13 Vehicle refrigeration

Refrigeration transport is much in demand to move frozen or chilled food from storage centres to shops and supermarkets. Thermally insulated body containers used for frozen and chilled food deliveries for both small rigid trucks and large articulated vehicles are shown in Figs 13.1 and 13.2 respectively. Refrigeration systems designed for motor vehicle trucks are basically made up of two parts supported on an aluminium alloy or steel frame. The condenser unit which is mounted outside the thermally insulated cold storage compartment, comprises a diesel engine (and optional electric motor) compressor, condenser coil, fan thermostat and accessories. The evaporator unit protruding inside the cold storage body contains the evaporator coil, evaporator fan expansion valve remote feeler bulb and any other accessories. For certain applications a standby electric motor is incorporated to drive the compressor when the truck is being parked at the loading or delivery site for a long period of time such as overnight, the electricity supply being provided by the local premises' mains power. Typical self-contained refrigeration unit arrangements incorporating an engine, compressor, evaporator, condenser, fans and any other accessories for small to medium and large frozen storage compartments are shown in Figs 13.3 and 13.4 respectively. Temperature control is fully automatic on a start-stop cycle. With small and medium size refrigeration systems the engine runs at full governed speed until the thermostat temperature setting is reached. It then automatically reduces speed and disconnects the magnetic or centrifugal clutch which stops the compressor. A slight increase of temperature will return the engine to full speed and again driving the compressor.

The cold storage compartment temperature for frozen food is usually set between — 22° C and — 25° C whereas the chilled compartment temperature is set between +3°C and +5°C.

13.1 Refrigeration terms (Fig. 13.5) To understand the operating principles of a refrigeration system it is essential to appreciate the following terms:

Refrigerant This is the working fluid that circulates though a refrigeration system and produces both cooling and heating as it changes state. The desirable properties of a refrigerant fluid are such that it flows through the evaporator in its vapour state, absorbs heat from its surroundings, then transfers this heat via the flow of the refrigerant

Air circulation

Overcab mounted refrigeration unit

Air circulation

Thermally insulated box body frozen/ cold storage compartment

Fig. 13.1 Overcab mounted self-contained refrigeration system for small and medium rigid trucks

Thermally insulated box body frozen/ cold storage compartment

Fig. 13.1 Overcab mounted self-contained refrigeration system for small and medium rigid trucks

Nose mounted refrigeration unit

Cold air distribution ducting

Nose mounted refrigeration unit

Cold air distribution ducting

Fig. 13.2 Nose mounted self-contained refrigeration system for large articulated truck

to the condenser; the refrigerant then condenses to a liquid state and in the process dissipates heat taken in by the evaporator to the surrounding atmosphere. Refrigerants are normally in a vapour state at atmospheric pressure and at room temperature because they boil at temperatures below zero on the celsius scale; however, under pressure the refrigerant will convert to a liquid state.

Saturated temperature (Fig. 13.5) This is the temperature at which a liquid converts into vapour or a vapour converts into liquid, that is, the boiling point temperature.

Saturated liquid (Fig. 13.5) This is a liquid heated to its boiling point, that is, it is at the beginning of vaporization.

Subcooled liquid (Fig. 13.5) This is a liquid at any temperature below its saturated (boiling) temperature.

Saturated vapour (Fig. 13.5) This is the vapour which is formed above the surface of a liquid when heated to its boiling point.

Fig. 13.3 Light to medium duty diesei engine and standby electric motor belt driven compressor refrigeration unit

Oil separator

(

/ s

s \

í \

z

)

(

- -—- I

Oil separator

Receiver

Coupling and clutch

Coupling and clutch

Evaporator coil

Evaporator coil

Condenser unit

Starter motor

Fig. 13.4 Heavy duty diesel engine shaft driven compressor refrigeration unit

In-line four cylinder diesel engine

Condenser unit

Starter motor

Vee cylinder compressor

Fig. 13.4 Heavy duty diesel engine shaft driven compressor refrigeration unit

In-line four cylinder diesel engine ai a.

Saturated temperature

Fig. 13.5 Illustrative relationship between the refrigerant's temperature and heat content during a change of state

Latent heat of evaporation (Fig. 13.5) This is the heat needed to completely convert a liquid to a vapour and takes place without any temperature rise.

Superheated vapour (Fig. 13.5) This is a vapour heated to a temperature above the saturated temperature (boiling point); superheating can only occur once the liquid has been completely vaporized.

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