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In a nutshell, the operation of popular refrigerators is based on a process of heat transfer from a cold source to a hot source. However, this process is not spontaneous: it takes an amount of external energy, which occurs in the form of work, for this transfer to be possible. For the sake of clarity, the cold source is the freezer and the hot source is the condenser (also called the radiator).
We will now analyze the thermodynamic cycles that occur during the operation of a refrigerator. For this, consider the figure below.
This graph represents the cycle involved by means of a PV diagram, divided into five processes. Obviously, this is the idealization of cycles, as no possible energy losses are foreseen, for example.
Let's look at what happens in each of the stages of the cycle.
* 1 - 2: adiabatic compression
By increasing the fluid pressure, the compressor reduces the volume. Since this process occurs very quickly, so that energy losses are minimal, we can consider it as an adiabatic process. The work that the compressor performs is responsible for increasing the internal energy of the fluid and, consequently, for raising its temperature.
* 2 - 3: isobaric cooling
The fluid begins to lose energy in the form of heat, and as the compressor keeps the pressure high and constant, the volume and temperature decrease.
* 3 - 4: condensation
Still in the condenser and under high pressure, the fluid loses some more energy in the form of heat. Because of this, the volume and temperature of the fluid decrease further and it changes from the gaseous state to the liquid. Importantly, until now, the fluid was in the gaseous state.
* 4 - 5: adiabatic expansion
Under high pressure, fluid flows through the capillary tube and at the outlet of the tube it expands. Since this expansion occurs very quickly, so that the fluid exchanges a small amount of energy (in the form of heat) with the neighborhood, we can consider the process as adiabatic. However, the pressure and temperature of the fluid decrease, and some of it vaporizes. Thus, at the outlet of the tube, the fluid appears as liquid droplets suspended in steam at low pressure. note: The low pressure of the capillary tube is an effect of compressor operation which draws gaseous fluid from this part of the circuit to compress it into the condenser.
* 5 - 1: Isobaric vaporization
In the evaporator, under low and constant pressure, the remaining droplets are vaporized, absorbing energy (in the form of heat) from the freezer. Upon exiting the evaporator, the fluid is fully in gaseous state and at low pressure, flowing into the compressor and repeating the cycle.