Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
267
To ensure that all air terminal units have sufficient pressure to
operate, a supply airflow control system is required. To monitor
duct static, static pressure sensors are installed near the end of the
supply duct. When VAV terminal unit dampers open, the static
drops in the supply duct. The static pressure sensor detects the
static pressure drop and the airflow control system increases the
supply fan output. The opposite occurs when the VAV terminal
unit dampers close.
Another VAV system feature is that the difference in airflow
between the supply and return fans (and not the position of the
outdoor air damper) determines the amount of minimum
outdoor air ventilation delivered by the supply system. For
example, Figure 1 shows a single duct VAV system for a building
with the fans on and outdoor air dampers at minimum position.
The design condition for the supply fan is 14.0 m
3 /s and the
return fan is 12 m 3 /s. The 2 m 3 /s difference is lost through
kitchen and restroom exhaust fans (0.7 m 3 /s) and exfiltration
(1.3 m 3 /s). Thus 2.0 m 3 /s of outdoor air must be brought in
through the supply fan to make up the difference. Depending
on mixed air system pressure drops, even is all dampers in the
mixed air system are fully open, the outdoor air volume will be
2.0 m
3 /s. 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.
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.
TYPES OF AIRFLOW SYSTEMS
An air handling system can provide heating, cooling,
humidification, and dehumidification as well as variable
quantities of outdoor air. Air handling systems can be classified
as single-path or dual-path. The single-path system has all
heating and cooling coils in series in a duct. The single duct
supplies all terminal equipment. The dual-path system has a
cooling coil in one duct and a heating coil (or just return air) in
another duct. Both ducts supply dual-duct terminal equipment
or multizone dampers.
These systems are further classified (ASHRAE 1996 HVAC
Systems and Equipment Handbook) as follows:
Single-Path Systems:
Single duct, constant air volume
Single zone systems
Reheat systems, single duct-variable air volume
Simple variable air volume
Variable air volume, reheat
Single duct, variable air volume-induction
Single duct, variable air volume-fan powered
Constant fan, intermittent fan
Dual-Path Systems:
Dual duct, single fan-constant air volume
Single fan, constant air volume reheat
Variable air volume
Multizone
The more common types of air handling systems are:
Single duct, variable air volume
Single duct, constant air volume
VARIABLE AIR VOLUME
A Variable Air Volume (VAV) system controls primarily the
space temperature by varying the volume of supply air rather than
the supply air temperature (Fig. 1). The interior zones of most
large buildings normally require only cooling because of
occupancy and lighting loads. Air terminal units serve these zones
and operate under thermostatic control to vary the airflow in the
individual spaces to maintain the required temperature. The
perimeter zones can have a varying load depending on the season
and exposure. Heating may be supplied via reheat coils that operate
under thermostatic control while air terminal units maintain
minimum airflow.
Airflow in the supply duct varies as the sum of the airflows
through each VAV terminal unit varies. In light load conditions,
the air terminal units reduce the airflow. As more cooling is
required, the units increase airflow. Air terminal units typically
have controls to limit maximum and minimum airflow and
compensate for variations in supply duct static.