<P> A refrigerator (colloquially fridge) is a popular household appliance that consists of a thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that 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 . Refrigeration is an essential food storage technique in developed countries . The lower temperature lowers the reproduction rate of bacteria, so the refrigerator reduces the rate of spoilage . 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 (37 to 41 ° F). A similar device that maintains a temperature below the freezing point of water is called a freezer . The refrigerator replaced the icebox, which had been a common household appliance for almost a century and a half . For this reason, a refrigerator is sometimes referred to as an icebox in American usage . </P> <P> The first cooling systems for food involved using ice . Artificial refrigeration began in the mid-1750s, and developed in the early 1800s . In 1834, the first working vapor - compression refrigeration system was built . The first commercial ice - making machine was invented in 1854 . In 1913, refrigerators for home use were invented . In 1923 Frigidaire introduced the first self - contained unit . The introduction of Freon in the 1920s expanded the refrigerator market during the 1930s . Home freezers as separate compartments (larger than necessary just for ice cubes) were introduced in 1940 . Frozen foods, previously a luxury item, became commonplace . </P> <P> Freezer units are used in households and in industry and commerce . Commercial refrigerator and freezer units were in use for almost 40 years prior to the common home models . The freezer - on - top - and - refrigerator - on - bottom style has been the basic style since the 1940s, until modern refrigerators broke the trend . A vapor compression cycle is used in most household refrigerators, refrigerator--freezers and freezers . Newer refrigerators may include automatic defrosting, chilled water, and ice from a dispenser in the door . </P>

When were freezers combined with refrigerators and sold on the market