Document 44MYZzBYMQyZeqJYdB19w4RNG

840 CHAPTER 53 ` Table 3 .... Results of Held Survey on Chimney Damage Type ot Coattruciioa of Reported Percent Chuaeeyi Damaged Chimneys without liners........... 4720 Chimneys with sewer tile con- struction.................................... 8410 Wilder metal coated with as- phalt chromate emulsion___ 5185 Steel coated with plastic........... 9200 Vitreous enamel on steel........... 614 Alnminrrm .................................. -50,000 .562.- 1E9 104 1.21 291 5.6 1928 21.0 No record .-- 228 `' 0/45 ' 1965 Guide And Data Book - AVAILABLE DRAFT FOR THE INDUSTRIAL CHIMNEY The available draft, for large chimneys and stacks has been estimated with apparent satisfaction in the past by means of formulas which in effect deduct an estimated fric tion loss from a theoretical draft determined as in Equation 2. .The friction loss can be estimated by means of one of the formulas available for ducts, such as the Fanning equation. This.procedure results in formulas for the available draft-as follows: The first terms represent the theoretical draft and the second terms represent the friction loss. ... For a cylindrical stack: (2) and for a rectangular stack: -in rrn ( * 2.86 O-00KMS>TMv* +---id-<' 3): tchere D, -- available draft, inches water. '>' H -- height of chimney above inlet^ feet. . B. -- eriiding barometric pressure, inches of mercury. ^ -- density of air at 0 F, 1 atmosphere pressure. fit -- density of-flue gas at 0 F, 1 'atmosphere presure. .7',;- temperature of atmosphere, Fahrenheit, absolute. T, -- average temperature of flue gas,, Fahrenheit, abso- . v lute. IF -- flue gas flow rate,.pounds per second. /"`coefficient of friction^ = length of friction duct' (approximately equal to H); - feet. ) .. . d -- minimum diameter of round chimney, feet. .: , x and y = length and width of cross-section of rectangular chimney, feet.- The following notes facilitate the use of Equations 2 and 3. -1.' The barometric pressure, represented by B,, is the actual pressure at the site of the chimney and not the pressure reduced to sea level datum. In general, the barometric pressure decreases approximately 0.1 in. Hg per 100 ft increase'in elevation. 1 ' *' 2. The tmil vxiqhi of a cubic foot of chimney gases at 0 F and sea level barometric pressure is given by the equation:. > V; p, - 0.131CG, '+ 0.0950* + 0.083N, ' /(4) Io this equation, COt, 0*, end IV* represent the percentages of the parts by volume of the carbon dioxide, .oxygen, and nitrogen contort, respectively. For ordinary' operating conditions, the value of p, may be assumed at 0.09. -i ' The .effect on-the chimney gas density, of .superheated water vapor.'resulting from moisture, and hydrogen in the fuel, or of any air infiltration in the chimney prop, is disregarded.* All though water vapor content is not disclosed by Oreataaalysis,' Fig. 6.... Typical Set of Operating Characteristics'of a Natural-Draft Chimney its presence tends to reduce the actual weight per cubic foot of chimney gases. 3. The atmospheric temperature is the actual observed tem perature of the outdoor air at the time the analysis of the op erating chimney is made. The mean atmospheric temperature m the temperate tone is approximately 62 F. 4. The chimney gas temperature decreases from the breeching connection to the top of tne chimney. This drop in temperature depends upon the material and construction of the chimney, its tifftitnewB or freedom from leaks, its area, its height, and the veloc ity of the gases through it. The same chimney will suffer different temperature losses, depending upon the capacity under which it is working'and the: variable atmospheric conditions. - No general equation covering all these variables has been suggested. It has been rmmtmwd by some authorities that the temperature drop will average about 1 F deg per foot of chimney height in steel chim neys, and } to i F deg per foot of height in brick lined, brick and concrete chimneys.11 ., 5/ The coefficient of friction between the chimney, gases and a sooted surface hiwjt been taken by many workers in this field as a constant Value of 0.016 for the conditions involved.'This value, of course,'would be less for a new,unlined steel chimney than for a brick or bride-lined chimney, but, io time, the inside surface of all chimneys, regardless of the materials of construction, becomes' covered with a layer of soot. Therefore, the coefficient of friction has been assumed the same for all types of chimneys, and gener ally constant for all conditions of operation. For reasons of sim- Slicity and convenience to the; reader, this constant value of .016 has been employed in the developmentof the various special equations and charts shownin this chapter. In important large chimney design, especially when the con struction or the materials are unusual, it is recommended that use be made of the Reynolds number1* in determining the fric tion factor, /. The foDowing problem illustrates the use of Equation 2. Example 1: Determine the available draft of'a natural draft chimney-80 ft is bright and 3 ft in diameter, operating under the following conditions: atmospheric temperature, 62 F; chim ney gas temperature, 500 F; - sea level atmospheric pressure, B, -- 29.92 in. Hg; atmospheric and chimney gas density, 0.0S63 and 0.09, respectively; coefficient of friction, 0.016; length of friction duct, 80 ft. The chimney discharges 3.15 lb of gases per Solution: Substituting these values in Equation 2 and reducing, (0.0863 0,09\ I X 29.92 X ( 522 960/ - 0.00126 X 3.15* X-960 X 0.016 X 80 3* X 29.92 X 0.09 = 0.507 - 0.024 - 0.483 Fig. 6 shows the variation-in the available draft of a typical 80 ft by 3 ft chimney operating under the general'conditions noted in Example 1. When the chimney is under static con- Chimneys and Draft - - 841 ae at i.o i.i 1.2 1.3 i. 1.9 1.0 1.7 i.e; >.9 u u u u WEIGHT OF CASES PER tOOO MTU RELEASED ClHOLDING MATER WKfl)-ta Heavy 08 (by wl) Fvel Aaatyte* Ught 08 (by wt) ; .: - /;, ' ' Soft Coat (by wtj To solse a typical example: Proceed horizontally from a _ Wright Flow Rate point to intersection with diameter line; from this intersection follow vertically to chimney height line; from this intersection follow horizontally to the nght to Avail able Draft scale. Starting froma point of Available Draft, take steps in reverse order. Fig. 7 .... Chimney Performance Chart C=0.856 3-0.023 H--0.097 ^-0-020' C--0.834 8-0.006 ' 0=0.013 N--0.147 - C--0.6958 S--0.0396 0-0.1069 H =0.0519 Ash-0.0012 *. N --0.0120 Ash-0.0938 Btu/lb --18,300 Btu/lb -19,580 Btu/lb --12,847 ditions and no gases are flowing, the available draft is equal Nat Gas Mfd Gas* to 0.507 in. of water, the theoretical intensity. As theamount , Notes of gases flowing increases, the available draft decreases (by vol) . (by vol) until it becomes zero at a gas flow of 14.6 lb per second, at which point the draft loss, due to friction, is equal to CH,=0.9416 CH, =0.282 * Do not use Mfd the theoretical intensity. The point of maximum draft and C*H*--0.0030 H-0.252 Gas data for zero capacity is called shutioff draft, or point of impending H-0.0170 CO-0.139 . ,, any gas except delivery, and corresponds to the point of shut-off head of a CO-0.0055 . I1L -0.047 with analysis centrifugal pump. The point of zero draft and maximum COi-0.0029 CO,--0.053 - similar -to that capacity is called the wide open point, and corresponds to 0-0.0030 N--0.221 shown. the wide open point of a centrifugal pump. A set of operat N-0.0270 b Specific gravity ing characteristics may be developed for any size chimney.. =0.678. operating under any set of conditions by substituting the . proper values in Equation 2 and then plotting the results in .. Btu/lb --22,330 Btu/lb-10,010* the manner shown in Fig. 6. \ Fig. 7 is atypical chimney performance chart, giving the available draft for various gas flow rates and sizes of chimney. This chart is based on an atmospheric -temperature of 62 F, an fig. 8 .... Graphical Evaluation and Rate of Hue Gas Flow from Percent CO* and Fuel Rate average chimney gas temperature of 500 F, a unit chimney - gas weight of 0.09 lb per cu ft, 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 point of entry of the Sue gases. These curves may be used for general operat ing conditions. For specific conditions, a new chart may be draft loss through the boiler is 0.28'in., and the draft loss in the breeching is 0.08 in.,' making a total draft requirement of 0.48 in. water. Tne average stack temperature is 500 F, and the ambient temperature is 62 F. 3- Solution: From Fig. 8, using the heavy oil curvejdetennine 1.03 . lb gases per 1000 Btu released at 12 percent CO*. Then: prepared from Equation 2. The usual problem encountered by the designer is the selec- tion of a chimney size, as illustrated by Example 2. Example S: Select a~natural dfaft.chimney for a boiler with, fuel input 73 gpb of 151,000 Btu per gal heavy oil, burned at 12 priceat CO*. Tne draft tension required in the firebox is 0.12 in.;` flue gases produced per second. Several combinations of chimney diameter and height wm satisfy the draft and fiue gas flow, re quirements. The dashed line in Fig. 7 indicates that a '36.in. diameter stack 80' ft high will be satisfactory. The use of a