Trane SYS-APM001-EN manual System Configurations, Subtracting a chiller in a VPF system

System Configurations

In his article, “Primary-Only vs. Primary- Secondary Variable Flow Systems,” Steven T. Taylor, principal of Taylor Engineering LLC, notes that unloading the active chillers before starting another produces warmer chilled water. Although the temperature increase seldom causes problems for comfort cooling, it may be unacceptable in industrial/process applications.17

Sequencing based on load

Some plant operators prefer to sequence chillers by comparing the actual system load with the total plant capacity that would result if a chiller is turned off. However, this method can be less reliable than one based on power draw because flow- and temperature-sensing devices require periodic recalibration to correct for drift.

SYS-APM001-EN

Controlling transient flows is mandatory, regardless of plant size. The number of chillers in the plant will not alter the degree of care needed to properly manage transient flow-rate changes because the transition from one operating chiller to two is inevitable in almost all plants.

Temporarily unload the operating chillers before starting the next one.

Reduce shock resulting from transient flows by unloading the operating chillers before opening an isolation valve to bring another chiller online.

You can accomplish this by imposing a demand limit of 50 to 60 percent on the operating chillers, or by raising the chilled water setpoint one to three minutes before the isolation valve actuates. (See sidebar.)

Open the chiller isolation valves slowly to encourage stable operation. How slowly? That depends. If the chiller controller can only handle a flow-rate change of 2 percent per minute, then the isolation valve must take 25 minutes to open… far too long for most applications. Besides helping with chiller stability, slow valve operation reduces the likelihood of valve-induced water hammer in the piping system.

With sophisticated chiller controls, a 30-percent-per-minute change in the rate of flow should work well in most applications. At this rate, the isolation valve will transition from fully closed to fully open in about two minutes.18

Like the bypass valve, be sure to select isolation valves that maintain a linear relationship between valve position and flow rate.

The bottom line is that control of VPF systems must be considered during system design.

Subtracting a chiller in a VPF system

Subtracting a chiller in a VPF system is not simple, either. It is important to devise a “stop” strategy that protects the chillers from short-cycling. Knowing when to stop a chiller (to provide sufficient downtime between chiller starts) often is more challenging than knowing when to start it. The most reliable way to do so—assuming that the VPF system is properly installed, calibrated, and maintained—is to monitor the power draw of the operating chillers. (See sidebar.) Most unit controllers measure running load amps (RLA) at regular intervals. The %RLA (actual RLA divided by design RLA) provides a good indication of the present chiller load.

Base the “stop” strategy for a multi-chiller plant with equally sized machines on the sum of the present %RLA for all chillers divided by the number of operating chillers minus one. If the result is less than the desired capacity for the operating chiller(s), then stop one of the machines.

For example, suppose that a plant consists of three equally sized chillers, each of which is presently running at 60 percent of full-load capacity. If one chiller is shut off, the two chillers still online would operate at approximately 90 percent of full-load capacity; (60% + 60% + 60%) / (3-1) = 90%. Having the remaining two chillers operating almost fully loaded risks the need to restart a chiller if the load increases.

Chiller System Design and Control 65