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American Society of Heating arid 'Ventilating Engineers Guide, 1935 Chapter 26---Chimneys and Draft Calculations A chimney for a gas-fired boiler or furnace should be constructed in accordance with the principles applicable to-other boilers. Where the wall forming a smoke flue is made up of less than an 8-in. thickness of brick, concrete, or stone, a burnt fire clay flue tile lining-should be used Care should be used that the lengths of flue tile meet properly with no openings at the joints. Cement mortar should be used for the entire chimney. Table 2. Minimum Round Chimney- Diameters for Gas Appliances (Inches) - -Height ot Chimney Frt ___. 100 . ,,Gas Consumption, in Thousands or Btu feb Hocb - 200 300 400 500 750 1000 1500 ' ' 2000 5 # at Name the principal advantages of natural draft. - b. Name the principal disadvantages of natural draft. Simplicity, reliability, freedom from mechanical, parts, low cost of maintenance, relatively long life, relatively low depreciation, operation with no power requirement. h Lack of flexibility, irregularity, dependence on surroundings, susceptibility to tem- ' paature changes. ' ' ' ............ , -^ 6 * How is mechanical draft created? By forced draft, by induced-draft fans, or by a Venturi chimney. 7 Distinguish between theoretical and available draft.___ - Theoretical draft is the difference in pressure inside and outside the base of a chimney when it is under operating temperatures but when there are no gases flowing. Available draft is less than theoretical draft by the friction loss due to the flow of gases through the chimney. 20 4.50 5.70 6.60 7.30 8.00 9.40' 10.50 12.35 13.85 40 60 4.25 4.10 5.50 5.35 6.40 7.10 6.20 . 6.90 -7.80 - 9.15 7.60 8.90 10.25 10.00 12.10 11.85 13.55 13.25 4 V 80 jj-100 4.00 3.90 5.20 5.00 6.00 5.90 6.70 7.35 6.50 . 7.20 8.65 8.40 9.75 9.40 11.50 11.00 12.85 12.40 V: - Table 2 gives the minimum cross-sectional diameters of round chim neys (in inches) for various amounts of heat supplied to the appliance, and for various chimney heights. This is in accordance with American Gas 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 gas requires 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. i I 8 Explain the term efficiency of a natural draft chimney. The efficiency of a chimney is the ratio of the work it does in moving gases to the theo retical amount of power it generates. 9 How is the available draft used in a heating plant? The available draft at the base of the chimney is used to overcome the loss in pressure through the grate, the fuel bed, the boiler passes, the breeching, and the chimney. 101 What are some of the factors that influence the draft loss through the fuel bed? Uniformity and size of coal, the amount of ash mixed with the fuel on the grate, thickness of fuel bed, rate of combustion, amount of air supply as related to the coal burning rate. 11 How does the volatile matter content affect the draft loss through the fuel bed? The higher the volatile content and the lower the fixed carbon content, the lower the draft loss. 12 In what cases will there he no fuel bed draft loss? In oil, gas, and powdered fuel firing the fuel is mixed and burned in suspension; con sequently, no measurable resistance is encountered in the combustion zone. PROBLEMS IN PRACTICE 1 What is .draft? Draft is* an unbalanced pressure` between the atmosphere and the passages in the ap paratus or construction through which the gases flow. 2 What two kinds of draft need be considered? Natural draft caused by temperature differences, and artificial draft caused by me chanical forcing. 3 What is the effective height of a chimney? The height from the grate level to the top of the chimney is the effective height in pro ducing natural draft. 13 Is it possible to state an average value for the draft loss through a boiler . and its setting? No. The draft loss varies widely and depends on many factors such as the size and type - of gas passageways. ' The manufacturer is usually able to supply such information. H Of what significance is the CO, content of stack gases in establishing draft loss? The COi content of the exit gases is a measure of the completeness of the combustion and the amount of excess air supplied. Low CO, indicates a high excess of air and hence a high draft loss. 15 What two effects does an economizer have on the draft loss? .-An economizer offers resistance to the flow of gases over the added surfaces; it lowers the temperature of the gases going to the chimney and therefore decreases the available draft. This decrease often necessitates the addition of forced draft. 4 What dual purpose does a tall chimney fulfill? A tall chimney primarily creates the necessary draft to move the air required for the combustion process and to move the products of combustion, and secondarily it dis charges the gases at a high elevation to prevent them from becoming a nuisance. 440 T6 What main provisions should be considered in good chimney construction? Chimneys should be air-tight and connected to only one smoke opening. The chimney P should be high enough above surroundings so the wind will not strike it at any angle 441