Trane SYS-APM001-EN Chiller selection requirements, System Configurations, Evaporator flow limits

System Configurations

Dispelling a common misconception

True or false: “Chillers operate more efficiently in a system with variable rather than constant primary flow because of the greater log mean temperature difference (LMTD).”

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 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.

In a system with variable primary flow:

•The convective heat transfer coefficient in the chiller evaporator decreases with a reduction in flow.

•Reduced flow decreases the overall heat-transfer effectiveness of the chiller evaporator.

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

secondary systems. The pressure drops previously satisfied by the distribution pumps are instead satisfied by the now larger primary-only pumps, permitting selection of larger, more efficient pumps (with efficiencies similar to those of the secondary pumps in a primary–secondary system).

VPF systems present building owners with several cost-saving benefits that are directly related to the pumps. The most obvious cost savings result from eliminating the constant flow primary pumps, which, in turn, avoids the material and labor expenses incurred with the associated piping connections, mechanical room space, and electrical service. Although the number of pumps is reduced, the sizes of both the pumps and the variable-frequency drives increase since the pumps must be sized to overcome the entire system’s pressure drop. This offsets some of the installed cost savings of having fewer pumps.

Building owners often cite pump-related energy savings as the reason they installed a VPF system. With the help of a software analysis tool such as System Analyzer™, TRACE™ 700, Chiller Plant Analyzer, or EnergyPlus, you can determine whether the anticipated energy savings justify the use of variable-primary flow in a particular application.

It may be easier to apply a variable-primary-flow system rather than a primary–secondary system to an existing constant-flow chilled-water plant. Unlike the primary–secondary design, the bypass can be positioned almost anywhere in the chilled-water loop and an additional pump is unnecessary.

Chiller selection requirements

Variable-flow systems require chillers that can operate properly when evaporator flow varies. Varying the water-flow rate through the chiller evaporator poses two control challenges for those who design and operate VPF systems:

1Maintaining the chiller flow rate between the minimum and maximum limits of the evaporator

2Managing transient flows without compromising stable operation, especially in multi-chiller plants

Evaporator flow limits

Select for a minimum evaporator-flow limit that is ≤60 percent of the chiller’s design flow rate. One benefit of VPF systems is reduced pumping energy. To realize this benefit, chilled water flow must not remain constant. As the flow decreases, it approaches the minimum flow rate of the chillers—so, how do we select for a minimum chiller flow rate that will result in the pump-energy savings?

The answer depends on the type of chiller, but generally speaking, lower is better because it extends the ability of a single chiller to operate at low loads without bypass flow. Most of the potential savings are realized by the time that the system flow rate decreases to 50 percent of design.

SYS-APM001-EN Chiller System Design and Control 57