Document ywbBd19EGMN95oagdpbYJrN6

494 CHAPTER 33 1960 Guide than lost heat. They may, therefore, be omitted from calcula tions of heat losses, or added to item 1. If there is CO in the flue gases, small amounts of unbumed hydrogen and hydrocarbons will probably also be present. The small losses due to incomplete combustion of these latter gases would also be included in item 7. Symbols used in Equations 12 to 16 inclusive are: hi hi A, A h. to. t, t, H, 1091.8 M w CO, CO, c heat loss in the dry chimney gases, Btu per pound of fuel. heat loss in water vapor from combustion of hy drogen, Btu per pound of fuel, heat loss in water vapor in combustion air, Btu per pound of fuel. heat loss from incomplete combustion of carbon, Btu per pound of fuel. heat loss from unburned carbon in the ash, Btu per pound of fuel. weight of dry Sue gas per pound of fuel (from Equation 10), pounds. m.ftn specific heat of flue gases at constant pressure (c, ranges from 0.242 to 0.254 for flue gas tempera tures from 300 F to 1000 F)p* Btu per pound, temperature of flue gases at exit of heating device, Fahrenheit. temperature of combustion air, Fahrenheit, percentage of hydrogen in fuel by weight from ulti mate analysis of fuel burned, enthalpy of saturated water vapor at a temperature of 70 F, Btu per pound. humidity ratio of combustion air, pounds of water vapor per pound of dry air. weight of combustion air per pound of fuel used, pounds, from Equations 2, 4, 5, 6, 7 and 8. percentages of CO, CO, in flue gases by volumeweight of carbon burned per pound of fuel cor rected for carbon in ash, pounda ge, - WjC. 100W (17) where C = percentage of carbon in the fuel by weight from the ultimate analysis. Wm = weight of ash and refuse, pounds. Ca = percent of combustible in ash by weight (combustible in ash is usually considered to be carbon). W -- weight of fuel used, pounds. Flue-gas losses for solid and liquid fuels, listed as items 2, 3 and 4 of the beat balance, may be determined with suffi cient precision for most purposes from curves shown in Fig. 4,' if CO* 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 the typical ultimate analyses shown in Table 1. The curves for medium-volatile bituminous coal may be used for high-volatile bituminous coal with negligi ble 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 heat balance is also a more simple procedure in that items 5 and 6 do not generally apply to gas installations. A series of typical alignment charts is combined in Fig. 5 Table 10.... Average Flue Gas Dew Point for Various Fuels* Typ* of Fuel Average Dew Point Reaper- 84 93 111 127 137 119 121 A. Vcatilatiat sir outlet regUte, for appliance room, 1 sq in. free area for each 1000 Btu per hr appliance input located above reliefopening of draft hood. Rou ter must not be blocked bgr drapes or other furnialiin**. B. Both register* must either face same large ventilated interior space or ex tend to each space by means of ducts. Vertical distance C/L to C/L of registers should be not lees than JVt ft. C- Suggest room eoeess door be not lea than 6 ft high by a width sufficient to provide for installation or removal of appliance. At least t ft borisontal clearance tlwuld be provided m front of appliance when eloeet doer is open, or IB in. wheD D. Combustion and ventilation air inlet register (or appliance room, 1 aq in. free are* (or each 1000 Btu per hr appliance input, located at or below combustion air inlet to appliance. Register most not be blocked by drapes or other furnish ings. E. Air cirenlsted by must be handled by duets which are seated to appliance ~--f and are entirely reparate from means provided for supplying combustion and ventilation ah. F. Spacing between draft hood and wall at least 0 in. fantes, approved for dener spacing). If flue products may be directed toward wall. 12 in. spacing G. No part cf appliance '--'"fl eloeer than 6 in. to wall (unless approved for closer spating). H. Flue should terminate above peak ainot and above nearby walls to assure satisfactory flue performance. Fig. 6 .... Illustration Showing Air Openings Necessary to Supply Air for Combustion When Appfiance is Installed in Confined Space for use in determining flue losses of items 2, 3 and 4 from common types of gas burning appliances. To determine flue losses place a 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 Fuels and Combustion 495 Table 11 .... Permanent Openings for Air Supply to Spaces Containing Fuel-Burning Equipment fiuet-Bemmg Cqaipmettf Pro* Area of Veofflah'ng Opening* Air Inlet Air Outlet Equipment in a large enclosure hav None required ca ing adequate air infiltration None required w Equipment in a large enclosure hav Total free area communicating with Total free area communicating with ing unusually tight construction outdoor air not less than sq in. outdoor air not less than M sq in. per g -S3 per (1000 Btu) (hr) furnace input (1000 Btu) (hr) furnace input 33 p. am "s S' oM o 6 Equipment in a small enclosure in a buildiDg having adequate air in filtration Equipment in a small enclosure in a building of unusually tight con struction Opening in party wall or door, (above draft hood) with free area of not less than 1 sq in. per (1000 Btu) (hr) furnace input Grilled opening connecting directly with outdoors, through duct if nec essary; free area not less than $4 sq in. per (1000 Btu) (hr) furnace in put Opening in a party wall or door, (below combustion cur inlet to appliance) with free area of not less than 1 sq in. per (1000 Btu) (hr) furnace input Grilled opening connecting directly with outdoor air, through duct if necessary. Area and location same as above- in g -5 <5 & Si 3 III ON Equipment in an enclosure having adequate air infiltration Equipment in an enclosure having unusually tight construction Total free area equivalent to not less than the combined ares of the flue outlets of all fuel-burning equip ment in the space Total free area equivalent to not less than twice the combined area of the flue outlets, of all fuel-burning equipment in the space None required None required * Operation of eihatat Iona, tatchea ventilation systems, fire place*, etc- may requirespacial consideration. ForreaidectaJ ckeeiinstallation, see Pig. 6. can then be computed with sufficient precision by application of Equation IS. Percent Combustion Efficiency =* Gross Btu of fuel \ _ ftotal flue losses per\ gas per cubic foot/ \ cubic foot fuel gas / ^ ^ Gross Btu of fuel gas per cubic foot --- Reference to Table 9 will show that ultimate CO, percent age values of fuel gases vary. While personal errors involved in 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 in flue gases. They become active whenever sufficient mois ture is present for the formation of sulfurous or sulfuric acid,14 and they lower the dew points of flue gases appreci ably. Therefore, unless heating equipment is designed for operation at flue-gas temperatures below the dew point, which is seldom the case, it is always advisable to maintain temperatures above this value in all parts of the appliance. Excessive spot temperatures in the combustion chamber or elsewhere, on the other hand, are also 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 example, specify that minimum spot heating surface temperatures during normal operation must neither fall below 178 F (50 F above aver age dew point) nor exceed 830 F to 1230 F on any portion of the heating surface, depending on the type and thickness of the 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 dissatisfaction, 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 10. 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 heat transmission to the water or air. The Bureau oj Mines Report of Investigations No. 3272" shows that the loss 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 fiMS 54" points out that, although the decrease in efficiency of an oil fired boiler, due to soot deposits, is relatively small, the attendant increase in stack temperature may be considerable. 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 a brush, or a vacuum cleaner. However, it has been found that copper chloride, lead chloride, tin chloride, zinc chloride, common salt and some other salts are par tially effective in removing soot from furnaces and boilers when properly used." A discussion of instruments and methods of evaluating smoke will be found in Chapter 44.