Control and optimization of central chilling systems - Intelligent Buildings and Building Automation - page 203

The control outlined is not difficult to achieve, but it is not achieved auto-
matically in practice. In practical operation, there needs to be certainty that
different chillers have the same chilled water supply and return temperatures.
That means each chiller should have the same supply temperature set-point
and the water supply temperature sensors should be well calibrated (or, at
least, have the same offset). The return water should be well mixed before
tee-off to individual chillers. Note that a common mixing return pipe is
recommended if a building has more than one return water pipe. When the
same chilled water supply and return temperatures are maintained, it needs
to be ensured that each chiller has the same water flow rate for chillers of the
same size or the flow rate of each chiller is proportional to its capacity. This
relies essentially on correct installation and commissioning.
9.8 Pump speed and sequence control of chilled water
systems
Similar to chillers in central chilling systems, multiple pumps are often used
in air-conditioning systems for circulation of chilled water, condenser water
or sea water. Therefore, a sequence control system is definitely needed to
provide control of the number of pumps put into operation to cope with
the changing cooling load from time to time. For variable speed pumps in
chilled water systems, a control system is needed to change the pump speed in
order to provide the required chilled water circulation. Both pump speed and
sequence control can save pump energy consumption. Details of the pump
speed and sequence control system are determined by the characteristics of
the pumps, matching between the pumps and the load side as well as the
designs of chilled water distribution systems.
9.8.1 Pump sequence control through chiller and pump interlock
For primary pumps in the constant primary-only systems, constant primary/
variable secondary systems and variable primary systems, the primary pump
and chiller combination is designed on a one-to-one matching basis and a
pump and its associated chiller are interlocked. In this case, sequence con-
trol of the pumps is accomplished by interlocking the on and off commands
for a pump with its associated chiller. Lead/lag timing of pumps should be
considered.
Starting of a pump that serves a chiller must take place at a time leading
the starting of the chiller compressor. The goal of this lead timing is to ensure
that there is already chilled water circulating in the evaporator whenever the
chiller operates. Stopping time of the pump, however, must take place at a
time lagging the stopping of the chiller. If the circulation of the chilled water
stops as soon as the chiller compressor is stopped, the residual refrigerant in
the evaporator may freeze the chilled water that stays in the evaporator tubes,
thus causing them to burst. Likewise, the condenser water for water-cooled