Danfoss VLT HVAC Basic Drive FC 100 Series Design Manual page 22

Introduction to VLT HVAC Ba...
2.5.21 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 common:
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 frequency until the design flow
rate is achieved.
Using a frequency converter to decrease the pump speed is very similar to trimming the pump impeller, except it doesn't
require any labor and the pump efficiency remains higher. The balancing 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 pump speed instead of requiring a new pump impeller.
Flowmeter
Frequency
converter
VLT HVAC Basic Drive Design Guide
F
Frequency
converter
®
MG.18.C2.02 - VLT is a registered Danfoss trademark
Flowmeter
F
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