COMPUTING RADIATION Computing Direct Radiation. It is a perfectly simple matter to compute the amount of radiation required to heat a room, by finding the probable loss of heat per hour, and dividing this by the heat given off by a square foot of radiating surface in the same time.
Numerous tests have shown that an ordinary cast-iron radiator gives off approximately 1.6 heat units per hour per degree difference in tem perature between the steam and the surrounding air. With low-pressure heating a square foot of direct radiation is commonly rated at about 250 H. 15. Glass transmits about 85 heat units per square foot per hour, with 70° inside and 0° out; and walls of ordinary thickness may be reckoned as trans mitting one-fourth as much heat.
The heat losses stated should be increased about 25 per cent for a north or west exposure, and about 15 per cent for an easterly exposure. An allowance should be made for reheating rooms that are lowed to cool down slightly at night. This may be done most con veniently by adding to the loss of heat through walls and glass a ber of heat units equal to 0.3 of the cubic contents of a room with two exposures, and 0.6 the contents of a room with a single exposed wall.
The way this works but may best be shown by a couple of ex amples: Suppose we have a room 16 feet square and 10 feet high, with two exposed walls facing respectively north and west, each having a window three feet 6 inches by 6 feet.
Exposure of room = (16 + 16)X 10e 320 sq. ft.
Glass surface -.2 X 21 sq. ft. .= 42 " " Net wall 278 sq. ft.
Equivalent glass surface (E. G. S.) of net wall — 278 _ 4 Approximately 70 sq. ft.
Actual glass surface a 42 " " Total E. G. S. Approximately 112 sq. ft.
Heat transmitted —112 sq. ft. X 85 heat units X 1.25 factor — 11,890 H. U. Allowance for reheating — 0.3 X cubic contents of 2,560 cu. ft. .= 768 " " Total heat loss to be made good by direct radiation 12,658 H. U.
This 12,658 heat units, divided by 250, the amount given off by one square foot of radiation in one hour, = 50 sq. ft. approximately, giving a ratio of 1 sq. ft. of radiating surface to 53 cubic feet of space.
Take as a second example a room with one exposure toward the east, the dimensions of the room being 14 by 14 by 10 feet, with one window 4 by 6 feet. Proceeding as before, The loss of heat through roofs and through ceilings to unheated attic spaces above may be allowed for conveniently, and with suffi ciently close approximation to the actual heat loss, by dividing the area of the roof by 10, and that of the ceiling by 20, to give the E. G. S.
In the case of a well-constructed plank roof, with paper or other material above that will prevent the leakage of air, the roof area may safely be divided by 15 to ascertain the E. G. S.
It is hardly necessary, as a rule, to allow for the loss of heat through a first floor to the basement when the latter is well enclosed and contains steam and return mains or is otherwise kept at a moderate temperature.
Computing Direct-Indirect Radiation. The most common meth od of computing the amount of direct-indirect radiation required, is to ascertain, in the manner described, the direct radiating surface necessary, and add to it approximately 25 per cent; that is, if a direct radiator of 100 square feet were found to be necessary to heat a given room, a direct-indirect radiator of 125 square feet would be required.
Computing Indirect Radiation. To compute the amount of in direct radiation necessary to heat a given room, about the simplest method to grasp is to compute, first, the direct radiation required, as previously explained, and then add 50 per cent to this amount, since it happens that, under average conditions of 70° inside and 0° outside, practically 11 times as much surface is required to heat a given space with indirect as with direct heating.
When a stated air supply is required, the loss of heat by ventilation must be computed, and a different method followed in ascertaining the amount of indirect radiation required. For example, take a 50 pupil schoolroom with the common compulsory allowance of 30 cubic feet of air per minute per pupil—equal to 1500 cubic feet per minute per room. Each cubic foot escaping up the vent flue at 70° b., when the outside temperature is zero, removes from the room 11 heat units; hence the total heat loss by ventilation per hour would be 60 X 1500 X 11 112,500 heat units. A standard schoolroom has about 720 square feet of exposure, of which not far from 180 square feet is glass, leaving a net wall of 540 square feet, which, divided by 4, gives 135 square feet equivalent glass surface. This, added to the actual glass, gives 315 square feet E. G. S., which, in turn, multiplied by 85 heat units X a factor of 1.25 for north or west, gives a total heat loss by transmission of 33,470 heat units approximately.
The combined loss of heat by transmission and ventilation amounts to 145,970 H. U.
With the greater air-flow through indirect heaters used in schools, the heat emitted per square foot per hour should exceed somewhat the amount given off by indirect radiators in residence work—namely, 400 H. U. To be on the safe side, allow 450 H. U. The total heat loss from the room, divided by this number, gives approximately 300 square feet as the surface required.