Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 174

controllers as the cooling load indicators of individual zones. The minimum
flow set-point is set to avoid performance deficiency in individual zones,
which may have different values for different zones and need to be selected
and tuned according to the actual design and situation of individual zones.
The upper flow rate set-point is a parameter used only for calculating the
relative load of a zone. Since different zones may have very different flow
ranges, the air flow rate set-point of each zone is normalized by using the
ratios of the set-point to the minimum flow set-point and upper flow set-
point respectively.
The maximum ratio to upper flow set-point among zones is selected as the
indicator of the relative load of the zone with the most critical thermal load.
The minimum ratio to minimum flow set-point among zones indicates the
most critical zone in terms of ventilation. A positive value of L PID indicates
that flow in a certain zone is too low and the supply air temperature set-
point needs to be increased. A large value of H PID indicates that the load of
a certain zone is high and the supply air temperature set-point needs to be
reduced. The outputs of the PID functions are rescaled within a pre-set range
to determine the temperature set-point. The change rate limit is applied to
the temperature set-point to ensure the stability of system control.
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