Document MG6Q8V2KJKGQgKE1k884g9VaM

Heating Ventilating Air Conditioning Guide 1938 As is the case with the complete combustion of almost all fuels, the products of combustion for gas are carbon dioxide (COi) and water vapor with just a trace of sulphur trioxide (SOj). Sulphur usually burns to the trioxide in the presence of an iron oxide catalyst. The volume of water vapor in 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 of 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 a good draft. Lack of chimney draft causes stagnation of the products of combustion in the chimney and results in the condensation of a large amount of the water vapor. A good chimney draft draws air into the chimney through the openings in the back-draft diverter, lowers the dew point of the mixture, and reduces the tendency of the water vapor to condense. 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 Table 4. Minimum Round Chimney Diameters for Gas Appliances (Inches) Chxmmbt Feet 100 Gab Consumption in Thousands op Btu per Houb 200 300 400 500 750 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 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. Local practice should be followed in the selection of the most appropriate flue materials. 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 diver.ter. 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 110 openings at the joints. Cement mortar should be used for the entire chimney. Table 4 gives the minimum cross-sectional diameters of round chim- 198 Chapter 10. Chimneys and Draft Calculations 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. 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 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. 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 beouning a nuisance. 5 What is the direct influence of the height on the design of a chimney? The immediate purpose of height is to provide that draft intensity under the conditions of chimney gas temperature such that it will be adequate to overcome all the frictional resistances of the installation, as well as to provide for the actual gas movement. 6 Of what importance is chimney cross-sectional area in stack design? The area should be as large as is economically feasible in order that the frictional loss for the chimney height should not destroy the effectiveness of the height-created draft in overcoming the necessary frictional resistances of the boiler and its flue connections. 7 Of what importance is the Reynolds number in chimney* design? It permits the selection of a more specific value of the chimney friction factor, rather than a general one, to correspond with conditions of size, nature of the gas, rate of gas flow, and condition of the surface. 8 a. Name the principle advantages of natural draft. b. Name the principle disadvantages of natural draft. fl. 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. 9 How is mechanical draft created? By forced draft, by induced-draft fans, or by a Venturi chimney. I 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. 199