Document 5kdr5D6JyXa5vx95rLQNqRpZD

278 CHAPTER 18 1965 Guide And Data Book 10 or 11, tile excess air quantity may be determined by sub tracting tin; quantity of dry flue gases which would result from stoichiometric combustion. Computations of-flue losses, are described in the next section on Energy Balance; Application of the preceding equations and tables are il lustrated by Examples 1 and S. Example 1: The analysis of the flue gases resulting from the burning of a natural gas is 10.0 percent CO*, 3.1 percent 0,. and 88.9 percent N*by volume.-The'analysis of the fuel is 90 percent CHi, 5 percent N, and 5 percent C*H by-volume; Find U the martmnm theoretical percent CO* and the. percent. excess air. Solution: From Equation 9, (100)000) V 1L8% CO* From Equation 8, Percent Excees Air (112 - 10J) X 90 102 10 Example t: For the analysis in Example l And, per cubic foot of fuel gas, the cubic feet of- dry air required for com bustion, the cubic feet of each constituent in the-flue gases,- and the total volume of dry and wet flue gases. Solution; From Equation 3 (or Table 6) .the volume of dry air required for combustion is: (9.53) (CH,) + (16.68)(C*H) - 9.53 X 0.90 + 16.68 X 0.05 - 9.41 cu ft/cu ft gas. ? - From Table 5, the constitutents per cubic foot of. flue gas are: Nitrogen, N*: . From methane = (09; CHi)(9-53 -- 20) -- 6.78 From ethane - (005 C^H.)(16.68 - 3.5) = 006 . Nitrogen in fuel. = ... 005 Nitrogen in excess air = 0.791 X 0162 X 9.41 = 120 . Total Nitrogen : 809 cu ft : Oxygen, ft : , Oxygen in excess air = 0209 X'0162 X 9.41 = 022 cu ft' Carbon dioxide. COi : From methane =. (09 CH)(10) = 090 . .i . From ethane- - .(005 CiH.)(40/20) - 0.10 = Total Carbon Dioxide 1O0 cu ft Water vapor. H0 (does not .appear in Oreat analysis): From methane * (0.9 CH)(2.0) * 1.8 : .Fromethane - (0.05C*H)(6.0/2.0), - 0.15 .. . - Total water vapor * 1.95 cu ft '7 Total volume of dry gas per cubic foot of gas: , 809 + 022 + 100= 10O1 eu ft Total volume of wet per cubic foot of gas (neglecting ' water vapor in combustion air): 1001 + 195 * 1196 cu ft ; V The cubic feet of dry flue gas per eubie foot of fuel gas may also be computed from Equation 11 as follows: ' - (100)000) . ^ENERGY BALANCE The ngiftl practice in analyzing the performance of- heat ing appliances is to. make an accounting, insofar as possible, of the disposition of all'heat units available in the' quantity of fuel burned. This accounting is called an energy balance. Various components of this balance,are generally, expressed in terms of Btu per pound of fuelburned, or as a percentage of its, heating value. CoOipyuenb. of special interest are listed as items 1 to 7:includve. . 1. Heat transferred to heating medium computed as the prod uct of the rate of flow and the enthalpy change. 2. Heat loss in- the dry chimney gases. *= - t,) (12) 3. Heat loss in water vapor in products formed by combustion. 9* " fWfci -- (*/)*! (13)` 4. Heat loss in water vapor in the air supplied. Ji"M1|(i)w-(il)t.) . 5. Heat loss from incomplete combustion. . !`-I0143C (co^Tco). (14) > 6. Heat loss from unhurried carbon in the ash or refuse. *- 14600 (m~c) - o 7. Radiation and all other unaccounted for losses. Radiation and * convection losses from .a heating appliance are not usually determined by diiect measurement.- For this reason they, together with any other losses not measured, are determined by subtracting the total of items 1 to. 6-from the heat of combustion of the fuel. If the. heating appliance is located within the heated spaced' however, raJitm and convection losses may be considered - as useful heat rather than lost heat. They may, therefore, be omitted from culmila- tions of heat losses, or added to item 1. If there is CO in the-flue gases, small amounts of-unburned -hydrogen hydrocarbons will probably also be ,present. The mmll lnsgp due to incomplete combustion of these latter -would also be included in item 7. Symbols used in Equations 12 to 16 inclusive are:. qi = heat loss in* the dry chimney gases,' Btu per pound' of fuel. qt = heat loss in water vapor .from combustion of hy drogen, -Btu per pound ot fuel. qt = heat loss in water -vapor in combustion air, Btu per pound of fuel. qi = heat. loss from incomplete combustion of carbon, Btu per pound of fuel. . qt heat loss from unbumed carbon in the' ash, Btu per pound of fuel. , ;. ! - (&()m " enthalpy of saturated steam at oombustion air! tern-. . perature, Btu per pound. 1 :;(A)r enthalpy of superheated steam at gas temperature, and 1 psia, Btu per pound.; * . . .- - (hj)u enthalpy, of saturated water at<air. temperature, Btu per pound. . ; . " weight of dry flue gas per pound of fuel (from Equa- , tion 10), pounds. Cfg ** mean specific heat of flue gases at constant pressure . - . (c, ranges from 0242 to 0-254.for.flue gas tempera tures from 300 F to 1000 F),' Btu per.pound. (, = temperature of flue gases at exit of heating device,! Fahrenheit. : U temperature of combustion air, Fahrenheit. - H* -- percentage of hydrogen in fuelby weight from ulti- ' mate analysis of fuel burned: < , . ,., - . M " humidity ratio'of combustion air, pounds of.water' .- vapor per pound of dry air: .. > fueb and Combustion 279 m 'a weight of combustion- air per pound of fuel used, ~~ pounds, from Equations 2, 4, 5, 6, 7 and 8. ' qq OO, a, percentages of CO, CO* in flue gases by volume. ' C *=' wei^t of carbon burned per pound of fuel corrected for carbon in ash, pounds. WC, '- W,C. <7 100w (17) where 0. " percentage of carbon in the fuel by weight from the - - ultimate analysis. y a weight of ash and refuse, pounds. Cpercent of combustible in ash by weight (combustible in ash is usually considered to be carbon). W = weight of fuel used, pounds. Flue-gas loren* for solid and liquid fuels, listed as.items 2, 3 and 4 of the heat balance, may be determined with suffi cient precision for most purposes from curves shown in Fig. 1, if COi content and temperature of flue gases are known. Values of the losses plotted for fuel oil were computed from the ultimate analysis of a typical fuel oil used in domestic burners, while those presented for the several ranks of coal were computed from typical ultimate analyses - shown in Table 1. Curves for medium-volatile bituminous coal may be used for high-volatile bituminous coal with negligible,error. Utilization of gaseous fuels, for numerous reasons, is gen erally a'more simple process than is the case with either solid or liquid fuels. Accordingly, the determination .'of a practical beat balance is also a more simple procedure ih.that items 5 and 6 do not generally apply to-gas installations. A series of typical alignment charts is combined in .Fig.. 2 for use-in determining flue losses of items-2, 3 and 4 from common types of gas burning appliances. To determine flue' losses place s straight edge extending from'the .corrected' temperature reading to the percent. CO* recorded. -Percent flue loss is indicated - where the straight edge intersects the flue loss column. The operating efficiency of a gas appliance can then be computed with sufficient precision by application of Equation 18. Percent Combustion Efficiency = / Gross Btu of fuel \ _ ftotal flue losses per\ \gns per cubic foot/ \ cubic foot fuel gas ) Gross Btu of fuel gas per cui^ic foot ^ Reference to Tabled will show that ultimate CO* percent-, age values of fuel gases -vary. While personal errors involved w CO*, temperature, and . chart. determinations, would doubtless more than offset any inaccuracies due to universal use of the alignment charts shown, precise laboratory work may require a more exact method. For more complete in formation the reader is referred to Combustion, 3rd Edition, and Gaseous Fuels, (published by American Gas Associa tion) and particularly to tables covering various properties of different commercial gases included in these publications. CONDENSATION AND CORROSION Sulfur dioxide or - sulfur trioxide, formed by the combus tion of sulfur in fuels, are the principal corroding substances m flue gases. They become active whenever sufficient mois ts is present for the formation of sulfurous or sulfuric *d-* Traces of vanadium are occasionally found in residual fuels. This element acts as a catalyst in accelerating the change of sulfur dioxide to the trioxide form.1* Excessive spot tem peratures in the combustion chamber or elsewhere, are also Table S .. Average Rue Gas Dew Point for.Various fueis Type of ftwf Average Dew Point Temper- Semi-Bituminous Coal....................................... Oil.................... ...............................................v Manufactured Gas...-........................................ Butane Gas (3200 Btu/eu ft)............................ Butane-Air Gas Mixture (535 Btu/cu ft). . . .84 137 124 121 destructive in that they may result in rapid oxidation of ordinary heating surfaces. American Standard Requirements for gas furnaces, floor furnaces, and recessed heaters, for ex ample, specify that minimum spot heating surface tempera tures during normal operation must neither fall below 178 F (50 F above average dew point) nor exceed 830 F to 1230 F on any portion of the heating surface, depending on the type and thickness of tiie metal. In any event it is usually desirable to maintain flue-gas temperatures within the limits indicated not only throughout the appliance, but in its connecting vent, flue, or chimney as well. Otherwise, excessive condensation and corrosion problems, with resultant customer dissatisfac tion, will in all-probability be the result. Average dew-point temperatures of flue gases resulting from the combustion of various fuels, when burned with the amount of excess air normally supplied to insure complete combustion are shown in Table 8. - SOOT The deposit of soot on the flue surfaces of a boiler or heater acts as an insulating layer over the surface, and reduces the beat transmission to the water or air. The Bureau of Mines Report of Investigations. No. 327211 shows that the los3 of seasonal efficiency is not so great as has been believed, and usually is not over 6 percent because the greater'part of the heat is transmitted through the com bustion' chamber surfaces. The Bureau of Standards Report BMS 54" points out that, although the decrease in efficiency of an oil fired boiler, due to soot deposits is relatively email, the attendant increase in stack temperature may be con siderable. Condensation frequently occurs during idle summer periods as a result of moisture in the air condensing on the cooler boiler surfaces. This condition/combined with surfaces coated with sulfur-bearing soot and fly ash can also result in sulfurous acid and accelerated corrosion. The soot accumulation clogs the flues, reduces the draft, and may prevent proper combustion. Soot can probably be most effectively removed by a jet of compressed air, by means of o brush, or a vacuum cleaner. However, it has been found that copper chloride, lead chloride, tin chloride, zinc chloride, and some other salts are partially effective in re moving soot from furnaces and boilers when properly used.11 These are preferably applied to the surfaces. A discussion of instruments and methods of evaluating smoke will be found in Chapter 16. AIR SUPPLY TO FUEL-BURNING EQUIPMENT All rooms or spaces containing boilers, furnaces, water heaters, or any other fuel-burning - equipment must be provided with a constant supply of combustion air at ade quate static pressure to insure proper combustion of the fuel.