Like the compressor in an
electric vapor compression cycle, the absorption system uses its "thermal"
compressor (consisting of the generator, absorber, pump and heat exchanger) to boil water
vapor (refrigerant) out of a lithium bromide/water solution and compress the refrigerant
vapor to a higher pressure. Increasing the
refrigerant pressure also increases its condensing temperature. The refrigerant vapor condenses to a liquid at this
higher pressure and temperature. Because this
condensing temperature is hotter than the ambient temperature, heat moves from the
condenser to the ambient air and is rejected. The
high-pressure liquid then passes through a throttling valve that reduces its pressure. Reducing its pressure also reduces its boiling
point temperature. The low-pressure liquid
then passes into the evaporator and is boiled at this lower temperature and pressure. Because the boiling temperature is now lower than
the temperature of the conditioned air, heat moves from the conditioned air stream into
the evaporator and causes this liquid to boil. Removing
heat from the air in this manner causes the air to be cooled.
The refrigerant vapor then passes into
the absorber where it returns to a liquid state as it is pulled into the lithium bromide
solution (the absorption process). The diluted
lithium bromide solution is pumped back to the generator.
Because lithium bromide (the absorbent) does not boil, water (the
refrigerant) is easily separated by adding heat. The
resultant water vapor passes into the condenser, the absorbent solution returns to the
absorber, and the process repeats.
|Although the process is similar to
conventional electric vapor compression systems, absorption cooling substitutes a
generator and absorber, called a thermal compressor, for an electric compressor. Efficiency and lower operating costs are achieved
through the use of a pump rather than a compressor and a heat exchanger to recover
and supply heat to the generator. Double-effect absorption cooling adds a second
generator and condenser to increase the refrigerant flow, and therefore the cooling
effect, for a fraction of the heat input of a single-effect system.