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838 CHAPTER 53 1965 Guide And Data-Book there were neither friction nor cooling of the gases as they traversed the chimney. After the ideal draft for a given chimney height and entering fiue-gas temperature from Fig. 3 has been obtained, and the appropriate efficiency value from Table 2 has been selected, the available draft can be com puted from Equation 1. Fig. 3 can be used in reverse in con junction with efficiency values from Table 2 to determine what chimney height is required to produce the required draft at rated output for a given heating appliance. Fig. 2 shows that the efficiency of a given chimney varies with the mass flow rate of gases and with the entering fiue-gas temperature. The efficiency also varies with the height for a given construction and depends somewhat oh the method used to connect the smokepipe to the chimney, and on 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 of metal have been selected from the litera- ture4,l,*-w and summarized in Table 2 for use. with Fig. 3 in determining the available draft produced by short chimneys of IS 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 C of Table 2, the efficiency values listed will yield the available draft for the range of mass flows, inlet flue-gas temperatures, and chimney height shown, within 10 percent. In the application of short chimneys, observance of toe following precautions will assist the designer in obtaining the highest draft practicable and may correct 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 metal. Insulation of the chimney liner in masonry chimneys re duces the heat loss from the flue gases and the infiltration of cold air. 3. Do not use an oversized chimney because larger 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 permit enough cold air to leak into the chimney to reduce toe draft ap preciably. 6. Avoid air ImH 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 beating device. 9. Ek> 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 toe chimney draft will not be required to do the added work of drawing combustion air into the closet from the surroundings. GAS APPLIANCE VENTING Gas-burning appliances approved by toe American Gas Association as vented equipment must be connected to an effective gas vent or chimney. The primary function of the gas vent or chimney is to safely remove the products of com bustion from the space being heated. Factory Built Chimneys Factory built chimneys are listed by Underwriters' Lab oratories for use with all gas appliances as well as all other types of fuel-burning equipment. These should be installed according to the manufacturers' instructions and their listing. Table 2 .... Efficiency of Short Chimneys A. Matonrf Chimneys Height h TT1 " Rue Cat How Rate Ib/hr Infernal liner Size, bw 7 (die) 7,7 i7 K 11 and 7,7 | 10 (dial 6 to 1 Efficiency, Percent 90 75-82 1 65-70 I 65-80 200 83-84 67-85 . 82-88 315 75-89 | 67-86 | 81-89 (Inlet Flue Gas Temp. 200 to 1000 F) fl. Unfnsutoted Metal Chimneys - rnI p 1__ 1 Height . 4 to 8 Roe Gas How Rote Ib/hr Infernal Diameter, far. 6 |8 Efficiency, Percent 90 90-100 j 85-95 200 83-83 315 | 70-80 . (Inlet Flue Gas Temp. 200 to 600 F) C Metal CUsMyi4 m Height CJr ! Uninsulated 1-in. Inwlotioa with dead air tpace around dibuney 1-in. gtsotation wflh 1-in. open air (pace around dieaney Efficiency, Percent 8 to 14 76 - 81 * Stx-trKh *imTmy not ---w"*^***^ far theee flow rates. b Internal die fl-ia.; Sue (U flow rate SO to 126 Ib/hr; inlet floe gta tetnp. 200 to 1000 F. Brick and Meta! Chimneys Masonry and metal chimneys may be used with gas ap pliances as well as all other types of fuel-burning equipment. Brick chimneys should be lined to aid prompt establishment of draft and to prevent damage to the chimney by the water vapor that condenses from the flue gases. Type B Gas Vents Type B gas vents are listed by Underwriters' Laboratories, and are recommended for venting all vented gas-burning ap pliances except incinerators, appliances which may be con verted readily to the use of solid or liquid fuel, and combina tion gas-oil burning appliances. Type B*W gas vents are listed by Underwriters' Labora tories, for use with A.GA. approved vented recessed heaters. Specific installation clearances may be found on listed type B and B-W vents in the current Gas & Off Equipment List publishedby Underwriters' Laboratories. Complete installation instructions for gas vents will be found in NFPA No. 54 or ASA Z21.30--1959 Standard far the Installation cf Gas Appliances and Gas Piping, (National Fire Protection Association or American Standards Associa- Chimneys and Draft 839 fig. A .... Capacity in Btu per Hour for Gas Vents or Chimneys Serving More Than One Appliance tion). The most recent report11 of the American Gas Associ ation should also be consulted. >The minimum flue or vent sizes adopted by the American Standards Association and the National Board of Fire Under writers in 1950 for gas appliances14 are shown in Fig. 4/ Additional provisions of ASA Standard Z21.30--1959 re lating to chimney size are: 1. The gas vent or chinmeyjwhen connected to a single appli ance shall oe not less than theorize of the draft hood outlet. 2. When more than one appliance is connected to a gas vent or fig. 5 .... Capacity of a Rectangular Rue or a SemiElliptical Flue, with Semi-GraHar Bids Having Its Minimum Width Equal to. the- Diameter of-a . Grcular Hue, Compared with the Capacity of the Grcular Flue chimney, the area of toe passageway "hall be not less than the area of toe largest vent connector plus 50 percent of the areas of additional vent connectors. 3. Any shape of gas vent may be used, provided its venting capacity is equal to toe capacity of round pipe for which it b sub stituted, and the minimum internal dimension of the gas vent b not leas than 2 in. Since Fig. 4 has been prepared for circular flues, relative capacities for rectangular and semi-elliptical flues" are shown in Fig. 5. Heating appliances designed to bum gas, as well as appli ances converted to gas burning, except those equipped with power-type burners and excepting conversion burner installa tions in excess of 400,000 Btu per hr input, are usually equipped with a draft hood attached to the flue outlet of toe appliance. This draft hood is required if the appliance is to meet the approval requirements of the American Gas Association and the American Standards Association, and is essential for safe operation. It is designed to prevent excessive chimney draft which would lower appliance efficiency, to prevent a blocked flue or a downdraft in the chimney from impairing combus tion, to provide a relief opening for toe products of combustion during downdraft or blocked-flue conditions, and to prevent spillage of the products of combustion to the space surround ing the appliance, if there is a chimney draft equivalent to that provided by a 3-ft chimney. Because the draft hood is H<*rigniH without moving parts, the relief opening is always open, and consequently some air is drawn into the chimney. This air lowers the gas temperature in the chimney, but it also lowers the dew point of the gases and tends to prevent condensation. The installation of conversion burner equipment in large boilers is usually made in accordance with regulations of the local gas company. In such installations a definite chimney draft may be required for proper combustion, and the fore going reference to the use of draft hoods would not apply. Sealed combustion chamber appliances are so constructed and installed that all air for combustion is drawn from out doors and all combustion products are discharged to the out ride atmosphere. With the air intake and combustion product discharge located adjacent to each other in the same pressure zone, location of the terminal on a ride wall does not usually present a problem. The products of complete combustion of gas are water va por and carbon dioxide. In toe case of manufactured gas, the presence of organic sulfur compounds, generally between 3 and 15 grains per hundred cubic feet, gives rise to minute percentages of sulfur dioxide and sulfur trioxide. The volume of water vapor in the flue products from natural or coke oven gas is about twice the volume of carbon dioxide. It is ex tremely important that the chimney be tight and resistant to corrosioq, not only from moisture, but alan from dilute sulfur trioxide. Clay linings with joints which prevent retention of mois- ^ ture, and linings made of noncorrosive materials, are advan tageous. The interior protection of unlined chimneys by ap plying an asphalt-chromate emulsion has been investigated by some gas companies, but toe material is not of current in-, terest because insurance companies do not favor potentially combustible coatings.14 An investigation of damage .occurring in 78,129 chimneys used for venting gas appliances in a severe climate has been made by the Minneapolis. Gas Company, with toe results shown in Table 3 taken from Reference 14. Advice regarding recommended practice and materials for flue connections and chimney linings can usually be obtained. from the local gas company and should be given careful[coh^ rideration.