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46 Chiller System Design and Control SYS-APM001-EN
System Configurations
Figure 28. Decoupled arrangement
The unrestricted bypass line hydraulically decouples, or separates, the
production and distribution pumps so that they cannot operate in a series
coupled pumping arrangement.
Although the two pumping systems are independent, they have three things
in common:
• bypass piping,
• no-flow static head (from the building water column), and
• water.
Changes in flows or pressures, due to variations in dynamic head or the
number of chillers operating, cannot cross the bypass line.
The extent of decoupling depends solely on the restriction (or lack of
restriction) in the bypass pipe. Total decoupling is accomplished only if the
bypass piping has zero pressure loss at any flow. Since this is not possible,
some insignificant pump coupling will exist. The important issue is to keep
the bypass piping free of unnecessary restrictions such as check valves (see
sidebar).
Check valves
Some designers recommend the
installation of a check valve in the
bypass line of a primary–secondary
system to eliminate the possibility of
deficit flow in the bypass line. The
premise is that if there is a system
problem (low temperature differential),
the check valve will put the primary and
secondary pumps in series and pump
more water through the chiller, thus
balancing the primary and secondary
flow requirements. This is not universally
accepted.
Coad
14
states:
One constraint is that if the system is
designed as variable flow and is
experiencing operating problems
related to low return water
temperatures, the solution is not in the
plant but rather in the load.
and
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.
~
Chiller 3
Chiller 2
Chiller 1
Bypass Line
Production
Distribution
VV Pump
CV Pump
CV Pump
CV Pump