The bypass valve may have one or two functions.
One function is to limit system pressure. The bypass
valve dumps supply water flow back to the pumps if the
system pressure becomes excessive. Another function
is to maintain a minimum flow through the pumps. This
is to prevent pump cavitation, which occurs when the
flow through an operating pump is restricted. A bypass
valve is used most commonly in systems where the enduse
equipment has throttling valves.
If the system has chilled water pumps that are
matched in capacity to the chillers, the procedure is
simple. Operate the same number of pumps and chillers.
Whenever a chiller is turned off, turn off the
corresponding pump and isolate the evaporator of the
chiller.
If the pump and chiller capacities are not matched,
use a pressure sensing control to control the number of
pumps that operate. How to determine the minimum
acceptable system pressure is explained below.
This method of control should eliminate the need
for the bypass valve under all or most conditions. Pump
cavitation is not a problem with this Measure, because
it reduces the number of pumps in operation. To make
sure that the bypass valve does not waste energy by
bypassing water unnecessarily, determine the maximum
safe pressure for the system. Set the bypass valve to the
highest allowable pressure.