Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 167

-100 per cent represents fully open position (i.e. maximum heating demand).
In this process, the DCV-based fresh air damper control is activated.
8.4.4 Stability of AHU sequential split-range strategies
In the simplest case of split-range control as shown in Figure 8.11, there exists
the problem of instability when the controller changes its mode between
heating and cooling coil controls due to the dead zone or rangeability of
valves, and the controllability when the valve is near closed position. When
the characteristics of heating and cooling coils are different, the control
parameters suitable for two processes are different. In the split-range strate-
gies presented in this chapter, multiple PIDs are used, which allow PIDs for
different processes to be tuned individually. However, as the PIDs can be
activated simultaneously even though only one is desired, the PIDs of other
processes affect the control of the running process.
When the control of the outdoor air damper is involved in the AHU
sequential split-range control, as shown in Figures 8.12 and 8.13, the prob-
lem is more obvious as the characteristics of the damper can be much more
different from that of coils and more changeover regions are involved. When
the AHU split-range control involves DCV control, the situation is more
complicated and the problem can be even more serious.
When AHU systems utilize economizer control to reduce energy consump-
tion, control difficulties often occur in the transient region between heating
and total free cooling, and at the transient region between total free cooling
and partial free cooling. Such control instability results in waste of energy
by using alternation modes and has a negative impact on comfort condi-
tions. In addition, it causes unexpected wear and tear on valves, dampers
and actuators.
A finite state machine (FSM) sequencing control strategy was introduced
by Seem et al. (1999) for AHUs to overcome the difficulties. Simply speak-
ing, the FSM strategy adopts certain delays in two transient regions, one
between heating mode and total free cooling mode, and the other between
total free cooling mode and partial free cooling mode, and assumes a dead
band temperature in the transient region between partial free cooling mode
and mechanical cooling mode plus minimum fresh air flow rate, when shift-
ing the control from one mode to another.
However, the problem of interference between PIDs for different processes
remains and the fixed control parameters cannot produce good performance
for the entire operating conditions. Gain scheduling is a technique that deals
with nonlinear processes, processes with time variation or situations where
the requirements on the control change with the operating conditions.
Generally, in the control strategy for AHU with economizer control, the
supply air temperature is utilized, with respect to the set-point temperature,
to determine control signals to the heating coil valve, the fresh air damper and
the cooling coil valve. When DCV control is adopted, the fresh air rate set-