Shell-type boilers are restricted in size to 14 feet (4.3 m) in diameter and an operating pressure of 300 lb./inch.2 (2.1 megapascals) by the stresses in the large-diameter shells and the necessity to design the flat-tube sheets for practicable thicknesses. They are best suited for low-pressure heating service and for portable use because the integrated structure can be transported easily and the generous reserve in water capacity requires minimum attendance.
The straight-tube boiler, often called the header-type boiler, has the advantage of direct accessibility for internal inspection and cleaning through hand holes, located opposite each tube end, in the headers. The steam-generating sections are joined to one or more steam-and-water drums located above and parallel or transverse to the boiler tube bank.
In bent-tube boilers, commonly referred to as drum-type boilers, the boiler tubes terminate in upper and lower steam and water drums that have few access openings. Although internal inspection is restricted, the tubes can be mechanically or chemically cleaned, and developments in water treatment and cleaning methods have overcome the early objections to the use of bent tubes.