Convection Section

Convection Section

The convection section is located in the cooler flue gas stream. It often contains rows of extended surface tubes to improve the efficiency of the furnace. The flue gases can be cooled to a very low temperature, but caution must be used to avoid going below the dew point of the flue gas with the metal temperature of the tubes or surface, since this could cause corrosion.

Extended Surfaces

Modern convection sections frequently use extended surface to increase the overall heat transfer efficiency of the fired heater. The most common extended surfaces are of the following types.

Segmented Fins:
These are usually one of the two types shown below.
High Frequency
Continuously Welded
SegHR1 SegHR2
Standard Frequency
Spot Welded
SegSR1 SegSR2

The standard frequency, spot welded, design is not used in fired heaters very often since this design is normally selected when using very thin, high density finning such as in a large heat recovery boiler. Most fired heater convections seldom use fins less than 0.049 inch thick. The standard frequency, spot welded fin also has a foot which presents a place where corrosion can occur if flue gases are corrosive or moisture is present.

Solid Fins:
These are the most popular fins for modern fired heaters.
High Frequency
Continuously Welded
Solid1 Solid2
Stud Fins:
These are used generally when the fuel is No. 6 or higher.
Stud1 Stud2

Thermal rating procedures for all these extended surface types are presented in section 4, Heat Transfer Concepts. Both segmented fin types are rated using the same formulas.