Control and optimization of central chilling systems - Intelligent Buildings and Building Automation - page 204

chillers must continue to flow through the condenser for a certain period of
time after the chiller compressor is stopped so that the residual hot refriger-
ant gas discharged by the compressor will condense into liquid state, thus
avoiding the build-up of excessively high pressure in the condenser.

9.8.2 Speed and sequence control of secondary pumps

In conventional constant primary/secondary variable pumping systems, con-
trol of the secondary pumps (constant or variable speed) is independent of
the chiller sequence control, as the flows of two systems are decoupled. The
primary concern in control of secondary pumps is to ensure that the number
of pumps put into operation have adequate capacity to deliver the required
amount of chilled water to the load side (i.e. cooling coils).
For sequence control of secondary pumps of constant speed, the controlled
variable could be the flow rate or the differential pressure of the chilled water
supplied to the load side. The signal of this variable is sensed by a flow meter
installed at the main supply or return pipe of the secondary system (as shown
in Figure 9.22). The differential pressure at the supply side or at a critical (or
most remote) zone may be measured. When using flow rate measurement for
control, the controller switches the pumps on or off according to the staged
flow rate setting values pre-set. The controller decides the required number
of pumps to be put into operation according to the flow rate demanded of
the load side.
Figure 9.23 illustrates the control schemes of multiple constant speed
pumps when the differential pressure at the supply side of the secondary
chilled water loop is used as the control variable. Pressure settings for the
switching points are pre-set according to the pumps' characteristic curves.

For speed control of
variable speed pumps

Coil

Differential
pressure set-point

Differential
pressure

Coil

Controller

Constant or variable
speed pumps

M

Modulating valve

Bypass

To chillers From chillers

Figure 9.22 Schematic diagram of sequence control for secondary pumps in constant
primary/secondary variable pumping systems.