It is true that the return water temperature in a properly operating VPF system remains constant as the amount of flow changes. It is also true that the LMTD can be increased by changing the production (primary) side of the chilled-water loop from constant to variable flow. But there are other facts to consider.

In a system with variable primary flow:

• The convective heat transfer coefficient decreases with a reduction in flow.
• Reduced flow decreases the overall heat-transfer effectiveness of the chiller evaporator.

In a system with constant primary flow:

• Entering-evaporator temperature and LMTD fall as the cooling load diminishes.
• The convective heat transfer coefficient, like the primary flow, remains constant despite reductions in load.

The net effect is that the power consumption for a given chiller is virtually the same whether the system’s primary flow is variable or constant.

Back to Weighing The Pros And Cons or return to the Table of Contents.