Document M4nL7LRwwMXQ6508arvLBm7qy

414 CHAPTER 16 1952 Guide Available- draft equal to, or greater than, that produced by the other three larger liners. When the flue gas rate was increased to 320 lb per hr, the three larger liners produced a little more draft than the smallest one for ehtCnhg flue gas temperatures above 600 F. Table 1 shows approximate values of the mass flow rates and flue-gas Velocities at the chimney inlet that produce the maximum available draft for masonry chimneys8 of several conventional sizes and with an effective height of 15 ft. This table shows that a 9-in. round chimney is best suited to mass flow rates from 130 to 170 lb per hr, a 9 X 9-in. chimney performs best for flow rates from about 200 to 300 lb per hr, and a 9 X 13in. and 12 in. round chimneys are best suited to flow rates above 300 lb per hr.. These results were obtained with clean chimneys, so that conclusions about chimney areas require some modification if soot deposits are taken into consideration. Soot deposits in chimneys reduce" the effective area of the finer and may in some cases entirely close the passage. Soot deposits are likely to be FIVE CAS FLOW-POUNDS PER HR Fig. 8. Graphical Evaluation op Rate of Flue Gas Flow from Percent CO, and Fuel Rate* * Average density gas 0.045 lb per cu ft, 62 F. Average weight oil 7.08 lb per gal, GO F. greater in the horizontal passages in the heating plant, the breeching, and the smokepipe, than in the vertical chimney finer. An increase in the liner diameter of one inch above that required for a clean chimney will probably make adequate allowance for soot deposits in all but the worst cases. Where smoky combustion is likely to occur and the mass flow rates on a clean chimney basis approach those listed in Table 1, the next larger commercial size finer should be used. Smoky combustion with oil-buining devices, particularly with vaporizing oil burners, is likely to be caused by inadequate draft which is often due to insufficient chimney height. For coal, inadequate chimney height or chimney area may be a contributing cause, but smoky combustion with coal is related to fuel characteristics and firing methods. Therefore, an increase in chimney area will usually hot cure smoky combustion of a coal-burning device, but can be expected to lengthen the interval between cleanings. New standard sizes of clay flue linings were recently developed by the industry and approved by the American Standards Association, in order to effect economies that can be derived from coordination of the dimensions of building .materials. These linings are known as modular clay flue Chimneys and Draft Calculations 415 Table 1. Approximate Flue-Gas Flow Rates for Maximum Available Draft in Masonrt Chimneys Nominal External Dimensions Internal Area op Liner, Sq In 9 in. Dia. 9 in. x 9 in. 12 in. Dia. 9 in. x 13 in. 38.5 49 78.5 77 200 Flue Gas Temperature at Chimnet Inlet, Fahb. 600 1000 200 600 .1000 Mass Flow Rate, LB/HR Flue Gas Velocity at Chimney Inlet, FPM 170 150 130 175 250 300 215 306 . 334 175 400 600 290 Above Above 150 320 320 295 Above Above 150 320 320 finings, and the dimensions and tolerances are summarized in ASA Stand ard A62.4-47. As the effective areas of these liners are somewhat smaller than those of the corresponding linings used previously, they cannot be used in certain municipalities where building codes specify minimum areas based on the older dimensions. ' DRAFT REQUIREMENTS OF APPLIANCES Typical flue-gas temperatures and drafts required at rated output for several kinds of domestic heating appliances11 are contained in Table 2. Chimney height and chimney area for cast iron boilers are specified in the I = B = R Testing and Rating Code of the Institute of Boiler and Radiator Manufacturers. Mechanically-fired devices such as oil burners and stokers are equipped with blowers, and therefore, the chimney is not required to overcome the resistance of the fuel bed or burner. Nevertheless, a draft in the firebox, of about 0.03 in. of water, is considered desirable so that any small openings in the firebox or flue passages will result in leakage of air inward, and not leakage of combustion products outward. Firebox leakage should be kept to a minimum, however, since such leakage adversely affects heating plant efficiency. Difficulty in obtaining enough draft for natural draft appliances is likely to occur for attic installations or in one-story houses without basements. Automatic oil-burning space heaters, floor furnaces, and warm air furnaces employing natural-draft vaporizing burners require a draft of 0.06 to 0.08 Table 2. Drafts Required by Typical Residential -Heating Devices or Appliances Device Draft, Inches Water Stack Tempera ture6 F Deg Space Heater, Oil Burning, Pot Burner....................... Warm Air Furnace, Oil Burning, Pot Burner....... Warm Air Furnace, Hand Fired........................ Floor Furnace, Oil Burning, Pot Burner... Mechanical Oil Burner, Less than 5 gph.................. Mechanical Oil Burner, More than 5 gph....... Cooking Stove, Solid Fuel................ . .'.......... Space Heater, Coal Burning........................ 0.06 to 0.08 0.06 0.06** 0.06 0.03* 0.04b 0.06b 1000 860 900 860 _ 400 900 * Draft in firebox. b For chestnut sired anthracite. c 18 in. from heater.