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The compressor is the heart of a refrigerant circuit based on the so-called “vapor compression cycle.” This thermodynamic cycle exploits the evaporation of a coolant within a closed loop piping circuit. Specifically, evaporation occurs in a heat exchanger called an evaporator, which absorbs energy from the surrounding air; This is then delivered to the food storage compartment or air-conditioned room by natural convection or forced by fans. The same applies to water as a medium pumped by the heat exchanger and then discharged into the storage tank for use by terminal units. Once evaporated, the coolant can no longer absorb considerable amounts of energy; therefore, it must be returned to the liquid state by condensation. Thus, the problem is having a “cold” environment sufficient to absorb the energy of the refrigerant, which naturally cannot be the same compartment or space that has just cooled. The compressor is then used to compress the coolant at a more significant pressure than in the evaporator (up to 8-10 times!) This allows the condensation process to proceed at a temperature compatible with a “cold” source readily available, typically outdoor air. Condensation is thus carried out at a high temperature (usually 35-55°C) within a heat exchanger, where the two fluids are outside air and refrigerant. The latter condenses and returns to the liquid state while the outside air is heated. The liquid refrigerant is still at high pressure when it leaves the condenser. An expansion device is thus needed to expand the liquid refrigerant and reduce its stress to the value at which evaporation occurs. The refrigerant has now returned to its initial state (liquid at low pressure and temperature) and can once again absorb energy from the air or water.
Therefore, the compressor has the function of circulating refrigerant inside the circuit, drawing it in as a gas from the evaporator, compresses it, and delivering it at higher pressure to the condenser. It provides volumetric compression, i.e., a progressive reduction in volume, using rotating or reciprocating systems. This mechanical work implies a significant increase in the gas temperature (at times above 100°C) and power consumption. Compressor power consumption depends on the difference between the two operating pressures. The refrigerant entering the compressor must be in the gaseous state, as liquids are notoriously incompressible. The compressor starts working when the unit needs to provide cooling and is usually activated via temperature control systems. Not all air-conditioning and refrigeration applications have the exact requirements in terms of capacity, noise, efficiency, and operating range, and as a result, there are different types of compressors. These essentially differ regarding how compression is achieved, with reciprocating compressors featuring a reciprocating movement to create compression, and rotary compressors, including rotary vane, scroll, screw, and centrifugal compressors, featuring a rotational movement to bring about reduction. 

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