Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 156

8.3.2 VAV terminal and room temperature control

The AHU of a VAV system supplies air usually at constant or nearly con-
stant temperature and humidity. If supervisory control is employed, it can
also supply variable temperature. Capacity is controlled to match cooling
load by varying the volume of air supplied to a zone. Usually, a VAV box is
provided in each zone which is connected to air supply duct(s). Such a unit
can regulate the amount of air that enters the space or room to maintain the
temperature both in heating and cooling seasons.
In terms of pressure, VAV boxes can be divided into pressure-dependent
and pressure-independent. According to power types, they can be classified
as induction types and fan-powered types. Fan-powered VAV boxes consist
of parallel arrangement and series arrangement.
In a pressure-dependent unit, the damper ( D-1 ) of the VAV box is control-
led directly by the temperature controller ( TC-1 ) using the measured space
temperature ( T1 ), as shown in Figure 8.17. This means, in effect, that the
pressure-dependent unit has no way of sensing change in the system pressure
that may change due to a number of boxes throttling down and reducing the
air flow of the system. These types of boxes depend entirely on the ability of
the central fan system to adjust the flow as the boxes close off or open up.
The result is that boxes near the fan get the majority of total air flow rate
during heavy loads or pull-down periods. Moreover, seriously over-tuned or
down-tuned phenomena should be very obvious because of the slow response
of the building system as the change of pressure is sensed only after the room
temperature is affected.
Figure 8.18 shows the control diagram of a pressure-independent VAV
box. An example of the implementation of the control strategy in a simplified
symbolic programming environment is shown in Figure 8.19. Temperature
control is based on resetting flow set-point using cascade control to eliminate
the effects of supply air pressure change on zone temperature control. In the

VAV box

D-1

Return air

Supply air

Occupied zone

T 1

M-1

Pre-set

TC-1

temperature

TC-1: Space air temperature controller
D-1: Damper of VAV box
M-1: Damper motor

Figure 8.17 Control diagram of a pressure-dependent VAV box.