Trane SYS-APM001-EN manual AFD on both chillers

Primary System Components

by reducing motor speed at “low-lift” conditions, when cooler condenser water is available.

Certain system characteristics favor the application of an AFD, including:

•A substantial number of part-load operating hours (for example, when an air- or water-economizer is not installed in the system)

•The availability of cooler condenser water (condenser-water reset)

•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. Performing a comprehensive energy analysis is the best method of determining whether an AFD is desirable. It is important to use actual utility costs, not a “combined” cost, for demand and consumption charges. It is also important to include drive maintenance and replacement costs, since the drive life is shorter than the chiller life. See “Energy and economic analysis of alternatives” on page 26.

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, or if the condenser water is close to its design temperature most of the time (e.g., in a hot and humid climate such as Miami).

Consider the following analysis of an 800-ton office building with two chillers. The analysis compares equally priced high efficiency or AFD- equipped chillers, as one or both of the chillers. Utility costs for the combined or “blended” rate are $0.10 per kWh and for the actual rate are $12 per kW and $0.06 per kWh.

Simple paybacks using the combined rate analysis show almost no difference between the two options (Table 2). However, when utility costs with an actual consumption and demand component are used, the difference between the alternatives is much more pronounced. The conclusion is that using actual energy rates matters a great deal.

Table 2. Analysis of high-efficiency chiller options with combined vs actual rates

Simple paybacks, humid climate	Combined rate	Actual rate
AFD on both chillers	7.2	12.7
High efficiency on both chillers	7.1	8.3
AFD on one chiller	6.1	10.8
High efficiency on one chiller	6.3	7.7

SYS-APM001-EN Chiller System Design and Control 17