Document xjOgLbDvwNg8QJbjKE4zkyRpJ

American Society of Heating and Ventilating Engineers Guide7i93j the casing above the level of the grate in the furnace. To accomnl' k this the shoe must be wide. ' . Tests of six different systems of cold air returns, Fig. 6, made at th University of Illinois2, resulted in the following conclusions: "e 1. In general, somewhat better room temperature conditions may be obtained k returning the air from positions near the cold walls. . D>' 2. Friction and turbulence in elaborate return duct systems retard the flow of a' and may seriously reduce furnace efficiency, and lessen the advantages of such a desim' 3. The cross-sectional duct area is not the only measure of effectiveness. Frictin and turbulence may operate to make the air flow out of all proportion to the vanV. duct areas. . ___ ous & 1 Afp./ \ : Area of Areo of f/o.Z . or Oirc/s SSP sp S9 wt o// cooes 03S sp in f^pcs oot cAoftpeo /VaS J ' Fig. 6. Arrangement of Cold Air Returns for Six Installations FURNACE CAPACITY The size of furnace should, of course, be such as will provide the necessary air heating- capacity, usually expressed ,in square inches of leader pipe area, and at the same time provide a grate of the proper area to burn the necessary fuel at a reasonable chihiney draft. The total leader pipe area required is easily obtained by finding the sum of the leader pipe areas as already designated. The grate area will depend on several factors of which four are very important. First of all, the air temperature at the register for which the plant has been designed must be determined. Usually, this tempera ture is taken as 175 F. Second in importance is the combustion rate, which must always correspond with the register air temperature, as is shown by reference to a set of typical furnace performance curves (Fig. 7) for a cast-iron circular radiator furnace with a 23-in:, diameter:grate and 50-in. diameter casing. The conditions shown on these curves which seem to . ..'Investigation of Warm-Air Furnaces and Heating Systems, Part N', by A. C. Willard, A. P. Kratx and V; S. Day (University of Illinois Engineering Experiment Station' Bulletin No. 189).' TTChapter 24^-Gravity Warm Air Furnace Systems' : mate nearest to the 175 F register warm-air temperature are: aPPr tjon rate, 7 lb; warm-air register temperature, 173 F; efficiency of * ( mace, 58.5 per cent. The third factor is efficiency, which, in turn, is *1* _Urt;on of the combustion rate varying with it as shown by the effiacurve of Fig. 7. The fourth factor is the heat value per pound of f"61?burned, which was 12,790 Btu. This is not shown on the curves since tras constant for all combustion rates. fi/5, --1--r--1--1--r Draft in Inches. 220000 aw- --1 -- - -- QOS k ^ZOO 000 ISO 000 fsfer Te/iipet-atore- 1. er- zzoA --- -- / - L * I ^ 160 000 X. "a7pa c/tt, *>% /<w| $/, I ,7l 80000 Grci/e tO/arle/e.rZ3 fn_ c%sin !atptver. SO /n. 4 70 o>s fflaencif 60& 4 -- Conwust/on Rote to tfi. per sp ft of Crateper fir k Fig. 7. Typical Performance Curves for a WarmAir Furnace and Installation in a Three Story Ten Leader Plant; Operating on Recirculated Air . From the relation existing among these factors it is found (Fig. 7) that the capacity of the furnace under test is 147,750 Btu per hour for the total grate, which gives the capacity at the furnace bonnet per square foot of grate as 51,200 Btu and per square inch of grate as'356 Btu per hour. Suppose it is desired to select a furnace to deliver air to the rooms at a register temperature approximating 160 F rather than 175 F. Referring to the curves, the relation is: combustion rate, 5.5 lb; register warm-air temperature, 160 F; and efficiency of the furnace, 62 per cent. Underthis condition the capacity of the furnace at the furnace bonnet per square foot of grate area is 43,200 Btu per hour, and per square inch of grate it is 300 Btu per hour. From these perfprmance yalues; the grate area for any 397