Document vB3drXjNkwJmn55GOoEzwka3b

American Society of Heating and Ventilating Engineers Guide, 1924-25 DETERMINING BOILER CAPACITY REQUIRED FOR A PARTICULAR INSTALLATION First determine from the boiler manufacturer whether his complete line of boilers has been tested according to the A. S. H. & V. E. Boiler Testing Code Revision of 1923 (See p. 65). Look for the evidence of such testing in his catalog also facts as to fuel capacity, evaporative power and on the smaller boilers, length of firing period, efficiency and draft requirements. The net amount of radiation should be increased by about 20 per cent for line losses to determine the total heating load on the boiler. The smaller the boiler, the greater the handicaps it usually must operate under, such as poor draft, dirty coal, sooty heating surface, incompetent attention, etc. To offset those handicaps on boilers used for residence heating, a factor of safety of 50 to 100 per cent should be figured above the total load in determining boiler capacity. In larger size installations the operating conditions are more favorable where a factor of safety of 25 to 75 per cent is ample. Under ordinary operating conditions any boiler should be able to develop at least its full rated capacity. IMPORTANCE OF DRAFT The capacity a boiler is capable of developing depends more upon the amount of draft available than upon any other factor. Assuming that a chimney is smoke tight and well built according to the Ordinance for Construction of Chimneys, 1921 (recommended by the National Board of Fire Underwriters and approved by the Society), the intensity of the draft depends upon the height of the chimney, and. the quantity or amount of draft depends principally upon the effective area of the chim ney. According to their height, heating chimneys are divided into three classes, the erratic, uncertain and reliable. Chimneys less than 36 ft. high are erratic in their action. The head produced by such a low height, is so small that the least unfavorable condition or interference practically puts the chimney out of commission. Chimneys between 36 ft. and 64 ft. in height are in the doubtful zone, sometimes good and sometimes bad. The head produced by this rela tively low height is frequently offset by slight unfavorable conditions that may be difficult to locate. Chimneys over 64 ft. in height are usually reliable, because as a rule the chimney is designed by an engineer and must be well built to sustain such a heavy load and the height is such as to produce considerable head or force to offset unfavorable weather con ditions, etc. Chimneys in this class produce about 0.009 in. draft per ft. of height in zero weather with 600 deg. in the stack according to the formula: where P. = draft pressure in inches of water. H = height of chimney in feet. To -- absolute temperature of outside air. T& = absolute temperature of stack gases. 58 For low pressure heating boilers, water heaters and warm air furnaces conservative modern practice in the matter of chimney sizes is in accord ance with the accompanying schedule, Table 50: TABLE 50. CHIMNEY SIZES Warm Air Furnace Capacity in Leader Pipe Sq. In. Steam Boiler Capacity Sq. Ft. . OP Radia tion Hot Water Heater Capacity Sq. Ft. OF Radia tion Nominal Dimen sions of FirbClay Lining In. Rectangular Flue round Flub Actual . Inside Dimensions of Fire Clay ' Lining In. Actual Area Sq. In. Effec tive Area Sq. In. Inside Diameter of Lining In. Effec tive Area Sq. In. Height in Ft. from Grate- 790 590 973 834*13 7x1134 81 70 35 1000 690 1140 10 79 900 1490 13x13 1134x1134 127 99 900 1100 1490 834x18 1820 654x1634 110 100 12 113 40 1700 2800 13x18 1134x1634 183 156 1940 3200 15 177 2130 3520 18x18 1554x1554 248 195 2480 3150 4090 5200 20x20 1734x1734 298 234 18 45 254' 50 4300 7100 20 314 5000 8250 24x24 21x21 441 4600 7590 20x24* 480 326 55 5570 9190 24x24 576 380 60 5580 9200 22 380 6980 11500 24 452 65 7270 12000 24x28 672 468 8700 14400 28x28 784 531 9380 15500 27 573 10150 16750 30x30 900 616 .10470 17250 28x32 . 896 635 11800 19500 30 707 70 14700 24300 33 855 17900 29500 36 1018 Dimensions below are for unlined rectangular flues. Chimneys recommended for larger boilers 15 to 250 hp. are propor tioned in accordance with the report made by a joint Committee of the American Boiler Manufacturers Association and Stoker Manufacturers Association and approved by these organizations. The sizes are given in Table 51. TABLE 51. HEIGHT OF STACK FOR AVERAGE INSTALLATIONS (Sea Level) Forced Draft Stokers Per Cent Rating..... ................. Draft Furnace........................... Friction Loss (Boiler) Friction Loss (Breeching)TM................ Total Draft Required........... Height of Stack (Ft.)...................... ; 100 0.15 0.18 0.10 0.43 80 150 0.15 0.4 0.10 0.65 112 200 0.15 0.65 0.10 0.90 145 250 0.15 0.9 0.10 1.15 178 300 0.15 1.20 0.10 1.45 220 59