How Do Absorption Systems Work?
An absorption system works in one of two ways:
Single-Effect Absorption Refrigeration Cycle
As shown in the illustration below, with a single-effect absorption refrigeration cycle, an absorber, generator, pump and recuperative heat exchanger replace the compressor. The cycle "begins" when high-pressure liquid refrigerant from the condenser passes through a metering device (1) into the lower-pressure evaporator (2) and collects in the evaporator pan or sump. The "flashing" that occurs at the entrance to the evaporator cools the remaining liquid refrigerant. Similarly, the transfer of heat from the comparatively warm system water to the now-cool refrigerant causes the latter to evaporate (2), and the resulting refrigerant vapor migrates to the lower-pressure absorber (3).
There, it is "soaked up" by an absorbent lithium-bromide solution. This process not only creates a low-pressure area that draws a continuous flow of refrigerant vapor from the evaporator to the absorber, but also causes the vapor to condense (3) as it releases the heat of vaporization picked up in the evaporator. This heat—along with the heat of dilution produced as the refrigerant condensate mixes with the absorbent—is transferred to the cooling water and released in the cooling tower.
Assimilating refrigerant dilutes the lithium-bromide solution and reduces its affinity for refrigerant vapor. To sustain the refrigeration cycle, the solution must be reconcentrated. This is accomplished by constantly pumping (4) dilute solution from the absorber to the generator (5), where the addition of heat boils the refrigerant from the absorbent. Once the refrigerant is removed, the reconcentrated lithium-bromide solution returns to the absorber, ready to resume the absorption process.
Meanwhile, the refrigerant vapor "liberated" in the generator migrates to the cooler condenser (6). There, the refrigerant returns to its liquid state as the cooling water picks up the heat of vaporization carried by the vapor. The liquid refrigerant's return to the metering device (1) completes the cycle.
Double-Effect Absorption Refrigeration Cycle
The double-effect absorption refrigeration cycle takes absorption to the next level. The easiest way to picture a double-effect cycle is to think of two single-effect cycles stacked on top of each other. The cycle on top is driven either directly by a natural gas or oil burner, or indirectly by steam. Heat is added to the generator of the topping cycle (primary generator), which generates refrigerant vapor at a relatively higher temperature and pressure. The vapor is then condensed at this higher temperature and pressure and the heat of condensation is used to drive the generator of the bottoming cycle (secondary generator), which is at a lower temperature.
If the heat added to the generator is thought to be equivalent to the heat of condensation of the refrigerant, it becomes clear where the efficiency improvement comes from. For every unit of heat into the primary generator, two masses of refrigerant are boiled out of solution, or generated: one in the primary generator and one in the secondary generator. In a single-effect cycle only one mass is generated.
Therefore, in a double-effect system, twice the mass flow of refrigerant is sent through the refrigerant loop per unit of heat input, so twice the cooling is delivered per unit of heat input. Using this approach a double-effect system has a coefficient of performance (COP) that is roughly twice that of a single-effect cycle. However, this simplifying assumption does not account for cycle inefficiencies and losses. In actuality, a single-effect system has a COP of about 0.65, and a double-effect system has a COP of about 1.0.
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