Document YrNaOK8yGzGXQ17KvGQbp4evD

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 7. Size of furnace necessary to supply the warm air required to overcome the heat toss from the building. This size should include square inches of leader pipe area which the furnace must supply. _ It is also desirable to call for a minimum bottom fire-pot diameter in inches, which is the nominal grate diameter. 8. Area and dimensions in inches of chimney and smoke pipe. If an unlined chimney is to be used, that fact should be made clear. The heat loss calculations should be made in accordance with the procedure outlined in Chapter 5, taking into consideration the trans mission losses as well as the infiltration losses. LEADER PIPE SIZES In a gravity circulating warm-air furnace system the size of the leader to a given room depends upon the temperature of the warm air entering the room at the register. A reasonable air temperature at the registers must, therefore, be chosen before the system can be designed. The National Warm Air Heating and Air Conditioning Association has ap proved an air temperature of 175 F at the registers as satisfactory for design purposes. At this temperature, the heat-carrying capacity (heat available above 70 F) per square inch of leader pipe per hour for first, second or third floors is shown by Fig. 1 at 175 F to be 105, 170 and 208 Btu, respectively. For average calculations, the values 111, 166 and 200 will simplify the work and may be satisfactorily substituted for these heat-carrying capacities. If H represents the total heat to be supplied any room, the resulting equations are: Leader areas for first floor, square inches = jyj = approximately 0.009H (1) Leader areas for second floor, square inches = = approximately 0.00617 (2) Leader areas for third floor, square inches = = approximately 0.00511 (3) Z\J\) In designing for a lower warm-air register temperature, say 160 F, the factors 111, 166 and 200 become 80, 140 and 166 (Fig. 1 at 160 F), and the resulting equations are: u Leader areas for first floor, square inches = = approximately 0.012H oU (4) Leader areas for second floor, square inches = - approximately 0.007H (5) .^ Leader areas for third floor, square inches = - approximately 0.006H (6) These equations are applicable to straight leaders from 6 to 8 ft in length. Longer leaders must be thoroughly insulated or the vertical stacks must be increased in area as discussed under wall stacks. If some provision is not made for these longer leaders, the air temperature may be much lower than anticipated and the room will not be properly heated. The values shown in Fig. 1 apply only to the case where the straight, leader pipe is 8 ft in length and is connected to stacks whose crosssectional area is approximately 75 per cent of that of the leader pipe. 346 CHAPTER 18. GRAVITY WARM AIR FURNACE SYSTEMS Any deviation from these conditions requires a modification of the con stants used in Equations 1, 2, and 3. The temperature drop in leaders of various lengths at three different register temperatures is shown in Fig. 2, and should be used to obtain new register temperatures, lower than 175 F, on which to base selections from the curves of Fig. 1, and thereby new constants for Equations 1, 2 and 3. Leader sizes should in general be not less than those obtained by Equations 1 to 3 nor should leaders less than 8 in. in diameter be used. In residences requiring a leader pipe area of 650 sq in. or less, it is advisable to use two or more leader pipes to rooms requiring more than the capacity of a 12 in. round pipe. It is not considered good commercial practice to specify diameters except in whole inches. The tops of all leaders should be at the same elevation as they leave the furnace bonnet, and from this point there should be a uniform up-grade of 1 in. per foot of run in all cases. Leaders over 12 ft in length should be avoided if possible. In cases where such leaders are required, the use of a larger size pipe, than is required by the application of the equations, smooth transition fittings, and duct insulation are recommended. 347