132 Principles of a vapourcompression cycle refrigeration system Fig 136

1 High pressure subcooled liquid refrigerant at a typical temperature and pressure of 30 °C and

10 bar respectively flows from the receiver to the expansion valve via the sight glass and drier. The refrigerant then rapidly expands and reduces its pressure as it passes out from the valve restriction and in the process converts the liquid into a vapour flow.

2 The refrigerant now passes into the evaporator as a mixture of liquid and vapour, its temperature being lowered to something like —10 °C with a corresponding pressure of 2 bar (under these conditions the refrigerant will boil in the evaporator). The heat (latent heat of evaporation) necessary to cause this change of state will come from the surrounding frozen compartment in which the evaporator is exposed.

Condenser coil

Saturated vapour 40 °C 10 bar (high pressure)

Discharge line

Superheated

(high pressure)

"X5

Superheated vapour 8°C 2 bar (low pressure)

Suction Evaporator line coil

(low pressure)

Saturated vapour -10 °C 2 bar (low pressure)

Condenser coil

Saturated vapour 40 °C 10 bar (high pressure)

Discharge line

Superheated

(high pressure)

"X5

Superheated vapour 8°C 2 bar (low pressure)

Suction Evaporator line coil

(low pressure)

Saturated vapour -10 °C 2 bar (low pressure)

Saturated liquid

(high pressure)

Liquid/vapour mixture

Liquid/vapour mixture 40 °C 10 bar

Subcooled liquid

Fig. 13.6 Refrigeration vapour-compression cycle

Saturated liquid

(high pressure)

Liquid/vapour mixture

Liquid/vapour mixture 40 °C 10 bar

Subcooled liquid

Fig. 13.6 Refrigeration vapour-compression cycle

3 As the refrigerant moves through the evaporator coil it absorbs heat and thus cools the space surrounding the coil. Heat will be extracted from the cold storage compartment until its pre-set working temperature is reached, at this point the compressor switches off. With further heat loss through the storage container insulation leakage, doors opening and closing and additional food products being stored, the compressor will automatically be activated to restore the desired degree of cooling. The refrigerant entering the evaporator tube completes the evaporation process as it travels through the evaporator coil so that the exit refrigerant from the evaporator will be in a saturated vapour state but still at the same temperature and pressure as at entry, that is, —10 °C and 2 bar respectively.

4 The refrigerant is now drawn towards the compressor via the suction line and this causes the heat from the surrounding air to superheat the refrigerant thus raising its temperature to something like 8 °C; however, there is no change in the refrigerant's pressure.

5 Once in the compressor the superheated vapour is rapidly compressed, consequently the superheated vapour discharge from the compressor is at a higher temperature and pressure in the order of 60 °C and 10 bar respectively.

6 Due to its high temperature at the exit from the compressor the refrigerant quickly loses heat to the surrounding air as it moves via the discharge line towards the condenser. Thus at the entry to the condenser the refrigerant will be in a saturated vapour state with its temperature now lowered to about 40 °C; however, there is no further change in pressure which is still therefore 10 bar.

7 On its way through the condenser the refrigerant saturated vapour condenses to a saturated liquid due to the stored latent heat in the refrigerant transferring to the surrounding atmosphere via the condenser coil metal walls. Note the heat dissipated to the surrounding atmosphere by the condenser coil is equal to the heat taken in by the evaporator coil from the cold storage compartment and the compressor.

8 After passing through the condenser where heat is given up to the surrounding atmosphere the saturated liquid refrigerant now flows into the receiver. Here the increased space permits a small amount of evaporation to occur, the refrigerant then completes the circuit to the expansion valve though the liquid line where again heat is lost to the atmosphere, and this brings the refrig erant's temperature down to something like 30 °C but without changing pressure which still remains at 10 bar.

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