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TRG-TRC010-EN 29
period three
Compressor Capacity Control
notes
An adjustable-frequency drive (AFD), or variable-speed drive, is another
device used to vary the capacity of a centrifugal compressor. AFDs are widely
used with fans and pumps, and with the advancement of microprocessor-based
controls for chillers, they are now being applied to centrifugal water chillers.
Using an AFD with a centrifugal chiller can degrade the chiller’s full-load
efficiency. It will, however, offer energy savings by reducing motor speed at
low-load conditions when cooler condenser water is available. An AFD also
controls the inrush current at start-up, reducing stress on the compressor
motor.
Certain system characteristics favor the application of an adjustable frequency
drive, including:
■ A substantial number of part-load operating hours
■ The availability of cooler condenser water
■ Chilled-water reset control
■ High electrical charges
Performing a comprehensive energy analysis is the best method of determining
if an adjustable-frequency drive is desirable. Depending on the application, it
may make sense to take the additional money needed to purchase an AFD and
use it to purchase a more efficient chiller instead.
Adjustable-Frequency Drives
power consumption
power consumption
load
load
inlet vanes only
inlet vanes only
AFD + inlet vanes
AFD + inlet vanes
Figure 42
trgtrc010_book.book Page 29 Wednesday, September 1, 1999 10:39 AM