Fig. 35 shows a common form of cast-iron boiler. It is made up of slabs or sections, each one of which is connected by nipples with headers at the sides and top. The top header acts as a steam drum and the lower ones act as mud drums; they also receive the water of condensation from the radiators. The gases from the fire pass backward and forward through flues and are finally taken off at the rear of the boiler. The ratio of heating to grate surface in this type of boiler ranges from 15 to 25 in the best makes. They are provided with the usual attachments, such as pressure gage, water glass, gage cocks and safety valve; a low-pressure damper regulator is furnished for operating the draft doors, thus keeping the steam pressure practically constant. A pressure of from 1 to 5 pounds is usually carried on these boilers depending upon the outside temperature. The usual setting is simply a covering of some kind of nonconducting material like plastic magnesia or asbestos, although some forms are enclosed in light brickwork. Fig. 36 shows one of this kind with part of the setting removed. In computing the required size we may proceed in the same manner as in the case of a furnace. For the best types we may assume a combustion of 5 pounds of coal per square foot of grate per hour, and an average efficiency of 60 per cent, which corresponds to 8,000 B. T. U. per pound of coal, available for useful work.

Sectional Boilers 100041

Fig. 35.

Sectional Boilers 100042

Fig. 36.

In the case of direct steam heating we have only to supply heat to offset that lost by radiation and conduction, so the grate area may be found by dividing the computed heat loss per hour by 8,000 which gives the number of pounds of coal, and this in turn divided by 5 will give the area of grate required. The most efficient rate of combustion will depend somewhat upon the ratio between the grate and heating surface. It has been found by experiment that about 1/4 of a pound of coal per hour for each square foot of heating surface gives the best results, so that by knowing the ratio of heating surface to grate area for any make of heater we may easily compute the most efficient rate of combustion and from it determine the necessary grate area.

For example - The heat loss from a building is 480,000 B. T. U per hour; we wish to use a heater in which the ratio of heating surface to grate area is 24, what will be the most efficient rate of combustion and the required grate area? 480,000/ 8,000 = 60 pounds of coal per hour, and 24/ 4 = 6, which is the best rate of combustion to employ, therefore 60/ 6 = 10, the grate area required.

Example. - The heat loss from a building is 168,000 B. T. U. per hour and the chimney draft is such that not over 3 pounds of coal per hour can be burned per square foot of grate. What ratio of heating to grate area will be necessary and what will be the required grate area?

The coal necessary is 168,000/ 8000 = 21 pounds per hour, requiring 7 square feet of grate surface at the given rate of combustion. Allowing 1/4 pound per square foot, we need 21/ 1/4 = 84 square feet of heating surface, and this gives a ratio of 84/ 7 = 1.2. Ans.

Cast iron sectional boilers are used for dwelling houses, small schoolhouses, churches, etc., where low pressures are carried. They are increased in size by adding more slabs or sections. After a certain length is reached the rear sections become less and less efficient, thus limiting the size and power.