INDIRECT STEAM HEATING As already stated, in the indirect method of steam heating, a special form of heater is placed beneath the floor, and encased in galvanized iron or in brickwork. A cold-air box is connected with the space beneath the heater; and warm-air pipes at the top are connected with registers in the floors or walls as already described for furnaces. A separate heater may be provided for each register if the rooms are large, or two or more registers may be connected with the same heater if the horizontal runs of pipe are short. Fig. 50 shows a section through a heater arranged for introducing hot air into a room through a floor register; and Fig. 51 shows the same type of heater connected with a wall register. The cold-air box is seen at the bottom of the casing; and the air, in passing through the spaces between the sections of the heater, becomes warmed, and rises to the rooms above.
Different forms of indirect heaters are shown in Figs. 52 and 53. Several sections con nected in a single group are called a stack. Some times the stacks are en cased in brickwork built up from the basement floor, instead of in gal vanized iron as shown in the cuts. This method of heating provides fresh air for ventilation, and for this reason is especially adapted for schoolhouses, hospitals, churches, etc. As pared with furnace heating, it has the advantage of being less affected by outside wind-pressure, as long runs of horizontal pipe are avoided and the heaters can be placed near the registers. In a large building where several furnaces would be required, a singb boiler can be used, and the n um ber of stacks increased to suit the existing conditions, thus making it necessary to run but a single fire. Another advan tage is the large ratio between the heating and grate surface as compared with a furnace; and as a result, a large quan tity of air is warmed to a mod erate temperature, in place of a smaller quantity heated to a much higher temperature. This gives a more agreeable quality to the air, and renders it less dry. Direct and indi rect systems are often com bined, thus providing the liv ing rooms with ventilation, while the hallways, corridors, etc., have only direct radiators for warming.
Types of Heaters. Various forms of indirect radiators are shown in Figs. 52, 53, 54, and 56. A hot-water radiator may be used for steam; but a steam radiator cannot always be used for hot water, a.
it must be especially designed to produce a continuous flow of water through it from top to bottom. Figs. 54 and 55 show the outside and the interior construction of a common pattern of indirect radiator designed especially for steam. The arrows in Fig. 55 indicate the path of the steam through the radiator, which is supplied at the right, while the return connection is at the left. The air-valve in this case should be connected in the end of the last section near the return.
A very efficient form of radiator, and one that is especially adapted to the warming of large volumes of air, as in schoolhouse work, is shown in Fig. 56, and is known as the School pin radiator. This can be used for either steam or hot water, as there is a continuous passage downward from the supply connection at the top to the return at the bottom. These sections or slabs are made up in stacks after the
manner shown in Fig. 57, which represents an end view of several sections connected together with special nipples.
A very efficient form of indirect heater may be made up of wrought-iron pipe joined together with branch tees and return bends.
A heater like that shown in Fig. 58 is known as a box coil. Its effi ciency is increased if the pipes are ataggered—that is, if the pipes in alternate rows are placed over the spaces between those in the row below.
Efficiency of Heaters. The efficiency of an indirect heater depends upon its form, the difference in temperature between the steam and the surrounding air, and the velocity with which the air passes over the heater. Under ordinary conditions in dwelling-house work, a good form of indirect radiator will give off about 2 B. T. U. per square foot per hour for each degree difference in tem perature between the steam and the entering air. Assum ing a steam pressure of 2 pounds and an. outside tem perature of zero, we should have a difference in tempera ture of about 220 degrees, which, under the conditions stated, would give an efficiency of 220 X 2 = 440 B. T. U. per hour for each square foot of radiation. By making a similar computation for 10 degrees be low zero, we find the efficiency to be 460. In the same manner we may calculate the efficiency for varying conditions of steam pressure and outside temperature. In the case of schoolhouses and similar buildings where large volumes of air and warmed to a moderate tern perature, a somewhat higher efficiency is obtained, owing to the in creased velocity of the air over the heaters. Where efficiencies of 440 and 460 are used for dwellings, we may substitute 600 and 620 for schoolhouses. This corresponds approximately to 2.7 B. T. U. per square foot per hour for a difference of 1 degree between the air and steam.
The principles involved in indirect steam heating are similar to those already described in furnace heating. Part of the heat given off by the radiator must be used in warming up the air-supply to the temperature of the room, and part for offsetting the loss by conduction through walls and windows. The method of computing the heating surface required, depends upon the volume of air to be supplied to the room. In the case of a schoolroom or hall, where the air quantity is large as compared with the exposed wall and window surface, we should proceed as follows: First compute the B. T. U. required for loss by conduction through walls and windows; and to this, add the B. T. U. required for the necessary ventilation; and divide the sum by the efficiency of the radiators. An example will make this clear.
Example. How many square feet of indirect radiation will be required to warm and ventilate a schoolroom in zero weather, where the heat loss by conduction through walls and windows is 36,000 B. T. U., and the air-supply is 100,000 cubic feet per hour? By the methods given under "Heat for Ventilation," we have