Danfoss VLT HVAC Drive FC 102 Design Manual page 37

Product Features
Water
Inlet
BASIN
Illustration 5.19 Drive Used with a Condenser Pump
For more information, consult the Danfoss supplier for the Condenser Pumps: Improving Condenser Water Pumping Systems
application note.
5.3.4.5 Primary Pumps
Primary pumps in a primary/secondary pumping system can maintain a constant flow through devices that encounter
operation or control difficulties when exposed to variable flow. The primary/secondary pumping technique decouples the
primary production loop from the secondary distribution loop. Decoupling allows devices such as chillers to obtain constant
design flow and operate properly while allowing the rest of the system to vary in flow. As the evaporator flow rate
decreases in a chiller, the water begins to become overchilled. As the water becomes overchilled, the chiller attempts to
decrease its cooling capacity. If the flow rate drops far enough, or too quickly, the chiller cannot shed its load sufficiently
and the low evaporator temperature safety trips the chiller, requiring a manual reset. This situation is common in large
installations, especially when 2 or more chillers in parallel are installed if primary/secondary pumping is not used.
®
VLT solution
A drive can be added to the primary system, to replace the throttling valve and/or trimming of the impellers, leading to
reduced operating expenses. Two control methods are common:
•
A flow meter installed at the discharge of each chiller can control the pump directly because the desired flow rate
is known and constant. Using the PID controller, the drive always maintains the appropriate flow rate, even
MG16C302
Design Guide
Frequency
converter
Flow or pressure sensor
Water
Outlet
Condenser
Water pump
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Throttling
valve
5 5
Supply
35