Diffusion absorption refigeration

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The ammonia/water solution flows to the generator where it is heated to 180 °C and some ammonia evaporates.

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The ammonia/water solution flows to the generator where it is heated to 180 °C and some ammonia evaporates.

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The vapor forms bubbles that push columns of liquid solution in the pump-tube up to the liquid–vapor separator.

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Ammonia vapor is separated out from the solution in the separator and then is purified at the rectifier, and is then liquefied in the air-cooled condenser.

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The weak solution is collected at the separator and returns to the absorber via a solution heat exchanger.

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The ammonia liquid passes to the evaporator, which is divided into two sections: a freezer and a food chiller.

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Ammonia is heavier than hydrogen and so at the higher part of the evaporator, in the freezer, there is less ammonia and more hydrogen. The evaporation temperature here is between −30 °C and -18 °C. The the partial pressure of ammonia is lower than the hydrogen.

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Because the evaporator is charged with hydrogen, and the density of ammonia is considerably greater than that of hydrogen, so the vapor mixture (ammonia and hydrogen) becomes heavier as ammonia continues to evaporate, so it drops from the freezer to the food chiller and enters the absorber.

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There is more ammonia in the lower part of the evaporator, at the food chiller, so its partial pressure is higher at this part. The evaporation temperature is higher, around -5 °C.

summary

In the absorber, ammonia is absorbed into the solution, the vapor becomes lighter, because there is less ammonia but more hydrogen, and thus it rises to the top. This causes a circulation of hydrogen in the evaporator and absorber.

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So, you see, the auxiliary gas, hydrogen, or helium, is used as a capping gas, creating low partial pressure of ammonia in the evaporator, providing pressure equalisation for ammonia between the condenser and evaporator.

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Its application is limited to small refrigerators only, with low refrigeration capacity, up to 200 W, because of the limited flow rate of bubble pump. Its efficiency is relatively poor, COP of around 0.2