Hot water and direct-fired machines can be controlled in a similar manner.
Other manufacturers may have other controls for capacity
control. These may control the internal fluid flow in the
machine. For example, the flow of weak (dilute) solution to
the concentrator is practical because at reduced load, less solution
needs to be concentrated.
48.31 CRYSTALLIZATION
The use of a salt solution for absorption cooling creates the
possibility of the solution becoming too concentrated and actually
turning back to rock salt. This is called crystallization
and may occur if the machine is operated under the wrong
conditions. For example, with some systems if the cooling
tower water is allowed to become too cold while operating at
full load, the condenser will become too efficient and remove
too much water from the concentrate. This will result in a
strong solution that has too little water. When this solution
passes through the heat exchanger, it will turn to crystals and
restrict the flow of the solution. If this is not corrected, a
complete blockage will occur and the machine will stop
cooling. Because this is a difficult problem to overcome
when it happens, manufacturers have developed various
methods to prevent this condition. One manufacturer uses
pressure drop in the strong solution across the heat exchanger
as a key to the problem. The action taken may be to
open a valve between the refrigerant circuit and the absorber
fluid circuit to make the weak solution very weak for long
enough to relieve the problem. When the situation is corrected,
the valve is closed and the system resumes normal operation.
Another manufacturer may shut the machine down
for a dilution cycle when overconcentration occurs.
Crystallization can occur for several reasons. Cold condenser
water is one cause, and a shutdown of the machine
due to power failure while operating at full load is another.
An orderly shutdown calls for the solution pumps to operate
for several minutes after shutdown to dilute the strong solution.
With a power failure, the shutdown is not orderly and
crystallization can occur.
Because the machine operates in a vacuum, atmospheric
air can be pulled into the machine at any point where a leak
occurs. Air in the system can also cause crystallization.
48.32 PURGE SYSTEM
The purge system removes noncondensables from the absorption
machine during the operating cycle.
All absorption systems operate in a vacuum. If there is
any source for a leak, the atmosphere will enter. A soft drink
bottle full of air in a 500-ton machine will affect the capacity.
The air will expand greatly when pulled into a vacuum.
These machines must be kept absolutely leak free. All piping
of the solutions have factory-welded connections wherever
possible. When the typical packaged absorption machine is
assembled at the factory, it is put through the most rigid of
leak tests, called the mass spectrum analysis. For this test, the
machine is surrounded with an envelope of helium. The system
is pulled into a deep vacuum and the exhaust of the vacuum
pump is analyzed with a mass spectrum analyzer for any
helium, Figure 48–62. Helium is a gas with very small molecules
that will leak in at any leak source.
Even with these rigid leak-check and welding procedures,
the system is subject to leaks developing during shipment to
the job site. Leaks may also develop after years of operation
and maintenance. When a leak occurs, the noncondensables
must be removed.
Absorption systems also generate small amounts of noncondensables
while in normal operation. A by-product of the
internal parts causes hydrogen gas and some other noncondensables
to form inside during normal operation. These are
held to a minimum with the use of additives, but they still
occur and it is the responsibility of the purge and the operator
to keep the machine free of these noncondensables.
Two kinds of purge systems are used on typical absorption
cooling equipment: the nonmotorized purge and the
motorized purge. The nonmotorized purge uses the system
pumps to create a flow of noncondensable products to a
chamber where they are collected and then bled off by the machine operator,