GE AF-600 FP Design And Installation Manual page 72
Fan & pump drive
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Application Set-up Examples
6.3.20 Secondary Pumps
Secondary pumps in a primary/secondary chilled water
pumping system are used to distribute the chilled water to
the loads from the primary production loop. The primary/
secondary pumping system is used to hydronically de-
couple one piping loop from another. In this case, the
primary pump is used to maintain a constant flow through
the chillers while allowing the secondary pumps to vary in
flow, increase control and save energy.
If the primary/secondary design concept is not used and a
variable volume system is designed, the chiller cannot
shed its load properly when the flow rate drops far
enough or too quickly. The chiller's low evaporator
temperature safety then trips the chiller, requiring a
manual reset. This situation is common in large instal-
lations, especially when two or more chillers are installed
in parallel.
6.3.21 The AF-600 FP Solution
While the primary-secondary system with two-way valves
improves energy savings and eases system control
problems, the true energy savings and control potential is
realized by adding adjustable frequency drives.
Figure 6.14
AF-600 FP Design and Installation Guide
With the proper sensor location, the addition of adjustable
frequency drives allows the pumps to vary their speed to
follow the system curve instead of the pump curve.
This results in the elimination of wasted energy and
eliminates most of the over-pressurization to which two-
way valves can be subjected.
As the monitored loads are reached, the two-way valves
close down. This increases the differential pressure
measured across the load and two-way valve. As this
differential pressure starts to rise, the pump is slowed to
maintain the control head also called setpoint value. This
setpoint value is calculated by summing the pressure drop
of the load and two-way valve together under design
conditions.
Please note that when running multiple pumps in parallel,
they must run at the same speed to maximize energy
savings, either with individual dedicated drives or one
adjustable frequency drive running multiple pumps in
parallel.
DET-768A
6
6
6-15
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