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:
• air leakage into the machine,
• low temperature condenser water, and
• 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.
Major absorption chiller manufacturers now incorporate devices that minimize the possibility of crystallization. These devices sense impending crystallization and shut the machine down after going through a dilution cycle. These devices also prevent crystallization in the event of power failure. A typical anti-crystallization device consists of two primary components: (i) a sensor in the concentrated solution line at a point between the concentrator and the heat exchanger, and (ii) a normally open, two-position valve located in a line connecting the concentrated solution line and the line supplying refrigerant to the evaporator sprays.
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