Danfoss VLT Series Design Manual page 26

2. Introduction to VLT HVAC Drive
2.7.20. 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 over-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 in parallel are installed if primary/ secondary
pumping is not utilized.
2.7.21. 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.
A frequency converter 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 com-
mon:
The first method uses a flow meter. Because the desired flow rate is known and is 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 frequency converter 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.
Using a frequency converter to decrease the pumps speed is very similar to trimming the pumps
impeller, except it doesn't require any labor and the pumps efficiency remains higher. The bal-
ancing contractor simply decreases the speed of the pump until the proper flow rate is achieved
and leaves the speed fixed. The pump will operate at this speed any time the chiller is staged on.
Because the primary loop doesn't have control valves or other devices that can cause the system
curve to change and the variance due to staging pumps and chillers on and off is usually small,
this fixed speed will remain appropriate. In the event the flow rate needs to be increased later in
the systems life, the frequency converter can simply increase the pumps speed instead of requiring
a new pump impeller.
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Flowmeter Flowmeter
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MG.11.B2.02 - VLT is a registered Danfoss trademark
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VLT HVAC Drive Design Guide