Principle of refrigeration
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Refrigeration refers to the artificial process of lowering the temperature of an object or space below ambient temperature and maintaining that temperature. Unlike natural cooling, which involves heat transferring spontaneously from a hotter to a cooler area, refrigeration reverses this process by moving heat from a low-temperature source to a high-temperature one. This reverse heat transfer requires external energy input, as dictated by the second law of thermodynamics. The machines used to perform this task are commonly called "refrigerators," and they often rely on substances known as refrigerants to facilitate the cooling process.
Refrigeration technology spans a wide range of temperatures, from just below ambient to nearly absolute zero. Based on temperature ranges, the industry typically divides refrigeration into three categories: ordinary refrigeration (above 120 K), low-temperature refrigeration (between 120 K and 4.2 K), and ultra-low temperature refrigeration (below 4.2 K). Common methods include vapor compression, absorption, gas expansion, and thermoelectric refrigeration. Each method has its own advantages and applications, with vapor compression being the most widely used in air conditioning systems.
The ideal refrigeration cycle, known as the inverse Carnot cycle, is a theoretical model that assumes no losses due to friction, heat transfer, or irreversibility. It consists of two isothermal and two isentropic processes. In this cycle, the refrigerant undergoes evaporation at a low temperature, absorbs heat, and then compresses, condenses, and expands to complete the loop. While this cycle is not practically achievable, it serves as a benchmark for evaluating real-world refrigeration systems.
In practice, actual cycles experience inefficiencies, leading to lower performance compared to the ideal case. To measure how close a real system is to the ideal, engineers use the coefficient of performance (COP), which compares the cooling effect to the work input. A higher COP indicates better efficiency. Understanding these principles is essential for students studying refrigeration, as it lays the foundation for practical engineering applications.