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400 CHAPTER 16 1954 Guide Mechanical Warm Air Furnaces Mechanical or forced warm air furnaces include fans or blowers as in tegral parts, for the purpose of circulating the air, and usually include air filters. Centrifugal fans with either backward or forward curved blades are the type most commonly used. Motors may be mounted on the fan shaft or connected to the fan by a belt drive. Adjustable pulleys are desirable to provide means of regulating the quantity of air distributed to the heated spaces. Either the motor load or the noise considerations may limit the maximum operating fan speed. Two-speed motors have given successful operating results. Motors and mountings must be carefully selected for quiet operation. Electrical conduit and water piping must not be fastened to, nor make contact with the fan housing. Filters Several types of filters are available for mechanical warm air furnace applications, and are discussed in Chapter 34. 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 percent average efficiency on an 8-hr test at a maximum resistance of 0.25 in. of water. Filter resistance rises rapidly with the accumulation of dirt, and may reduce the air circulation over heating surfaces. In domestic fur naces, the maximum velocity, based on nominal filter area, should not exceed 300 fpm. Fuel Utilization A combustion rate of from 5 to 8 lb of coal per (square foot of grate) (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 the ratio may- be 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 a 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. Furnaces for burning oil fuel are usually designed for blow-through instah lations so that the pressure in the air space is higher than that in the combustion chamber or flues. The National Warm Air Heating and Av Conditioning Association has prepared a Tentative Code for Testing and Rating of Oil-Fired Furnaces. Compact fan-fumace-burner units are available, suitable for basement, closet, or attic installations. ,,t Gas-fired forced air furnaces should conform in construction and per formance to A.(j.A. Approval Requirements. - Heavy Duty Fan Furnaces Fan furnaces for large commercial and industrial buildings, churches, schools, etc., are available in sizes ranging from 300,000 to 6,000,000 Btu per (hour) (unit). Heavy duty furnace heaters may be arranged in bat tery combinations of one or more units. .^ Most manufacturers of heavy duty furnaces rate their furnaces in Heating Boilers, Furnaces, Space Heaters 401 per hour, and also in the number of square feet of heating surface. Con servative practice indicates that at no time in the heating-up period should the furnace surface be required to emit more than an average of 3500 Btu per square foot. A higher rate of heat emission tends to increase the heat loss up the chimney, and raise fuel consumption, to shorten the life of the furnace, and to overheat the air. The ratio of heating surface to grate area of furnaces for this type of work should never be less than 30 to 1 and, as indicated previously, may run as high as 50 to 1. Control of temperature is secured through (1) controlling the quantity of heated air entering the room, (2) using mixing dampers, or (3) regulating the fuel supply. . The design of heavy duty fan furnace heating systems is in many re spects similar to that of the central fan heating systems described in Chap ter 30. Ducts are designed by the method outlined in Chapter 32. MATERIALS AND CONSTRUCTION Cast-Iron Furnaces Cast-iron furnaces are made in a multiplicity of designs or shapes. For solid fuels they are.generally of round sectional construction, the sections being.cemented or bolted together. Various types of radiators for second ary convection heat transfer are employed. Such radiators are of the circular, doughnut type, or tubular type. Cast-iron is frequently used in the construction of gas or oil-fired fur naces, designs varying considerably with two general types in common use: multi-sectional type, and those with single combustion chambers having auxiliary secondary surface. Cast-iron furnaces are made in capacities ranging from those for small insulated residence application with inputs of 40,000 Btu per hour or less, to capacities as large as 600,000 Btu per hour. Cast-iron furnaces are usually constructed with a minimum sectional thickness of f in., and effectively resist high temperatures and corrosion. They usually have a fairly large heat capacity because of their mass, which Provides a distinct fly wheel or carry-over heating effect. Steel Furnaces Formed sheet steel construction is frequently used in furnace design. Welding, riveting, or both are used to join the formed metal. The use f steel castings, however, is rare, because of the cost, and because high stresses are not encountered in normal furnace construction. Types of design employed vary greatly, although perhaps the most common type consists of a drum and circumferential or rear radiator. Steel gas furnaces *ay also be sectional in design, or may be combinations of common com bustion chambers and sectional or tubular radiation surfaces connected to a "be gas collector. Steel furnaces are made in capacities ranging from 40,000 Btu per hour to capacities as large as 600,000 Btu. Steel furnaces have low heat capacities a result of their relatively low mass and, therefore, deliver heat rapidly b demand. FURNACE RATING air furnaces are generally rated in Btu per hour output at the net (point of heat generation) or at the register (point of heat delivery).