COMPUTING RADIATION Computing Direct Radiation. The process of computing hot water radiating surface is precisely the same as that explained for ascer taining the amount of steam radiation required for a given case, with this important exception: the hot-water radiators give off only about as much heat per square foot as is emitted by a steam radiator; hence calculations must be based on an allowance of 150 heat units per square foot of direct radiating surface per hour, instead of 250 heat units used in connection with steam-heating work.
It has been stated that direct-indirect hot-water radiators are rarely used. In case, however, it is desired to compute the amount of this class of radiation for a given service, proceed as explained for steam heating, but allow only I as much heat emitted per square foot as that given off by steam radiators.
Computing Indirect Radiation. With indirect hot-water radia tion in connection with the open-tank system, the radiators must be deeper than for steam heating, in order properly to heat the air.
The greater depth retards the flow of air; and since the water is at a much lower temperature than steam, the heating capacity of indirect extended-surface hot-water radiators should be taken at not far from 300 heat units per square foot per hour, as against 400 or more heat units for indirect steam radiation.
To compute the amount of radiation required, proceed as ex plained for indirect steam heating; that is, compute the amount of direct radiation as pointed out under the preceding heading, then add not less than 60 per cent to this amount, to ascertain the indirect radi ating surface required.
This method, though perhaps crude, has the advantages of being simple and of affording a check on the work, since one soon knows by experience about what the ratio should be to heat a room of given size by direct radiation. For example, take a room with 3000 cubic feet, to heat which the ratio for direct radiation should be, say, 1 square foot to 30 cubic feet, giving a 100-square foot radiator. Adding 60 per cent for indirect radiation, gives 160 square feet, or a ratio of 1 square foot to a little less than 20 cubic feet of space.
Indirect hot-water radiators with extended pins or ribs will, with the open-tank system, give off not far from 250 to 300 heat units per hour per square foot of extended surface.