2. The refrigerator
From a thermodynamic viewpoint, a refrigerator is based on a simple rule: “when two substances are in contact, the hot one supplies heat to the cold one up to equilibrium”. In a refrigerator, the transfer of heat proceeds by evaporation-condensation of a volatile fluid: the refrigerant. Heat is absorbed locally by the evaporation of the fluid, transported by the gas phase, and released from this gas by condensation outside as human and mammalian bodies do. The expulsed sweat cools the surface and the interior of the body by evaporation in the atmosphere until perspiration is no longer necessary.
The transfer of heat from a hot medium to a cold one is spontaneous and rather slow. In contrast, heat exchanges from a cold medium to a hot one cannot be spontaneous. Some energy must be supplied. It is the role of the compressor present in air-conditioning machines and refrigerators. For humans, there is no compressor and the pressure in tissues is the sole driving force to push water outside. A schematic representation of a refrigerator is shown in Figure 1. There are four main stages to control the inner temperature:
2.1 Evaporation
From 1 to 2 in Figure 1, the volatile refrigerant enters the inner evaporator as a liquid. Going through this multi-tube device, the liquid absorbs part of the heat present in the closed space, including that of foods in it. As a result, the liquid is turned to gas at the outlet of the evaporator while the temperature inside the closed space decreases slightly.
2.2 Compression
From point 2 to point 3, the gas is pressurized in the compressor where it is super-heated by compression and then transferred to the multi-tube condenser located outside, generally in the back.