Secondary Pumps; The Vlt Solution - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

Product Overview

3.1.22 Secondary Pumps

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 1 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 concept is not used in the design
of a variable volume system 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 2
or more chillers in parallel are installed.

3.1.23 The VLT Solution

While the primary-secondary system with 2-way valves
improves energy savings and eases system control
problems, the true energy savings and control potential is
realized by adding frequency converters.
Frequency
converter
Illustration 3.16 Secondary Pumps
MG18C702
Design Guide
Frequency
converter
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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 overpressurization that 2-way valves
can be subjected to.
As the monitored loads are reached, the 2-way valves close
down. This increases the differential pressure measured
across the load and the 2-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 summing the pressure drop of the load
and the 2-way valve together under design conditions.
NOTICE
When running multiple pumps in parallel, they must run
at the same speed to maximize energy savings, either
with individual dedicated frequency converters or 1
frequency converter running multiple pumps in parallel.
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