Basic Refrigeration Cycle

Basic Vapor Compression Cycle, the Carnot CycleFour main elements are required to complete the basic refrigeration cycle, a compressor, a condenser, a metering device (expansion valve in this case), and an evaporator, all labeled below in the system diagram along with connecting components.

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1. Super-heated vapor enters the compressor.
2. After compressing the gas, Super-heated vapor leaves the compressor.
3. The first part of the condenser de-superheats the gas.
4. The bulk of the condenser removes heat from the gas as it condenses. (Fan cooling)
5. The last part of the condenser sub-cools the liquid below the condensing temperature
6. The liquid passes through the expansion valve where the pressure is reduced to the evaporator pressure.
7. Refrigerant droplets rapid evaporate and absorb the heat from the air being forced across by the fan.
8. The last part of the evaporator super-heats the vapor before it continues back to the compressor.

This cycle is based on the Carnot theory, which is a theoretical construct that describes the maximum possible efficiency of a thermodynamic system doing work can achieve, developed in 1824 by Sadi Carnot. No existing system can match the Carnot system of perfect efficiency.
Below is what the perfect system, Carnot system, looks like compared to a practical system on a diagram called a “Pressure-Enthalpy Diagram”. (Enthalpy, for this brief introduction, can be considered energy).

​The numbers in the above diagram coincide with the description numbered 1-8 above for each stage of the cycle and the refrigerant condition.
It stands out, that the practical cycle is more outside the saturated region and rises above and below the Carnot cycle. The differences are driven by the efficiency of the physical equipment and materials used to apply the process.
The lower line, dubbed 7, is the evaporator line, it is lower because the material used in evaporators isn’t 100% effective at transferring the heat. Evaporators must be designed below the cycle temperature that is wished to achieve.
 Same as for the condenser, location 3,4, and 5, but in the opposite direction, condensers must run hotter than the theoretical cycle to work.
From point 1 to point 2 is compression, on the Carnot cycle, the compression takes place with no losses, on the practical cycle, it extends out past the Carnot due to inefficiency that generates heat and volume losses.
Point 5 must extend into the liquid region to ensure that the expansion valve function correctly.
At point 6, The Carnot cycle would deliver liquid at 100% quality to the evaporating cycle, where the practical system must accept liquid at a quality of 12-18% (vapor fraction) this reduces the over-all refrigeration attainable per cycle, which is less efficient.
That wraps up the basic refrigeration vapor compression cycle. More details will be explored in later posts.