Danfoss VLT HVAC Basic Drive FC 100 Series Design Manual page 21

Introduction to VLT HVAC Ba... VLT HVAC Basic Drive Design Guide
2.5.20 Primary Pumps
2 2 Primary pumps in a primary/secondary pumping system can be used to 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. This 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 chilled water begins to become over-chilled. As this happens, 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 chiller's low evaporator temperature safety trips the chiller requiring a manual reset. This situation
is common in large installations especially when two or more chillers in parallel are installed if primary/ secondary pumping
is not utilized.
®
20 MG.18.C2.02 - VLT is a registered Danfoss trademark