Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 172

position control demands as the indicator of relative load of the individual
VAV terminals as shown in Figure 8.28.
The local DDC control signals can be conveniently collected by the super-
visory controller when the AHU and VAV control stations are integrated in
a single network. The static pressure is adjusted just allowing that the VAV
damper with the highest relative cooling load among all the VAV terminals
is controlled to be very close to fully open position at any time, in order to
ensure that all the individual zones are supplied with sufficient air and the
static pressure is controlled at its lowest allowable level.
When a large number of VAV terminals are associated with one AHU, a
certain limited number of VAV dampers may be allowed to be fully open (or
over the threshold) by setting the parameter ( N max ). Therefore, the first N max
largest damper positions are neglected and the ( N max +1 )st is used as the maxi-
mum control damper position by the controller. When the maximum damper
position is over a threshold, which is set as the set-point of the PID, one PID
function is activated to increase the static pressure set-point. By contrast, the
other PID function is activated to reduce the static pressure set-point when
the maximum damper position is below the threshold. A hysteresis is added
as a 'dead band' of set-points to increase the stability of the control strategy.
The static pressure set-point is determined by rescaling the difference between
the outputs of two PID functions within a pre-set pressure range.
In some applications, the summations of the demanded flow set-points and
the actual measured flow rates of VAV boxes are used as the indicator. If the
summation of actual measured flow rates of VAV boxes is less than that of
the set-points, the pressure set-point is not high enough to deliver sufficient

VAV damper

Supply

Return

Occupied zone

F

T

A

Air flow
set-point

Air flow
controller

Temperature
controller

VAV control
signal from
other zones

Static pressure set-point reset

D

Maximum PID (-)

Function

Static pressure
set-point

N max

Differential

D set,H

PID (+)
Function

D set
D hys

Threshold
settings

Change rate
limit

Re-scale

D set,L

Figure 8.28 Schematic of VAV static pressure set-point reset strategy.