Trane SYS-APM001-EN System Issues and Challenges, Low ΔT Syndrome, Amount of Fluid in the Loop

System Issues and Challenges

Low ΔT Syndrome

For many years the “low ΔT syndrome” debate has raged.27, 28 The symptom of the problem is that, in large systems, return-water temperature is too low, thus not allowing the chillers to fully load. Many system operators simply turn on more pumps and chillers to satisfy flow requirements, which wastes energy. For primary-secondary systems, some system designers advocate putting a check valve in the bypass line—thus putting chilled-water primary and secondary pumps in series and varying the flow through chiller evaporators. Other designers install primary pumps that are larger than necessary and “over pump” chillers at part-load conditions. These solutions are all “band-aids” and do not treat the source of the problem. Coad14 points to the fact that a properly operating hydraulic system will work as designed and explains the fallacies in the check valve and over pumping approaches. (Refer to the “Check valves” sidebar on page 46.) Taylor29 recommends that a number of mitigating procedures be implemented to eliminate the problem. They include:

•Eliminating three-way valves

•Ensuring that airside control is not causing the problem

•Properly maintaining the system, including regular air filter changes, coil cleaning, control calibration, and proper setpoints

Before applying band-aid approaches in an attempt to “fix” symptoms such as low ΔT syndrome, ensure that the system is operating properly using some or all of the procedures Taylor29 discusses. In addition to these procedures, simply reducing the chilled-water supply temperature will have the effect of raising the system return-water temperature in systems using two-way valves.

Amount of Fluid in the Loop

Two questions must be answered when determining how much fluid is necessary to maintain proper chilled-water-system control:

•How fast can the specific chiller respond to changing conditions?

•How fast can the system respond to changing conditions?

The amount of fluid the loop requires to operate properly is related to the larger of these two answers. Note that both answers describe an amount of time.

Required Volume = Flow Rate × Loop Time

Where:

•Required Volume = the amount of fluid in the coil, pipes, evaporator barrel, storage tank, etc., in gallons [liters]

SYS-APM001-EN Chiller System Design and Control 79