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 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.
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 setpoint value is calculated by summing the pressure
drop of the load and two way valve together under design conditions.
NB!
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 drive running multiple pumps in parallel.
VLT
®
6000 HVAC Series
MG.61.B5.02 - VLT
®
is a registered Danfoss trademark 23
Introduction to HVAC
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