System Controls

Table 17. VFDs and centrifugal chillers performance at 90% load

ECWT

2 Chillers*

1 Chiller

Difference

 

 

 

 

85°F

306.4

268.0

-38.4

 

 

 

 

80°F

268.0

238.0

-30.0

 

 

 

 

75°F

230.8

210.6

-20.2

 

 

 

 

70°F

195.2

185.7

-9.5

 

 

 

 

65°F

160.3

164.3

+4.3

Note: Data shows only chiller power. * Load equally divided.

If the chiller and tower capabilities are conducive to this strategy, the location and load profile determine if, when, and for how long the right conditions might occur. Determine the optimum control sequence for the entire plant by performing a detailed energy analysis of each component. Base the analysis on realistic load profiles and ambient conditions, and account for the energy used by all ancillary equipment.

For VPF systems, there will likely not be enough system flow to allow more chillers than necessary to operate without requiring bypass to stay above the chillers’ minimum flows.

Condenser-Water System Control

Minimum refrigerant pressure differential

Every chiller requires a certain refrigerant pressure differential between the evaporator and condenser in order to operate. The chiller must develop its pressure differential within a manufacturer-specified time or its controls will shut it off. During some start-up conditions, this pressure differential may be hard to produce within the time limitation.

An example of such a condition is an office building that has been unoccupied during a cool, clear, fall weekend. The tower sump water is at 40°F [4.4°C]. Monday is sunny and warm, which requires a chiller to be turned on. Since the chiller is lightly loaded and the tower sump is large, the pressure differential cannot be reached before the chiller turns off. If the condenser flow rate for a given chiller can be reduced, this scenario is less likely to occur. The lower flow rate increases the leaving condenser-water temperature, which increases the condenser-refrigerant temperature and refrigerant pressure.

SYS-APM001-EN

Chiller System Design and Control

89

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Trane SYS-APM001-EN manual Condenser-Water System Control, Minimum refrigerant pressure differential, Chillers Difference

SYS-APM001-EN specifications

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