Ventilation - Honeywell AUTOMATIC CONTROL Engineering Manual

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 40,000 cfm. Therefore, the
supply fan is sized for 40,000 cfm and, with 5,000 cfm exhaust,
the return fan is sized for 35,000 cfm.
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 40,000 cfm with a diversity of 75 percent.
This means that the supply fan is sized for only 30,000 cfm.
Similarly the return fan is sized for 25,000 cfm instead of 35,000
cfm 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.
If this is done on a VAV system at design load, as the VAV
boxes require less cooling and less airflow, the supply fan
capacity reduces, and the inlet pressure to the supply fan
becomes less negative as the fan unloads. As the filter inlet
pressure becomes less negative, less OA is drawn into the system
which is unacceptable from a ventilation and IAQ perspective.
VAV systems therefore require design considerations to prevent
non-economizer occupied mode ventilation from varying with
the cooling load. This may be accomplished in several ways.
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
The dampers may be set at design load as for a constant air
volume system, and the filter inlet pressure noted. Then the
noted filter inlet negative pressure can be maintained by
modulating the return air damper. Keeping this pressure constant
keeps the OA airflow constant. This method is simple but
requires good maintenance on the OA damper and linkage,
positive positioning of the OA damper actuator, and the
balancing person to provide the minimum damper position and
the pressure setpoints.
Another positive method is to provide a small OA injection
fan set to inject the required OA into the AHU mixing box
during occupied periods (the OA damper remains closed). The
fan airflow quantity may be controlled by fan speed adjustment
or inlet damper setting and sensed by an airflow measuring
station for closed loop modulating control. This basic method
is positive and relatively maintenance free, but like the pressure
control method, it requires a balancing person to make
adjustments. This closed loop control method is more costly
and requires keeping the airflow pickup/sensor clean, but it
allows simple setpoint entry for future adjustments.
A minimum OA damper may be provided for the occupied
OA airflow requirement. An airflow measuring station in the
minimum OA duct is required to modulate the minimum OA
damper in sequence with the RA damper to maintain a constant
volume of OA. This method is more costly than the first method,
but it allows convenient software setpoint adjustments.
Theoretically, the mixing dampers may be modulated to
maintain a constant OA volume during occupied periods using
an OA duct airflow measuring station. Since, in these examples,
the OA duct is sized for 100 percent OA, the minimum is usually
20 to 25 percent of the maximum. The airflow velocity at
minimum airflow is extremely low, and velocity pressure
measurement is usually not practical. Hot wire anemometer
velocity sensing at these velocities is satisfactory, but costly,
and requires that the sensing element be kept clean to maintain
accuracy. (Filtering the entering OA is helpful.) A smaller,
minimum OA damper and duct may also be used to assure
adequate airflow velocity for velocity pressure measurement.
If a return fan and volumetric tracking return fan control are
used, and no relief/exhaust dampers exist, or if the RA damper
is closed and the MA damper is open during occupied non-
economizer modes, the OA volume equals the SA volume minus
the RA volume. This method is simple and low cost but is only
applicable when building exhaust and exfiltration meets
minimum ventilation requirements.
Where minimum OA only is provided (no economizer
dampers), variations of any of these methods may be used. See
the Air Handling System Control Applications section for
further information.
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ENGINEERING MANUAL OF AUTOMATION CONTROL