Finned tubes reduce the size of the heat exchanger. For air heating applications, it is recommended to use heat exchangers with finned tubes instead of bare tubes.
A heat exchanger is a device that ensures efficient heat transfer from one fluid to another, when the fluids are separated by a solid wall so that they never mix.

In an air heater, steam is used to heat the incoming air. Here steam condenses to give off its heat to the air thereby raising the air temperature.

The formula for rate of convective heat transfer is given as

q = hA(T S − T b )
A is the surface area of heat transfer

TS is the surface temperature

Tb is the temperature of the fluid

As can be seen, the heat transfer rate is a function of area of heat exchange. This area is the measure of the total heated surface in contact with air.

The co-efficient of heat transfer for air is low,and it thus requires larger heat exchanger areas to achieve good heat transfer rates.

Finned tube heat exchangers greatly increase the area of heat exchange (typically 1.5 times). Thus for the same heat transfer rates finned tube heat exchangers are also more compact as compared to bare tube heat exchangers.