Document VKa2XZgvmb3D6bjZvKBb9dJwo

454 CHAPTER 17 1958 Guide DETERMimNG RESIDENTIAL CHIMNEY SIZES The flue sizes for small residential chimneys are governed by the National Building Code of the National Board of Fire Underwriters for gas-burning appliances. Chimney areas for liquid- and solid-fuel-buming devices are selected primarily to meet the requirements of local building codes, but these requirements are not determined by any rigorous formula based on physical principles. By calculating the available draft for the chimney in question, and comparing it with the performance values of the heating unit, (either natural or forced draft type) at the desired output, it is possible to de termine whether the chimney is adequate in height for the particular heating unit it serves. For calculations where the fuel rate and the percentage CO, are the only known factors, the flue-gas rate can easily be determined for coal-, oil and gas from Fig. 9. By entering Fig. 9 at the percentage COi, moving verti- FLUE CAS FLOW-POUNOS PER HR Fig. 9. Graphical Evaluation of Rate of Flue Gas Flow from Percent CO* and Fuel Rate* * Average density gas 0.0845 lb per cu ft, 62 F. Average weight oil 7.08 lb per gal, 60 F. cally to the curve for the type of fuel, and then moving horizontally to the fuel rate, the flue-gas rate in pounds per hour may be determined for any fuel.7 Any of the described methods of determining available draft may be used, but a graphical solution to the problem may be had for the 8 X 8-in. chimney from Figs. 6, 7 and 9. This solution can be best explained by a numerical example. Example S: Determine whether a 13-ft, 8 x 8-in. nominal-size flue is sufficient fora coal-heating unit rated at 0.03 in. of water draft at 400 F inlet temperature, fuel rate being 10 lb per hr of bituminous coal, with 10 percent COj. Solution: From Fig. 9, a flue-gas rate of approximately 180 lb per hr is obtained. The available draft for a 180 lb per hr fuel rate, and an inlet temperature of 400 F, obtained from Fig. 6, is 0.056 in. of water. This indicates that the chimney is ade quate. The selection of chimney areas for liquid- and solid-fuel-buming devices is difficult because of the variability in efficiency of different models, the possibility that soot on the lining will restrict the chimney area, and the variation in combustion air requirements of different solid fuels. Figs. 6 and 7 show that a given chimney produces a maximum available draft and Chimneys and Draft Calculations 455 a efficiency for some intermediate mass flow rate for any selected inlet flue-gas temperature. For mass flow rates lower than the optimum the greater'cooling of the gases in the chimney causes lower available draft whereas for mass flow rates above the optimum the greater friction losses' reduce the available draft.. A chimney for a given heating system should probably be designed to operate at its point of maximum efficiency and "invimiiTn available draft for its full rated output. A chimney would have an accelerating effect10 on the combustion rate of a solid-fuel burning device if it were operating to the left of the optimum point in Fig. 6 because an additional increment in mass flow rate would increase the available draft a small amount and tend to increase the mass flow still more. On the other hand, a chimney operating to the right of the optimum point in Fig. 6 would tend to decelerate the combustion rate for any small increase in mass flow rate. Data are not complete for -the selection of proper chimney areas for heating plants of different capacities, but some information on the effect of cross-section area on the. capacity of masonry chimneys is provided by Table 2. Approximate Flue-Gas Flow Rates for Maximum Available Draft in Masonrt Chimneys Nominal External Liner 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 Chimney Inlet, Fahr. 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 recent tests8,11 on several chimneys with liners having nominal outside dimensions: 9 in. diameter, 12 in. diameter, 9X9 in., and 9 X 13 in. These tests showed that for flue gas rates up to 200 lb per hr, and entering flue gas temperatures from 200 to 1000 F, the 9-in round liner provided an 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 entering flue gas temperatures above 600 F. Other data9 show that metal chimneys with an internal diameter of 6 in. should not be used for mass flow rates greater than 130 lb per hr. Table 2 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 13m. 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 liner and _ may in some cases entirely close the passage Soot deposits are likely to be