R-12

Fig. 1 Refrigerant-12 Temperature/Pressure:

LATENT HEAT OF VAPORIZATION
Most automotive systems are charged with R-12, with a vaporization temperature of -30°C (-21.7°F) under standard sea level pressure. Vaporization temperature will vary as pressure varies. The pressure-to-pressure temperature relationship of R-12 is shown in the illustration.

R-12 has a constant pressure-to-temperature relationship. If either value is known the other can be determined without measurement. The vaporization temperature of R-12 increases as pressure increases (illustration). For example, at 345 kPa (5O psi) the vaporization temperature is 13°C (55°F). At 517 kPa (75 psi) vaporization occurs at 24°C (75°F).

Cooling and dehumidification is best accomplished when system pressure in the evaporator is 2O7 kPa (3Opsi) at -1.1°C (3O°F).

LIQUIFICATION
The R-12 used in an automotive air conditioning system is reused. The heat absorbed during vaporization must, therefore, be dispelled and the vapor must be converted to a liquid. This requires the use of four components:
^ Condenser
^ Compressor
^ Suction Accumulator/Drier
^ Fixed Orifice Tube

Liquification Of A Gas:

The fixed orifice tube is used to restrict the flow of refrigerant and change it from a liquid under high pressure to one under low pressure enroute to the evaporator. The illustration shows how refrigerant changes from a gaseous to liquid state.

For the A/C system to function properly, the refrigerant must transfer all heat absorbed from the air inside the vehicle to the air outside the vehicle. For this to happen more readily, the pressure and temperature of the refrigerant must be raised.

The compressor works against the fixed orifice tube located between the condenser and evaporator and provides the necessary increase in temperature and pressure. Refrigerant is then pumped out of the compressor and into the condenser as a high-pressure vapor.

The higher temperature accompanying the increase in pressure generated by the compressor does not increase the heat contained in the vapor. The BTU's involved are dissipated in the cooling process which converts the high-pressure vapor input to the condenser into a high-pressure liquid output.

Heat transfers from the refrigerant, causing condensation. Thus, the vapor reverts to its liquid form and leaves the condenser under high pressure as it flows toward the fixed orifice tube.