Document pm3XDz3vMaOK867b1DzjER8nw

Heating Ventilating Air Conditioning Guide 1939 The grate area will depend on several factors of which four are v important. First of all, the air temperature at the register for wVu the plant has been designed must be determined. Usually, this temDe ture is taken at 175 F. Second in importance is the combustion rat* which must always correspond with the register air temperature, as is shn by a set of typical furnace performance curves (Fig. 7) for a cast ir^" circular radiator furnace with a 23 in. diameter grate and 50 in. diarne?"' casing. The third factor, is efficiency, which is a function of the co 61 bustion rate, and varies with it as shown by the efficiency curve of Fie1? The fourth factor is the heat value per pound of fuel burned, which w 12,790 Btu. This is not shown on the curves since it was constant for mi combustion rates. a| Chapter 19- Gravity Warm Air Furnace Systems . the furnac_e__i_s 62 .per cent. Under this condition the capacity at the .^furnace, bboonnnneet tp^e^er "square .g 300 foot of gtu per jg,rra| ttej,eisre4q3U,2ir0e0d Barteuapoefr thher agnradtepeinr square i*-** -- 1,25 H square inches in this case will be -'30Q = 0.0042 H. It should be ted that a larger grate area is required if the furnace is to deliver air ' at a lower register temperature. typical performance curves shown in Fig. 7 are not applicable to <2 No. 1 No. 2 Fig. 6. Arrangement of Cold Air Returns for Six Installations It may be noted from Fig. 7 that for this particular furnace a register temperature of 175 F was accompanied by a combustion rate of approxi mately 7.5 lb per sq ft per hr, a capacity at the bonnet of 152,000 Btu per hr and a furnace efficiency of 58 per cent. Under these conditions the capacity at the bonnet per square foot of grate was equivalent to a value of 52,800 Btu per hr and per square inch of grate was equivalent to 367 Btu per hr. If it is desired to use these curves to select a furnace to deliver air at 175 F register temperature in a house where the total heat Joss is H Btu per hour and the loss between the furnace and the registers is 0.25 H Btu per hour, the area of the grate in square inches will be ^fg7g = 0.0034 H. If, on the other hand, it is desired to select a furnace to deliver air at 160 F register temperature, the combustion rate is 5.5 lb and the efficiency 390 Fig. 7. Typical Performance Curves for a Warm-Air Furnace and Installation in a Three-Story Ten Leader Plant, Operating on Recirculated Air all furnaces and hence for ordinary design purposes the values recom mended in the Standard Code3 should be used. The equation for a furnace having a ratio of heating surface to grate area of 20 to 1 is equal to: ,, GXpXfXEiXEtX 0.866 . Standard Code Regulating the Installation of Gravity Warm Air Heating Systems in Residences. Tbra code has been sponsored by the National Warm Air Heating and Air Conditioning Association, the National Association of Sheet Metal Contractors, and the American Society of Heating and Ventilating Engineers. It is recommended that the installation of all gravity warm air heating systems in residences -- be governed by the provisions of this code, the ninth edition of which may be obtained from the National Warm Air Heating and Air Conditioning Association, 60 W. Broad St., Columbus, Ohio. 391