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Saturday, October 24, 2009

Absorption Refrigeration Primer

I’m new to RVing and am wondering exactly how does a refrigerator run off LP?
Eugene, (Washington, DC)

Eugene, of all the appliances found in today’s recreation vehicle, the one that is probably the most perplexing to the RVer, is the absorption refrigerator. Understanding the theory of absorption and contemplating why it takes heat in order to make cold can be baffling at best. Then toss in the fact that this is all accomplished with no moving parts and is done silently.

Here’s a quick lesson: Today’s RV refrigerator can use three energy sources; 12-volts DC, 120-volts AC, and LP gas. All three energy sources are utilized as heat producers. The 12-volt DC and 120-volt AC electricity are used to power a heater element, while the LP is burned at an open burner. The main component of any RV absorption refrigerator and the one that is most visible on the back of the refrigerator is the cooling core. All cooling units consist of four major component parts: Boiler, condenser, evaporator and absorber.

Through these four main components circulate the pressurized and sealed contents of the cooling unit in an endless cycle during the time the refrigerator is turned operating. All cooling units are sealed and, therefore, are not field repairable. Specialized machinery and unique material handling make it mandatory that if a cooling unit is determined to be at fault in a non-working refrigerator, it must be replaced. Inside the cooling unit, added in precise amounts during manufacture are; water, liquid ammonia, hydrogen gas and a compound chemical called sodium chromate.

As temperature and pressure act on the contents, they undergo various states of evaporation and condensation, all within the confines of the sealed cooling unit. Because of the caustic nature of liquid ammonia, the sodium chromate is added to protect the insides of the pipes during the refrigeration cycle.

When heat is applied to the boiler portion of the cooling unit, the ammonia and water begin to boil. Bubbles of ammonia gas are produced which rise into the percolator tube along with an accumulation of weak ammonia and water. As the ammonia and water solution pass into the tube, the ammonia vapor continues into the water separator. Any water vapor reaching this point is condensed and falls back into the boiler section leaving dry ammonia vapor to pass to the condenser. As air circulates over the condenser fins from the outside, it removes heat from the ammonia vapor inside causing it to condense into a liquid which flows to the low temperature evaporator.

The low temperature evaporator portion of the cooling unit is positioned in the wall or shelf of the freezer section of the refrigerator. The evaporator is internally supplied with hydrogen gas which passes across the surface of the incoming ammonia and subsequently causes the vapor pressure to allow the liquid ammonia to evaporate. The evaporation of the ammonia removes heat from the evaporator section through the walls of the freezer, and through the tubing, removing heat from the freezer section of the refrigerator, including any food stored there. The net result is that through the theory of absorption and its principles, the RV refrigerator isn’t making cold, but simply removing heat. Cold, therefore, is simply the absence of heat.

From the low temperature evaporator, any remaining remnants of liquid ammonia and hydrogen gas are passed lower into the high temperature evaporator which is positioned in the lower section of the refrigerator. This continues to remove and transfer heat from inside the box to the outside, but not as much heat is removed. That’s why it is warmer in the food section, or lower portion, than in the upper freezer section.

After the ammonia and hydrogen gas pass through the evaporator, the contents flow to the absorber section. Upon entering the upper portion of the absorber, a continuous trickle of weak ammonia solution comes into contact with the mixed ammonia and hydrogen gas which readily absorbs the ammonia from the mixture, freeing the hydrogen gas and allowing it to rise back through the absorber coil and to the evaporator section.

The hydrogen gas, effectively, moves back and forth between the absorber and the evaporator sections. The strong ammonia solution produced in the absorber flows down to the absorber vessel (the large tank-looking container located below the absorber coils) where it is held, mixed with water, and fed into the boiler section, and the process starts all over. Study hard, there might be a test!


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