As you can see from this description, the absorption
process is not complicated. The only moving parts are the
pump motors and impellers.
Some other features are added to the cycle to make it more
efficient. One of these is shown in Figure 48–56. It is a heat
exchange between the cooling tower water and the absorber
solution in the absorber. This heat exchange removes heat
that is generated when the water vapor is absorbed into the
absorber solution.
Figure 48–57 is a heat exchanger between the dilute and the
concentrated solution. This heat exchange serves two purposes;
it preheats the dilute solution before it enters the concentrator
and precools the concentrated solution before it enters the absorber
section. It is much like the heat exchange between the
suction and liquid line on a household refrigerator. Without this
heat exchange, the machine would be less efficient.
Some manufacturers have developed a two-stage absorption
machine that uses a higher pressure steam or hot water,
Figure 48–58. The steam pressure may be 115 psig or hot
water at 370°F. These machines are more efficient than the
single-stage machine mentioned earlier.
48.27 SOLUTION STRENGTH
The concentration strength of the solutions determines the
ability of the machine to perform. The wider the spread between
the dilute and the strong solution, the more capacity
the machine has to lower the pressure to absorb the water
from the evaporator. If the concentrated solution becomes
too concentrated, the solution will become rock salt. If this
occurs, the flow of concentrated solution may be partially or
totally blocked, which causes the machine to stop cooling.
The solution is said to have “become solid” or “crystallized,”
and service procedures must be performed in order to dissolve
the hard salt crystals. This is a delicate balance of water
and Li-Br. Adjusting the strengths of these solutions is the
job of the start-up technician. Some machines are shipped
with no charge and the charge is added in the field. The Li-
Br is shipped in steel drums to be added to the machine. The
estimated amount of Li-Br is added first. Distilled water is
used as the refrigerant charge and added next. When the
charge is adjusted, the technician calls it the trim. You charge
a compression cycle system and trim an absorption system.
Typically, a technician trims the machine with the approximate
correct amount of Li-Br and water, then starts the machine. The machine is then gradually run up to full load.
Full load may be determined by the temperature drop across
the evaporator and full steam pressure, which is normally 12
to 14 psig (hot water or direct-fire equipment would be
similar). When full load is obtained, the technician pulls a
sample of dilute and strong solution and measures the specific
gravity using a hydrometer,