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Heating Ventilating Air Conditioning Guide 1939 fan will be capable of delivering air against pressures that may not have been foresee during the design of the duct system. ^eo 12. Select a furnace capable of delivering heat at the register outlets equal to th total heat loss of the structure to be heated. e The following formula may be used for coal burning furnaces: where G= H fXpXEi X . [1 + 0.02 (R - 20)] (5) G -- required grate area, square feet. H = total heat loss from building, Btu per hour. / = calorific value of coal, Btu per pound. p = combustion rate in pounds of fuel per square foot of grate per hour. Ei = furnace efficiency based on heat available at bonnet. Ei = efficiency of transmission based on ratio of heat delivered at register to heat available at bonnet. R = ratio of heating surface to grate area. In practice it is customary to use the following constants: / = 12,000 (for specific values, see Table 5, Chapter 9). p = 7.5 lb. Ei = 0.65 lower efficiency must be used with highly volatile solid fuel. E, = 0.85. The foregoing procedure for determining the size of the furnace to be used applies to continuously heated buildings. 13. Although intermittently heated buildings usually have their heat losses computed according to the standard rules for determining such losses, these rules do not take into account the heat which will be absorbed by the cold material of the building after the air is raised in temperature. This heat absorption must be added to the normal heat loss of the building to determine the load which the heating plant must carry through the warming-up process, ft is customary to increase the normal heat loss figure by. from 50 to 150 per cent depending upon the heat capacity of the construction material, the higher percentage applying to materials of high heat capacity such as concrete and brick. Fan furnace systems are well adapted for heating intermittently heated buildings as these systems do not require the warming of intermediate piping, radiators, or convectors, the generation of steam, or the heating of hot water. 14. Follow the same methods for an oil furnace as for coal where a conversion unit is to be used, making sure that the ratio of heating surface to grate area exceeds 20 to 1. If it does not, a size larger furnace should be selected. Use the manufacturer's Btu ratings of furnaces designed for exclusive use with oil, and select a burner with liberal excess capacity. ,N 15. The selection of the proper size gas furnace for a constantly heated building can be easily made by using the following American Gas Association formifla: where H ft = 0.9 H = total heat loss from building, Btu per hour. R = official A.G.A. output rating of the furnace, Btu per hour. (6) In the case of converted warm air furnaces a slightly different procedure is necessary, as the Btu input to the conversion burner must be selected rather than the furnace out put. The proper sizing may be done by means of the following formula: 7 = 1.59 H (7) 408 Chapter 20. Mechanical Warm Air Furnace Systems where l = Btu per hour input. The factor 1.59 is the multiplier necessary to care for a 10 per cent heat loss in the distributing ducts and an efficiency of 70 per cent in the conversion burner. 16 Specify location and type of all dampers in both supply air and return air sides f system Specify controls including location of all thermostats. Arrange for proper mtrol of humidifying equipment. HEAVY DUTY FAN FURNACES Fan furnaces for large commercial and industrial buildings are available in sizes ranging from 400,000 to 3,000,000 Btu per hour per unit. Heavy duty heaters may be arranged in combinations of one or more units in a : battery. A few possible arrangements are shown in Figs. 6 to 9 in clusive. 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 on 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 21. Ducts are designed by the method outlined in Chapter 29. HUMIDIFICATION Mechanical warm air systems offer a means of proportioning and distributing moisture-bearing air; consequently, during the winter months humidifiers may be employed to deliver water vapor to the fan-driven air stream in proper amounts to produce a more humid atmosphere, with increased comfort for people and increased life for household furnishings.. Temperatures and relative humidities should be governed within the limits of the-generally accepted standards. See Chapters 3 and 25 for more detailed information on this point. In earlier types of furnaces, water evaporating pans were usually placed in the cool portions of the air stream, but modern types usually locate them in air which has been heated by contact with the heating surfaces. To change water into vapor capable of being carried in an air stream as part of the mixture, about 1000 Btu per pound are required. Without the addition of this heat, termed the latent heat of evaporation, water injected into the air will be carried along in the form of tiny globules until it falls out of the stream or is deposited upon some surface. Furthermore, when dry air is in contact with water for a sufficient length of time without the presence of a sizable body of water or a source other than air from 409 /