The outer wall or casing of the duct is usually fabricated from 3/16-inch steel plate, reinforced against warping. (API-560 and HTR-MS-1350 call for 3/16-inch plate as the standard). Prevailing design guidelines call for sealing the heater casing plate by welding, in order to prevent air and water infiltration.
Note that ¼-inch plate may be the economic choice when the extra stiffness offsets the need for extensive, small stiffening braces. The vendor normally specifies the casing thickness, but you can discuss the choice with them if you have any doubts.
Because air infiltration can seriously affect furnace performance and control, furnace design should minimize air infiltration, particularly at viewing port covers, header compartment doors, and tube sheets. Casing penetrations should be minimized.