REFRIGERATOR

A refrigerator referred to as a fridge is a common household appliance that consists of a thermally insulated compartment and a heat pump which transfers heat from the inside of the fridge to its external environment so that the inside of the fridge is cooled to a temperature below the ambient temperature of the room. Cooling is a popular food storage technique in developed countries and works by decreasing the reproduction rate of bacteria. The device is thus used to reduce the rate of spoilage of foodstuffs.
A refrigerator maintains a temperature a few degrees above the freezing point of water. Optimum temperature range for perishable food storage is 3 to 5 °C. A similar device which maintains a temperature below the freezing point of water is called a freezer.The refrigerator is a relatively modern invention among kitchen appliances. It replaced the icebox, which had been a common household appliance for almost a century and a half prior. For this reason, a refrigerator is sometimes referred to as an icebox.
refrigerators keep things cold because of the nature of heat. The Second Law of Thermodynamics essentially states that if a cold object is placed next to a hot object, the cold object will become warmer and the hot object will become cooler. A refrigerator does not cool items by lowering their original temperatures; instead, an evaporating gas called a refrigerant draws heat away, leaving the surrounding area much colder. Refrigerators and air conditioners both work on the principle of cooling through evaporation.

A refrigerator consists of two storage compartments - one for frozen items and the other for items requiring refrigeration but not freezing. These compartments are surrounded by a series of heat-exchanging pipes. Near the bottom of the refrigerator unit is a heavy metal device called a compressor. The compressor is powered by an electric motor. More heat-exchanging pipes are coiled behind the refrigerator. Running through the entire system is pure ammonia, which evaporates at -32 Celsius. This system is closed, which means nothing is lost or added while it is operating. Because liquid ammonia is a powerful chemical, a leaking refrigerator should be repaired or replaced immediately. The refrigeration process begins with the compressor. Ammonia gas is compressed until it becomes very hot from the increased pressure. This heated gas flows through the coils behind the refrigerator, which allow excess heat to be released into the surrounding air. This is why users sometimes feel warm air circulating around the fridge. Eventually the ammonia cools down to the point where it becomes a liquid. This liquid form of ammonia is then forced through a device called an expansion valve. Essentially, the expansion valve has such a small opening that the liquid ammonia is turned into a very cold, fast-moving mist, evaporating as it travels through the coils in the freezer. Since this evaporation occurs at -32 degrees Celsius, the ammonia draws heat from the surrounding area. This is the Second Law of Thermodynamics in effect. Cold material, such as the evaporating ammonia gas, tend to take heat from warmer materials, such as the water in the ice cube tray.

As the evaporating ammonia gas absorbs more heat, its temperature rises. The cool ammonia still draws heat from the warmer objects in the fridge, but not as much as the freezer section. The ammonia gas is drawn back into the compressor, where the entire cycle of pressurization, cooling and evaporation begins.