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r HEATING VENTILATING AiR CONDITIONING GUIDE 1941 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 at 175 F. Second in importance is the combustion rate which must always correspond with the register air temperature, as is shown by 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 third factor is efficiency, which is a function of the com bustion rate, and varies with it as shown by the efficiency curve of Fig. 7. The fourth factor is the heat value per pound of fuel burned, which was 12,790 Btu. This is not shown on the curves since it was constant for allcombustion rates. CHAPTER 18. GRAVITY WARM AIR FURNACE SYSTEMS of the furnace is 62 per cent. Under this condition the capacity at the furnace bonnet per square foot of grate is 43,200 Btu per hour and per couare inch of grate is 300 Btu per hour, the required area of the grate in 94 i 25 H square inches in this case will be '^qq = 0.0042 H. It should be noted that a larger grate area is required if the furnace is to deliver air at a lower register temperature. The typical performance curves shown iri Fig. 7 are not applicable to Fig. 6. Arrangement of Cold Air Returns for Six Installations It may be hotted 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 squarte foot per hour, a capacity at the bonnet of 152,000 Btu per hour and a furnace efficiency of 58 per cent. Under these con ditions the capacity at the bonnet per square foot of grate was equivalent to a value of 52,800 Btu per hour and per square inch of grate was equi valent to 367 Btu per hour. 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 loss 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 = 0.0034 H. oo7 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 352 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 H~ 144 Standard Gravity Code for the Design and Installation of Gravity Warm Air Heating Systems in Residences. This code bag been sponsored by the National Warm A ir Heatingand Air Conditioning Associ ation, the National 'Association oj 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 tenth edition of which may be obtained from the National Warm Air Heating and Air Conditioning Association, 5 E. Long St., Columbus, Ohio. 353