Finned-tube coils are found in many heat exchange processes that exchange gas to liquid. Finned-tube coils have two important parts that convey heat transfer. The first is the coil tubes, which also are called the "primary surface." The second is the finned outside surface, which also is known as the "secondary surface". The fins on a coil accomplish two major functions. First, they straighten the air out as it comes in contact with the entering-air side of the tube coil surface. Second, they exchange heat transfer of their own. The heat transfer is taken from the tube to the fin, thus raising the overall capacity of the coil. This is the reason the fins are called a secondary heat transfer surface.