Convection Section Extended Surface Tubes
Extended surface is either in the form of studs (cylindrical projections welded to the outside of the tube) or fins. Fins are helically wound, with continuous edge (not serrated), and are continuously welded to the tube. Their purpose is to increase the outside heat transfer coefficient, which is usually the controlling (smaller) one.
Where Used. Extended surface is not used on shock or shield tubes (the first two rows of the convection section if exposed to direct radiation from the firebox). Otherwise, extended surface may be used provided that fin or stud tips will not be overheated. See Section 233 for estimation of fin tip temperature. In heaters with very hot fireboxes, e.g. steam reformers, the first several rows of convection tubes are bare.
Gas Fired Heaters. Fin density should not exceed 5 fins/inch, height should not exceed 1 inch and minimum thickness is 0.05 inches for low sulfur fuel gas.
Thickness should be increased if sulfur content of fuel is higher. The choice between 0.75 inch height (the usual minimum) and 1 inch height is usually determined by fin tip temperature.
Oil Fired Heaters. Fin density should not exceed 2½ fins/inch, height should not exceed 0.75 inch and minimum thickness is 0.1 inches. Studs may be used, but check for the availability of studded tubes before making a commitment. The availability may be limited and therefore affect the heater delivery. They should be of 0.5 inch minimum diameter and 1.0 inch maximum height.
Extended Surface Material. See Section 3.2 of API-560 and Model Specification HTR-MS-1350.