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.