It is common practice to allow roughly for the loss of heat from uncovered mains, branches, and risers, by adding about 25 per cent to the actual direct radiating surface in radiators and coils.
Equivalent in direct radiation of 800 sq. ft. of indirect v 800X 1 = 1,200 " " Equivalent in direct radiation, in rooms at 70°, of 1,500 sq. ft.
in rooms at 50° _ }; X 1,500v 1,700 " " Total equivalent D. R. S. (direct radiating surface) exposed in 70° air — 3,900 sq. ft.
Add 25 per cent of actual surface to allow approxi mately for piping = 825 " " Total equivalent D. R. S., or Boiler Rating — 4,725 sq. ft.
Grate Surface and Heating Capacity. It is advisable always to check the catalogue ratings of boilers as follows, when selecting one for a given service: Suppose the Direct Radiating Surface, including piping, is 3,000 square feet. One square foot, it may be assumed, will give off about 250 heat units in one hour—a heat unit being the amount of heat necessary to raise the temperature of 1 pound of water 1 degree Fahren heit. A pound of coal may safely be counted on to give off to the water in the boiler 8,000 heat units. Now, 3,000 sq. ft. X 250 heat units 8,000 heat units, gives the amount of coal burned per hour; and this, divided by the square feet of grate, gives the rate of combustion per square foot per hour. Suppose in this case, the grate has an area of 3000X 250 15 sq. ft.; then 15 ° pounds coal burned per square foot of grate surface per hour. This is not a high rate for boilers of this size, though for ordinary house-heating boilers the rate should not exceed 5 pounds; and for small heaters having 2 to 4 square feet of grate, the rate should be as low as 3 to 4 pounds per square foot of grate per hour. Otherwise, more frequent attention will be required than it is convenient to give to the operation of such small boilers. This is where depth of fire-box plays an important part, for, with a shallow fire, the coal quickly burns through, necessitating frequent firing.
Coal Consumption. For house-heating boilers a fair maximum rate of combustion is 5 pounds per square foot of grate per hour. In many residences it is the custom to bank the fire at night, when the rate will fall to, say, 1 pound. In cold weather, then, one square foot of grate would burn 5 pounds of coal for each of 16 hours, and 1 pound during each of the remaining 8 hours, a total of 80 + 8 = 88 pounds.
In many sections of the country, the average outside temperature ing the heating season is about 40°; and since the heat required is proportional to the difference in temperature between indoors and 70° — outside, the average coal consumption would be — _ 4 of the maximum in zero weather.
With a heating season of 200 days, the coal burned on one square foot of grate would be 200 X 4 X 88 = 7,600 pounds in round num bers, corresponding to an average rate throughout the season of 7,600 pounds • 1.6 pounds approximately. 200 days X 24 hrs.
A method of approximating the coal consumption for a given amount of radiating surface, designed to maintain a constant tem perature in rooms of 70° day and night, would be to multiply the surface (which, for example, take at 1,000 square feet, including allowance for mains) by 250 heat units—the amount given off by a square foot per hour—and then multiply the product by 4, as explain ed above, to allow for average conditions. This gives 1,000 X 250 X 4, which, divided by 8,000 heat units per pound of coal, gives the weight of coal required per hour; and this, multiplied by the hours per season, gives the total consumption.
Non-Conducting Coverings. It is customary to cover cast-iron sectional boilers with non-conducting material composed as a rule chiefly of asbestos or magnesia applied in a coating 11 to 2 inches thick, the exterior being finished hard and smooth.
Exposed basement piping in first-class work is covered with sec tional covering j inch to 1 inch thick, according to the character of the work.
The loss of heat through fairly good coverings, is not far from 20 per cent of the loss from a bare pipe, which, with low-pressure steam, is approximately 2 heat units per square foot per hour for each degree difference in temperature between the steam and the surround ing air.