process that requires the use of very low temperature applications, such as food
warehousing, ice manufacturing, pharmaceuticals and quick freezing applications,is
a natural fit for a natural gas refrigeration system. Its reliability and economical
value make it the technology of choice.
units use the same cooling process as electric systems, but a gas engine is used
in lieu of an electric motor. Engine-driven units are environmentally friendly
and are available in a variety of sizes from about 50 tons to several thousand
tons of cooling capacity. Some engines can run up to 5,000 equivalent full-load
hours between maintenance. The most distinguishing factor about natural gas industrial
refrigeration units is their ability to cool to extremely low temperatures. On
average, they reach temperatures 60° lower than conventional space conditioning
equipment - as low as minus 10-20°F.
cycle is a single stage compression cycle that consists of four elements: an evaporator,
compressor, condenser and expansion device.
the evaporator, heat is absorbed from the hot source to the cold refrigerant causing
it to boil. The low-pressure gas leaves the evaporator and returns to the compressor
(A to B). The compressor raises the pressure of the refrigerant to a point where
it can be condensed to a liquid by use of ambient air or other means (e.g. cooling
tower water)to absorb the heat given off by thecondensing refrigerant.
the refrigerant vapor to condense at a relatively high temperature, permitting
the use of ambient air or other means (e.g. cooling tower water)to absorb the
heat given off by the condensing refrigerant.
removal occurs from points C to D. The high-pressure gas enters the condenser
and leaves it as a high-pressure liquid at D. The high-pressure liquid is then
throttled across an expansion device to deliver, at point A, a low-pressure /
temperature liquid and a small amount of flash gas (to cool the remaining liquid),
ready to absorb the heat from the object(s) to be refrigerated.
Compound refrigeration cycles are the most common cycles employed efficiently
providing refrigeration within the temperature range from -70°F (-56.7°C)
evaporation temperature (ET) to -10°F (-23.3°C). Early compound refrigeration
systems used two or more independent compressors, each with its own prime mover.
The advent of internally compounding, reciprocating compressors (by manifolding
a bank of low-pressure [evaporator temperature] cylinders through an intercooler
and into a bank of high-pressure cylinders) reduced the need for multiple compressors.
of screw compressors to refrigeration in the 1960s dramatically changed refrigeration
options. Screw compressors can easily accommodate side loads which have intermediate
temperature requirements within a refrigeration system to provide useful cooling
and improve cycle efficiency.
A compound cycle requires less work input than a single stage system. Therefore,
where economically feasible and operationally sound, compound systems are more
efficient at medium temperatures and below.
Cycle Vapor Compression
can also accommodate two levels of cooling. Compound systems can incorporate an
evaporator operating at the pressure level indicated by points G & H and by
merely increasing the size of the high-stage compressor to accommodate the increased
gas flow at point G.
A cascade system is utilized when there are operating and/or capital cost advantages
to using two (or more) different refrigerants within the same system.
for example, use two different refrigerants because the thermophysical properties
of a low temperature refrigerant allow smaller components and lower energy input.
These low temperature refrigerants exhibit low specific volumes at low temperatures.
size and type are two of the major considerations that determine the best system
Cascade systems should be considered when evaporator temperature reaches -50 or
below, as the refrigerant characteristic variations begin to become extreme.