Trane SYS-APM001-EN manual Managing transient water flows

System Configurations

Small packaged chillers typically offer less design flexibility than larger machines. It may not be possible to select a small packaged chiller with a minimum flow rate of less than 60 percent of the design system flow… but don’t let this deter you from designing a VPF plant that includes small packaged chillers. Remember, pump power drops with the cube of the reduction in flow, so even a modest 20 percent decrease in flow results in a 50 percent pump energy reduction. A 40 percent flow reduction yields an 80 percent pump energy reduction. The key to making variable flow with limited flow turndown work properly is devising a plant layout and sequencing strategy that accommodates the chiller’s minimum evaporator-flow limit.

Managing transient water flows

The second requirement of the selected chillers is proper control during “transient flows.” This situation refers to the hydraulic effects caused by an isolation valve when it opens (before the associated chiller starts) or closes (after the chiller stops). To illustrate what happens, let’s look at an example.

Assume that the two-chiller VPF system in Figure 37 is designed for a 16°F [8.9°C] ΔT and that it delivers 40°F [4.4°C] chilled water. The temperature of the return water remains relatively constant at 56°F [13.3°C], provided that the coils and two-way valves function properly. Only Chiller 1 operates when the cooling load is low; the isolation valve for Chiller 2 remains closed.

As the cooling load increases, the pump controller increases the rate of chilled water flow through the system. Chiller 2 starts when Chiller 1 can no longer produce 40°F [4.4°C] water. Opening the isolation valve for Chiller 2 almost instantly reduces the flow rate through Chiller 1 by half (Table 14), which effectively doubles the ΔT. Chiller 1’s controller will unload the machine as quickly as possible, but in the interim, it will attempt to produce a 32°F ΔT [0°C] and cool the water to 24°F [-4.4°C]. If the chiller cannot unload quickly enough, built-in fail-safes should stop and lock out the chiller before damage occurs… but at the expense of satisfying the cooling load. The system can be designed and operated to keep this scenario from occurring. This information is provided in the following sections.

Table 14. Flow-rate changes that result from isolation-valve operation

	Number of operating chillers
	1	2	3	4	5
Flow-rate reduction when an isolation valve	50%	33%	25%	20%	17%
opens*

*Flow-rate reduction is expressed as a percentage of the actual chilled water flow rate prior to transition:

%flow-rate reduction = 1– number of chillers operating number of chillers operating +1

Select for the greatest tolerance to large changes in flow rate. The objective

is to simplify system control by minimizing the need for “supplemental” demand limiting or valve control as chillers come online. Chillers that are well-suited for variable primary flow can tolerate and respond to rapid flow-

SYS-APM001-EN Chiller System Design and Control 59