Document 4JR7V2kdeEZe2Y2EbkdNqBv8j

American Society of Heating and Ventilating Engineers Guide, 1934 are made with welded or riveted seanis. The proper design of the furnace depends largely on the kind of fuel to be burned. Accordingly, various manufacturers are making special units for coal, oil and gas. Each type of fuel requires a distinct type of furnace for highest efficiency and econ omy, substantially as follows: 1. Coal Burning: a. Bituminous--Large combustion space with easily accessible secondary radiator or flue travel. h. Anthracite or cake--Large fire box capacity and liberal secondary heating surfaces. 2. Oil Burning: . Liberal combustion space. . Long fire travel and extensive heating surface. 3. Gas Burning: a. Extensive heating surface. b. Close contact between flame and heating surface. A combustion rate of from 5 to 8 lb of coal per square foot of grate per hour is recommended for residential heaters. A higher combustion rate is Chapter 23--Mechanical Warm Air and Fan Furnace Systems but they may also be constructed of brick. Galvanized-iron casings should be lined with black-iron liners, extending from the grate level to the top of the furnace and spaced from 1 in. to in. from the outer casing. It is generally believed that either brick or sheet metal casings should be equipped with baffles to secure impingement of the air to be heated against the heating surfaces. Brick furnace casings should be sup plied with access doors for inspection. Most furnace casings are sized for gravity flow of air, aiid where a fan is to be used, many manufacturers recommend the use of special baffles to restrict the free area within the casing and to force impingement of the air against the heating surfaces. The method of making these baffles for furnaces with top horse-shoe radiators and for furnaces with back crescent radiators is illustrated in Fig. 1. Either square or round casings may be used. Where square casings are used, the corners must be baffled to reduce the net free area and to force impingement of air against the heating surfaces. Fig. 2 shows the usual method of baffling square furnace casings for fan-furnace work. Fig. 1. Usual Method of Baffling Round Casings for Fan Furnace Work A. Liner, 1 in. from casing. B. Hole to vent baffle. C- Baffle, closed top and bottom. D. Outer casing. permissible with larger furnaces for buildings other than residences, depending upon the ratio of grate surface to heating surface, firing period, and available draft. Where oil fuel is used, care must be exercised in selecting the proper size and type of burner for the particular size and type of furnace used. It is recommended, especially where sectional furnaces are used, that the system be designed for blow-through installations, so that the furnace shall be under external pressure, in order to minimize the possibility of leakage of the products of combustion into the air circulating system. In residential furnaces for coal burning, the ratio of heating surface to grate area will average about 20 to 1; in commercial sizes it may run as high as 50 to 1, depending on fuel and draft. Furnaces, may be installed singly, each furnace with its own fan, or in batteries of any number of furnaces, using one or more fans. Casings are usually constructed of galvanized iron, 26-gage or heavier, ' 306 ( Fig. 2. Method of Baffling Square Furnace Casing for Fan Furnace Work jf. Baffle, closed top and bottom. B. Liner, 1 in. from /casing. C. Outer casing. D. Hole to vent baffle. The hood or bonnet of the casing above the furnace should be as high as basement conditions will allow, to form a plenum chamber over the top of the furnace. This tends to equalize the pressure and temperature of the air leaving the bonnet through the various openings. It is generally con sidered advisable to take off the warm air pipes from the side of the bonnet near the top, as this method of take-off allows the use 6f a higher bonnet and thus provides a larger plenum chamber. Fig. 3 illustrates a complete fan-furnace installation showing location of fan, furnace, filters, plenum chamber and method of take-off of warm air pipe. FANS AND MOTORS Centrifugal type fans are most commonly used, and these may be equipped with either backward or forward curved blades. Low tip speed is desirable for the elimination of air noise,-especially where forward curved blades are'used. Motors may be mounted on the fan shaft or outside of the fan with belt connection. Multi-speed motors or pulleys 307