312E, 312F, 313F, 316E, 316F, 318E, 318F, 320E, 320F, 323E, 323F, 324E, 325F, 326F, 329E, 329F, 330F, 335F, 336E, 336F, 340F, 349E, 349F, 352F, 374F and 390F Excavators and 336E MHPU and 349E MHPU Mobile Hydraulic Power Units Air Conditioning and Hea Caterpillar

Illustration 1	g02307934
(1) Evaporator coil (A) Refrigerant from the expansion valve (B) Vaporized refrigerant to the compressor

The expansion valve delivers a controlled amount of liquid refrigerant to evaporator coil (1). The liquid refrigerant has low temperature and low pressure. Since the pressure of the liquid refrigerant is low, the temperature of saturation of the refrigerant is low. At the temperature of saturation, the refrigerant changes from a liquid into a vapor. The temperature of saturation of a fluid increases when the pressure of a fluid increases.

Fins on the evaporator coil are exposed to the air that is inside of the cab. The heat from the air that is inside of the cab is absorbed by the evaporator. The evaporator transfers the heat to the refrigerant. The amount of heat that is required to reach the temperature of saturation is called the latent heat of vaporization. When this heat is absorbed by the refrigerant, the temperature of the refrigerant will increase. When the temperature of the refrigerant reaches the temperature of saturation, the refrigerant will change from a liquid into a vapor. The vaporized refrigerant will leave the evaporator. The vaporized refrigerant will carry away the heat from the cab. The vaporized refrigerant will return to the compressor when the refrigerant leaves the evaporator. This process is repeated continuously.

Moisture in the air condenses by cooling. This results in small amounts of water that attaches to the outside surface of the evaporator. When water freezes on the evaporator coil, the performance of the cooling system will decrease. Adequate drainage for this water must be present in order to prevent freezing water on the evaporator coil.

The amount of refrigerant that is supplied to the evaporator is adjusted by the expansion valve. In order to make precise adjustment, the evaporator requires less of a drop in refrigerant pressure. Therefore, decreasing the drop in pressure is one element that fully utilizes the ability of the evaporator.