undergoes various changes throughout the vapor compression cycle and it is in the evaporator where it actually produces the cooling effect. The evaporator is usually a closed insulated space where the refrigerant absorbs heat from the substance or food to be cooled.
The cooling effect is produced by the refrigerant rotating continuously in the refrigerating or vapor compression cycle. The refrigerant gets compressed and superheated in the compressor
and then loses heat in the condenser
. In the throttling valve
the pressure and temperature of the refrigerant is reduced suddenly and drastically. The low temperature liquid refrigerant enters the evaporator and produces the chilling effect for a refrigerator
and cooling effect for an air-conditioner
The space comprising the evaporator is an enclosed space. For instance, in the case of a household refrigerator, the small enclosed freezer section has an evaporator embedded into it. In the case of the deep freezer the evaporator is enclosed in the space where ice or ice cream is to be made. The evaporator section of refrigerators is usually insulated by using insulating materials like polyurethane foam (PUF).
The low temperature refrigerant flowing through the evaporator absorbs heat from the food, substance or any other enclosed space and gets converted into a gaseous state as its temperature rises. This is then sucked by the compressor
, which compresses it, keeping the cycle of refrigerant continuous.
In the case of air-conditioners the evaporator is also called the cooling coil. Usually the fan would pass the hot room air over the evaporator coil, which is chilled, hence the air gets cooled. This air is then supplied into the room, where it creates the cooling effect by absorbing heat from the room.
Evaporators are of various types. Evaporators used for industrial refrigeration and air-conditioning purposes are very large and also called chillers. They are usually made in the form of shell and tube types with two possible arrangements: namely, dry expansion evaporators and flooded evaporators. In dry expansion evaporators the refrigerant usually flows through the tube side while the liquid to be chilled flows through the shell side. The flooded system is used where large quantities of liquids have to be cooled to extremely low temperatures. Since the load in such cases is very high, a large quantity of refrigerant flows through these evaporators. In flooded evaporators the refrigerant will usually pass through the shell side while the liquid to be chilled will pass through the tube side.
For smaller and home purposes there are three types of evaporators: bare-tube type, plate-surface type and finned evaporators:
- In bare-tube evaporators the refrigerant flows through the bare-tube and the fluid to be chilled flows directly over it.
- Plate-surface evaporators are used in household refrigerators. These evaporators are formed by welding together two plates that have grooves on their surface. When they are welded, the closed grooves form a sort of the tubing through which the refrigerant flows.
Finned evaporators are commonly used in window, split and packaged air-conditioners. They are in the form of a copper coil over which several fins are welded to increase the cooling area of the evaporator. Hot air passes over this evaporator and gets chilled as it enters the room.