Causes of Crystallization Some absorption chillers are notorious for "freezing up" or crystallizing.
The basic mechanism of failure is simple enough – the lithium bromide
solution becomes so concentrated that crystals of lithium bromide form and
plug the machine (usually the heat exchanger section).
The most frequent causes are:
1. Air leakage into the machine,
2. Low temperature condenser water, and
3. Electric power failures.
The first two are actually very similar
since they both drive the heat input up to the point that crystallization
can occur. Whether air leaks into the machine or the condenser water
temperature is too low, the water vapor pressure in the absorption chiller
evaporator has to be lower than normal to produce the required cooling. This
forces the heat input to the machine to be higher to increase the solution
concentration. Air leakage into the machine can be controlled by designing
the machine with hermetic integrity and routinely purging the unit using a
vacuum pump.
Excessively cold condenser water (coupled
with a high load condition) can also cause crystallization. While reducing
condenser water temperature does improve performance, it could cause a low
enough temperature in the heat exchanger to crystallize the concentrate.
Sudden drops in condenser water temperature could cause crystallization. For
this reason, some of the early absorption chillers were designed to produce
a constant condenser water temperature. Modern absorption chillers have
special controls that limit the heat input to the machine during these
periods of lower condenser water temperatures.
Power failures can cause crystallization as
well. A normal absorption chiller shutdown uses a dilution cycle that lowers
the concentration throughout the machine. At this reduced concentration, the
machine may cool to ambient temperature without crystallization. However, if
power is lost when the machine is under full load and highly concentrated
solution is passing through the heat exchanger, crystallization can OCCUR.
The longer the power is out, the greater the probability of crystallization.
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