Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 148

needed to control supply air humidity in cooling seasons. However, humidi-
fication is necessary in dry seasons, and humidification control can be used
alone, which was discussed in Section 8.1. Humidity controller HC-1 with
supply air humidity sensor ( H1 ) can regulate steam valve ( V-3 ) according
to the supply air humidity set-point set by the controller HC-2 using space
humidity sensor H2 . The supply air temperature and fresh air control can
be achieved with sequential split-range control. However, in many practical
applications, the fresh air intake is set constant or controlled manually and
the control is less complicated. In many applications, only cooling (coil) or
heating (coil) is provided, and the situation is even easier to control.
The control of a situation with heating and cooling coils (without outdoor
air control) is discussed below followed by discussion of the control strategy
of the situation with a 'complete set-up'. It is important to notice that the
sequential split-range control strategies for the control of heating and cooling
coils and the control of heating and cooling coils plus outdoor air damper
are the same as that used in VAV systems.
In the simplest case when only one cooling coil or heating coil is used,
the control is simply a reconfigured cascade control. In the other two cases,
the control is a combination of cascade control and sequential split-range
control. The control strategy of the supply air temperature set-point reset
controller ( TC-2 ) is similar in all the CAV control cases. It can generate sup-
ply air temperature set-points using the measured space temperature ( T2 ). In
practical applications, PI or PID function is usually employed in the control-
ler. Figure 8.10 illustrates the control strategy of the principle of a simple
supply air temperature set-point reset controller. As the cooling load increases
(or heating load reduces), the PI(D) output increases and the set-point of
the supply air is reduced linearly, which is the simplest option and a good
choice for control. The upper limit ( Tset max ) and lower limit ( Tset min ) are set
according to system design or practical requirements, which are basically the
limits that operators or designers have allowed, taking into account various

Tset max

Tset min

(Cooling load increases)

100%

0% Output of PID control

Figure 8.10 Control strategy of CAV supply air set-point reset controller.