The efficiency of a radiator, that is, the B. T. U. which it gives off per square foot of surface per hour, depends upon the difference in temperature between the steam in the radiator and the surrounding air, the velocity of the air over the radiator, and the quality of the surface, whether smooth or rough. In ordinary low-pressure heating the first condition is practically constant, but the second varies somewhat with the pattern of the radiator. An open design which allows the air to circulate freely over the radiating surfaces is more efficient than a close pattern and for this reason a pipe coil is more efficient than a radiator.

In a large number of tests of cast iron radiators, working under usual conditions, the heat given off per square foot of surface per hour, for each degree difference in temperature between the steam and surrounding air, was found to vary from about 1.3 to 1.7 B. T. U. The temperature of steam at 3 pounds pressure is 220 degrees, and 220 - 70 = 150, which may be taken as the average difference between the temperature of the steam and the air of the room, in ordinary low-pressure work. If we take the mean of the above results, that is, 1.5 we shall have 150 X 1.5 = 225 B. T. U. as the efficiency of an average cast iron radiator. A circulation coil made up of pipes from 1 to 2 inches in diameter will easily give off 300 B. T. U. under the same conditions, and a shallow pipe radiator of standard height may be safely counted upon to give 260. These efficiencies are lower than are given by some engineers, but if the sizes are taken from trade catalogues it is not safe to go much above these figures. If the radiator is to be used for warming rooms which are to be kept at a temperature above or below 70 degrees, the radiating surface may be changed in the same proportion as the difference in temperature between the steam and the air.

Fig. 42.

For example - if a room is to be kept at a temperature of 60° the efficiency of the radiator becomes 160/150 X 225 = 240; that is the efficiency varies directly as the difference in temperature between the steam and the air of the room. It is not customary to consider this unless the steam pressure should be raised to 10 or 15 pounds or the temperature of the rooms changed 15 or 20 degrees from the normal.

From the above it is easy to compute the size of radiator for any given room. First compute the heat loss per hour by radiation and conduction, in the coldest weather, then divide the result by 225 for cast iron radiators, 260 for pipe radiators and 300 for pipe coils. It is customary to make the radiators of such size, that they will warm the rooms to 70 degrees in the coldest weather. This varies a good deal in different localities, even in the same state, and the lowest temperature for which we wish to provide must be settled upon before any calculations are made. In New England and through the Middle and Western States it is usual to figure on warming a building to 70 degrees when the outside temperature is from zero to 10 degrees below.

The makers of radiators publish in their catalogues, tables giving the square feet of heating surface for different styles and heights, and these can be used in determining the number of sections required for all special cases.

If pipe coils are to be used, it becomes necessary to reduce square feet of heating surface to linear feet of pipe; this can be done by means of the factors given below.

 Square feet of heating surface X { 3 = linear ft. of 1" pipe 2.3 = " " 1 1/4" " 2 = " " 1 1/2" " 1.6 = " " 2" "

The size of radiator is only made sufficient to keep the room warm after it is once heated, and no allowance is made for "warming up," that is, the heat given off by the radiator is just equal to that lost through walls and windows. This condition is offset in two ways - first, when the room is cold, the difference in temperature between the steam and air of the room is greater and the radiator is more efficient, and second the radiator is proportioned for the coldest weather so that for a greater part of the time it is larger than necessary. This last condition is one of the disadvantages of direct steam heating; if steam is on the radiator at all it will give off the same amount of heat regardless of the outside temperature.

Fig. 43.

Example. - A schoolroom is to be warmed with circulation coils of 1 1/4-inch pipe. The heat loss is 30,000 B. T. U. per hour; what length of pipe will be required?

The heating surface of a pipe coil will be

30,000 = 100 square feet.

Therefore we need

2.3 X 100 = 230 linear feet of 11/4-inch pipe.

Location. Radiators should be placed in the coldest part of the room if possible, as under windows or near outside doors. In living rooms it is often desirable to keep the windows free, in which case the radiators may be placed at one side. Circulation coils are run along the outside walls of a room under the windows. Sometimes the position of the radiators is decided by the necessary location of the pipe risers, so that a certain amount of judgment must be used in each special case as to the best arrangement to suit all requirements.