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Two Stage Absorption Chillers


The energy efficiency of absorption can be improved by recovering some of the heat normally rejected to the cooling tower circuit. A two-stage or two-effect absorption chiller accomplishes this by taking vapors driven off by heating the first stage concentrator (or generator) to drive off more water in a second stage. Many absorption chiller manufacturers offer this higher efficiency alternative.

Notice that two separate shells are used. The smaller is the first stage concentrator. The second shell is essentially the single stage absorption chiller from before, containing the concentrator, condenser, evaporator, and absorption chiller. The temperatures, pressures, and solution concentrations within the larger shell are similar to the single-stage absorption chiller as well.

Steam at pressures typically in the l25 - 150 psig range is brought into the stainless steel tubes of the first stage concentrator causing the solution there to boil. The pressure at which boiling occurs and the pressure of the released refrigerant vapor is approximately 5 psig (20 psia). The partially concentrated solution from this first stage flows through the high temperature heat exchanger where it is cooled by the lower temperature dilute solution returning from the concentrator. This concentrate then passes into the lower pressure second stage concentrator where the vapors from the first stage take it to the final desired concentration levels. This second stage operates at a pressure of 0.1 atmosphere (1.5 psia).

The reuse of the vapors from the first stage generator makes this machine more efficient than single stage absorption chillers, typically by about 30%.

Two-stage absorption chillers are typically driven by high-pressure (60 to 130 psig) steam, direct-fired with natural gas or #2 fuel oil, or using hot exhaust gas from combustion engines.

 



 

 




 

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