Document x55j3rajmrr0OLY0eQ98Jk8ME

American Society of Heating and Ventilating Engineers Guide, 1936 area of the leader, the designer must make allowances for all other conditions. 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. It is not considered good commercial practice to specify diameters except too * :ni i i i i W/th constant ii heart mpo , . to necrren or, -cv, i/c/i/tc?.r.o /* >r/ rr \ l III I I AH xtar/rx t req/jten 8-0".to t c boot to 7L eacters. boot ' /o ' Lt t < z 4 S' s' `k V tn- /, f/ * / s & 0.80 S/nq/e tVa// Stack Doab/e Hkf/t Stack, 7 / '/ / 6 7/7 Le ter tnq/e IVa/t Stack ''Doubte /1/a //1 terck ^ 0.70 ! it I TT! 1~ A t/stac'ks COt77C ed '/ft t the best s/na/e watt stack I 0.60 o //f Siiffy fO/n. anet S-/n. Lenafar '79s/s. ___i c o.t 0.2 o.3 o.4 as ae az as a9 /.o katr'o -Stack Area to L eaater Area Fig. 3. Relative Heating Effect of Stacks at Constant Heat Input to Furnace Note.--Exterior surface of all ducts is bright tin except at joints where asbestos^sealing strips are used.. in whole inches. rThe 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 are to be avoided or should receive very special attention. PROPORTIONING WALL STACKS The wall stack for an upper floor should be made not less than 70 per cent of the area of the leader which has been selected from Fig. 1. So long as the leader is short and straight as was the case for Fig. 1, 420 Chapter 24---Gravity Warm Air Furnace Systems such a practice is probably justified, since the loss (Fig. 3) in capacity occasioned by the smaller stack is not very serious for stacks having areas in excess of 70 per cent of the leader area. For leaders over 8 ft in length or for leaders which are not straight, the ratio of stack area to leader area should be greater than 70 per cent in order to offset the greater temperature losses (Fig. 2) in the longer leader. In gravity circulating systems, this stack to leader area ratio is a very important consideration. Specific data for a great variety of cases are presented in Figs. 4 and 5 and the designer should check the stack to leader com binations with the nearest comparable case as shown in these figures. Any second-floor stack supplying heat to a room whose heat loss is 9,000 Btu or more (see Figs. 4 and 5 which show that high temperatures are necessary if rooms of more than 9,000 Btu requirement are heated by one stack each in 4-in. studding) should be run within 6-in. studded walls or should have multiple stacks. Stack sections, wherever possible, should be changed from the thin rectangular to the more nearly square shape. REGISTER SIZES The registers used for discharging warm air into the rooms should have free or net area hot less than the area of the leader in the same run of piping. The free area should be at least 70 per cent of the gross area of the register. No upper-floor register should be wider horizontally than the wall stack, and it should be placed either in the baseboard or side wall, if this ca.n be done without the use of offsets. First-floor registers may be of the baseboard Or floor type, with the former location preferred. RECIRCULATING DUCTS AND GRILLES . The ducts through which air is returned to the furnace should be designed to minimize friction and turbulence. They should be of ample area, in excess of the total area of warm-air pipes, and at all points where the air stream must change direction or shape, streamline fittings should be employed. Horizontal ducts should pitch at least 3^2 in. per foot upward from the furnace. The recirculating grilles (or registers) should have a free area at least equal to the ducts to which they connect, and their free area should never be less than 50 per cent of their gross area. The location and number of return grilles will depend on the size, details and exposure-of the house. Small compactly built houses may'frequently be adequately served by a single return effectively placed in a central hall. More often it is desirable to have two or more returns, provided, however, that in two-story residences one return must be placed to effectively receive the cold air returning by why of the stairs. Where a divided system of two or more returns is used, the grilles must be placed to serve the maximum area of cold wall or windows. Thus in rooms .having only small windows the grille should be brought as close to the furnace as possible, but if the room has a bay window, French doors, or other large.sources of cooling or leakage of cold air, the grille, should be placed close by, so as to collect the cool air and prevent drafts. When long ducts of this type are employed they must be made 421