Document ZRRLb4LyoQ8yXZ6N8jw80ZGd

HEATING VENTILATINC AIR CONDITIONING GUIDE 1944 for the use of coal, oil and gas'. Each type of fuel requires a distinct type of furnace for highest efficiency and economy, substantially as listed herewith: * 1. Coal Burning: o. Bituminous--Large combustion space with easily accessible secondary radiator or flue travel. b. Anthracite or coke--Large firebox capacity and liberal secondary heating surfaces. c. Stoker firing--Adequate firebox height, liberal heating surface. 2. Oil Burning: a. Liberal combustion space. b. Long fire travel and extensive heating surface. 3. Gas Burning: a. Low flue resistance. b. Extensive heating surface. c. Close contact between combustion products 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 furnaces. A higher combustion rate is permissible with larger furnaces for buildings other than residences, depending upon the ratio of grate surface to heating surface, firing period, and available draft. 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. 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. The systems are usually designed for blow-through installations so that the furnace is under external pressure. The National Warm Air Heating and Air Conditioning Association has adopted a Tentative Code for Testing and Rating of Oil-Fired Furnaces. Compact fan-furnace-burner units are available, suitable for basement, closet, or even attic installations. Gas fired forced air furnaces should conform in construction and per formance to A.S.A. Approval Requirements2. Furnace Casings Casings are usually constructed of galvanized iron or fireproof insulating boards, but they may also be constructed of brick. Galvanized iron casings should be lined with sheet iron liners, extending from the grate level to the top of the furnace and spaced from 1 in. to 1in. from the outer casing. Casings for commercial or heavy duty furnaces, if built of galvanized iron, should be insulated with fireproof insulating material at least 2 in. thick. In general, either brick or sheet metal casing should be equipped with baffles to secure impingement of the air to be heated against the heating surfaces. Brick furnace casings should be supplied with access doors for inspection. R^"irements Central Heating Gag Appliances Z 21.13-1943. 372 CHAPTER 20. MECHANICAL WARM AIR FURNACE SYSTEMS Where a fan is to be used with a furnace casing sized for gravity air flow, some form of baffling must be employed to restrict the free area within the casing and to force impingement of the air against the heating surfaces. Where square casings are used, the corners must be baffled. 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 of a higher bonnet and thus provides a larger plenum chamber. FANS AND MOTORS Centrifugal fans are the type most commonly used, and these may be equipped with either backward or forward curved blades. Motors may be mounted on the fan shaft of outside of the fan for belt drive. Adjust able pulleys are desirable to provide a factor of safety and to allow for increased air circulation, provided the motor is adequate to carry the load imposed at maximum fan speed. Two-speed motors have given successful operating results and are recommended: Special attention should be given to the problem of noise elimination. The metal duct connection to and from the furnace casing and fan housing should be broken by strips of canvas. Motors and their mountings must be carefully selected for quiet operation. Electrical conduit and water piping must not be fastened to, nor make contact with fan housing. The installation of a fan directly under a cold air grille is usually not recom mended. (See also Chapter 33.) FILTERS Several types of filters are available for mechanical warm air furnace applications and are discussed in Chapter 29. For maximum efficiency and life under operating conditions, filters should not be subjected to a temperature in excess of 150 F. Filters should have at least 80 per centaverage efficiency on an 8 hr test. Filter resistance rises rapidly with the /accumulation of dirt, and may reduce the air circulation over heating surfaces. For domestic- furnaces, the maximum velocity should not exceed 300 fpm'based on nominal filter area. AIR DISTRIBUTION The conditions of comfort obtained in a room are greatly influenced by the type of register used and the locations of the supply registers and return grilles. In general it has been found that changes in the type, air velocity, and location of the supply register affect the room conditions much more than the changes in the location of the return grilles. One method is to locate the supply register near the floor so that the warm air . from the register blankets a cold wall, and mixes with the cold air dropping off from the exposed walls and glass. Another method is to locate the supply openings near the floor on the inside wall and the return openings 373