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:
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.
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.