Trane SYS-APM001-EN manual Adding a chiller in a VPF system, Flow-rate-fluctuation examples

System Configurations

flow nears the maximum limit for the operating chiller(s), another machine must be brought online. Similarly, as the system load and flow decrease, chillers must be shut down to reduce the need for bypass water flow.

Adding a chiller in a VPF system

The simplest way to control a VPF system is to monitor the leaving- evaporator water temperature and allow the operating chiller(s) to load almost fully before bringing the next chiller online. As long as the system can maintain the target temperature, there is no need to activate another chiller.

When the operating chiller(s) no longer provide enough cooling, the plant controller should start the next chiller. For example, when the temperature exceeds the design setpoint by a certain amount (for example, 1.5°F [0.8°C]) for a set time (for example, 15 minutes) an additional chiller starts.

One caveat: The next chiller should start before the chilled water flow reaches the maximum limit of the operating chiller(s), even if the operating chillers are not yet fully loaded. (This would rarely happen; most pipe velocity limits are below that of the chillers.)

As chillers are brought online, flow rates may fluctuate substantially, and this occurs quite often in a system with two chillers. All systems, with any number of chillers, will find the most difficult transition when adding or subtracting the second chiller. An example will help explain the challenges.

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Table 16. Flow-rate-fluctuation examples

	Design	Minimum	Maximum
	flow rate	flow rate	flow rate
	gpm [L/s]	gpm [L/s]	gpm [L/s]
Chiller 1	960 [60.6]	576 [36.3]	2,110 [133.1]
Chiller 2	1,440 [90.8]	675 [42.6]	2,474 [156.1]

At a point in time, Chiller 1 is active and has 1,100 gpm [69.4 L/s] flowing through its evaporator. It can no longer satisfy the required system supply temperature. What happens if Chiller 2’s valve is opened with no other action taken? If we assume that pressure drops are equal, 550 gpm [34.7 L/s] will flow through each chiller. This means Chiller 1’s flow rate drops by 50 percent as fast as Chiller 2’s isolation valve opens (probably beyond what its controls can respond to) and we are below each chiller’s minimum flow rate. Be aware that this could become an issue since chiller controls may protect the chiller by shutting it off. The result is that some combination of pump speed, bypass valve control, and slow-acting valves at the chillers must do two things:

•Keep change in flow rate within the equipment limitations

•Keep each chiller’s flow rate above its minimum

64 Chiller System Design and Control SYS-APM001-EN