Danfoss VLT HVAC Drive FC 102 Design Manual page 39

Product Features
5.3.4.6 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, which increases 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 2 or more chillers in parallel are
installed.
®
VLT solution
While the primary/secondary system with 2-way valves improves energy and system control, using drives increases the
energy savings and control potential further. With the proper sensor location, the addition of drives allows the pumps to
match their speed to the system curve instead of the pump curve, which eliminates wasted energy and most of the over-
pressurization to which 2-way valves can be subjected.
As the monitored loads are reached, the 2-way valves close down, increasing 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 setpoint value is calculated by summing the pressure drop of the load and 2-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 drives, or 1 drive running multiple pumps in parallel.
Frequency
converter
Illustration 5.21 Drives Used with Secondary Pumps in a Primary/Secondary Pump System
For more information, consult the Danfoss supplier for the Secondary Pumps: Improving Secondary Pumping in Pri/Sec System
application note.
MG16C302
Design Guide
Frequency
converter
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