Handbook of air conditioning and refrigeration / Shan K

9.50 CHAPTER NINE
with centrifugal chillers in the United States. Applications of air or gas expansion refrigeration systems
are limited in aircraft and cryogenics. Vapor compression systems dominate in both comfort
and processing air conditioning.
Vapor compression refrigeration systems can also be classified according to
● The type of compressor used, such as reciprocating, rotary, scroll, screw, or centrifugal systems.
● The type of evaporator: direct-expansion (DX) cooler or liquid chillers. In a DX cooler system,
space air is directly cooled and dehumidified by the expanded and evaporated refrigerant in a DX
coil. A liquid chiller is a factory-assembled refrigeration package in which chilled water is
produced in its evaporator. Air is then cooled and dehumidified by chilled water in air-handling
units (AHUs) or in fan coils.
● The size (tonnage in capacity ) of the refrigeration system, such as small (� 2.5 ton, or 8.8 kW),
medium, and large (� 75 tons or 264 kW).
For a refrigeration system, the type of compressor, system size, and whether a DX coil or a water
chiller is used are often interrelated. A centrifugal chiller is often a large-tonnage refrigeration system.
A small refrigeration system with a cooling capacity of less than 2.5 tons (8.8 kW) is often an
air-cooled DX cooler using a reciprocating, rotary, or scroll compressor. Refrigeration systems can
therefore be primarily classified as follows:
● DX (direct-expansion) cooler systems:
Air-cooled, reciprocating DX coolers
Air-cooled, scroll DX coolers
Air-cooled rotary DX coolers
● Liquid chillers
Centrifugal chillers
Screw chillers
Scroll chillers
Reciprocating chillers
● Liquid overfeed systems
● Multistage systems
● Absorption systems
● Air or gas expansion systems
Among these, DX coolers have a share of about 70 percent based on the air conditioned floor
area in commercial buildings in 1992. For air conditioned homes, more than 90 percent use
DX coolers. Chillers have a share of about 28 percent of floor space in air conditioned commercial
buildings. According to the U.S. domestic market research 1997 estimate provided by
BSRIA/Ducker in Air Conditioning, Heating and Refrigeration News, September 22, 1997, for a
total number of nearly 18,460 chillers sold, the large-tonnage centrifugal and screw chillers with a
capacity � 75 tons (264 kW) had a share of 28 percent in units and a value of about $505 million.
Reciprocating, screw, and scroll chillers with a capacity � 75 tons (264 kW) using step unloading
had a share of 61 percent in units and a value of about $380 million. Absorption and gas-engine
chillers had a value of about $66 million.
Refrigeration systems are also classified according to their evaporating temperatures T ev as
Low-temperature systems
Medium-temperature systems
High-temperature systems
� 40°F � T ev � 0°F (�40°C � T ev ��18°C)
0°F �T ev � 32°F (�18°C � T ev � 0°C)
T ev � 32°F (T ev � 0°C)