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TRG-TRC016-EN 5
period one
Types of Water Chillers
notes
These various types of compressors are discussed in detail in the Refrigeration
Compressors Air Conditioning Clinic.
The capacity of a centrifugal chiller can be modulated using inlet guide vanes
(IGV) or a combination of IGV and a variable-speed drive (adjustable-frequency
drive, AFD). Variable-speed drives are widely used with fans and pumps, and as
a result of the advancement of microprocessor-based controls for chillers, they
are now being applied to centrifugal water chillers.
Using an AFD with a centrifugal chiller will degrade the chiller’s full-load
efficiency. This can cause an increase in electricity demand or real-time pricing
charges. At the time of peak cooling, such charges can be ten (or more) times
the non-peak charges. Alternatively, an AFD can 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.
Certain system characteristics favor the application of an adjustable-frequency
drive, including:
n A substantial number of part-load operating hours
n The availability of cooler condenser water
n Chilled-water reset control
Chiller savings using condenser- and chilled-water-temperature reset, however,
should be balanced against the increase in pumping and cooling-tower energy.
This is discussed in Period Four. Performing a comprehensive energy analysis
is the best method of determining whether an adjustable-frequency drive is
desirable. It is important to use actual utility costs, not a “combined” cost, for
demand and consumption charges.
Depending on the application, it may make sense to use the additional money
that would be needed to purchase an AFD to purchase a more efficient chiller
instead. This is especially true if demand charges are significant.
Variable-Speed Drives
variable
variable
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speed
speed
drive
drive
Figure 7