Handbook of air conditioning and refrigeration / Shan K

REFRIGERATION SYSTEMS: ABSORPTION 14.9
Reverse Parallel Flow. As shown in Fig. 14.3, the solution from the absorber is pumped through
a low-temperature heat exchanger and flows into a low-temperature generator. After that, the solution
splits into two streams. One flows in sequence through the high-temperature heat exchanger,
high-temperature generator, and high-temperature heat exchanger again, and then combines with
the other stream discharged from the low-temperature generator. The combined stream flows
through the low-temperature heat exchanger and returns to the absorber. A reverse parallel flow system
is one of the most energy-efficient operating cycles today. It also has a more compact design.
Flow of Solution and Refrigerant
FIGURE 14.4 Purge unit in a lithium bromide absorption chiller. (Source:
ASHRAE Handbook 1998, Refrigeration. Reprinted with permission.)
For a double-effect, direct-fired, reverse parallel flow absorption chiller, the absorption cycle shows
the properties of the solution at various concentration, temperatures, and pressures, as seen in
Fig. 14.5. Sometimes the refrigerant, water vapor, is combined with the solution; and sometimes the
refrigerant and solution are separated.
Three kinds of LiBr solution are used during operation: concentrated solution, partly concentrated
solution, and diluted solution. Concentrated solution has a higher concentration of LiBr and
is created after all the refrigerant has been boiled off from the solution. Concentrated solution is
used to absorb the water vaporized in the evaporator. The solution that leaves the low-temperature
generator is partly concentrated. Diluted solution has a lower concentration of LiBr and is the combination
of aqueous LiBr solution and absorbed water vapor.
An absorption refrigeration system can also be divided into three regions according to the pressure
of the refrigerant and solution. The high-pressure region includes the high-temperature generator;
the intermediate-pressure region comprises the low-temperature generator and condensers; and
the low-pressure region includes the absorber and evaporator. Because the evaporator and absorber
as well as the low-temperature generator and the condenser are both connected by vapor passages