Document G5Jb4Qb2q2dgGwVOgQd9vpp87

352 CHAPTER 18 ' 1946- Guide b. According to fuels for which the furnaces are designed - these are coal hand-fired or stoker-fired, oil, gas, or wood. c. According to .materials of construction--they are cast-iron, low carbon steel, and occasionally high temperature steel alloys. d. According to design or construction, such as drum and radiator, tubular, hori zontal, etc. Gravity Warm Air Furnaces A gravity furnace is one in which the motive head producing air flow' depends upon the difference in density between the heated air leaving the top of the casing and the cooled air entering the bottom of the casing. Since this gravity head is relatively low, the furnace must have low in ternal resistance to the flow of air and relatively large areas must be available for free circulation within the furnace casing. It is common practice to provide approximately 50 per cent free air area through gravity type furnaces. Furnaces for gravity type systems are available in designs suitable for. central heating, pipeless furnace, or unit floor furnace installations. Booster fans are sometimes used in conjunction with gravity design systems, to increase air circulation. 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 impinge ment of the air against the heating surfaces. Where square casings are used, the corners must be baffled. -. Mechanical Warm Air Furnaces Mechanical or forced warm air furnaces include fans or blowers as ' integfal parts for the purpose of circulating the air and usually includeair filters. .. Fans and Motors 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 aiej desirable to provide means of regulating the quantity of air distributed! to the heated spaces. Either the, motor load or the noise, may limit the- maximum operating fan speed. Two-speed motors have given successful operating results and are recommended.-. 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 33. '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 cent 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 furnaces, 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 / Heating Boilers and Furnaces' 353 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 l-, .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 installations so that the pressure in the,air space is higher than that in the combustion chamber or flues. The National Warm Air Heating and Air Conditioning Association has prepared 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 performance to A.G.A. Approval Requirements. Heavy Duly Fan Furnaces Fan furnaces for large commercial and industrial buildings, churches, schools, etc., are available in sizes ranging from 300,000 to 3,000,000 Btu per (hour) (unit).' Heavy duty furnace heaters may be arranged in battery combinations of one or more units. Most manufacturers of heavy duty furnaces rate their furnaces in Btu 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) regu lating the, fuel'supply. The design of heavy duty fan furnace heating systems is in many respects similar to that of .the central fan heating systems described in* Chapter 43. Ducts are designed by the method outlined in Chapter 4L 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.secondary 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, furnaces, 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