Document G6wkRNz3BwR66z1yrOYya4Lmv

American Society of Heating and,Ventilating Engineers. Guide, 1935 1. The barometric pressure varies inversely as the altitude of the plant above sea level Fig. 5 gives the barometric pressure corresponding to various elevations as computed from the equation: '' ' - OQ 00 ' - , ' ` . . = 62,737 log ' (3) where ' Ei -- altitude of plant above sea level, feet. 1 In general, the barometric pressure decreases approximately 0.1 in. of mercury per 100 ft increase in elevation. 2.-The unit weightofa cubic fool of. chimney gasesat.O. Fand sea leyel_barometric - pressure.is given by the equation: > ........... ' = 0.131CO, + 0.095 Oi +:0.083. N,.... (4). ^Chapter 26--Chimneys and.Draft Calculations . 7 The amount.of gases flouring and being discharged is, of course, equal to the amount -generated.in.the combustion chamber of the boiler.' The total products of "'mbSbn'may be computed from the equation: ' where .-. : v.': w _ CgGWtp , 3600 Cg =' pounds of fuel burned per. square foot of grate surface per hour. ' G'= total grate surface of boilers, square feet. = 'total weight of products of combustion per pound of fuel. (5) In this equation COt, ft and ,N* represent, the percentages of the parts by weight of the carbon dioxide, oxygen and nitrogen content, respectively, of the gas analysis. For ordinary operating conditions, the value of We may be assumed at 0.09. . . .. 3. 'The atmospheric temperature is the actual observed temperature of the outside air at the time the analysis of the operating chimney is made. The mean atmospheric temperature in the temperate zone is approximately 62 F. ., - 4. The chimney gas temperature does not vary appreciably from the gasitempefature as it leaves the'breeching and enters the chimney,' For average operating conditions, the chimney gas temperature will vary between 500 F and 650 F except in the case when economizers and recuperators are used, when the temperature will Vary between'300 F and 450 F. If a chimney has been properly constructed, properly lined and has no iir infiltration due to open joints, the temperature of the gases throughout the chimney will not differ appreciably from the foregoing figures. In most up-to-date heating plants, the temperature may be read from instruments or ascertained from a pyrometer. 5. The coefficient offriction between the chimney gases and a sooted surface has been found to be approximately 0.016. This factor, of course, will be much less for a new unlined steel stack than fora brick or brick-lined chimney, but in time the inside surface of all chimneys regardless of the material of which they are constructed becomes covered with a layer of soot and the coefficient of friction should be the same for all types of chimneys. 6. The length of the friction duct is the vertical distance'between, the bottom of the breeching opening and the top of the chimney. Ordinarily this distance is approximately equal to the height of the chimney above the grate level. 428 ' .Fig. ,6 is a typical chimney performance chart giving the available draft interisities for various amounts' of gases flowing and sizes of chimney. This chart is based on an atmospheric temperature of 62 F, a chimney gas temperature of 500 F, a unit chimney gas weight of 0.09 lb per cubic foot, sea level atmospheric pressure, a coefficient of friction of 0.016, and a friction duct length equal to the height of the chimney above the grate level. These curves may be used for general operating conditions.. For specific operating conditions, a new chart should be constructed from. Equation !,, .. ........................ uas.oeen the usual custom,.and still is to a lamentably great extent, ro. select the required size of a natural draft chimney from a table of 429