Fin surface area of a heating or cooling coil is called:

The term "secondary surface area" refers specifically to the fin surface area of a heating or cooling coil. In heat exchangers and similar equipment, fins are utilized to enhance the surface area for heat transfer, making them an essential component for improving efficiency. The fin surface area, which is not part of the primary surface area directly exposed to the fluid, is significant for optimizing the thermal performance of coils. This is why it's distinguished as the secondary surface area; it contributes to the overall heat exchange process by maximizing the contact area for heat transfer between the coil and the fluid passing over it.

The other terms do not accurately capture the role of fins in this context. Primary surface area typically refers to the main area where heat exchange occurs, directly in contact with the fluid. Tube surface area is focused on the cylindrical portions of the coil, and heater surface area might suggest a broader area without specifically addressing the fins, which are crucial for maximizing heat transfer efficiency in coils.