Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 173

air flow to the air-conditioned zones. In cases where pneumatic controllers
are used for VAV boxes or the VAV controllers are not integrated with the
AHU controllers, it is not possible for the AHU controller to have the infor-
mation on the control of VAV boxes, so some 'near optimal' control could be
adopted. The optimal set-point can be set as a function of the total supply air
flow rate according to experience or site monitoring or site tests. When the
total supply air flow rate reduces, the pressure set-point can be reduced.

8.6.2 Supply air temperature set-point reset

The air flow rate to a zone is reduced by closing down the VAV damper when
the load of a zone is low. Under low partial load, the total flow rate of a
zone may be reduced to be very low in order to meet the reduced load. The
reduction in total flow rate results in significant savings in fan power. On
the other hand, a low ventilation flow rate may cause deficiencies in system
performance (e.g. poor mixing of supply air and room air, inadequate room
ambient air circulation and dumping). When a minimum limit of the total
flow rate is used, a space may be overcooled under low partial load if the
supply air temperature is low.
A proper resetting of the supply air temperature allows the VAV system to
avoid poor ventilation and save fan power. A strategy for online reset of the
supply air temperature set-point is illustrated in Figure 8.29. The strategy
utilizes the air flow rate set-points of the pressure-independent VAV box

VAV damper

Supply

Return

Occupied zone

F

T

A

Air flow
set-point

AHU supply air
temperature set-point

Air flow
controller

Temperature
controller

From other
zones

AHU supply air temperature set-point reset

Ratio to high
flow setting

M up,set

H PID

Maximum

PID (-)
Function

Change rate
limit

N max
N min

Ratio to low
flow setting

PID (-)
Function

Minimum Re-scale

M min,set

From other
zones

L PID

RM min,H
RM min,L

RM min,set
RM min,hys

Threshold
settings

PID (+)
Function

Differential

Figure 8.29 Schematic of AHU supply air temperature set-point reset strategy.