Environmental Engineering Reference
In-Depth Information
Critical point
Gas cooling
Discharge
pressure
Evaporation
Enthalpy
Figure 8.6
Transcritical refrigeration cycle for R-774 (carbon dioxide), where heat rejection takes place at
pressures above the critical point.
The main disadvantage of having refrigeration equipment working according to a
transcritical cycle is high pressures, which require specially designed equipment in contrast to
the cascade arrangement that can use standard refrigeration parts. Transcritical equipment can
operate in theory by cooling the gas with atmospheric air; however, in practice it does not
work well in hot climates, so the gas cooling step is often accomplished by using water, which
can be an advantage because hot water is needed in most cases.
Air cycle refrigeration
Air cycle refrigeration is not a new concept. It was originally developed in the 1800s for the
transportation of frozen food in ships, but then was replaced quickly by chlorofluorocarbon
compression equipment in the 1930s. At present, air cycle refrigeration is in use to provide air
conditioning in passenger aircrafts.
After it was discovered that hydrofluorocarbons have a high GWP, air cycle refrigeration
gained renewed interest. Moreover, the working fluid and air has zero impact on the environ-
ment and is absolutely free.
In contrast with compression refrigeration, which relies on the latent heat of evaporation of
the working fluid to capture heat in the evaporator, in air cycle systems the working fluid (air)
does not experience a phase change. Instead, it works by the principle that when a compressed
gas is allowed to expand, the drop in pressure produces a drop in its temperature as well. But
before air can be expanded it needs to be compressed. This is achieved with a centrifugal
compressor in one or in multiple steps with intermediate cooling steps (Fig. 8.7).
Rejected heat is captured with water via a heat exchanger, and the hot water used for
heating applications. The end of the cycle consists of expanding the compressed air, which
is normally accomplished by using a turbine instead of a throttle valve. Expansion through
a turbine produces an instantaneous drop in the air temperature and at the same time
mechanical energy that can be used to run an intermediate compression step (Pelsoci, 2001;
Spence, 2007).
