Danfoss FC 101 Design Manual page 26

Product Overview
2.6.22 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 2
or more chillers in parallel are installed.
2.6.23 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
realised by adding frequency converters.
Illustration 2.16 Secondary Pumps
24
®
VLT HVAC Basic Drive FC 101 Design Guide
Frequency
converter
3
3
Frequency
converter
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, 2-way valves
can be subjected too.
As the monitored loads are reached, the 2-way valves close
down. This increases the differential pressure measured
across the load and 2-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.
NOTICE
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.
MG18C502 - Rev. 2013-09-06
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