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Gas Technologies: Thermally Activated Absorption Chillers

Commercially proven absorption cooling systems, ranging in size from 3 to 1,700 tons, are readily available today. These systems come as stand-alone chillers or as chillers with integral heating systems.
The basic operating principle of an absorption chiller is the same as that of a conventional vapor compression chiller, namely, that cooling is provided by evaporating a refrigerant. However, large absorption systems are different in that they:

  • Use water rather than standard refrigerants.
  • Operate at low pressure/vacuum conditions, rather than at moderate to high pressure.
  • Use heat rather than a compressor as their driving force.

Thermally activated absorption chillers operate on the principle that some materials will absorb other materials even when both are in liquid form. For example, ordinary table salt pulls water vapor out of the air, absorbing it and making it difficult to pour. Lithium bromide water solution is a liquid substance that absorbs water.

One of the major differences between a
conventional vapor compression cycle and an absorption cycle is the refrigerant used. Chlorofluorocarbons, or CFCs, have been the most popular refrigerants for mechanical refrigeration systems; however, distilled water is used as the refrigerant in most large commercial absorption systems.

Unlike conventional mechanical compression systems, absorption cycles need a second fluid, the lithium bromide water solution, which is nontoxic. Because lithium bromide (the absorbent) does not boil, water (the refrigerant) is easily separated from it by adding heat.

Gas absorption systems feature several advantages over conventional electric systems.

1. Lower Operating Costs
2. No Ozone-Damaging Refrigerants
3. Safer, Quieter Operation
4. Smaller Total Space Requirements Compared to an Electric Chiller with Separate Boiler
5. High Reliability
6. Low Maintenance

All water-cooled absorption systems on the market today use water as the refrigerant and a lithium bromide solution as the absorbent material. A typical air-cooled absorption chiller uses ammonia as the refrigerant and water as the absorbent material.

How They Work
From a refrigeration standpoint, the absorption cycle shown in Figure 2 is identical to the vapor compression cycle (Figure 1). The only differences are the components contained in the dotted box on each figure and the refrigerants used. The dotted box shown in the vapor compression cycle (Figure 1) is drawn around the compressor. In the absorption cycle (Figure 2), this dotted box is drawn around a group of components that are sometimes referred to collectively as the thermal compressor, because they serve the same purpose as a compressor, namely, they take in low-pressure refrigerant and create high-pressure refrigerant.

The cycle begins when the refrigerant leaves the condenser as a high pressure liquid. On the way to the evaporator, the refrigerant flows through an expansion valve that drastically lowers the operating pressure. Once inside the evaporator (step 1), heat is absorbed and the low-pressure liquid "boils" and is vaporized.

The absorption process uses vaporization to produce a cooling effect, but the work restoring the refrigerant is done differently than the vapor compression cooling cycle. The vapor returning to the absorber, (step 2) is absorbed by a liquid (an "absorbent") just as alcohol absorbs water. The resulting solution can then be pressurized by a simple motor-driven pump. Then, by using a gas-fueled generator (boiler) (step 3) to heat the solution, the two fluids can be separated. The liquid absorbent is cycled back to pick up more refrigerant. The high pressure refrigerant vapor is condensed to a liquid releasing its heat to the outdoors (step 4) and sent back to the evaporator to produce more cooling.

The Single-Effect Absorption Cycle
A single-effect absorption cycle, which has just been described on the previous page, can be direct-fired with natural gas or fuel oil, or it can be indirectly powered by hot water or steam. The cycle can be driven by a relatively low-grade heat source, so it is very effective for heat-recovery applications.

The figure below is a schematic representation of a single-effect chiller. This diagram better illustrates how an absorption unit actually looks. Heat exchangers for large commercial units are usually tube bundles contained in tubular-shaped pressure vessels.

The Double-Effect Absorption Cycle
The figure below shows the double-effect absorption cycle. Double-effect absorption machines add a second generator and condenser that operate at a higher temperature. The higher-temperature generator (1) is called the first stage-generator. Refrigerant vapor is recovered from the first-stage generator in the higher-temperature condenser (2). The refrigerant vapor is then condensed at a higher temperature and the heat from this condensation process is used to desorb additional refrigerant (water) from a lower-temperature, second-stage generator (3).

This additional refrigerant increases the cooling effect in the evaporator for the same heat input because the mass flow of refrigerant through the evaporator has increased. This increased refrigerant flow results in a 40% performance increase. Another way of thinking of this process is that the double-effect cycle can produce the same refrigerant flow rate, or cooling effect, as the single-effect cycle for a fraction of the heat input. It is clear why the coefficient of performance (COP) of absorption chillers ranges from 0.60 to 0.70 for indirect-fired single-effect systems, to about 1.20 for indirect-fired double-effect units.

There are three types of commercially available double-effect absorption machines. The series flow cycle gets its name from the fact that the lithium bromide water solution flows in series, first to the primary generator and then to the secondary generator. The other two double-effect cycles are the parallel-flow cycle and the reverse-flow cycle. The parallel-flow cycle gets its name from the fact that the solution stream is split after the low-temperature heat exchanger and the solution flows to the two generators through parallel paths. The reverse cycle sends the weak solution to the secondary generator before proceeding on to the primary generator.

GAX Absorption Technology
Generator absorber heat exchanger (GAX) technology, a new entry into the HVAC market for residential and light-commercial applications, differs from conventional electric technologies because a thermal compressor, consisting of a generator and an absorber, replaces the motor/compressor. GAX uses water and ammonia as working fluids to avoid harmful CFCs and HCFCs. The GAX cycle heats or cools the conditioned space; in the heating mode, it is less sensitive to ambient temperatures than electric systems and therefore requires less back-up heat at low ambience. Driven by a gas burner, GAX reduces the use of expensive peak-load electricity. Also, the GAX should require no more maintenance than the occasional burner cleanings required for all furnaces.

Common Processes:
Thermoplastics Molding, Equipment Cooling