Document kmzdn1nNoLZnek7V6pBMdNjxy

422 CHAPTER 17 1954 Guide# and the amount of exposure to the outdoor air. Experimental data are;f 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 literature8 7 '8'9 and summarized in. Table 1 for use with.1; Fig. 8 in determining the available draft produced by short chimneys of/ 15 ft in height or lower. In parts A and B of Table 1 the value of efficiency chosen from the range cited should be increased as the chimney inlet temper- ' ature increases over the applicable range. In part C of Table 1 the effi- Table 1. Efficiency of Short Chimneys A. Mabon-ht Chimneys Height h ft Rate lb/br Internal Liner Size, in. Dia. 7 7x7 7 x 11 and - Dia. 10 , l_U 5 to 15 Efficiency, Percent 90 200 75-82 65-76 83-84 67-85 65-80 82-88 c& 315 75-89 67-86 81-89 (Inlet Flue Gaa Temp 200 to 1000)%' -------------------------- izate B. Uninsulated Metal Chimneys jfInternal Diameter, in. Flue Gas Flow m Height k ft Rate lb/hr jj -C J IfEfficiency, Percent 4 to 8 90 200 315 90-100 I 85-95 83-88 70-80 fis3ft 00*)$I (Inlet Flue Gas Temp. 200 to I C. Metal Chimneys^ WIWI ueau an space around chimney 1-in. insul*6on, with 1-in. Opw-I air space around/ j chimney *6 SInixte-irnncahl cdhiaim. 6n-einy; nflouet rgeacsomflomwernadteed5f0otroth1e2s5elbf/lohwr; riantleest. flue gas temp. 200 to 1000 F. ciency values listed will yield the available draft for the range of mass' flows, inlet flue gas temperatures, and chimney height shown within-10- percent. -si'#* In the application of short chimneys, observance of the following cautions will assist in obtaining the highest practicable draft and may,cui> unsatisfactory operation of the chimney and heating plant in certain m# stances: v|! 1. Use a minimum length of horizontal smokepipe between the heater 2. cInhsimulnaetey.the smokepipe and the chimney itself, if made of metal. Insulatj. of the chimney liner in masonry chimneys reduces the heat loss from gases and the infiltration of cold air. 3. Do not use an oversized chimney because larger chimneys produce eooline of the flue gases. ` Chimneys and Draft Calculations 423 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 heat ers since most barometric dampers permit 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 for an intermittently-fired heating device. 9. Do not operate a kitchen exhaust fan unless 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. co2 -PE* CENT Fig. 9. Graphical Evaluation op Rate op 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. 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-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 neating unit it serves. For calculations where the fuel rate and the percentage COi are the only anown factors, the flue-gas rate can easily be determined for coal, oil and sas from Fig. 9. By entering Fig. 9 at the percentage CO,, moving vertiaily to the curve for the type of fuel, and then moving horizontally to the fuel ue"8as rate in pounds per hour may be determined for any uj\ny f the described methods of determining available draft may be chi ' a graphical solution to the problem may be had for the 8 X 8-in. mney from Figs. 6, 7 and 9. This solution can be best explained by a dermal example.