Refrigerators use similar basic concepts, whether powered by electricity or by fossil fuels like natural gas. Both approaches cool storage spaces with a sealed heat exchange system that circulates a liquid refrigerant. When the refrigerant boils, the change to vapor robs heat from the local environment, dropping the refrigerated compartment's temperature. Electric systems control a refrigerant by mechanical compression, but gas-powered refrigerators use absorption refrigeration, a method that works without a noisy compressor.
Different liquids boil and change to vapor at different temperatures. Boiling points also change with pressure. Water, for example, boils at a lower temperature as altitude increases. Other liquids such as freon or ammonia boil at much lower temperatures than water. Even if a liquid boils at a temperature near freezing, the refrigerant absorbs energy from its cold environment as it changes to vapor. Refrigerant coils boil refrigerants under low pressure inside the cold storage compartment, absorbing heat. Hot vapor moves into a different coil, under higher pressure, to condense back to liquid and shed heat into the outside air.
To keep coolant circulating, an electric refrigerator uses a compressor to force liquid refrigerant into the evaporator or cooling coil. Gas-powered, or absorption, refrigerators use a liquid to absorb and circulate the refrigerant. In many absorption refrigerators, water acts as the absorbent and ammonia becomes the refrigerant. Recirculating ammonia vapors dissolve in the water within an absorption chamber, releasing heat. The water and ammonia mixture travels to a generator tank. Gas flame heats steam coils inside the tank to evaporate the ammonia from the water. Cooling stages condense the ammonia vapor into pure liquid ammonia, the system's refrigerant. Water circulates back to the absorption compartment.
In the evaporator coil located in the walls of the refrigeration compartment, the liquid ammonia boils and ammonia vapor collects heat. Hot ammonia vapor circulates through an expansion valve into the higher pressure condenser coil. In the condenser coil, the hot vapor exchanges heat with the air flowing over the outside of the sealed metal tubing. Cooling ammonia flows back to the absorption chamber. Both the condenser coil and the absorption chamber vent heat to the outside air. If the system can't shed the excess heat because of dirty coils or lack of ventilation, the cooling effect stops.
Gas-powered refrigeration systems might need several hours of operation before beginning efficient cooling. Absorption refrigerators depend upon gravity to circulate ammonia. If not installed perfectly level, ammonia flow can stop, halting refrigeration. Gas-powered refrigerators can use hydrogen gas to enhance the evaporation of ammonia in the evaporator coil. Both the hydrogen gas and the ammonia circulate within sealed systems, but leaks create serious hazards. A punctured coil can release explosive hydrogen, or toxic ammonia gas that can destroy lung tissue and burn skin. Gas flames can also produce toxic fumes if not adjusted to burn with a clean blue flame.
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