Danfoss VLT HVAC Basic Drive FC 100 Series Design Manual page 23

Introduction to VLT HVAC Ba...
2.5.22 Secondary Pumps
2 2 Secondary pumps in a primary/secondary chilled water pumping system 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, when the flow rate drops far
enough or too quickly, the chiller cannot shed its load properly. 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 in
parallel are installed.
2.5.23 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 frequency converters.
With the proper sensor location, the addition of frequency converters 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, two-way valves can be
subjected too.
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 set-point 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 frequency converter running multiple pumps in parallel.
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VLT HVAC Basic Drive Design Guide
Frequency
converter
3
3
Frequency
converter
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