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Process Description
Some liquids like water have a great tendency for absorbing
large amount of certain vapors (NH3) and reduce the total volume quite. The absorption chiller refrigeration system differs basically from vapor compression system only in the
method of compressing the refrigerant.
In the absorption refrigerating system, the compressor is
replaced with an absorber, generator and pump. Figure 6.7 shows the schematic
diagram of a vapor absorption refrigeration system.
Ammonia vapor is extracted from the NH3 strong solution at high pressure in the
generator by an external heating source. In the rectifier, the water vapor
which carried with ammonia is removed and only the dried ammonia gas enters
into the condenser, where it’s condensed.
The pressure and temperature of cooled NH3 solution is then
reduced by a throttle valve below the temperature of the evaporator. The NH3
refrigerant at low temperature enters the evaporator and absorbs the required
heat from it, then leaves it as saturated vapor.
The low
pressure NH3 vapor is then passed to the absorber, where it’s absorbed by the NH3
weak solution which is sprayed also in the absorber as shown in Fig.6.7. After
absorbing NH3 vapor by the weak NH3 solution (aqua–ammonia), the weak NH3
solution becomes strong solution and then it is pumped to the generator passing
through the heat exchanger.
In the pump, the pressure of the strong solution increases
to generator pressure. In the heat exchanger, heat form the high temperature
weak NH3 solution is absorbed by the strong NH3 solution coming from the
absorber.
As
NH3 vapor comes out of the generator, the solution in it becomes weak. The weak
high temperature NH3 solution from the generator is then passed through the
throttle valve to the heat exchanger. The pressure of the liquid is reduced by
the throttle valve to the absorber pressure.
Why Do We Use Ammonia/Water ?
Because most commercial and industrial refrigeration
applications occur at temperatures below 32 F and many are 0 F. As a result, a
fluid which is not subject to freezing at these temperatures is required. So
the lithium bromide/water cycle is no longer able to achieve this conditions, because
water is used for the refrigerant.
Also the required heat input temperatures must be at least
230oF. It should also be remembered that the required evaporation temperature
is 10 to 15oF below the process temperature.
Use of ammonia/water equipment in conjunction with
geothermal resources for commercial refrigeration applications is influenced by
some of the same considerations as space cooling applications. Figure 13.5
illustrates the most important of these. As refrigeration temperature is
reduced, the required hot water input temperature is increased.
Figure 13.6 suggests a minimum hot water temperature of
275oF would be required. For example, for a +20oF cold storage application, a
5oF evaporation temperature would be required.
For geothermal
resources that produce temperatures in this range, it is likely that small
scale power generation would be competing consideration unless cascaded uses
are employed. Figure 13.7 indicates another consideration for refrigeration
applications. The COP for most applications is likely to be less than 0.55.
Figure 13.8, shows these two cycles is
substantially higher COP, over a much broader range of generator input temperatures
than the conventional lithium/bromide cycles. The superior performance is
achieved by operating the chiller input stage at constant temperature, rather
than constant pressure as in conventional systems. This has the effect of
reducing the thermodynamic irreversibilities in the absorption cycle (Wahlig,
1984).
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