Specifications
2.7.23 Secondary Pumps
Secondary pumps in a primary/secondary chilled water
pumping system are used to distribute the chilled water to
the loads from the primary production loop. The primary/
secondary pumping system is used to de-couple one
piping loop from another hydronically. In this case, the
primary pump maintains a constant flow through the
chillers, allowing the secondary pumps to vary 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 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.
2.7.24 The VLT Solution
While the primary/secondary system with 2-way valves
improves energy and system control, using frequency
converters increases the energy savings and control
potential further.
With the proper sensor location, the addition of frequency
converters allows the pumps to match their speed to the
system curve instead of the pump curve.
This eliminates wasted energy and most of the over-
pressurization, two-way valves can be subjected to.
As the monitored loads are reached, the 2-way valves close
down, increasing 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 setpoint value is
calculated by summing the pressure drop of the load and
two way valve together under design conditions.
NOTICE
When running multiple pumps in parallel, they must run
at the same speed to increase energy savings, either
with individual dedicated frequency converters, or one
frequency converter running multiple pumps in parallel.
Frequency
converter
Frequency
converter
CHILLER
CHILLER
3
3
P
130BB454.10
Illustration 2.16 Frequency Converters used with Secondary Pumps in a Primary/Secondary Pump System
Introduction VLT® HVAC Drive FC 102 Design Guide
MG16C102 - Rev. 2013-08-20 29
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