134 Vapourcompression cycle refrigeration system with reverse cycle defrosting

A practical refrigeration system suitable for road transportation as used for rigid and articulated vehicles must have a means of both cooling and

Reverse expansion valve - cold (closed)

Condenser coil Remote feeler bulb

Remote feeler bulb

/ Evaporator coil

Fins

Condenser

1 Fins

Evaporator fan

Remote feeler bulb

/ Evaporator coil

Fins

Condenser

1 Fins

Evaporator fan

Check valve open cvo®

Check valve open cvo®

(a) Refrigeration cycle

Fig. 13.15 (a and b) Refrigeration system with reverse cycle defrosting

Reverse expansion valve - hot (open)

Condenser coil \ Remote feeler bulb

Remote feeler bulb Evaporator coil

Fins7

Fins

Reverse expansion valve - hot (open)

Condenser coil \ Remote feeler bulb

Remote feeler bulb Evaporator coil

Fins

Fins7

Discharge port

Discharge port

Receiver

(b) Heating and defrost cycle defrosting the cold compartment. The operation of such a system involving additional valves enables the system to be switched between cooling and heat/defrosting, which will now be described.

13.4.1 Refrigeration cooling cycle (Fig. 13.15(a)) With the pilot solenoid valve de-energized and the compressor switched on and running the refrigerant commences to circulate through the system between the evaporator and condenser.

Discharge line pressure from the right hand compressor cylinder is transferred via the pilot valve to the reverse cycle valve; this pushes the slave piston and valves inwards to the left hand side into the 'cooling' position, see Fig. 13.15(a). Low pressure refrigerant from the receiver flows via the open check valve (1), sight glass and drier to the thermostatic expansion valve where rapid expansion in the valve converts the refrigerant to a liquid/vapour mixture. Low pressure refrigerant then passes through the evaporator coil where it absorbs heat from the cold storage compartment: the refrigerant then comes out from the evaporator as low pressure saturated vapour. Refrigerant now flows to the compressor suction port via the reverse cycle valve and suction pressure valve as superheated vapour. The compressor now converts the refrigerant to a high pressure superheated vapour before pumping it to the condenser inlet via the oil separator and reverse cycle valve; at this point the refrigerant will have lost heat to the surroundings so that it is now in a high pressure saturated vapour state. It now passes through the condenser where it gives out its heat to the surrounding atmosphere; during this process the high pressure refrigerant is transformed into a saturated liquid. Finally the main liquid refrigerant flows into the receiver via the open check valve (4) where there is enough space for the remaining vapour to condense. This cycle of events will be continuously repeated as the refrigerant is alternated between reducing pressure in the expansion valve before passing through the evaporator to take heat from the cold chamber, to increasing pressure in the compressor before passing through the condenser to give off its acquired heat to the surroundings. Note check valves (1) and (4) are open whereas check valves (2), (3) and (5) are closed for the cool cycle.

13.4.2 Heating and defrosting cycle

With constant use excessive ice may build up around the evaporator coil; this restricts the air movement so that the refrigerant in the evaporator is unable to absorb the heat from the surrounding atmosphere in the cold storage compartment, therefore a time will come when the evaporator must be defrosted.

Heating/defrosting is achieved by temporarily reversing the refrigerant flow circulation so that the evaporator becomes a heat dissipating coil and the condenser converts to a heat absorbing coil.

To switch to the heat/defrosting cycle the pilot solenoid valve is energized; this causes the solenoid valve to block the discharge pressure and connect the suction pressure to the servo cylinder reverse cycle valve, see Fig. 13.15(b). Subcooled high pressure liquid refrigerant is permitted to flow from the receiver directly to the now partially opened reverse thermostatic expansion valve (due to the now hot remote feeler bulb's increased pressure). The refrigerant expands in the reverse expansion valve and accordingly converts to a liquid/vapour; this then passes through the condenser via the open check valve (3) in the reverse direction to the normal refrigeration cycle and in the process absorbs heat from the surroundings so that it comes out as a low pressure saturated vapour. The refrigerant then flows to the compressor suction port via the reverse cycle valve and suction pressure valve but due to the high surrounding atmospheric temperature it is now superheated vapour. The compressor then transforms the low pressure superheated vapour into a high pressure superheated vapour and discharges it to the evaporator via the oil separator and reverse cycle valve. Hence the saturated vapour stream dissipates its heat through the tubing walls to the ice which is encasing the tubing coil until it has all melted. The refrigerant at the exit from the evaporator will now be in a saturated liquid state and is returned to the receiver via the open check valve (2), sight glass, and open check valve (5) for the heating/defrosting cycle to be repeated. Note during the refrigeration cycle the condenser's reverse expansion valve and remote feeler bulb sense the reduction in temperature at the exit from the condenser, thus the corresponding reduction in internal bulb pressure is relayed to the reverse expansion valve which therefore closes during the defrosting cycle. Defrosting is fully automatic. A differential air pressure switch which senses any air circulation restriction around the evaporator coil automatically triggers defrosting of the evaporator coil before ice formation can reduce its efficiency. A manual defrost switch is also provided.

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