Document EdK9vgodJ7pBvZ3XXV42vwGon

536 CHAPTER 36 1960 Guide *Dmt*W front teaperutvre ploh. Liner 8 x 8 in. ovtdde, 6% * 6% bade. Height 13 ft. Fig. 7.... Effect of Gas Flow on Chimney Efficiency* chimneys can be used, with slight error, for chimneys made of shale tile, concrete block, or cinder block* CHIMNEY SIZES FOR HEATING AND PROCESS BOILERS Automatically Fired Intermittent Operation The 1958 Editions of the l~B^R Testing and Rating Code and the SBl Rating Code both publish information governing minimum dimensions of chimneys. The curves which show heights, and the tabulation which shows cross sectional dimensions were e-aloniatAd from Equa tions 3 and 4, by substituting commonly used. values for densities, pressures, and temperatures, and then applying appropriate allowances for warming up, friction, and un usual chimney conditions. The curves and tabulation follow in Fig. 8 and Table 1. They provide a simple and quick method for determining chimney dimensions for automatically fired, intermittently operated, boilers when the firing rate, stack temperature, and required draft are known. Example l. Select the chimney height and area for an auto matically fired, intermittently operated boiler under the follow ing conditions: Draft loss thru boiler Draft required in firebox 0.082 in. water 0.020 in. water Total draft required at smoke outlet 0.102 in. water Firing Rate ' 1L3 gph Gross Stack Temperature 590 F Solution: Refer to Fig. 8 and find a height of 25 ft at the intersection of horixoataf0.102 draft with 590 F ordinate. Refer to Table 1 at burner capacity nearest to 11.3 gph and find^ inside dimensions of tile liner to be 1314 x 1314 in. or 15 SHORT CHIMNEYS The application of heating systems to one-story houses without basements requires special consideration because the chimneys in such houses are often made so low in height for architectural reasons that they produce insufficient draft for some kinds of heating systems. Gas-burning devices do not necessarily require a chimney for proper combustion of the gas, but the chimney must be of adequate sue and height to carry the flue gases out of the building once it is undesirable and contrary to American Standards to have the products of combustion discharged in the living space. If a naturaldraft oil-burning device is operated with insufficient draft because of a short chimney, smoky combustion usually oc curs, pulsations are possible in the combustion chamber, the chimney or smokepipe may become blocked with soot, and the capacity of the heating equipment may be reduced to the point of inadequacy. Insufficient draft is not ordinarily the cause of smoky combustion in coal-burning heaters, but too short a chimney can cause slow pickup of the fire and insuffi cient heating capacity of the device. Based on the amount of draft usually required for preseatday heating systems at rated output, any chimney with an effective height of less than 15 ft above the center line of the thimble should be regarded as a short chimney. In short chimneys every precaution should be taken to attain the highest possible average temperature in the chimney and the lowest practicable friction loss. Short chimneys of conven tional construction are likely to produce from 60 to 90 per cent of the ideal draft and, therefore, the draft would only be increased from 10 to 40 percent if all friction and cooling could be eliminated. The available draft for short chimneys can perhaps be determined most readily by using Equation 13, Fig. 9, and Table 2. Fig. 9 shows the relationship between effective chim ney height, inlet flue-gas temperature, and the ideal draft that would be produced for two values of outdoor tempera ture if there were neither friction nor cooling of the gases as they traversed the chimney. After obtaining the ideal draft for a given chimney height and entering flue-gas temperature from Fig. 9 and selecting the appropriate efficiency value from Table 2, the available draft can be computed from Equation 13. Fig. 9 can be used in reverse in conjunction with efficiency values from Table 2 to determine what chimney height is required to produce the required draft at rated out put for a given heating appliance. Fig. 7 shows that the efficiency of a gjven chimney varies with the mass flow rate of gases and with the entering flue gas temperature. The efficiency also varies with the height for a given construction and depends somewhat on the method used to connect the smokepipe to the chimney and the amount of exposure to the outdoor air. Experimental data are not sufficiently complete to tabulate efficiencies that take into account all of these variables. However, recommended values of efficiency for several sizes of domestic chimneys of masonry materials and of metal have been selected from the literature4, T> * * and summarized in Table 2 for use with Fig. 9 in determining the available draft produced by short chim neys of 15 ft in height or lower. In parts A and B of Table 2 the value of efficiency chosen from the range cited should be increased as the chimney inlet temperature increases over the applicable range. In part G of Table 2 the efficiency val ues listed will yield the available draft for the range of flows, inlet flue-gas temperatures, and chimney height shown within 10 percent. In the application of short chimneys, observance of the following precautions will assist in obtaining the highest draft practicable and may cure unsatisfactory operation of the chimney and heating plant in certain instances: 1. Use a minimum length of horizontal smokepipe between the heater and chimney. 2. Insulate the smokepipe and the chimney itself, if made of Chimneys and Draft Calculations metal. Insulation of the chimney liner in masonry chim neys reduces the heat loss from the fiue gases and the in filtration of cold air. 3. Do not use an oversized chimney because huger chimneys produce greater cooling of the flue gases. 4. Avoid downdrafts by proper construction above the roof or by the use of a suitable chimney cap. 5. Avoid the use of barometric dampers in the smokepipe of natural-draft heaters since most barometric dampers per mit enough cold air to leak into the chimney to reduce the draft appreciably. 6. Avoid air leaks in the smokepipe and chimney. 7. Avoid connecting more than one heating device to a single chimney. 8. Use construction materials of low heat capacity if the chimney draft must increase quickly fox an intermittentlyfired heating device. 9. Do not operate a kitchen.exhaust fan unl^ an air intake of ample area is used to prevent lowering the house pressure. 10. Avoid tight utility closets so the chimney draft will not be required to do the added work of drawing combustion air into the closet from the surroundings. DETERMINING 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-burning devices are selected primarily to meet the requirements of local building codes, but these 537 requirements are not determined by any rigorous formula based on physical principles. By calculating the available draft for the chimney in ques tion, and comparing it with the performance values of the heating unit (either natural or forced draft type) at the de sired output, it is possible to determine 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. 10. By entering Fig. 10 at the percentage CO,, moving vertically 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/ 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 10. This solutipn can be best explained by a numerical example. Example 8: Determine whether a 13-ft, 8 x 8-in. nominal-size flue is sufficient for a coal-heating unit rated at 0A3 in. of water draft at 400 F inlet temperature, fuel rate being 10 lb per hi of bituminous coal, with 10 percent CO*. Solution; From Fig. 10, a fiue 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 adequate. The selection of chimney areas for liquid- and solid-fuelburning devices is difficult because of the variability in effi-