Both the boiler and fireplace are enclosed in fire-brick. Boilers may be divided roughly into two general classes: water-tube, and fire-tube boilers. The distinction between the two is that in water-tube boilers water flows through the tubes and the fire is on the outside of the tubes, while the conditions are reversed in the case of fire-tube boilers.
Fig. 158. - Return Tubular Boiler.
The most widely used of all boilers in America and England is the return tubular boiler (Fig. 158). This is a closed vessel made of steel or iron, simple in construction, and easy to clean and repair. The first horizontal tubular boilers were ordinary iron storage tanks, 30 or 40 ft. long and 48 to 56 in. in diameter. This type of boiler frequently exploded at the girth seam over the fires, and 50 or 60 lbs. was considered high pressure.
Steel instead of iron is used in the construction of modern boilers. Although the average diameter of boilers has increased only slightly and the average length has even decreased, the modern type is capable of carrying three times as high a pressure as the old type. The diameters of modern boilers range from 48 to 69 in., and the lengths from 16 to 20 ft., but a boiler carrying a pressure as high as 150 lbs. per square inch is not at all uncommon; some carry even a much higher pressure.