The Absorption Refrigerator
... or (How to apply heat to cool things down)
At the rear of the modern absorption RV refrigerator, there is a maze of steel tubing called the cooling unit. It is a self-contained, sealed system containing ammonia, hydrogen, water and a corrosion resisting agent. (usually sodium chromate)
In the lower portion of the cooling unit there is a bulb shaped container, called the absorber, holding a solution of ammonia and water, which is connected by a passageway to the siphon pump. The siphon pump is similar in operation to the center pipe of a coffee percolator. A heat source is applied (by a gas flame or an electric heating element) at the bottom of the siphon pump causing the ammonia/water solution to "boil" and form large gas bubbles. These bubbles push the ammonia/water solution to the top of the siphon pump where the now gaseous ammonia continues upward and the water separates out to flow down to a point where it is reused later.
The ammonia gas enters the finned condenser at the top of the cooling unit, where heat is dissipated to the atmosphere. As a result of this cooling effect, the ammonia vapor condenses to a liquid form and gravity takes over. The liquid ammonia flows down to the evaporator tube located inside the freezer compartment, where it mixes with pure hydrogen gas, again allowing the ammonia to "boil". It takes heat to produce this change of state (liquid ammonia to vaporous ammonia) and this heat is extracted from the freezer compartment and the food contained within.
The weight of the ammonia/hydrogen mixture carries it down to the absorber bulb at the bottom of the cooling unit, where the water in the system absorbs the ammonia. The released hydrogen (a very light gas) rises through the absorption tube passing over the water that is running down from the siphon pump (discussed above) and the remaining ammonia is absorbed. Therefor pure hydrogen is available again at the evaporator and the water/ammonia mix in the absorber bulb can continue the cycle.
This is the basic operation of the absorption cooling unit. Other components are involved to control the temperature settings.
See: Refrigerator Repair