Document ppkxER7o8eOgzGpNrEo25mQjj

444 CHAPTER 17 1957 Guide 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. 8, and Table . 1. Fig. 8 shows the rela tionship between effective chimney height, inlet flue gas temperature, and the ideal draft that would be produced for two values of outside temperature 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. 8 and selecting the appropriate efficiency value from Table' 1, the available draft can be computed from Equation 13. Fig. 8 can be used in reverse in conjunction with efficiency Fiq. 8. Ideal Dbaft fob Shobt Chiunets at Outside Temperatubes of 0 F and 60 F. values from Table 1 to determine what chimney height is required to produce the required draft at rated output for a given heating appliance. Fig. 7 shows that the efficiency of a given chimney varies with the mass flow rate of gases and with, the entering flue gas temperature. The effi ciency 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 au. 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 literature-7 8-9 and summarized in Table 1 for use with. Fig. 8 in determining the available draft produced by short chimneys pi 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 temp^ri ature increases over the applicable range. In part C of Table 1 the etn' ciency values listed will yield the available draft for the range of ma; flows, inlet flue gas temperatures, and chimney height shown within ,l.,_>. percent. . In the application of short chimneys, observance of the following P1*" Chimneys and Draft Calculations 445 cautions will assist in obtaining the highest practicable draft and may cure unsatisfactory operation of the chimney and heating plant in certain in stances: 1. Use a minimum length of horizontal smokepipe between the heater and chimney. 2. Insulate the smokepipe and the chimney itself, if made of metal. Insulation of the chimney liner m masonry chimneys reduces the heat loss from the flue gases and the infiltration of cold air. Table 1. Efficiency of Shobt Chimneys A. Masonbt Chimneys T~| " __ 1^ Height k ft Flue Gas Flow Hate lb/br Internal I4iwt Sixe, in. Dia. 7 7x7 7 x 11 and Dia. 10 5 to 15 Ef xciency, Pencent 90 75-82 65-76 65-80 200 83-84 67-85 8-88 315 75-89 67-86 81-89 (Inlet Flu Gas Temp 200 to 1000) B. Uninsulated Metal Chimnetb -- lO Height h ft Flue Gas Flow Rate lb/hr -C Internal Diameter, in. 8| 8 J Efficiency Percent 4 to 8 90 90-100 85-95 200 a 83-88 315 a 70-80 (Inlet Flue Gas T amp. 200 to 600 F) C. Metal Chimneys** [IT I.in. insulation 1-in. insulation ^ Height k ft Uninsulated with dead air with 1-in. open space around air spaoe around chimney chimney Efficiency, Percent 8 to 14 76 85 k IntS2i *Sin,J17y I0* recommended for these flow rates, internal dia. 6-in; flue gas flow rate SO to 126 Ib/hr; inlet flue gas temp. 200 to 1000 F. 81 nrxuFUvbruf,an oversised cnimney because larger chimneys produce greater cooung of the flue gases. * ^?1LidoT?drafts by proper construction above the roof or by the use of a suitable chimnev cap. the / barometric dampers in the smokepipe of natural-draft heat- rS1JlcefnostY aron?efcric dampers permit enough cold air to leak into the 6 A * reduce the draft appreciably. 7 A V^ air leaks in the smokepipe and chimney. 8 Us co"nect.in8 more than one heating device to a single chimney. in^JioM chon.mater!aIs of ,ow beat capacity if the chimney draft must 9. Do n6 qmckly f?r an intermittently-fired heating device. tn oprate a kitchen exhaust fan unless an air intake of ample area is used 10. A ow.e.nn8 the house pressure. adripH 1,1 l u.?H*ty closets so the chimney draft will not be required to do the ea work of drawing combustion air into the closet from the surroundings.