SYS-APM001-EN
Chilled-Water System Issues
64
One solution that has been suggested is to install a check valve in the
plant common pipe or decoupling line circuit ... However, to be
realistic, all that the check valve can do is assure that no water
bypasses the chillers, which in turn has the effect of increasing flow in
the chiller. Thus with the installation of the check valve must be an
algorithm to either slow down system pump(s) when flow increases
beyond the design maximum for the on chiller or turn on additional
chillers. Thus the check valve solution offers its own problems and
many designers feel very uncomfortable with forcing pumps into
series operation without the benefit of understanding the full impact
thereof.
This manual does not recommend the use of check valves in the bypass line.
Failure recovery
Figure 27 Failure recovery
With all the varied approaches available to potential customers, it sometimes
seems
that the main idea gets lost. People purchase chilled-water plants to
reliably produce chilled water to satisfy another need, such as comfort or
process cooling. Therefore when the plant operates there must be a process in
place to recover from the failure of a chilled- or condenser-water pump, cooling
tower, or chiller. Failure recovery should be integral to the chilled-water plant
control system. Refer to Figure 27. If the control sequence tries to start Chiller 2
and its pump, but the pump has an electrical malfunction, the sequence should
immediately lock out Chiller 2 and its pump. The control system should then
attempt to start Chiller 3 and its pump. At the same time, the control system
should send a pump malfunction alarm to the system operator.
Alternative energy sources
Energy source redundancy is receiving increased attention due to rate
variations and reliability issues brought about by the deregulation of electrical
utilities across the country. There are two basic approaches to meet these
issues:
n
Provide an alternate source of internally-generated electricity to serve a
system in the event of general failure or an unacceptably high temporary
cost of electricity (time-of-day rates).
n
Provide an alternate source of chilled-water producing capability, possibly
fueled by natural gas, another fossil fuel, or even using low cost electricity
as an alternative energy source.
Chiller 3 Start
Chiller 2 Unavailable
Chiller 1 Running
Bypass Line
Return
Supply