Page:Steam heating and ventilation (IA steamheatingvent00monrrich).pdf/37

 CHAPTER III.—STEAM-HEATING APPARATUS.

Boilers.—The questions of boiler design, construction, setting, etc., involve so many considerations requiring careful scientific study, that the entire problem is omitted from this work and the reader is referred to the valuable treatises devoted exclusively to the subject. There is, however, one kind of boiler which should be given some special mention in this place. This is the self-contained cast-iron sectional boiler which is used only for small, low-pressure gravity-heating systems in residences. These boilers are built up in sections of various shapes bolted together, the joints being kept tight merely by the pressure caused by the bolts. On account of their material, as well as the method of construction, they are not adapted for anything but very low pressures. For the larger plants in which cast-iron boilers are used, it is better to install two small ones than to attempt to use one large one, as it gives better economy in operation, and there is less danger of accident to small boilers than to those with large castings.

In selecting these boilers it should always be borne in mind that the demands of commercial competition cause them to be very generally overrated by their makers, so that in choosing sizes from catalogues it is advisable to make considerable reduction from the rated capacities. Mr. James R. Willett, in a pamphlet on the heating and ventilating of residences, gives the accompanying table for the sizes of cast-iron boilers. Where boilers of the kinds used for steam power are employed in simple heating plants of large size of which there are to-day but few—there is an old—rule for proportioning the size, to the effect that there must be 1 square foot of boiler surface to 10 square feet of radiating surface. This rule when applied to a boiler with a well-designed furnace, stack and setting should be very conservative. It is much better to estimate the steam consumption of the entire plant (see Chapter V., page 60 for steam consumption of radiation) and calculate the boiler requirements according to the rules of boiler design. The designs of the cast-iron heating boilers are innumerable and