Primary Pumps - Danfoss VLT Series Design Manual

2. Introduction to the VLT HVAC Drive

2.6.20. Primary pumps

Primary pumps in a primary/secondary pumping system can be used to maintain a constant flow
through devices that encounter operation or control difficulties when exposed to variable flow.
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The primary/secondary pumping technique decouples the "primary" production loop from the
"secondary" distribution loop. This allows devices such as chillers to obtain constant design flow
and operate properly while allowing the rest of the system to vary in flow.
As the evaporator flow rate decreases in a chiller, the chilled water begins to become overly chilled.
As this happens, the chiller attempts to decrease its cooling capacity. If the flow rate drops far
enough, or too quickly, the chiller cannot shed its load sufficiently and the chiller's low evaporator
temperature safety trips the chiller, requiring a manual reset. This situation is common in large
installations, especially when two or more chillers are installed in parallel and primary/secondary
pumping is not utilized.
2.6.21. The VLT solution
Depending on the size of the system and the size of the primary loop, the energy consumption of
the primary loop can become substantial.
An adjustable frequency drive can be added to the primary system to replace the throttling valve
and/or trimming of the impellers, leading to reduced operating expenses. Two control methods
are common:
The first method uses a flow meter. Because the desired flow rate is known and constant, a flow
meter installed at the discharge of each chiller can be used to control the pump directly. Using
the built-in PID controller, the adjustable frequency drive will always maintain the appropriate flow
rate, even compensating for the changing resistance in the primary piping loop as chillers and
their pumps are staged on and off.
The other method is local speed determination. The operator simply decreases the output fre-
quency until the design flow rate is achieved.
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MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide