Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 150

Heating zone Cooling zone

100%

Heating valve Cooling valve

0%

(Outdoor air temp. increases)

-100% 0% 100%

(A)

Output of PID controls

Heating valve

0~100%

Supply air
temperature

Set-point Rescale Rescale

0~100%

Supply air temperature
feedback controller

Wide open when u c =100%
Closed when u c ≤ 0%

Cooling valve

-100~0%

Rescale

0~100%

Wide open when u h =100%
Closed when u h ≤ 0%

(B)

Heating valve

Figure 8.11 Sequential split-range control strategy for supply air temperature control
of AHUs (outdoor air control optimization not included).

Figure 8.12 illustrates the split-range control strategy and Figure 8.13
provides more detailed description of the relationship between the split-range
control output ( u ) of the supply air temperature feedback controllers and the
control signals u c , u h and u d (corresponding to fresh air damper control) in the
strategy in this configuration. The combined controller output scale ranges
from -100 to 200 per cent in the strategy as illustrated in the figures.
When the combined output of the feedback controls is between 100 and
200 per cent, it will be rescaled to 0 to 100 per cent to activate the cooling
coil valve. Here 100 per cent corresponds to the closed position of the cool-
ing coil valve, meaning no mechanical cooling, and 200 per cent corresponds
to the fully open position, meaning maximum mechanical cooling. At this
mode, the heating coil valve is closed. When the return air enthalpy is larger
than or equal to the fresh air enthalpy, economizer cycle control (the fresh