DUCTS AND FLUES Areas of Ducts and Flues. When indirect radiation is installed primarily for heating, ventilation being a secondary consideration, it is desirable to make the flues somewhat smaller in proportion to the heating surface than is done with steam heating. If the flues are made too large, the flow through the radiators will be too rapid, and the air will not get hot enough. It costs far more in fuel to heat with a large volume of moderately warmed air than with a smaller volume of hotter air.
The backs and sides of flues in exposed walls should be covered with non-conducting material.
Flue Velocities. The flue velocities will be somewhat lower than with steam heating, because of the lower temperature of the air. Reasonable allowance would be 250, 350, 400, and 450 feet per minute for the first, second, third, and fourth floors respectively.
Heating Water. The size of heater or steam coil necessary to heat water may be very readily determined on the heat-unit basis, if one knows the volume of water to be heated, the number of degrees its temperature is to be raised, and the time during which the heating must be done.
For example, what size of heater would be required to heat 300 gallons of water in 6 hours from 60° to 160°? In one hour 50 gals. would be heated 100° F.; and since one gal. weighs 81 lbs., 50 X 8+} X 100 - 41,667 heat units would be required.
Small heaters may be counted on to transmit to the water about 7000 heat units per pound of coal burned. The rate of combustion should be assumed to be from 3 to 6 pounds per square foot of grate per hour, according to the amount of attendance it is convenient to give.
With a 4-pound rate, 28,000 heat units would be furnished per square foot of grate surface per hour for heating the water. Therefore the heat units per hour necessary to raise the temperature of the water— via., 41,667—divided by 28,000, gives the number of square feet of grate surface required, which is equal to about 11 corresponding to a diameter of 16i inches.
To determine the size of steam boiler and coil required to heat a large volume of water in a tank, proceed as follows: Take, for ample, a 24,000-gallon tank, the water in which is to be heated from 45° to 75° in 10 hours. Now 24,000 gals. X 8i pounds X 30° rise in
temperature — 6,000,000 heat units, or 600,000 heat units per hour. Assuming 8000 heat units to be utilized per pound of coal burned at, say, a 7f-pound rate, one square foot of grate will supply 60,000 heat units per hour; hence, 10 square feet of grate surface will be required. There will, however, be a certain loss of heat from the tank by radiation, conduction, and evaporation; therefore, not less than, say, 12 square feet should be used in order to provide a reasonable margin. As to the size of steam coil required, a square foot of pipe surface surrounded by circulating water may be assumed to transmit to the water not far from 100 heat units per degree difference in temperature between the steam and the water in contact with the pipe.
Assume the steam temperature to be 230°, corresponding to a trifle more than 5 pounds gauge pressure. When the water in the tank is cold, the condensation of steam in the coil will be much more rapid than when the surrounding water becomes warmer. The average temperature of the water during the 10-hour period is 60°; but the water leaving the pipe and in contact with the upper half of its surface is at a considerably higher temperature than the main body of water in the tank; therefore, with natural circulation, it is well to make ample allowance for the effect of this skin of warm water surrounding the steam coils, and to assume that they will not give off more than 3 as much heat as that corresponding to the difference in temperature be tween the steam and the water in the tank, based on 100 heat units per degree difference as stated above.
In other words, allow only 663—or, in round numbers, 70—heat units per hour per degree difference in temperature between the steam and the water in the tank.
If the difference in temperature is 230°— 70° = 160°, on the basis stated, one square foot of coil would give off 70 X 160 = 11,200 heat units per square foot per hour; and since 600,000 heat units must be supplied to the water, a 53-square foot coil or slightly larger would he required, equal to about 122 ft. of 1k-inch pipe.