Document kaEDQdo2520Odr1k48X3eOZMb

American Society of Heating and Ventilating Engineers Guide, 1935 plant, they may add as much as 20 or 30 ft to the height. The decrease in the temperature of the gases after they have passed through the economizer has an extremely important effect on the performance of. a natural draft chimney and also upon the performance of a fan. CONSTRUCTION DETAILS 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 of warm-air furnace is connected affect the operation of the entire heating system. Most residence chimneys are built of brick and may be either lined or unlined, but in either case the walls must be air-tight and there should be only one smoke opening into the chimney. Cleanout, if provided, must be absolutely airtight when closed. The walls of brick chimneys shall be not less than 3M in. thick (width of a standard size brick) aiid shall be lined with fire-clay flue lining. Fire-clay flue linings shall be manufactured from suitable refractory clay, either natural or compounded, arid shall be adapted to withstand high-temperatures and the action of flue gases. They shall be of standard commercial thickness, but not less than M in. All fire-clay flue linings shall `meet the standard specification of the Eastern Clay Products Association. The flue sections shall be set in special mortar, and shall have the joints struck smooth on the inside. The masonry shall be built around each section of fining as it is placed, and all spaces between masonry and linings shall be completely filled with mortar. No broken flue lining shall be used. Flue lining shall start at least 4 in. below the bottom of smokepipe intakes of flues, and shall be continued the entire heights of the flues and project at least 4 in. above the chimney top to allow for a 2 in. projection of lining. The wash or splay shall be formed of a rich cement mortar. To improve the draft the wash surface should be concave wherever practical. Flue lining may be omitted in brick chimneys, provided the walls of the chimneys are not less than 8 in. thick, and that the inner course shall be a refractory clay brick. All brickwork shall be laid in spread mortar, with all joints push-filled. Exposed joints both inside and outside shall be struck smooth. No plaster lining shall be permitted. Chimneys shall extend at least 3 ft above flat roofs and 2 ft above the ridges of peak roofs when such flat roofs or. peaks are within-30 ft. of the chimney. The chimney shall be high enough so that the wind from any direction shall not strike the top of the chimney from an angle above the horizontal. The chimney shall be properly capped with stone, terra cotta, concrete, cast-iron, or other approved material: but no such cap or coping shall decrease the flue area. There shall be but one connection to the flue to which the boiler or furnace smokepipe is attached. The boiler or furnace smoke-pipe shall be thoroughly grouted into the chimney and shall not project beyond the inner surface of the flue lining. The size or area of flue lining or of brick flue for warm-air furnaces depends on height of chimney and capacity of heating system. For chimneys not less than 35 ft in height above grate line, the net internal dimensions of lining should be at least 7 x 11M infor a total leader pipe area up to 790 sq in. Above 790 and up to 1,000 sq in. of leader pipe area the lining should be at least 1IM x 11M in. inside. In case of brick fluesnot less than 35 ft in height with no. linings, the internal dimensionS: should be.at, j?ast 8 x 12 in. up to 790 sq in. of leader area, and at least 12 x 12 in. for leader capacities'up to 1,000 sq in. Chimneys under 35 ft in height are unsatisfactory in operation and hence should be avoided. CHIMNEYS FOR GAS HEATING The burning of gas differs from the burning, of coal in that the force which supplies the air for combustion of the gas comes largely from die pressure of the gas in the supply pipe, whereas air is supplied to a bed of 438 , Chapter 26--Chimneys and Draft Calculations ^burning coal by the force of the chimney draft. If, with a coal-burning Vboiler, the draft is poor, or if the chimney is stopped, the fire is smothered 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 comv- |,Ustion 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. i. A study of a typical back-draft diverter (Fig. 9) shows that partial or complete chimney stoppage will merely cause some of the products of cbribustion to be vented out into the boiler room, but will not interfere -with combustion. In facfi gas-designed appliances must perform safely z^:\ r i" ^ Fig. 9. Typical Back-Draft Diverter . under such a condition to be approved by the American Gas Association V Laboratory. Other functions of the back-draft diverter are to protect the .< burner and pilot from the effects of down-drafts, and to neutralize the effects of variable chimney drafts, thus maintaining the appliance ef. - ficiency at a substantially constant value. Converted boilers or furnaces, :f?as well as gas-designed appliances, should be provided with back-draft -diverters. 41/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 -Pfwith just a trace of sulphur trioxide (SOs). Sulphur usually burns to the /^tpoxide in the presence of an iron oxide catalyst! The volume of water ^japor-itt-the flue products is about twice the volume of the carbon dioxide ".when coke oven or natural gas is burned. Because of the large quantity 91 water vapor which is formed by the burning of gas, it is quite important , that all gas-fired central heating plants be connected to a chimney having : ,:\?pd draft. Lack of chimney draft causes stagnation of the products of cpmbustion in the chimney and results in the condensation of a large ...'amount of the water vapor. A good chimney draft draws air into the w,!mney through the openings in the back-draft diverter,, lowers the. dew -iftjwint of the mixture, and reduces the tendency of the water vapor to ^condense. 439