Danfoss VLT HVAC Basic Drive FC 101 Design Manual page 24

Introduction to VLT® HVAC B...
2.6.22 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 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.
2.6.23 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.
Frequency
converter
Illustration 2.16
®
VLT HVAC Basic Drive Design Guide
Frequency
converter
3
3
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MG18C402 - VLT is a registered Danfoss trademark
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.
NOTE
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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