Variable-Speed Drives
variable- speed drive
Figure 7
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:
nA substantial number of part-load operating hours
nThe availability of cooler condenser water
nChilled-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.
