Handbook of air conditioning and refrigeration / Shan K

Chiller Minimum Performance
Design Considerations
REFRIGERATION SYSTEMS: CENTRIFUGAL 13.35
ASHRAE/IESNA Standard 90.1-1999 mandates the minimum efficiency requirements for watercooled
electrically operated centrifugal chillers as follows:
Size Minimum efficiency Efficiency as of 10/29/2001
�150 tons 3.8 COP, 3.9 IPLV 5.0 COP, 5.0 IPLV
�150 tons and
�300 tons
4.2 COP, 4.5 IPLV 5.55 COP, 5.55 IPLV
�300 tons 5.2 COP, 5.3 IPLV 6.1 COP, 6.1 IPLV
Minimum efficiency requirements do not apply for chillers in low-temperature applications where
design leaving fluid temperature is �40°F (4.4°C).
Power Consumption. The following are effective means to reduce the power consumption, in
kW/ton (COP), as well as to enhance the minimum performance of centrifugal chillers:
● Raise the temperature of the chilled water leaving the evaporator T el by using a T el reset at reduced
loads under the condition that space relative humidity does not exceed 65 percent.
● Lower the condenser water entering temperature T ce as low as the chiller manufacturer permits.
● If the fouling factor on the water side of the condensing surface exceeds 0.00025 h�ft 2 �°F/Btu
(0.000044 m 2 �°C/W), reduce the fouling factor by using a brush cleaning system or some other
means, especially in location where the outdoor air is heavily contaminated.
● Increase the surface area of the evaporator and condenser if they are cost-effective.
Many manufacturers of centrifugal chillers now offer different sizes of evaporators and condensers
to match a specific size of compressor. The designer can thus select the centrifugal chiller that
meets the designated minimum performance.
Equipment Sizing. Although a single chiller is less costly than multiple chillers, there is a danger
of total loss of service when the chiller fails to operate or needs maintenance. Multiple chillers are
usually the best choice for a large chiller plant. A plant installed with three chillers ensures a 66
percent of capacity even if one of the chillers fails.
If continuity of service is critical, a standby chiller that has a capacity equal to the largest chiller
in the refrigeration plant should be provided. A smaller chiller used during light-load conditions, especially
in after-hours operation, in addition to several equally large chillers is often beneficial. The
refrigeration load always increases as building usage grows. The possibility of future growth should
be considered.
Plant Location and Layout. Generally, the central plant should be located near the system load as
well as near to the sources of utilities. For a high-rise building, the ideal location is often the hub of
the building, near shopping and public areas. If space at the hub is not available, the chiller plant
may be located in the basement, on an intermediate floor, or on the top floor, depending on the type
of building and the architectural design.
Equipment should be laid out in an orderly arrangement for efficient piping. Vertical and lateral
clearances must be maintained. Equipment may be 10 to 12 ft (3 to 3.6 m) in height. Provision of
access platforms and ladders to valves and piping should be considered in the design stage. A codespecified
clearance of 3 to 5 ft (0.9 to 1.5 m) from electrical panels and devices must be maintained.