Trane TRG-TRC016-EN Control of a Heat-RecoveryChiller, notes, System Variations, period three

period three

System Variations

notes

Control of a Heat-Recovery Chiller

percent maximumpressuredifferential

120

100

80

60

0

				le
			b				n
		a
	t					o
s
					ti
n				a			)	B
u		e
			r			e
	p
o					g
				r
		u
	s
	(

C

unloadingwith

constant enteringhot- water temperature

50

percent load

A

unloadingwith constant leavinghot- water temperature

100

Figure 79

The control of a heat-recovery centrifugal chiller, although seemingly simple, is critical to reliable chiller operation. Typically, either the temperature or the flow of the water entering the standard condenser is modulated to meet the capacity required by the heat-recovery condenser.

Controlling heat-recovery capacity based on the temperature of the hot water leaving the heat-recovery condenser can cause operational problems for a centrifugal chiller. This is explained best by using a map of centrifugal- compressor operation (see Figure 79). Control based on the temperature of the water leaving the heat-recovery condenser causes the condenser-to-evaporator pressure differential to remain relatively high at all loads (line A to B). High pressure differentials at low cooling loads increases the risk of a centrifugal compressor operating in its unstable region, commonly known as surge.

The preferred method is to control heat-recovery capacity based on the temperature of the hot water entering the heat-recovery condenser. This allows the condenser-to-evaporator pressure differential to decrease as the chiller unloads (line A to C), thereby keeping the centrifugal chiller from surging and resulting in more stable operation. If high leaving-hot-water temperatures are required at low-cooling-load conditions, another method to prevent surge is to use hot gas bypass on the centrifugal chiller.

For other types of chillers that are not prone to surge, operating at these high pressure differentials at low cooling loads may cause the chiller to consume more energy than it recovers in the form of heat.

TRG-TRC016-EN 71