Finned-Tube Evaporators
Finned-tube evaporators are made using a prime-surface bare-tube coil and installing metal plates or fins to increase the heat transfer surface area. Much of the air circulating across a bare-tube evaporator passes through the relatively large spaces between adjacent tubes. Consequently, a large proportion of the air does not come into contact with die coil surface. The air that does not come into contact with the coil does not give up its heat so its temperature does not change and it returns to the space unconditioned. When fins are added to a bare-tube coil, they extend into the spaces between the tubes, thereby acting as heat collectors. These fins are secondary surfaces that permit more of the air to come into contact with the cold evaporator. Fins transfer thermal energy from the mass of air that would not ordinarily come into contact with the prime surface. This design allows the evaporator to be made dimensionally smaller while still providing the same heat transfer capability as a larger prime-surface evaporator.
To be effective, the fins must be connected to the tubing in a manner that ensures that good thermal conductivity is maintained throughout the life of the evaporator. In some designs, the fins are soldered or tack welded directly to the tubing. In most designs, aluminum fins are slipped over the end of the tubing so they are aligned in their correct position. Once they are all properly spaced, a mandrel having a die that is slightly larger than the tubes inside diameter is pulled through the tube. The die expands the tube against the collars of the fins, establishing a tight, mechanical bond that permits excellent heat transfer throughout the life of the evaporator.
The spacing between the fins of diese evaporators varies based on the intended application. Densities of 1 to 14 fins per inch are commonly used in HVAC/R applications. When forced convection (fans) air circulation is used in applications in which the evaporator operates above 32° F (0°C), fin density is generally about 7 fins per inch. Evaporators designed for below freezing temperature applications have fewer fins per inch. This strategy provides a larger space between the fins to minimize the restriction of air circulation that occurs as frost forms on the heat transfer surfaces. Similarly, in applications operating below freezing temperatures with high latent loads, the fin density is reduced to limit the formation of ice on coils. Evaporator coils designed for use in air-conditioning and other applications where operating temperatures are too high for the formation of frost on the heat transfer surfaces may have as many as 14 fins per inch. The greater the fin density, the greater the quantity of latent heat removed from the air stream. It is important that the resistance to air flow is minimized in natural convection evaporator applications. Therefore, fin spacing is wider than that used by coils employing fans.
Key Words: Finned Tube, Fin Tubes, Extruded Fin Tubes, Embedded Fin Tube, L Fin tube, Tubos Aletados, Finned-Tube Evaporators