How Everything Works
How Everything Works How Everything Works

Question 920

How does ammonia refrigeration work?
There are actually two answers to this question. First, like the more modern chlorofluorocarbon (Freon) and hydrofluorocarbon refrigerants, ammonia (NH3 converts easily from a gas to a liquid near room temperature. If you squeeze ammonia to high density, it will release heat and convert to a liquid. If you let it expand to low density, it will absorb heat and convert to a gas. A compressor-based ammonia refrigeration unit makes use of that easy convertibility. First, it uses a compressor to squeeze the ammonia gas outside the refrigerator. The hot dense ammonia gas that leaves the compressor enters a condenser, where it releases heat to its surroundings and condenses to a cool ammonia liquid. This liquid enters the refrigerator and passes into an evaporator, where it's allowed to expand into a gas and it absorbs heat from its surroundings. The gas then returns outside the refrigerator to repeat this cycle again and again.

But there is a second type of ammonia refrigerator that makes use of an absorption cycle—ammonia dissolves extremely well in cool water but not so well in hot water. In an absorption cycle refrigerator, a concentrated solution of ammonia in water is heated in a boiler until most of the ammonia is driven out of the water as a high-pressure gas. This hot, dense ammonia gas then enters a condenser, where it gives up heat to its surroundings and becomes a cooler liquid. The liquid ammonia then enters a low-pressure evaporator, where it evaporates into a cold gas. This evaporation process draws heat out the evaporator and refrigerates everything nearby. Finally, the ammonia gas must be returned to the boiler to begin the process again. That return step makes use of the absorption process, in which the ammonia gas is allowed to dissolve in relatively pure, cool water. The gas dissolves easily in this water and thus maintains the low pressure needed for evaporation to continue in the evaporator. The now concentrated ammonia solution flows to the boiler where the ammonia is driven back out of the water and everything repeats.


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