Application Considerations

part of those jobs. See “Energy and economic analysis of alternatives” on page 26.

Number of chillers

The number of chillers to install is a function of redundancy requirements and first cost. In general, the more chillers installed, the higher the initial cost. Therefore, many small systems only use one chiller. Most chillers in the 20 through 200 ton range use multiple compressors with multiple refrigeration circuits and provide a reasonable level of cooling redundancy. The only system controls installed on a single chiller installation may be a clock and ambient lockout switch to enable and disable the chilled-water system. If only one chiller is used, a system that varies the flow rate through the chiller can be quite simple to operate. Minimum and maximum flows and maximum rate of change for the flow would still need to be addressed (see “Variable- Primary-Flow Systems” on page 55).

As systems get larger, the owner may require more redundancy, leading them to install multiple chillers. Some designers use 200 tons as the maximum job size for a single chiller.

When there is more than one chiller, there are many more system control decisions to be made including:

•enabling the second chiller,

•turning the second chiller off, and

•failure recovery.

Two-chiller plants require higher system control intelligence than single chiller plants. Sequencing logic, discussed in “System Configurations” on page 42, varies based on system configuration, and failure recovery is discussed on page 95.

Parallel or series

Parallel configurations are more common than series configurations. (See “Parallel Chillers” on page 42.) In chiller systems with an even number of chillers, there are advantages to putting them into a series configuration, especially if low or variable water flow is desired. This offers the benefits of better system efficiency and higher capacity because the upstream chiller produces water at a warmer temperature. Series chillers should not be applied with low system ΔTs, because the maximum flow through the chillers may be reached. Efforts to eliminate the so-called “Low ΔT syndrome” (page

79)must be addressed for both configurations. The energy and control

requirements of series chillers are covered in “Series Chillers” on page 44.

Part load system operation

For small chilled-water systems, especially those with only one chiller, part load system energy use may be dominated by ancillary equipment, especially in a constant flow system. At low loads, constant speed pumps and tower fans constitute a much larger portion of the chiller plant energy

Chiller System Design and Control

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