What is the difference between primary and secondary chilled water pumps.

Primary pumps are constant flow and they pump water in a circuit around the chiller. Secondary pumps are variable flow and they pump water from the primary circuit to the building as required.

If the load in the building is high secondary pumps will pump more water from primary circuit in to the building and if the load is low they will pump less water.

Reason for primary and secondary pumps scheme is to save energy because secondary pumps will use less energy at part load.

Primary/Secondary systems save energy but are more difficult to design than constant flow systems.