STEAM BOILERS AND CONNECTIONS Small Cast-Iron Boilers. For small low-pressure steam heating jobs, boilers made up of very few sections are used. Two types are illustrated in Figs. 1 and 2. The ratings of such boilers range, as a rule, from about 200 square feet to 800 square feet. These figures and those following are intended to give merely a general idea of the capacities of boilers of various types. There is no hard and fast rule governing the mat ter, manufacturers vary ing greatly in their prac tice. The ratings men tioned are given in the number of square feet of direct radiation the boiler is rated to supply, with steam at from 3 to 5 pounds' pressure when the radiators are sur rounded by air at 70° F.
Boilers similar, in a general way, to the one illustrated in Fig. 3 are used for jobs somewhat larger than the boilers above described would be adapted to. These boilers have grates ranging generally from 18 inches to 36 inches diameter, and are rated from about 300 square feet to 1,600 square feet, or more.
The boilers above described have the disadvantage of not being capable of having their grate surface increased by adding sections, ao may readily be done with boilers having vertical sections.
Cast-Iron Boilers with Vertical Sections. Boilers for jobs having Anywhere from 500 to 5,000 square feet of surface, or more, are made np of vertical sections, as in Fig. 4, connected either by slip nipples or by drums and nipples with long screws and lock-nuts.
Very many slip-nipple boilers are now being manufactured, find mg favor with fitters owing to the ease with which they can be erected.
The larger sizes of vertical sectional boilers are often made up of two sets of sections placed opposite each other, as shown in Fig. 5. Such boilers are rated up to 6,000 square feet and over.
Arrangement of Grates. Certain makers, in order to avoid mak ing patterns for a boiler with a wide grate, secure the necessary grate sur face by adding to the length. For ordinary low-pressure heating, the efficiency of any grate over 6 feet in length falls off very rapidly, owing to the difficulty of properly caring for the fire. Six feet should be
considered about the limit for the length of a grate in a low-pressure boiler.
Not long ago few portable boilers with grates wider than 36 inches were manufac tured. Now, boilers with 42-inch, 48-inch, a n d even wider grates, are common.
Selection of Boilers. It is well in selecting a boiler, to see that the proportion of heating surface to grate surface is not less than 16 to 1, and in large boilers not less than 20 to 1; that the fire-box is deep, so that ample coal may be put on to burn through the night; that the grate is not too long for convenient firing and cleaning; that there is ample steam space; and that the water line is not broken into too many small areas involv ing the likelihood that water will be lifted by rapid evaporation and wet steam result. See to it, also, that the ash-pit is deep, and that the grate is of a design that will permit convenient operation of the boiler.
On large jobs, it is better, as a rule, to use two boilers. One must remember that a plant must be designed for the coldest weather; and since the average temperature during the heating season is, in many Northern sections, not far from 40°, one of a pair of boilers will be sufficient under average conditions to do the work with economy; whereas a single, large boiler, during a good part of the heating sea son, would have to be run with draft, checked and under very unfavor able conditions as to economy. It is almost as poor economy to have too large a boiler as to have one too small, for, if run with the feed-door open or drafts closely checked, incom plete combustion takes place.
Boilers for Soft Coal. Some boilers for burning soft coal are arranged with a perforated pipe or duct discharging heated air above the fire to make the combustion more complete and thus diminish the amount of smoke given off. This arrangement is of somewhat doubt ful utility, since it is difficult to heat the air properly, and to regulate its admission.