Heat Transfer Process Design Thermodynamics

Typical Vapor Compression Refrigeration (VCR) cycle

A majority of big refrigeration systems in use nowadays use the Vapor Compression Refrigeration (VCR) cycle. VCR uses mechanical energy (through a compressor) as the driving force for refrigeration.

We will try to present below the basic characteristics of VCR cycle:

Figure 1 - Typical schematic of a Vapour Compression Refrigeration (VCR) cycle

Evaporation Stage of refrigeration

From points 1 to 2 on figure-1, low-pressure liquid refrigerant in the evaporator absorbs heat from its surrounding environment, usually air, water or some other liquid and gets evaporated into gas. As such, the refrigerant is slightly superheated at the outlet of the evaporator. The evaporating liquid absorbs heat from the surroundings, thus performing the cooling or refrigeration duty for the surrounding air, water or other medium. This is where the refrigeration actually occurs.

Compression of the refrigerant

From point 2 to 3 on figure-1, the superheated vapor from evaporator enters the compressor where its pressure is increased due to compression. The temperature also typically increases, since a part of the energy put into the compression process is transferred to the refrigerant.

Condensation of pressurized refrigerant

From point 3 to point 4 on figure-1, the pressurized and superheated gas from compressor outlet is sent to a condenser. The initial part of the cooling process (3-3a in figure-2) de-superheats the gas before it is then turned back into liquid (3a-3b in figure-2). The cooling for this process is usually accomplished by use of air or water. A further reduction in temperature happens in the pipe work and liquid receiver (3b-4 on figure-2), so that the refrigerant liquid is sub-cooled as it enters the expansion device. This step is where the heat absorbed from process fluid at the evaporation stage is vented out to atmosphere, as indicated in figure-2.

Expansion of the subcooled and highly pressurized refrigerant

From point 4 to point 1 on figure-1, The high-pressure sub-cooled liquid passes through the expansion device, which reduces its pressure as well as controls the flow into the evaporator. The process is repeated from now in cycle.

Figure 2 - Typical representation of Vapour Compression Refrigeration (VCR) cycle in pressure-enthalpy diagram

 


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