This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
90. The method of computing the amount of radiator surface required for any given surface, is as follows: Having ascertained the amount of heat to be supplied, in heat units per hour, the next thing to be done is to find the difference in temperature between the air to be heated and the heating fluid which is to be used in the radiators. If hot water is used, the temperature taken should be the average of its temperatures at entering and leaving the radiator. The coefficient of emission may then be found by referring to the preceding tables. The coefficient, or number which corresponds to the given difference of temperature, and to the kind of radiator which most nearly resembles the variety . to be used, should be multiplied by the difference in temperature in degrees. The product will be equal to the total emission of heat per square foot, per hour, which may be expected. The area of radiator surface required may then be found by dividing the total amount of heat required per hour by the emission from 1 square foot as computed.
A certain building requires a supply of heat amounting to 200,000 heat units per hour, and it is to be heated by steam having a temperature of 220°. The radiators are to be of the Detroit loop variety, 40 inches high, and are to heat by the direct system. How many square feet of radiator surface will be required to maintain the air in the building at 70°?
The difference in temperature between the heating agent and the air is 220° - 70° = 150°. The coefficient of emission for that difference of temperature is given in Table 11 as 1.63 for massed surfaces and 1.85 for a single row of tubes, 40 inches high. The efficiency of the radiator named will be somewhere between these, and the coefficient may be taken as 1.75. Then, the area of radiator surface required will be,
91. For direct heating, a greater amount of heat will be required. Of course, no fresh hot air can be introduced unless an equal amount of air be expelled from the room at the same time. Consequently, all of the heat contained in the fresh-air current below 70° (or the desired temperature of the room) will be lost by passing off with the spent air-that is, by ventilation. The fresh-air current must be heated from 20° to 50° hotter than the desired temperature of the room, so that, in cooling down to that temperature, it will give off an amount of heat sufficient to make good the loss by conduction through the walls, windows, etc.
Thus, in using a current of fresh air having a temperature of 110°, to maintain a room at 70°, the external temperature being zero, 70/110 of the heat imparted to the current will be lost by ventilation, and only 40/110 will be available to compensate for the loss of heat from the room through the windows and walls.