When liquid refrigerant reaches the evaporator, it vaporizes (or boils) and absorbs heat from the freezer. Sometimes the evaporator is referred to as a cooling coil. Due to the compressor suction power on the outlet of the evaporator and the fact that the capillary tube (with its very small inside diameter) is placed on the inlet side of the evaporator, the pressure in the evaporator is reduced to an average of 3.5 psi in an ambient temperature of 70°F (see fig. 1). It is these low pressures that cause the refrigerant to boil and absorb the surrounding heat during its change of state (essentials 1 and 3).
Four of the most common types of evaporators are illustrated above and in figures 3, 4, and 5. Figure 5 shows the cycle-defrost type of evaporator. This is basically a flat aluminum plate with a cooling coil in it. About three-fourth of this plate is in the freezer compartment while the remainder extends into the fresh-food compartment. Since fewer loops of the coil are in the fresh-food compartment, only a small portion of the cold air is produced there but adequate to maintain the proper temperature.
Frost-free refrigeration units are equipped with fan-forced circulation evaporators in which a fan increases the airflow through the coil. This type of evaporator cools the refrigerator cabinet or freezer very rapidly.
Manual-defrost refrigerators have a shell-type evaporator, which is located in the top of the cabinet. The shell type is similar to the evaporators used in the cycle-defrost type, which is a flat plate, but bent into a boxlike configuration (see fig. 15). Frozen food and ice trays are kept inside the shell while the fresh food in the rest of the cabinet is kept at the proper temperature by the cold air emitted from it. Shell-type evaporators are almost always found in small office-type or inexpensive refrigerators.
Normally, as a safety measure, an accumulator is installed at the outlet of all evaporators (the small cylinder in fig. 15) to trap any unvaporized refrigerant. It prevents liquid refrigerant from getting into the compressor, causing serious damage.
(This can be evidenced by a loud knocking when the compressor runs.) The liquid refrigerant trapped there will get a chance to vaporize before entering the compressor.