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American Society of Heating and Ventilating Engineers Guide ** that would be furnished for coat-fired boilers. In determining the hecare must be taken to design a stack that will give a sufficient draft the maximum requirements of the boiler, but no more. In this wav efficiency is safeguarded, and the danger of a draft which will draw large an excess of air, is eliminated. Table 7, based on a similar table calculated by C. R. Weymm . (TransactionsA.S.M.B., Vol. 34,1912), from actual test data, will befoun to give satisfactory results under the assumed conditions as given. . CHIMNEYS FOR GAS FIRED APPLIANCES The burning of gas differs from the burning of coal in that the f0r, which supplies the air for combustion of the gas comes largely from tM pressure of the gas in the supply pipe, whereas air is supplied to a bed cl burning coal by the force of the chimney df"aft. If, with a coal-burning Table 7. Maximum Capacities op Stacks in Boiler Horsepower for Oil Fuat i1 ..,_ji. Figures represent nominal rated horsepower; sizes as given are based on 50 per cent overloads asd centrally located stacks, short direct dues and ordinary operating efficiencies. boiler, the draft is poor, of if the chimney is stopped, the fire is smothered and the combustion rate reduced. In a gas boiler or furnace such a condition would interfere with the combustion of the gas, but the gas would continue to pass to the burners and the resulting incomplete com bustion would produce a dangerous condition. In order to prevent incom plete combustion from insufficient draft, all gas-fired boilers and furnaces should have a back-draft diverter in the flue connection to the chimney. A study of the drawing of a typical back draft diverter will make it plain that a stoppage in the chimney will cause the combustion products to come out into the boiler room, but there will be no interference with the complete combustion of the gas. Another feature in the construction, of the back-draft diverter is that a down draft from the chimney will have practically no effect upon the combustion of the gas, due to the baffle deflecting the draft into the boiler room. As is the case with the complete combustion of almost all fuels, the products of combustion for gas are carbon dioxide (CO,) and water vapor 238 Chapter 14--Draft and Chimneys Ejhjtiusi. a trace of sulphur trioxide (SO,). Sulphur usually burns to the Kggde in the presence of an iron oxide catalyst. The volume of water Wis the flue products is about twice the volume of the carbon fiifde when coke oven or natural gas is burned. Because of the farce r Quantity of water vapor which is formed by the burning of gas, it is quite yggrtant that all gas-fired central heating plants be connected to a PfgSfmiey having a good draft. Lack of chimney draft causes stagnation lt products of combustion in the chimney and results in the con- -Sensation Sensatiaonir omi,nfnattoo lattrhhgoeee acchhmiimmounnneetyyottfhhtrrhooeuuggwhha^tetthhreevaoopppoeernn. iinnAggssgoimnodtuhcreehimbnaecyk-ddrraafftt fi'^jer, lowers the dew-point of the mixture, and reduces the tendency _-- ' divAAelirecctehhwrii,mmaltonneweer yyevraffsoop-rro--raa--t-ogg--aa-c-ss-o--nffiidrreeeddnsbbeo.oiifleerr oorr ftuurrnnaaccee ssnhoouumld wbe co_n_s_t_ru__c_te__d..i.n accoccordance with ^the -p"r"in'c'i"plleess aDppDlliiccaable to other boilers. Where the tfall forming a smoke flue is made up of less than an 8 in. thickness of b&r1icJk, concrete or stone, a burnt fire clay flue tile lining should be used Corupwrmiecn*-i,nssg--hhsoouuallddt be used thatjthe lengths of flue tile btheeujsoeindttsh.atCmeme eienut gmui&ortar should meet properly with nc bhpe used for the ennttiirree chimney. Tabus 8. Minimum Round Chimney Diameters for Gas Appliances (Inches) 100 4.50 4.25 4.10 4.00 3.90 Gas CotrauHWHW w Thousands or B.t.d. ran Horn 200 5.70 5.50 5.35 5.20 5.00 300 6.60 6.40 6.20 6.00 5.90 400 7.30 7.10 6.90 6.70 6.50 SCO 8.00 7.80 7.60 7.35 7.20 750 9.40 9.15 8.90 8.65 8.40 1000 10.50 10.25 10.00 9.75 9.40 1500 12.35 12.10 11.85 11.50 11.00 2000 13.85 13.55 13.25 12.85 12.40 Table 8 gives the minimum cross-sectional diameters of round chim beys (in inches) for various amounts of heat supplied to the appliance, and for various chimney heights. This is in accordance with American Gtu,Association recommendations. ' The flue connections from a gas-fired boiler or furnace to the chimney should be of a non-corrosive material. In localities where the price of gas1requires the use of highly efficient appliances, the material used for the'flue connection not only should be resistant to the corrosion of water, but. should resist the corrosion of dilute solutions-of. sulphur trioxide in water. Sheet aluminum, as well as some other materials, seems to serve this, purpose very well.' , ;1 CONSTRUCTION OF CHIMNEYS ;..For general data on the construction of chimneys reference should be -made to the Standard Ordinance for Chimney Construction of the ^National Board of Fire Underwriters. Briefly summarized, these provisions are as follows for heating boilers and furnaces: .--.The construction, location, height and area of the chimney to which a heating boiler or warm-air furnace is connected affect the operation of the entire heating system. Most "ldence chimneys are built of brick and may be either lined or unlined, but in either 239 s