Introduction to the Refrigeration Evaporators
It is in the evaporators where the actual cooling effect takes place in the refrigeration and the air conditioning systems. For many people the evaporator is the main part of the refrigeration system and they consider other parts as less useful. The evaporators are heat exchanger surfaces that transfer the heat from the substance to be cooled to the refrigerant, thus removing the heat from the substance. The evaporators are used for wide variety of diverse applications in refrigeration and air conditioning processes and hence they are available in wide variety of shapes, sizes and designs. They are also classified in different manner depending on the method of feeding the refrigerant, construction of the evaporator, direction of air circulation around the evaporator, application and also the refrigerant control.
In the domestic refrigerators the evaporators are commonly known as the freezers since the ice is made in these compartments. In case of the window and split air conditioners and other air conditioning systems where the evaporator is directly used for cooling the room air, it is called as the cooling coil. In case of large refrigeration plants and central air conditioning plants the evaporator is also known as the chiller since these systems are first used to chill the water, which then produces the cooling effect.
In the evaporator the refrigerant enters at very low pressure and temperature after passing through the expansion valve. This refrigerant absorbs the heat from the substance that is to be cooled so the refrigerant gets heated while the substance gets cooled. Even after cooling the substance the temperature of the refrigerant leaving the evaporator is less the than the substance. The refrigerant leaves the evaporator in vapor state, mostly superheated and is absorbed by the compressor.
Types of Evaporators or Classification of the Evaporators
In the large refrigeration and air conditioning plants the evaporator is used for chilling the water. In such cases shell and tube type of heat exchangers are used as the evaporators. In such plants the evaporators or the chillers are classified as:
1) Dry expansion type of evaporators
2) Flooded type of the evaporators
In case of the dry expansion type of chillers or evaporators the flow of the refrigerant to the evaporators is controlled by the expansion valve. The expansion valve allows the flow of the refrigerant depending on the refrigeration load. In case of the shell and tube type of evaporators the refrigerant flows along the tube side, while the substance to be chilled (usually water or brine) flows long the shell side. In case of the flooded the evaporator is filled with the refrigerant and constant level of the refrigerant is maintained inside it. In these evaporators or the chillers the refrigerant is along shell side while the substance to be chilled or freezer flows along the tube side of the heat exchanger.
Though this classification is also applicable to the domestic refrigerators and the air conditioners, the evaporators used in these systems are classified based on their construction. The evaporators are classified based on the construction as:
1) Bare tube evaporators
2) Plate surface evaporators
3) Finned evaporators
The bare tube evaporators are the simple copper coil evaporators over which the substance to be cooled flows. The plate surface evaporators are commonly used in the household refrigerators. These evaporators are also in the form of coil which is attached to the plate. The finned evaporators are also made of copper coil with fins on the external surface as well on the internal surface. In the next articles we shall see all these evaporators in greater details.
1) Book: Principle of Refrigeration by Roy J.. Dossat, 4th Edition, Printice Hall
1) Book: Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan, Second Edition, Tata Mc-Graw-Hill Publishing Company Limited