Types of Evaporators
Different types of evaporators are used in different types of refrigeration applications and accordingly they have different designs. The evaporators can be classified in various ways depending on the construction of the evaporator, the method of feeding the refrigerant, the direction of circulation of the air around the evaporator, etc. Here we have classified the evaporators based on their construction.
Classification of the Evaporators Based on the Construction
The evaporators used for the refrigeration and the air conditioning applications have different types of construction depending on the application. Based on their construction the various types of evaporators are:
1) Bare Tube Evaporators
The bare tube evaporators are made up of copper tubing or steel pipes. The copper tubing is used for small evaporators where the refrigerant other than ammonia is used, while the steel pipes are used with the large evaporators where ammonia is used as the refrigerant. The bare tube evaporator comprises of several turns of the tubing, though most commonly flat zigzag and oval trombone are the most common shapes. The bare tube evaporators are usually used for liquid chilling. In the blast cooling and the freezing operations the atmospheric air flows over the bare tube evaporator and the chilled air leaving it used for the cooling purposes. The bare tube evaporators are used in very few applications, however the bare tube evaporators fitted with the fins, called as finned evaporators are used very commonly.
2) Plate Type of Evaporators
In the plate type of evaporators the coil usually made up of copper or aluminum is embedded in the plate so as so to form a flat looking surface. Externally the plate type of evaporator looks like a single plate, but inside it there are several turns of the metal tubing through which the refrigerant flows. The advantage of the plate type of evaporators is that they are more rigid as the external plate provides lots of safety. The external plate also helps increasing the heat transfer from the metal tubing to the substance to be chilled. Further, the plate type of evaporators are easy to clean and can be manufactured cheaply.
The plate type heat exchangers can be easily formed into various shapes as per the requirement. Thus in the household refrigerators and the deep freezers, where they are used most commonly, they can be converted into the box shape to form the closed enclosure, where various food can be kept in the frozen state. The plates can also be welded together forming the bank of the plate type of evaporators that can be used the larger evaporators of higher capacities.
Plate types of evaporators provide excellent shelves in the freezers and similar applications. They can be used as the as the partitions in the freezers, frozen food display cases, ice cream cabinets, soda fountains and others. Due to various advantages and flexibility offered by the plate type of evaporators, they are used extensively.
Plate Type of Evaporators
3) Finned Evaporators
The finned evaporators are the bare tube type of evaporators covered with the fins. When the fluid (air or water) to be chilled flows over the bare tube evaporator lots of cooling effect from the refrigerant goes wasted since there is less surface for the transfer of heat from the fluid to the refrigerant. The fluid tends to move between the open spaces of the tubing and does not come in contact with the surface of the coil, thus the bare tube evaporators are less effective. The fins on the external surface of the bare tube evaporators increases the contact surface of the of the metallic tubing with the fluid and increase the heat transfer rate, thus the finned evaporators are more effective than the bare tube evaporators.
The fins are the external protrusions from the surface of the coil and they extend into the open space. They help removing the heat from the fluid that otherwise would not have come in contact with the coil.
For the fins to be effective it is very important that there is very good contact between the coil and the fins. In some cases the fins are soldered directly to the surface of the coil and in other cases the fins are just slipped over the surface of the fins and then they are expanded thus ensuring a close thermal contact between the two. Tough the fins help increase the heat transfer, rate, adding them beyond certain numbers won’t produce any additional benefits, hence only certain number of fins should be applied on the external surface of the tube.
The finned evaporators are most commonly used in the air conditioners of almost all types like window, split, packaged and the central air conditioning systems. In these systems the finned evaporator is popularly known as the cooling coil. The hot room air flows over the finned evaporator or the cooling coil, gets chilled and enters the room to produce the cooling effect. In window a/c the finned evaporators is located behind the beautifully looking grill. In the wall mounted split unit it located behind the front grill of the indoor unit.
To further increase the effectiveness of heat transfer from the evaporators, the tubing are also given internal fins. These fins are made by forming different internal cross section shapes at the time of the manufacture of the tubing. Some methods of internal and external fins are shown in the figure below.
4) Shell and Tube types of Evaporators
The shell and tube types of evaporators are used in the large refrigeration and central air conditioning systems. The evaporators in these systems are commonly known as the chillers. The chillers comprise of large number of the tubes that are inserted inside the drum or the shell. Depending on the direction of the flow of the refrigerant in the shell and tube type of chillers, they are classified into two types: dry expansion type and flooded type of chillers. In dry expansion chillers the refrigerant flows along the tube side and the fluid to be chilled flows along the shell side. The flow of the refrigerant to these chillers is controlled by the expansion valve. In case of the flooded type of evaporators the refrigerant flows along the shell side and fluid to be chilled flows along the tube. In these chillers the level of the refrigerant is kept constant by the float valve that acts as the expansion valve also.
Reference and Images Courtesy
1) Book: Principle of Refrigeration by Roy J.. Dossat, 4th Edition, Printice Hall