Danfoss VLT FC 102 Series Design Manual page 37

Introduction to VLT® HVAC D...
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 hydraulically 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, the chiller cannot shed its
load properly when the flow rate drops far enough or too quickly. 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 are
installed in parallel.
2.7.24 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 adjustable frequency drives.
With the proper sensor location, the addition of adjustable frequency drives 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 to.
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 up the pressure drop of the load and two-way
valve under design conditions.
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 running multiple pumps in parallel.
Frequency
converter
Figure 2.22 The VLT Solution
MG11BC22
Design Guide
Frequency
converter
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