Document VjOkBXdBp1qjd3X44e82zxM8N

American Society of Heating and Ventilating Engineers Guide, 1936 Table 3. Minimum Round Chimney Diameters for Gas Appliances (Inches) Height or Gas Consumption m Thousands or Btu peb Hour Chxunet Fezt 100. 200 300 400 500 750 i 1000 1500 2000 20 4.50 5.70 6.60 7.30 8.00 9.40 10.50 12.35 13.85 40 4.25 5.50 6.40 7.10 7.80 9.15 10.25 12.10 13.55 60 4.10 5.35 6.20 6.90 7.60 8.90 10.00 11.85 13.25 80 4.00 5.20 6.00 6.70 7.35 8.65 9.75 11,50 12.85 100 3.90 . 5.00 5.90 6.50 7.20 8.40 9.40 11.00 12.40 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. When condensation in a chimney proves troublesome, it may be necessary to provide a drain to a dry well or sewer. The cause of the excessive condensation should be investigated and remedied if possible. This may be done by raising the flue temperature slightly or increasing the size of the back-draft diverter. The protection of unlined chimneys has been investigated and the results indicate that after the loose material has been removed, the spraying with a water emulsion of asphaltchromate provides an excellent protection. 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 3 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 accprdance with American Gas Association recommendations. PROBLEMS IN.PRACTICE 1 What are the principle factors influencing the intensity of natural draft? The intensity of natural; draft depends largely upon the height of chimney above the grate bar level and the temperature difference between the chimney gases and the atmosphere. 2 What two kinds of draft need be considered? Natural draft caused by temperature differences, and artificial draft caused by me chanical forcing. 3 B 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; 468 Chapter 26--Chimneys and Draft Calculations 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. 5 a. Name the principal advantages of natural draft. b. Name the principal disadvantages of natural draft. a. Simplicity, reliability, freedom from mechanical parts, low cost of maintenance, relatively long life, relatively low depreciation, operation with no power requirement. b. Lack of flexibility, irregularity, dependence on surroundings, susceptibility to tem perature 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. 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. 10 What arfe 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 The higher the volatile content and the lower the fixed carbon content, the lower the draft loss. _ 12 In what cases will there be 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. 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. 14 Of what significance is the C02 content of stack gases in establishing draft loss? e The C02 content of the exit gases is a measure of the completeness of the combustion and the amount of excess air supplied. Low C02 indicates a high excess of air and hence a high draft loss.