Secondary Pumps - Danfoss VLT Series Design Manual

2. Introduction to the VLT HVAC Drive

2.6.22. 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
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system is used to hydraulically decouple 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 installations, especially when two or more chillers are installed in
parallel.
2.6.23. The VLT 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 ad-
justable frequency drives.
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 adding the pressure drop of the load and the two-way valve under design conditions.
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NOTE
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
drive running multiple pumps in parallel.
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide