Most kitchen appliances, such as the coffee maker, dishwasher and blender, are conveniences, but not the refrigerator. It's a necessity, and it has been as long as people have needed to store food. Since ancient times, people have used ice boxes and evaporative tanks to cool food, and they have immersed food in streams and wells, kept it in cellars and used techniques such as salting, pickling, smoking and drying in an effort to preserve it, according to History Magazine.
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Ice boxes came in vogue in the early 1800s and became increasingly popular throughout the century, but active vapor condensation and evaporation also made its debut in the middle part of that century. At first, it was used primarily by the beer and meat industries, but by the 1920s, household refrigerators were becoming common and ice boxes becoming all but obsolete. Today, the refrigerator is an essential amenity in every household, and while there are many brands, they all work by the same principle and are constructed in essentially the same way.
Parts of Refrigerator and Function
The modern refrigerator, like an ice box, is basically a sealed, insulated compartment but instead of a block of ice, it uses the circulation of a volatile liquid, called a refrigerant, to keep the insides cool. A compressor pumps the refrigerant through a series of coils, pressurizing it and condensing it into a liquid. A tiny aperture called the expansion valve connects the condenser coils to another set of coils — the evaporative coils — and when the liquid sprays through this valve, it vaporizes, which is an endothermic process that draws heat out of the refrigerator compartment.
The condenser coils may be behind the refrigerator compartment, underneath it or on top, and they are often placed in front of an exhaust fan to dissipate the heat generated by condensation. The evaporative coils, on the other hand, are inside the freezer compartment and may or may not be hidden by a panel, depending on the model. The freezer and refrigerator compartments are separated, and a fan blows cold air through a duct that connects them.
Besides shelves, trays, drawers, lights and handles, the other parts of a refrigerator are electronic controls that monitor interior conditions and order the compressor on and off to maintain a preset temperature. The controls also regulate the fan that transfers cold air from the freezer to the refrigerator. The last component of a refrigerator is the door gasket, which forms an airtight seal that prevents cold air from escaping.
Refrigerator Body Material
A typical refrigerator body has three layers. The core, which is made of sheet metal, is sandwiched between an interior cabinet, which is usually made of polystyrene plastic, and an outer shell, made of stainless steel, plastic or sometimes painted sheet metal. Packed inside these layers is the insulation, which was absent in the earliest models but is standard equipment in contemporary ones.
The first insulation material was fiberglass, which was replaced by polyurethane foam in the 1980s, according to Assembly Magazine. Polyurethane foam is denser and more structurally stable than fiberglass, so the change allowed manufacturers to incorporate thinner refrigerator body material into sleeker and more energy-efficient designs. Insulation is the No. 1 determinant as to whether or not a particular model receives an Energy Star rating.
In the refrigerator manufacturing process, the core, interior cabinet and outer shell are molded, formed and connected together, and polyurethane foam is injected into the space between them. Besides hardening into a dense substructure that provides stability, polyurethane is an excellent adhesive that bonds the layers together.
Refrigerants Through the Years
No discussion of refrigerators would be complete without mentioning refrigerants, the volatile liquids that course through the coils of the refrigeration system. A refrigerant must possess certain physical and thermodynamic properties to be effective. In days when most refrigerators were used for commercial purposes, ammonia and carbon dioxide were the chemicals of choice. In the early days of domestic refrigerators, a number of hazardous chemicals were employed, including sulfur dioxide, methyl chloride and methylene chloride, the latter being the noxious chemical in paint strippers.
DuPont developed Freon, a chlorofluorocarbon (CFC), in the 1930s, and it soon replaced other refrigerants because it is chemically inert and has a superior capacity to transfer heat. Freon made refrigerators safer, but unfortunately, it turned out to have a major drawback that led to CFCs being banned. Because CFCs are broken down only by ultraviolet light, they migrate into the upper atmosphere, where they destroy the ozone layer.
CFCs were replaced by hydrochlorofluorocarbons (HCFCs), which are chemically similar but do not contain chlorine, the chemical that causes the ozone layer damage. These also turned out to have drawbacks in that they fill the atmosphere with fluorine, which is a greenhouse gas, so they too have been replaced by another class of fluorocarbons called hydrofluoroolefin (HFO). HFOs also release fluorine gas into the atmosphere, but at a lower rate, and the search continues for a refrigerant with even less environmental impact.
Copper Coils Are Best for Conducting Heat
The coils that conduct refrigerant from the compressor to the expansion valve into the evaporative coils and back to the compressor are made of copper. It has one of the best thermal conductivities of any metal, and it's is malleable enough to form into tubing. The expansion valve is usually made of brass, and the coil system is often supported by a steel grid. The compressor is a metal pump, and the exhaust fan has metal rotors and fins made of thin sheet metal. All the coil connectors are brass.
What's Behind the Door?
When you open the refrigerator door, you usually have to give it a little tug, and that's because you have to break the airtight seal the rubber gasket forms between the door and the inner cabinet. Modern refrigerators don't have the locking handles that their forebears did, and the gasket is responsible for keeping the door closed. To this end, the rubber may be blended with a ferromagnetic metal that holds the door firmly against a steel strip, which may be hidden behind the plastic interior lining.
The trays inside the cabinet are usually made of glass, and they sit on metal or plastic supports or on ridges molded into the interior lining. Drawers are molded from plastic, and the handles may be metal or, on less expensive models, plastic. If the freezer compartment is inside the refrigerator, it usually has a thin metal door to keep the freezer separated from the refrigerator, and an inexpensive plastic fan circulates air between the two compartments.
Refrigerators equipped with an ice maker have a plastic box in the freezer to hold the ice, while the ice maker has fins made from thin sheet metal. Models with water dispensers require tubing to conduct the water, and these are usually made of polybutylene or, on older models, copper.