Improvement in burner design has largely reduced the steam consumption, though to a greater degree in steam than in air atomizing burners. Recent experiments show that a good steam atomizing burner will require approximately 2 per cent of the total steam generated by the boiler operated at or about its rated capacity. This figure will decrease as the capacity is increased and is so low as to be practically negligible, except in cases where the question of loss of feed water is all important. There are no figures available as to the actual steam consumption of mechanical atomizing burners but apparently this is small if the requirement is understood to be entirely apart from the steam consumption of the apparatus producing the forced blast.

Capacity of Burners—A good steam atomizing burner properly located in a well-designed oil furnace has a capacity of somewhat over 400 horse power. This question of capacity of individual burners is largely one of the proper relation between the number of burners used and the furnace volume. In some recent tests with a Babcock & Wilcox boiler of 640 rated horse power, equipped with three burners, approximately 1350 horse power was developed with an available draft of .55 inch at the damper or 450 horse power per burner. Four burners were also tried in the same furnace but the total steam generated did not exceed 1350 horse power or in this instance 338 horse power per burner.

From the nature of mechanical atomizing burners, individual burners have not as large a capacity as the steam atomizing class. In some tests on a Babcock & Wilcox marine boiler, equipped with mechanical atomizing burners, the maximum horse power developed per burner was approximately 105. Here again the burner capacity is largely one of proper relation between furnace volume and number of burners.

Furnace Design—Too much stress cannot be laid on the importance of furnace design for the use of this class of fuel. Provided a good type of burner is adopted the furnace arrangement and the method of introducing air for combustion into the furnace are the all important factors. No matter what the type of burner, satisfactory results cannot be secured in a furnace not suited to the fuel.

The Babcock & Wilcox Co. has had much experience with the burning of oil as fuel and an extended series of experiments by Mr. E. H. Peabody led to the development and adoption of the Peabody furnace as being most eminently suited for this class of work. Fig. 29 shows such a furnace applied to a Babcock & Wilcox boiler, and with slight modification it can be as readily applied to any boiler of The Babcock & Wilcox Co. manufacture. In the description of this furnace, its points of advantage cover the requirements of oil-burning furnaces in general.

The atomized oil is introduced into the furnace in the direction in which it increases in height. This increase in furnace volume in the direction of the flame insures free expansion and a thorough mixture of the oil with the air, and the consequent complete combustion of the gases before they come into contact with the tube heating surfaces. In such a furnace flat flame burners should be used, preferably of the Peabody type, in which the flame spreads outward toward the sides in the form of a fan. There is no tendency of the flames to impinge directly on the heating surfaces, and the furnace can handle any quantity of flame without danger of tube difficulties. The burners should be so located that the flames from individual burners do not interfere nor impinge to any extent on the side walls of the furnace, an even distribution of heat being secured in this manner. The burners are operated from the boiler front and peepholes are supplied through which the operator may watch the flame while regulating the burners. The burners can be removed, inspected, or cleaned and replaced in a few minutes. Air is admitted through a checkerwork of fire brick supported on the furnace floor, the openings in the checkerwork being so arranged as to give the best economic results in combustion.



With steam atomizing burners introduced through the front of the boiler in stationary practice, it is usually in the direction in which the furnace decreases in height and it is with such an arrangement that difficulties through the loss of tubes may be expected. With such an arrangement, the flame may impinge directly upon the tube surfaces and tube troubles from this source may arise, particularly where the feed water has a tendency toward rapid scale formation. Such difficulties may be the result of a blowpipe action on the part of the burner, the over heating of the tube due to oil or scale within, or the actual erosion of the metal by particles of oil improperly atomized. Such action need not be anticipated, provided the oil is burned with a short flame. The flames from mechanical atomizing burners have a less velocity of projection than those from steam atomizing burners and if introduced into the higher end of the furnace, should not lead to tube difficulties provided they are properly located and operated. This class of burner also will give the most satisfactory results if introduced so that the flames travel in the direction of increase in furnace volume. This is perhaps best exemplified by the very good results secured with mechanical atomizing burners and Babcock & Wilcox marine boilers in which, due to the fact that the boilers are fired from the low end, the flames from burners introduced through the front are in this direction.

Operation of Burners—When burners are not in use, or when they are being started up, care must be taken to prevent oil from flowing and collecting on the floor of the furnace before it is ignited. In starting a burner, the atomized fuel may be ignited by a burning wad of oil-soaked waste held before it on an iron rod. To insure quick ignition, the steam supply should be cut down. But little practice is required to become an adept at lighting an oil fire. When ignition has taken place and the furnace brought to an even heat, the steam should be cut down to the minimum amount required for atomization. This amount can be determined from the appearance of the flame. If sufficient steam is not supplied, particles of burning oil will drop to the furnace floor, giving a scintillating appearance to the flame. The steam valves should be opened just sufficiently to overcome this scintillating action.

Air Supply—From the nature of the fuel and the method of burning, the quantity of air for combustion may be minimized. As with other fuels, when the amount of air admitted is the minimum which will completely consume the oil, the results are the best. The excess or deficiency of air can be judged by the appearance of the stack or by observing the gases passing through the boiler settings. A perfectly clear stack indicates excess air, whereas smoke indicates a deficiency. With properly designed furnaces the best results are secured by running near the smoking point with a slight haze in the gases. A slight variation in the air supply will affect the furnace conditions in an oil burning boiler more than the same variation where coal is used, and for this reason it is of the utmost importance that flue gas analysis be made frequently on oil-burning boilers. With the air for combustion properly regulated by adjustment of any checkerwork or any other device which may be used, and the dampers carefully set, the flue gas analysis should show, for good furnace conditions, a percentage of CO_{2} between 13 and 14 per cent, with either no CO or but a trace.

In boiler plant operation it is difficult to regulate the steam supply to the burners and the damper position to meet sudden and repeated variations in the load. A device has been patented which automatically regulates by means of the boiler pressure the pressure of the steam to the burners, the oil to the burners and the position of the boiler damper. Such a device has been shown to give good results in plant operation where hand regulation is difficult at best, and in many instances is unfortunately not even attempted.

Efficiency with Oil—As pointed out in enumerating the advantages of oil fuel over coal, higher efficiencies are obtainable with the former. With boilers of approximately 500 horse power equipped with properly designed furnaces and burners, an efficiency of 83 per cent is possible or making an allowance of 2 per cent for steam used by burners, a net efficiency of 81 per cent. The conditions under which such efficiencies are to be secured are distinctly test conditions in which careful operation is a prime requisite. With furnace conditions that are not conductive to the best combustion, this figure may be decreased by from 5 to 10 per cent. In large properly designed plants, however, the first named efficiency may be approached for uniform running conditions, the nearness to which it is reached depending on the intelligence of the operating crew. It must be remembered that the use of oil fuel presents to the careless operator possibilities for wastefulness much greater than in plants where coal is fired, and it therefore pays to go carefully into this feature.

Table 48 gives some representative tests with oil fuel.

TABLE 48

TESTS OF BABCOCK AND WILCOX BOILERS WITH OIL FUEL

Pacific Light Pacific Light Miami Copper and Power and Power Company Plant Company Company Los Angeles, Miami, Cal. Redondo, Cal. Arizona Rated Capacity Horse of Boiler Power 467 604 600 Duration of Test Hours 10 10 7 7 10 4 Steam Pressure by Gauge Pounds 156.4 156.9 184.7 184.9 183.4 189.5 Temperature of Degrees Feed Water F. 62.6 61.1 93.4 101.2 157.7 156.6 Degrees of Degrees Superheat F. 83.7 144.3 103.4 139.6 Factor of Evaporation 1.2004 1.2020 1.2227 1.2475 1.1676 1.1886 Draft in Furnace Inches .02 .05 .014 .19 .12 .22 Draft at Damper Inches .08 .15 .046 .47 .19 .67 Temperature of Degrees Exit Gases F. 438 525 406 537 430 612 Flue CO{2} Per Cent 14.3 12.1 Gas O Per Cent 3.8 6.8 Analysis CO Per Cent 0.0 0.0 Oil Burned per Hour Pounds 1147 1837 1439 2869 1404 3214 Water Evaporated per Hour from from and at Pounds 18310 27855 22639 40375 21720 42863 212 Degrees Evaporation from and at 212 Degrees per Pounds 15.96 15.16 15.73 14.07 15.47 13.34 Pound of Oil Per Cent of Rated Capacity Pounds 113.6 172.9 108.6 193.8 104.9 207.1 Developed B. t. u. per Pound of Oil B. t. u. 18626 18518 18326 18096 18600 18600 Efficiency Per Cent 83.15 79.46 83.29 76.02 80.70 69.6

Burning Oil in Connection with Other Fuels—Considerable attention has been recently given to the burning of oil in connection with other fuels, and a combination of this sort may be advisable either with the view to increasing the boiler capacity to assist over peak loads, or to keep the boiler in operation where there is the possibility of a temporary failure of the primary fuel. It would appear from experiments that such a combination gives satisfactory results from the standpoint of both capacity and efficiency, if the two fuels are burned in separate furnaces. Satisfactory results cannot ordinarily be obtained when it is attempted to burn oil fuel in the same furnace as the primary fuel, as it is practically impossible to admit the proper amount of air for combustion for each of the two fuels simultaneously. The Babcock & Wilcox boiler lends itself readily to a double furnace arrangement and Fig. 30 shows an installation where oil fuel is burned as an auxiliary to wood.



Water-gas Tar—Water-gas tar, or gas-house tar, is a by-product of the coal used in the manufacture of water gas. It is slightly heavier than crude oil and has a comparatively low flash point. In burning, it should be heated only to a temperature which makes it sufficiently fluid, and any furnace suitable for crude oil is in general suitable for water-gas tar. Care should be taken where this fuel is used to install a suitable apparatus for straining it before it is fed to the burner.



GASEOUS FUELS AND THEIR COMBUSTION

Of the gaseous fuels available for steam generating purposes, the most common are blast furnace gas, natural gas and by-product coke oven gas.

Blast furnace gas, as implied by its name, is a by-product from the blast furnace of the iron industry. This gasification of the solid fuel in a blast furnace results, 1st, through combustion by the oxygen of the blast; 2nd, through contact with the incandescent ore (Fe_{2}O_{3} + C = 2 FeO + CO and FeO + C = Fe + CO); and 3rd, through the agency of CO_{2} either formed in the process of reduction or driven from the carbonates charged either as ore or flux.

Approximately 90 per cent of the fuel consumed in all of the blast furnaces of the United States is coke. The consumption of coke per ton of iron made varies from 1600 to 3600 pounds per ton of 2240 pounds of iron. This consumption depends upon the quality of the coal, the nature of the ore, the quality of the pig iron produced and the equipment and management of the plant. The average consumption, and one which is approximately correct for ordinary conditions, is 2000 pounds of coke per gross ton (2240 pounds) of pig iron. The gas produced in a gas furnace per ton of pig iron is obtained from the weight of fixed carbon gasified, the weight of the oxygen combined with the material of charge reduced, the weight of the gaseous constituents of the flux and the weight of air delivered by the blowing engine and the weight of volatile combustible contained in the coke. Ordinarily, this weight of gas will be found to be approximately five times the weight of the coke burned, or 10,000 pounds per ton of pig iron produced.

With the exception of the small amount of carbon in combination with hydrogen as methane, and a very small percentage of free hydrogen, ordinarily less than 0.1 per cent, the calorific value of blast furnace gas is due to the CO content which when united with sufficient oxygen when burned under a boiler, burns further to CO_{2}. The heat value of such gas will vary in most cases from 85 to 100 B. t. u. per cubic foot under standard conditions. In modern practice, where the blast is heated by hot blast stoves, approximately 15 per cent of the total amount of gas is used for this purpose, leaving 85 per cent of the total for use under boilers or in gas engines, that is, approximately 8500 pounds of gas per ton of pig iron produced. In a modern blast furnace plant, the gas serves ordinarily as the only fuel required. Table 49 gives the analyses of several samples of blast furnace gas.

TABLE 49

TYPICAL ANALYSES OF BLAST FURNACE GAS

- - CO{2} O CO H CH{4} N - - Bessemer Furnace 9.85 0.36 32.73 3.14 .. 53.92 Bessemer Furnace 11.4 .. 27.7 1.9 0.3 58.7 Bessemer Furnace 10.0 .. 26.2 3.1 0.2 60.5 Bessemer Furnace 9.1 .. 28.7 2.7 0.2 59.3 Bessemer Furnace 13.5 .. 25.2 1.43 .. 59.87 Bessemer Furnace[47] 10.9 .. 27.8 2.8 0.2 58.3 Ferro Manganese Furnace 7.1 .. 30.1 .. .. 62.8[48] Basic Ore Furnace 16.0 0.2 23.6 .. .. 60.2[48] - -

Until recently, the important consideration in the burning of blast furnace gas has been the capacity that can be developed with practically no attention given to the aspect of efficiency. This phase of the question is now drawing attention and furnaces especially designed for good efficiency with this class of fuel are demanded. The essential feature is ample combustion space, in which the combustion of gases may be practically completed before striking the heating surfaces. The gases have the power of burning out completely after striking the heating surfaces, provided the initial temperature is sufficiently high, but where the combustion is completed before such time, the results secured are more satisfactory. A furnace volume of approximately 1 to 1.5 cubic feet per rated boiler horse power will give a combustion space that is ample.

Where there is the possibility of a failure of the gas supply, or where steam is required when the blast furnace is shut down, coal fired grates of sufficient size to get the required capacity should be installed. Where grates of full size are not required, ignition grates should be installed, which need be only large enough to carry a fire for igniting the gas or for generating a small quantity of steam when the blast furnace is shut down. The area of such grates has no direct bearing on the size of the boiler. The grates may be placed directly under the gas burners in a standard position or may be placed between two bridge walls back of the gas furnace and fired from the side of the boiler. An advantage is claimed for the standard grate position that it minimizes the danger of explosion on the re-ignition of gas after a temporary stoppage of the supply and also that a considerable amount of dirt, of which there is a good deal with this class of fuel and which is difficult to remove, deposits on the fire and is taken out when the fires are cleaned. In any event, regardless of the location of the grates, ample provision should be made for removing this dust, not only from the furnace but from the setting as a whole.

Blast furnace gas burners are of two general types: Those in which the air for combustion is admitted around the burner proper, and those in which this air is admitted through the burner. Whatever the design of burner, provision should be made for the regulation of both the air and the gas supply independently. A gas opening of .8 square inch per rated horse power will enable a boiler to develop its nominal rating with a gas pressure in the main of about 2 inches. This pressure is ordinarily from 6 to 8 inches and in this way openings of the above size will be good for ordinary overloads. The air openings should be from .75 to .85 square inch per rated horse power. Good results are secured by inclining the gas burners slightly downward toward the rear of the furnace. Where the burners are introduced over coal fired grates, they should be set high enough to give headroom for hand firing.

Ordinarily, individual stacks of 130 feet high with diameters as given in Kent's table for corresponding horse power are large enough for this class of work. Such a stack will give a draft sufficient to allow a boiler to be operated at 175 per cent of its rated capacity, and beyond this point the capacity will not increase proportionately with the draft. When more than one boiler is connected with a stack, the draft available at the damper should be equivalent to that which an individual stack of 130 feet high would give. The draft from such a stack is necessary to maintain a suction under all conditions throughout all parts of the setting. If the draft is increased above that which such a stack will give, difficulties arise from excess air for combustion with consequent loss in efficiency.

A poor mixing or laneing action in the furnace may result in a pulsating effect of the gases in the setting. This action may at times be remedied by admitting more air to the furnace. On account of the possibility of a pulsating action of the gases under certain conditions and the puffs or explosions, settings for this class of work should be carefully constructed and thoroughly buckstayed and tied.

Natural Gas—Natural gas from different localities varies considerably in composition and heating value. In Table 50 there is given a number of analyses and heat values for natural gas from various localities.

This fuel is used for steam generating purposes to a considerable extent in some localities, though such use is apparently decreasing. It is best burned by employing a large number of small burners, each being capable of handling 30 nominal rated horse power. The use of a large number of burners obviates the danger of any laneing or blowpipe action, which might be present where large burners are used. Ordinarily, such a gas, as it enters the burners, is under a pressure of about 8 ounces. For the purpose of comparison, all observations should be based on gas reduced to the standard conditions of temperature and pressure, namely 32 degrees Fahrenheit and 14.7 pounds per square inch. When the temperature and pressure corresponding to meter readings are known, the volume of gas under standard conditions may be obtained by multiplying the meter readings in cubic feet by 33.54 P/T, in which P equals the absolute pressure in pounds per square inch and T equals the absolute temperature of the gas at the meter. In boiler testing work, the evaporation should always be reduced to that per cubic foot of gas under standard conditions.

TABLE 50

TYPICAL ANALYSES (BY VOLUME) AND CALORIFIC VALUES OF NATURAL GAS FROM VARIOUS LOCALITIES

- - - - - - + Locality of Well H CH_{4} CO CO_{2} N O Heavy H_{2}S B. t. u. Hydro- per carbons Cubic Foot Calcul- ated[49] + - - - - - - + Anderson, Ind. 1.86 93.07 0.73 0.26 3.02 0.42 0.47 0.15 1017 Marion, Ind. 1.20 93.16 0.60 0.30 3.43 0.55 0.15 0.20 1009 Muncie, Ind. 2.35 92.67 0.45 0.25 3.53 0.35 0.25 0.15 1004 Olean, N. Y. 96.50 0.50 2.00 1.00 1018 Findlay, O. 1.64 93.35 0.41 0.25 3.41 0.39 0.35 0.20 1011 St. Ive, Pa. 6.10 75.54 Trace 0.34 18.12 1117 Cherry Tree, Pa. 22.50 60.27 2.28 7.32 0.83 6.80 842 Grapeville, Pa. 24.56 14.93 Trace Trace 18.69 1.22 40.60 925 Harvey Well, Butler Co., Pa. 13.50 80.00 Trace 0.66 5.72 998 Pittsburgh, Pa. 9.64 57.85 1.00 23.41 2.10 6.00 748 Pittsburgh, Pa. 20.02 72.18 1.00 0.80 1.10 4.30 917 Pittsburgh, Pa. 26.16 65.25 0.80 0.60 0.80 6.30 899 + - - - - - -



When natural gas is the only fuel, the burners should be evenly distributed over the lower portion of the boiler front. If the fuel is used as an auxiliary to coal, the burners may be placed through the fire front. A large combustion space is essential and a volume of .75 cubic feet per rated horse power will be found to give good results. The burners should be of a design which give the gas and air a rotary motion to insure a proper mixture. A checkerwork wall is sometimes placed in the furnace about 3 feet from the burners to break up the flame, but with a good design of burner this is unnecessary. Where the gas is burned alone and no grates are furnished, good results are secured by inclining the burner downward to the rear at a slight angle.

By-product Coke Oven Gas—By-product coke oven gas is a product of the destructive distillation of coal in a distilling or by-product coke oven. In this class of apparatus the gases, instead of being burned at the point of their origin, as in a beehive or retort coke oven, are taken from the oven through an uptake pipe, cooled and yield as by-products tar, ammonia, illuminating and fuel gas. A certain portion of the gas product is burned in the ovens and the remainder used or sold for illuminating or fuel purposes, the methods of utilizing the gas varying with plant operation and locality.

Table 51 gives the analyses and heat value of certain samples of by-product coke oven gas utilized for fuel purposes.

This gas is nearer to natural gas in its heat value than is blast furnace gas, and in general the remarks as to the proper methods of burning natural gas and the features to be followed in furnace design hold as well for by-product coke oven gas.

TABLE 51

TYPICAL ANALYSES OF BY-PRODUCT COKE OVEN GAS

+ + + + -+ CO{2} O CO CH{4} H N B.t.u. per Cubic Foot + + -+ -+ + + + + 0.75 Trace 6.0 28.15 53.0 12.1 505 2.00 Trace 3.2 18.80 57.2 18.0 399 3.20 0.4 6.3 29.60 41.6 16.1 551 0.80 1.6 4.9 28.40 54.2 10.1 460 + + -+ -+ + + + + + +

The essential difference in burning the two fuels is the pressure under which it reaches the gas burner. Where this is ordinarily from 4 to 8 ounces in the case of natural gas, it is approximately 4 inches of water in the case of by-product coke oven gas. This necessitates the use of larger gas openings in the burners for the latter class of fuel than for the former.

By-product coke oven gas comes to the burners saturated with moisture and provision should be made for the blowing out of water of condensation. This gas too, carries a large proportion of tar and hydrocarbons which form a deposit in the burners and provision should be made for cleaning this out. This is best accomplished by an attachment which permits the blowing out of the burners by steam.



UTILIZATION OF WASTE HEAT

While it has been long recognized that the reclamation of heat from the waste gases of various industrial processes would lead to a great saving in fuel and labor, the problem has, until recently, never been given the attention that its importance merits. It is true that installations have been made for the utilization of such gases, but in general they have consisted simply in the placing of a given amount of boiler heating surface in the path of the gases and those making the installations have been satisfied with whatever power has been generated, no attention being given to the proportioning of either the heating surface or the gas passages to meet the peculiar characteristics of the particular class of waste gas available. The Babcock & Wilcox Co. has recently gone into the question of the utilization of what has been known as waste heat with great thoroughness, and the results secured by their installations with practically all operations yielding such gases are eminently successful.

TABLE 52

TEMPERATURE OF WASTE GASES FROM VARIOUS INDUSTRIAL PROCESSES

+ -+ + -+ -+ Waste Heat From Temperature[50] Degrees + -+ -+ Brick Kilns 2000-2300 Zinc Furnaces 2000-2300 Copper Matte Reverberatory Furnaces 2000-2200 Beehive Coke Ovens 1800-2000 Cement Kilns 1200-1600[51] Nickel Refining Furnaces 1500-1750 Open Hearth Steel Furnaces 1100-1400 + -+ -+ + -+

The power that can be obtained from waste gases depends upon their temperature and weight, and both of these factors vary widely in different commercial operations. Table 52 gives a list of certain processes yielding waste gases the heat of which is available for the generation of steam and the approximate temperature of such gases. It should be understood that the temperatures in the table are the average of the range of a complete cycle of the operation and that the minimum and maximum temperatures may vary largely from the figures given.

The maximum available horse power that may be secured from such gases is represented by the formula:

W(T-t)s H. P. = ———- (23) 33,479

Where W = the weight of gases passing per hour, T = temperature of gases entering heating surface, t = temperature leaving heating surface, s = specific heat of gases.

The initial temperature and the weight or volume of gas will depend, as stated, upon the process involved. The exit temperature will depend, to a certain extent, upon the temperature of the entering gases, but will be governed mainly by the efficiency of the heating surfaces installed for the absorption of the heat.

Where the temperature of the gas available is high, approaching that found in direct fired boiler practice, the problem is simple and the question of design of boiler becomes one of adapting the proper amount of heating surface to the volume of gas to be handled. With such temperatures, and a volume of gas available approximately in accordance with that found in direct fired boiler practice, a standard boiler or one but slightly modified from the standard will serve the purpose satisfactorily. As the temperatures become lower, however, the problem is more difficult and the departure from standard practice more radical. With low temperature gases, to obtain a heat transfer rate at all comparable with that found in ordinary boiler practice, the lack of temperature must be offset by an added velocity of the gases in their passage over the heating surfaces. In securing the velocity necessary to give a heat transfer rate with low temperature gases sufficient to make the installation of waste heat boilers show a reasonable return on the investment, the frictional resistance to the gases through the boiler becomes greatly in excess of what would be considered good practice in direct fired boilers. Practically all operations yielding waste gases require that nothing be done in the way of impairing the draft at the furnace outlet, as this might interfere with the operation of the primary furnace. The installation of a waste heat boiler, therefore, very frequently necessitates providing sufficient mechanical draft to overcome the frictional resistance of the gases through the heating surfaces and still leave ample draft available to meet the maximum requirements of the primary furnace.

Where the temperature and volume of the gases are in line with what are found in ordinary direct fired practice, the area of the gas passages may be practically standard. With the volume of gas known, the draft loss through the heating surfaces may be obtained from experimental data and this additional draft requirement met by the installation of a stack sufficient to take care of this draft loss and still leave draft enough for operating the furnace at its maximum capacity.

Where the temperatures are low, the added frictional resistance will ordinarily be too great to allow the draft required to be secured by additional stack height and the installation of a fan is necessary. Such a fan should be capable of handling the maximum volume of gas that the furnace may produce, and of maintaining a suction equivalent to the maximum frictional resistance of such volume through the boiler plus the maximum draft requirement at the furnace outlet. Stacks and fans for this class of work should be figured on the safe side. Where a fan installation is necessary, the loss of draft in the fan connections should be considered, and in figuring conservatively it should be remembered that a fan of ample size may be run as economically as a smaller fan, whereas the smaller fan, if overloaded, is operated with a large loss in efficiency. In practically any installation where low temperature gas requires a fan to give the proper heat transfer from the gases, the cost of the fan and of the energy to drive it will be more than offset by the added power from the boiler secured by its use. Furthermore, the installation of such a fan will frequently increase the capacity of the industrial furnace, in connection with which the waste heat boilers are installed.

In proportioning heating surfaces and gas passages for waste heat work there are so many factors bearing directly on what constitutes the proper installation that it is impossible to set any fixed rules. Each individual installation must be considered by itself as well as the particular characteristics of the gases available, such as their temperature and volume, and the presence of dust or tar-like substances, and all must be given the proper weight in the determination of the design of the heating surfaces and gas passages for the specific set of conditions.

[Graph: Per Cent of Water Heating Surface passed over by Gases/Per Cent of the Total Amount of Steam Generated in the Boiler against Temperature in Degrees Fahrenheit of Hot Gases Sweeping Heating Surface

Fig. 31. Curve Showing Relation Between Gas Temperature, Heating Surface passed over, and Amount of Steam Generated. Ten Square Feet of Heating Surface are Assumed as Equivalent to One Boiler Horse Power]

Fig. 31 shows the relation of gas temperatures, heating surface passed over and work done by such surface for use in cases where the temperatures approach those found in direct fired practice and where the volume of gas available is approximately that with which one horse power may be developed on 10 square feet of heating surface. The curve assumes what may be considered standard gas passage areas, and further, that there is no heat absorbed by direct radiation from the fire.

Experiments have shown that this curve is very nearly correct for the conditions assumed. Such being the case, its application in waste heat work is clear. Decreasing or increasing the velocity of the gases over the heating surfaces from what might be considered normal direct fired practice, that is, decreasing or increasing the frictional loss through the boiler will increase or decrease the amount of heating surface necessary to develop one boiler horse power. The application of Fig. 31 to such use may best be seen by an example:

Assume the entering gas temperatures to be 1470 degrees and that the gases are cooled to 570 degrees. From the curve, under what are assumed to be standard conditions, the gases have passed over 19 per cent of the heating surface by the time they have been cooled 1470 degrees. When cooled to 570 degrees, 78 per cent of the heating surface has been passed over. The work done in relation to the standard of the curve is represented by (1470 - 570) / (2500 - 500) = 45 per cent. (These figures may also be read from the curve in terms of the per cent of the work done by different parts of the heating surfaces.) That is, 78 per cent - 19 per cent = 59 per cent of the standard heating surface has done 45 per cent of the standard amount of work. 59 / 45 = 1.31, which is the ratio of surface of the assumed case to the standard case of the curve. Expressed differently, there will be required 13.1 square feet of heating surface in the assumed case to develop a horse power as against 10 square feet in the standard case.

The gases available for this class of work are almost invariably very dirty. It is essential for the successful operation of waste-heat boilers that ample provision be made for cleaning by the installation of access doors through which all parts of the setting may be reached. In many instances, such as waste-heat boilers set in connection with cement kilns, settling chambers are provided for the dust before the gases reach the boiler.

By-passes for the gases should in all cases be provided to enable the boiler to be shut down for cleaning and repairs without interfering with the operation of the primary furnace. All connections from furnace to boilers should be kept tight to prevent the infiltration of air, with the consequent lowering of gas temperatures.

Auxiliary gas or coal fired grates must be installed to insure continuity in the operation of the boiler where the operation of the furnace is intermittent or where it may be desired to run the boiler with the primary furnace not in operation. Such grates are sometimes used continuously where the gases available are not sufficient to develop the required horse power from a given amount of heating surface.

Fear has at times been expressed that certain waste gases, such as those containing sulphur fumes, will have a deleterious action on the heating surface of the boiler. This feature has been carefully watched, however, and from plants in operation it would appear that in the absence of water or steam leaks within the setting, there is no such harmful action.



CHIMNEYS AND DRAFT

The height and diameter of a properly designed chimney depend upon the amount of fuel to be burned, its nature, the design of the flue, with its arrangement relative to the boiler or boilers, and the altitude of the plant above sea level. There are so many factors involved that as yet there has been produced no formula which is satisfactory in taking them all into consideration, and the methods used for determining stack sizes are largely empirical. In this chapter a method sufficiently comprehensive and accurate to cover all practical cases will be developed and illustrated.

Draft is the difference in pressure available for producing a flow of the gases. If the gases within a stack be heated, each cubic foot will expand, and the weight of the expanded gas per cubic foot will be less than that of a cubic foot of the cold air outside the chimney. Therefore, the unit pressure at the stack base due to the weight of the column of heated gas will be less than that due to a column of cold air. This difference in pressure, like the difference in head of water, will cause a flow of the gases into the base of the stack. In its passage to the stack the cold air must pass through the furnace or furnaces of the boilers connected to it, and it in turn becomes heated. This newly heated gas will also rise in the stack and the action will be continuous.

The intensity of the draft, or difference in pressure, is usually measured in inches of water. Assuming an atmospheric temperature of 62 degrees Fahrenheit and the temperature of the gases in the chimney as 500 degrees Fahrenheit, and, neglecting for the moment the difference in density between the chimney gases and the air, the difference between the weights of the external air and the internal flue gases per cubic foot is .0347 pound, obtained as follows:

Weight of a cubic foot of air at 62 degrees Fahrenheit = .0761 pound Weight of a cubic foot of air at 500 degrees Fahrenheit = .0414 pound ———————————— Difference = .0347 pound

Therefore, a chimney 100 feet high, assumed for the purpose of illustration to be suspended in the air, would have a pressure exerted on each square foot of its cross sectional area at its base of .0347 x 100 = 3.47 pounds. As a cubic foot of water at 62 degrees Fahrenheit weighs 62.32 pounds, an inch of water would exert a pressure of 62.32 / 12 = 5.193 pounds per square foot. The 100-foot stack would, therefore, under the above temperature conditions, show a draft of 3.47 / 5.193 or approximately 0.67 inches of water.

The method best suited for determining the proper proportion of stacks and flues is dependent upon the principle that if the cross sectional area of the stack is sufficiently large for the volume of gases to be handled, the intensity of the draft will depend directly upon the height; therefore, the method of procedure is as follows:

1st. Select a stack of such height as will produce the draft required by the particular character of the fuel and the amount to be burned per square foot of grate surface.

2nd. Determine the cross sectional area necessary to handle the gases without undue frictional losses.

The application of these rules follows:

Draft Formula—The force or intensity of the draft, not allowing for the difference in the density of the air and of the flue gases, is given by the formula:

/ 1 1 D = 0.52 H x P - - - (24) T T_{1}/

in which

D = draft produced, measured in inches of water, H = height of top of stack above grate bars in feet, P = atmospheric pressure in pounds per square inch, T = absolute atmospheric temperature, T_{1} = absolute temperature of stack gases.

In this formula no account is taken of the density of the flue gases, it being assumed that it is the same as that of air. Any error arising from this assumption is negligible in practice as a factor of correction is applied in using the formula to cover the difference between the theoretical figures and those corresponding to actual operating conditions.

The force of draft at sea level (which corresponds to an atmospheric pressure of 14.7 pounds per square inch) produced by a chimney 100 feet high with the temperature of the air at 60 degrees Fahrenheit and that of the flue gases at 500 degrees Fahrenheit is,

/ 1 1 D = 0.52 x 100 x 14.7 - - - = 0.67 521 961 /

Under the same temperature conditions this chimney at an atmospheric pressure of 10 pounds per square inch (which corresponds to an altitude of about 10,000 feet above sea level) would produce a draft of,

/ 1 1 D = 0.52 x 100 x 10 - - - = 0.45 521 961 /

For use in applying this formula it is convenient to tabulate values of the product

/ 1 1 0.52 x 14.7 - - - T T_{1}/

which we will call K, for various values of T_{1}. With these values calculated for assumed atmospheric temperature and pressure (24) becomes

D = KH. (25)

For average conditions the atmospheric pressure may be considered 14.7 pounds per square inch, and the temperature 60 degrees Fahrenheit. For these values and various stack temperatures K becomes:

Temperature Stack Gases Constant K 750 .0084 700 .0081 650 .0078 600 .0075 550 .0071 500 .0067 450 .0063 400 .0058 350 .0053

Draft Losses—The intensity of the draft as determined by the above formula is theoretical and can never be observed with a draft gauge or any recording device. However, if the ashpit doors of the boiler are closed and there is no perceptible leakage of air through the boiler setting or flue, the draft measured at the stack base will be approximately the same as the theoretical draft. The difference existing at other times represents the pressure necessary to force the gases through the stack against their own inertia and the friction against the sides. This difference will increase with the velocity of the gases. With the ashpit doors closed the volume of gases passing to the stack are a minimum and the maximum force of draft will be shown by a gauge.

As draft measurements are taken along the path of the gases, the readings grow less as the points at which they are taken are farther from the stack, until in the boiler ashpit, with the ashpit doors open for freely admitting the air, there is little or no perceptible rise in the water of the gauge. The breeching, the boiler damper, the baffles and the tubes, and the coal on the grates all retard the passage of the gases, and the draft from the chimney is required to overcome the resistance offered by the various factors. The draft at the rear of the boiler setting where connection is made to the stack or flue may be 0.5 inch, while in the furnace directly over the fire it may not be over, say, 0.15 inch, the difference being the draft required to overcome the resistance offered in forcing the gases through the tubes and around the baffling.

One of the most important factors to be considered in designing a stack is the pressure required to force the air for combustion through the bed of fuel on the grates. This pressure will vary with the nature of the fuel used, and in many instances will be a large percentage of the total draft. In the case of natural draft, its measure is found directly by noting the draft in the furnace, for with properly designed ashpit doors it is evident that the pressure under the grates will not differ sensibly from atmospheric pressure.

Loss in Stack—The difference between the theoretical draft as determined by formula (24) and the amount lost by friction in the stack proper is the available draft, or that which the draft gauge indicates when connected to the base of the stack. The sum of the losses of draft in the flue, boiler and furnace must be equivalent to the available draft, and as these quantities can be determined from record of experiments, the problem of designing a stack becomes one of proportioning it to produce a certain available draft.

The loss in the stack due to friction of the gases can be calculated from the following formula:

f W*W C H [Delta]D = ————- (26) A*A*A

in which

[Delta]D = draft loss in inches of water, W = weight of gas in pounds passing per second, C = perimeter of stack in feet, H = height of stack in feet, f = a constant with the following values at sea level: .0015 for steel stacks, temperature of gases 600 degrees Fahrenheit. .0011 for steel stacks, temperature of gases 350 degrees Fahrenheit. .0020 for brick or brick-lined stacks, temperature of gases 600 degrees Fahrenheit. .0015 for brick or brick-lined stacks, temperature of gases 350 degrees Fahrenheit. A = Area of stack in square feet.



This formula can also be used for calculating the frictional losses for flues, in which case, C = the perimeter of the flue in feet, H = the length of the flue in feet, the other values being the same as for stacks.

The available draft is equal to the difference between the theoretical draft from formula (25) and the loss from formula (26), hence:

f W*W C H d^{1} = available draft = KH - ————- (27) A*A*A

Table 53 gives the available draft in inches that a stack 100 feet high will produce when serving different horse powers of boilers with the methods of calculation for other heights.

TABLE 53

AVAILABLE DRAFT

CALCULATED FOR 100-FOOT STACK OF DIFFERENT DIAMETERS ASSUMING STACK TEMPERATURE OF 500 DEGREES FAHRENHEIT AND 100 POUNDS OF GAS PER HORSE POWER

FOR OTHER HEIGHTS OF STACK MULTIPLY DRAFT BY HEIGHT / 100

- -+ Horse Power Diameter of Stack in Inches + - - - - - - - - - - - - - - - - - - 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 132 144 - - - - - - - - - - - - - - - - - -+ 100 .64 200 .55 .62 300 .41 .55 .61 400 .21 .46 .56 .61 500 .34 .50 .57 .61 600 .19 .42 .53 .59 700 .34 .48 .56 .60 .63 800 .23 .43 .52 .58 .61 .63 900 .36 .49 .56 .60 .62 .64 1000 .29 .45 .53 .58 .61 .63 .64 1100 .40 .50 .56 .60 .62 .63 .64 1200 .35 .47 .54 .58 .61 .63 .64 .65 1300 .29 .44 .52 .57 .60 .62 .63 .64 .65 1400 .40 .49 .55 .59 .61 .63 .64 .65 .65 1500 .36 .47 .53 .58 .60 .62 .63 .64 .65 .65 1600 .31 .43 .52 .56 .59 .62 .63 .64 .65 .65 1700 .41 .50 .55 .58 .61 .62 .64 .64 .65 1800 .37 .47 .54 .57 .60 .62 .63 .64 .65 1900 .34 .45 .52 .56 .59 .61 .63 .64 .64 2000 .43 .50 .55 .59 .61 .62 .63 .64 2100 .40 .49 .54 .58 .60 .62 .63 .64 2200 .38 .47 .53 .57 .59 .61 .62 .64 2300 .35 .45 .52 .56 .59 .61 .62 .63 2400 .32 .43 .50 .55 .58 .60 .62 .63 2500 .41 .49 .54 .57 .60 .61 .63 2600 .47 .53 .56 .59 .61 .62 .64 .65 2700 .45 .52 .55 .58 .60 .62 .64 .65 2800 .44 .59 .55 .58 .60 .61 .64 .65 2900 .42 .49 .54 .57 .59 .61 .63 .65 3000 .40 .48 .53 .56 .59 .61 .63 .64 3100 .38 .47 .52 .56 .58 .60 .63 .64 3200 .45 .51 .55 .58 .60 .63 .64 3300 .44 .50 .54 .57 .59 .62 .64 3400 .42 .49 .53 .56 .59 .62 .64 3500 .40 .48 .52 .56 .58 .62 .64 3600 .47 .52 .55 .58 .61 .63 3700 .45 .51 .55 .57 .61 .63 3800 .44 .50 .54 .57 .61 .63 3900 .43 .49 .53 .56 .60 .63 4000 .42 .48 .52 .56 .60 .62 4100 .40 .47 .52 .55 .60 .62 4200 .39 .46 .51 .55 .59 .62 4300 .45 .50 .54 .59 .62 4400 .44 .49 .53 .59 .62 4500 .43 .49 .53 .58 .61 4600 .42 .48 .52 .58 .61 4700 .41 .47 .51 .57 .61 4800 .40 .46 .51 .57 .60 4900 .45 .50 .57 .60 5000 .44 .49 .56 .60 + - - - - - - - - - - - - - - - - - -

FOR OTHER STACK TEMPERATURES ADD OR DEDUCT BEFORE MULTIPLYING BY HEIGHT / 100 AS FOLLOWS[52]

For 750 Degrees F. Add .17 inch. For 700 Degrees F. Add .14 inch. For 650 Degrees F. Add .11 inch. For 600 Degrees F. Add .08 inch. For 550 Degrees F. Add .04 inch. For 450 Degrees F. Deduct .04 inch. For 400 Degrees F. Deduct .09 inch. For 350 Degrees F. Deduct .14 inch.

[Graph: Horse Power of Boilers against Diameter of Stack in Inches

Fig. 33. Diameter of Stacks and Horse Power they will Serve

Computed from Formula (28). For brick or brick-lined stacks, increase the diameter 6 per cent]

Height and Diameter of Stacks—From this formula (27) it becomes evident that a stack of certain diameter, if it be increased in height, will produce the same available draft as one of larger diameter, the additional height being required to overcome the added frictional loss. It follows that among the various stacks that would meet the requirements of a particular case there must be one which can be constructed more cheaply than the others. It has been determined from the relation of the cost of stacks to their diameters and heights, in connection with the formula for available draft, that the minimum cost stack has a diameter dependent solely upon the horse power of the boilers it serves, and a height proportional to the available draft required.

Assuming 120 pounds of flue gas per hour for each boiler horse power, which provides for ordinary overloads and the use of poor coal, the method above stated gives:

For an unlined steel stack—

diameter in inches = 4.68 (H. P.)^{2/5} (28)

For a stack lined with masonry—

diameter in inches = 4.92 (H. P.)^{2/5} (29)

In both of these formulae H. P. = the rated horse power of the boiler.

From this formula the curve, Fig. 33, has been calculated and from it the stack diameter for any boiler horse power can be selected.

For stoker practice where a large stack serves a number of boilers, the area is usually made about one-third more than the above rules call for, which allows for leakage of air through the setting of any idle boilers, irregularities in operating conditions, etc.

Stacks with diameters determined as above will give an available draft which bears a constant ratio of the theoretical draft, and allowing for the cooling of the gases in their passage upward through the stack, this ratio is 8. Using this factor in formula (25), and transposing, the height of the chimney becomes,

d^{1} H = ——- (30) .8 K

Where H = height of stack in feet above the level of the grates, d^{1} = available draft required, K = constant as in formula.

Losses in Flues—The loss of draft in straight flues due to friction and inertia can be calculated approximately from formula (26), which was given for loss in stacks. It is to be borne in mind that C in this formula is the actual perimeter of the flue and is least, relative to the cross sectional area, when the section is a circle, is greater for a square section, and greatest for a rectangular section. The retarding effect of a square flue is 12 per cent greater than that of a circular flue of the same area and that of a rectangular with sides as 1 and 1-1/2, 15 per cent greater. The greater resistance of the more or less uneven brick or concrete flue is provided for in the value of the constants given for formula (26). Both steel and brick flues should be short and should have as near a circular or square cross section as possible. Abrupt turns are to be avoided, but as long easy sweeps require valuable space, it is often desirable to increase the height of the stack rather than to take up added space in the boiler room. Short right-angle turns reduce the draft by an amount which can be roughly approximated as equal to 0.05 inch for each turn. The turns which the gases make in leaving the damper box of a boiler, in entering a horizontal flue and in turning up into a stack should always be considered. The cross sectional areas of the passages leading from the boilers to the stack should be of ample size to provide against undue frictional loss. It is poor economy to restrict the size of the flue and thus make additional stack height necessary to overcome the added friction. The general practice is to make flue areas the same or slightly larger than that of the stack; these should be, preferably, at least 20 per cent greater, and a safe rule to follow in figuring flue areas is to allow 35 square feet per 1000 horse power. It is unnecessary to maintain the same size of flue the entire distance behind a row of boilers, and the areas at any point may be made proportional to the volume of gases that will pass that point. That is, the areas may be reduced as connections to various boilers are passed.



With circular steel flues of approximately the same size as the stacks, or reduced proportionally to the volume of gases they will handle, a convenient rule is to allow 0.1 inch draft loss per 100 feet of flue length and 0.05 inch for each right-angle turn. These figures are also good for square or rectangular steel flues with areas sufficiently large to provide against excessive frictional loss. For losses in brick or concrete flues, these figures should be doubled.

Underground flues are less desirable than overhead or rear flues for the reason that in most instances the gases will have to make more turns where underground flues are used and because the cross sectional area of such flues will oftentimes be decreased on account of an accumulation of dirt or water which it may be impossible to remove.

In tall buildings, such as office buildings, it is frequently necessary in order to carry spent gases above the roofs, to install a stack the height of which is out of all proportion to the requirements of the boilers. In such cases it is permissible to decrease the diameter of a stack, but care must be taken that this decrease is not sufficient to cause a frictional loss in the stack as great as the added draft intensity due to the increase in height, which local conditions make necessary.

In such cases also the fact that the stack diameter is permissibly decreased is no reason why flue sizes connecting to the stack should be decreased. These should still be figured in proportion to the area of the stack that would be furnished under ordinary conditions or with an allowance of 35 square feet per 1000 horse power, even though the cross sectional area appears out of proportion to the stack area.

Loss in Boiler—In calculating the available draft of a chimney 120 pounds per hour has been used as the weight of the gases per boiler horse power. This covers an overload of the boiler to an extent of 50 per cent and provides for the use of poor coal. The loss in draft through a boiler proper will depend upon its type and baffling and will increase with the per cent of rating at which it is run. No figures can be given which will cover all conditions, but for approximate use in figuring the available draft necessary it may be assumed that the loss through a boiler will be 0.25 inch where the boiler is run at rating, 0.40 inch where it is run at 150 per cent of its rated capacity, and 0.70 inch where it is run at 200 per cent of its rated capacity.

Loss in Furnace—The draft loss in the furnace or through the fuel bed varies between wide limits. The air necessary for combustion must pass through the interstices of the coal on the grate. Where these are large, as is the case with broken coal, but little pressure is required to force the air through the bed; but if they are small, as with bituminous slack or small sizes of anthracite, a much greater pressure is needed. If the draft is insufficient the coal will accumulate on the grates and a dead smoky fire will result with the accompanying poor combustion; if the draft is too great, the coal may be rapidly consumed on certain portions of the grate, leaving the fire thin in spots and a portion of the grates uncovered with the resulting losses due to an excessive amount of air.

[Graph: Force of Draft between Furnace and Ash Pit—Inches of Water against Pounds of Coal burned per Square Foot of Grate Surface per Hour

Fig. 34. Draft Required at Different Combustion Rates for Various Kinds of Coal]

Draft Required for Different Fuels—For every kind of fuel and rate of combustion there is a certain draft with which the best general results are obtained. A comparatively light draft is best with the free burning bituminous coals and the amount to use increases as the percentage of volatile matter diminishes and the fixed carbon increases, being highest for the small sizes of anthracites. Numerous other factors such as the thickness of fires, the percentage of ash and the air spaces in the grates bear directly on this question of the draft best suited to a given combustion rate. The effect of these factors can only be found by experiment. It is almost impossible to show by one set of curves the furnace draft required at various rates of combustion for all of the different conditions of fuel, etc., that may be met. The curves in Fig. 34, however, give the furnace draft necessary to burn various kinds of coal at the combustion rates indicated by the abscissae, for a general set of conditions. These curves have been plotted from the records of numerous tests and allow a safe margin for economically burning coals of the kinds noted.

Rate of Combustion—The amount of coal which can be burned per hour per square foot of grate surface is governed by the character of the coal and the draft available. When the boiler and grate are properly proportioned, the efficiency will be practically the same, within reasonable limits, for different rates of combustion. The area of the grate, and the ratio of this area to the boiler heating surface will depend upon the nature of the fuel to be burned, and the stack should be so designed as to give a draft sufficient to burn the maximum amount of fuel per square foot of grate surface corresponding to the maximum evaporative requirements of the boiler.

Solution of a Problem—The stack diameter can be determined from the curve, Fig. 33. The height can be determined by adding the draft losses in the furnace, through the boiler and flues, and computing from formula (30) the height necessary to give this draft.

Example: Proportion a stack for boilers rated at 2000 horse power, equipped with stokers, and burning bituminous coal that will evaporate 8 pounds of water from and at 212 degrees Fahrenheit per pound of fuel; the ratio of boiler heating surface to grate surface being 50:1; the flues being 100 feet long and containing two right-angle turns; the stack to be able to handle overloads of 50 per cent; and the rated horse power of the boilers based on 10 square feet of heating surface per horse power.

The atmospheric temperature may be assumed as 60 degrees Fahrenheit and the flue temperatures at the maximum overload as 550 degrees Fahrenheit. The grate surface equals 400 square feet.

2000 x 34-1/2 The total coal burned at rating = ——————- = 8624 pounds. 8

The coal per square foot of grate surface per hour at rating =

8624 —— = 22 pounds. 400

For 50 per cent overload the combustion rate will be approximately 60 per cent greater than this or 1.60 x 22 = 35 pounds per square foot of grate surface per hour. The furnace draft required for the combustion rate, from the curve, Fig. 34, is 0.6 inch. The loss in the boiler will be 0.4 inch, in the flue 0.1 inch, and in the turns 2 x 0.05 = 0.1 inch. The available draft required at the base of the stack is, therefore,

Inches Boiler 0.4 Furnace 0.6 Flues 0.1 Turns 0.1 —- Total 1.2

Since the available draft is 80 per cent of the theoretical draft, this draft due to the height required is 1.2 / .8 = 1.5 inch.

The chimney constant for temperatures of 60 degrees Fahrenheit and 550 degrees Fahrenheit is .0071 and from formula (30),

1.5 H = ——- = 211 feet. .0071

Its diameter from curve in Fig. 33 is 96 inches if unlined, and 102 inches inside if lined with masonry. The cross sectional area of the flue should be approximately 70 square feet at the point where the total amount of gas is to be handled, tapering to the boiler farthest from the stack to a size which will depend upon the size of the boiler units used.

Correction in Stack Sizes for Altitudes—It has ordinarily been assumed that a stack height for altitude will be increased inversely as the ratio of the barometric pressure at the altitude to that at sea level, and that the stack diameter will increase inversely as the two-fifths power of this ratio. Such a relation has been based on the assumption of constant draft measured in inches of water at the base of the stack for a given rate of operation of the boilers, regardless of altitude.

If the assumption be made that boilers, flues and furnace remain the same, and further that the increased velocity of a given weight of air passing through the furnace at a higher altitude would have no effect on the combustion, the theory has been advanced[53] that a different law applies.

Under the above assumptions, whenever a stack is working at its maximum capacity at any altitude, the entire draft is utilized in overcoming the various resistances, each of which is proportional to the square of the velocity of the gases. Since boiler areas are fixed, all velocities may be related to a common velocity, say, that within the stack, and all resistances may, therefore, be expressed as proportional to the square of the chimney velocity. The total resistance to flow, in terms of velocity head, may be expressed in terms of weight of a column of external air, the numerical value of such head being independent of the barometric pressure. Likewise the draft of a stack, expressed in height of column of external air, will be numerically independent of the barometric pressure. It is evident, therefore, that if a given boiler plant, with its stack operated with a fixed fuel, be transplanted from sea level to an altitude, assuming the temperatures remain constant, the total draft head measured in height of column of external air will be numerically constant. The velocity of chimney gases will, therefore, remain the same at altitude as at sea level and the weight of gases flowing per second with a fixed velocity will be proportional to the atmospheric density or inversely proportional to the normal barometric pressure.

To develop a given horse power requires a constant weight of chimney gas and air for combustion. Hence, as the altitude is increased, the density is decreased and, for the assumptions given above, the velocity through the furnace, the boiler passes, breeching and flues must be correspondingly greater at altitude than at sea level. The mean velocity, therefore, for a given boiler horse power and constant weight of gases will be inversely proportional to the barometric pressure and the velocity head measured in column of external air will be inversely proportional to the square of the barometric pressure.

For stacks operating at altitude it is necessary not only to increase the height but also the diameter, as there is an added resistance within the stack due to the added friction from the additional height. This frictional loss can be compensated by a suitable increase in the diameter and when so compensated, it is evident that on the assumptions as given, the chimney height would have to be increased at a ratio inversely proportional to the square of the normal barometric pressure.

In designing a boiler for high altitudes, as already stated, the assumption is usually made that a given grade of fuel will require the same draft measured in inches of water at the boiler damper as at sea level, and this leads to making the stack height inversely as the barometric pressures, instead of inversely as the square of the barometric pressures. The correct height, no doubt, falls somewhere between the two values as larger flues are usually used at the higher altitudes, whereas to obtain the ratio of the squares, the flues must be the same size in each case, and again the effect of an increased velocity of a given weight of air through the fire at a high altitude, on the combustion, must be neglected. In making capacity tests with coal fuel, no difference has been noted in the rates of combustion for a given draft suction measured by a water column at high and low altitudes, and this would make it appear that the correct height to use is more nearly that obtained by the inverse ratio of the barometric readings than by the inverse ratio of the squares of the barometric readings. If the assumption is made that the value falls midway between the two formulae, the error in using a stack figured in the ordinary way by making the height inversely proportional to the barometric readings would differ about 10 per cent in capacity at an altitude of 10,000 feet, which difference is well within the probable variation of the size determined by different methods. It would, therefore, appear that ample accuracy is obtained in all cases by simply making the height inversely proportional to the barometric readings and increasing the diameter so that the stacks used at high altitudes have the same frictional resistance as those used at low altitudes, although, if desired, the stack may be made somewhat higher at high altitudes than this rule calls for in order to be on the safe side.

The increase of stack diameter necessary to maintain the same friction loss is inversely as the two-fifths power of the barometric pressure.

Table 54 gives the ratio of barometric readings of various altitudes to sea level, values for the square of this ratio and values of the two-fifths power of this ratio.

TABLE 54

STACK CAPACITIES, CORRECTION FACTORS FOR ALTITUDES

_____________ Altitude R R^{2/5} Height in Feet Normal Ratio Barometer Ratio Increase Above Barometer Reading R*R in Stack Sea Level Sea Level to Diameter Altitude ___ ___ ____ __ ___ 0 30.00 1.000 1.000 1.000 1000 28.88 1.039 1.079 1.015 2000 27.80 1.079 1.064 1.030 3000 26.76 1.121 1.257 1.047 4000 25.76 1.165 1.356 1.063 5000 24.79 1.210 1.464 1.079 6000 23.87 1.257 1.580 1.096 7000 22.97 1.306 1.706 1.113 8000 22.11 1.357 1.841 1.130 9000 21.28 1.410 1.988 1.147 10000 20.49 1.464 2.144 1.165 ___ ___ ____ __ ___

These figures show that the altitude affects the height to a much greater extent than the diameter and that practically no increase in diameter is necessary for altitudes up to 3000 feet.

For high altitudes the increase in stack height necessary is, in some cases, such as to make the proportion of height to diameter impracticable. The method to be recommended in overcoming, at least partially, the great increase in height necessary at high altitudes is an increase in the grate surface of the boilers which the stack serves, in this way reducing the combustion rate necessary to develop a given power and hence the draft required for such combustion rate.

TABLE 55

STACK SIZES BY KENT'S FORMULA

ASSUMING 5 POUNDS OF COAL PER HORSE POWER

Height of Stack in Feet Side of Equiva- Dia- Area lent meter Square 50 60 70 80 90 100 110 125 150 175 Square Inches Feet Stack Inches Commercial Horse Power 33 5.94 106 115 125 133 141 149 30 36 7.07 129 141 152 163 173 182 32 39 8.30 155 169 183 196 208 219 229 245 35 42 9.62 183 200 216 231 245 258 271 289 316 38 48 12.57 246 269 290 311 330 348 365 389 426 460 43 54 15.90 318 348 376 402 427 449 472 503 551 595 48 60 19.64 400 437 473 505 536 565 593 632 692 748 54 66 23.76 490 537 580 620 658 694 728 776 849 918 59 72 28.27 591 646 698 747 792 835 876 934 1023 1105 64 78 33.18 700 766 828 885 939 990 1038 1107 1212 1310 70 84 38.48 818 896 968 1035 1098 1157 1214 1294 1418 1531 75 Height of Stack in Feet Side of Equiva- Dia- Area lent meter Square 100 110 125 150 175 200 225 250 Square Inches Feet Stack Inches Commercial Horse Power 90 44.18 1338 1403 1496 1639 1770 1893 2008 2116 80 96 50.27 1532 1606 1713 1876 2027 2167 2298 2423 86 102 56.75 1739 1824 1944 2130 2300 2459 2609 2750 91 108 63.62 1959 2054 2190 2392 2592 2770 2939 3098 98 114 70.88 2192 2299 2451 2685 2900 3100 3288 3466 101 120 78.54 2438 2557 2726 2986 3226 3448 3657 3855 107 126 86.59 2697 2829 3016 3303 3568 3814 4046 4265 112 132 95.03 2970 3114 3321 3637 3929 4200 4455 4696 117 144 113.10 3554 3726 3973 4352 4701 5026 5331 5618 128 156 132.73 4190 4393 4684 5131 5542 5925 6285 6624 138 168 153.94 4878 5115 5454 5974 6454 6899 7318 7713 150

Kent's Stack Tables—Table 55 gives, in convenient form for approximate work, the sizes of stacks and the horse power of boilers which they will serve. This table is a modification of Mr. William Kent's stack table and is calculated from his formula. Provided no unusual conditions are encountered, it is reliable for the ordinary rates of combustion with bituminous coals. It is figured on a consumption of 5 pounds of coal burned per hour per boiler horse power developed, this figure giving a fairly liberal allowance for the use of poor coal and for a reasonable overload. When the coal used is a low grade bituminous of the Middle or Western States, it is strongly recommended that these sizes be increased materially, such an increase being from 25 to 60 per cent, depending upon the nature of the coal and the capacity desired. For the coal burned per hour for any size stack given in the table, the values should be multiplied by 5.

A convenient rule for large stacks, 200 feet high and over, is to provide 30 square feet of cross sectional area per 1000 rated horse power.

Stacks for Oil Fuel—The requirements of stacks connected to boilers under which oil fuel is burned are entirely different from those where coal is used. While more attention has been paid to the matter of stack sizes for oil fuel in recent years, there has not as yet been gathered the large amount of experimental data available for use in designing coal stacks.

In the case of oil-fired boilers the loss of draft through the fuel bed is partially eliminated. While there may be practically no loss through any checkerwork admitting air to the furnace when a boiler is new, the areas for the air passage in this checkerwork will in a short time be decreased, due to the silt which is present in practically all fuel oil. The loss in draft through the boiler proper at a given rating will be less than in the case of coal-fired boilers, this being due to a decrease in the volume of the gases. Further, the action of the oil burner itself is to a certain extent that of a forced draft. To offset this decrease in draft requirement, the temperature of the gases entering the stack will be somewhat lower where oil is used than where coal is used, and the draft that a stack of a given height would give, therefore, decreases. The factors as given above, affecting as they do the intensity of the draft, affect directly the height of the stack to be used.

As already stated, the volume of gases from oil-fired boilers being less than in the case of coal, makes it evident that the area of stacks for oil fuel will be less than for coal. It is assumed that these areas will vary directly as the volume of the gases to be handled, and this volume for oil may be taken as approximately 60 per cent of that for coal.

In designing stacks for oil fuel there are two features which must not be overlooked. In coal-firing practice there is rarely danger of too much draft. In the burning of oil, however, this may play an important part in the reduction of plant economy, the influence of excessive draft being more apparent where the load on the plant may be reduced at intervals. The reason for this is that, aside from a slight decrease in temperature at reduced loads, the tendency, due to careless firing, is toward a constant gas flow through the boiler regardless of the rate of operation, with the corresponding increase of excess air at light loads. With excessive stack height, economical operation at varying loads is almost impossible with hand control. With automatic control, however, where stacks are necessarily high to take care of known peaks, under lighter loads this economical operation becomes less difficult. For this reason the question of designing a stack for a plant where the load is known to be nearly a constant is easier than for a plant where the load will vary over a wide range. While great care must be taken to avoid excessive draft, still more care must be taken to assure a draft suction within all parts of the setting under any and all conditions of operation. It is very easily possible to more than offset the economy gained through low draft, by the losses due to setting deterioration, resulting from such lack of suction. Under conditions where the suction is not sufficient to carry off the products of combustion, the action of the heat on the setting brickwork will cause its rapid failure.



It becomes evident, therefore, that the question of stack height for oil-fired boilers is one which must be considered with the greatest of care. The designer, on the one hand, must guard against the evils of excessive draft with the view to plant economy, and, on the other, against the evils of lack of draft from the viewpoint of upkeep cost. Stacks for this work should be proportioned to give ample draft for the maximum overload that a plant will be called upon to carry, all conditions of overload carefully considered. At the same time, where this maximum overload is figured liberally enough to insure a draft suction within the setting under all conditions, care must be taken against the installation of a stack which would give more than this maximum draft.

TABLE 56

STACK SIZES FOR OIL FUEL

ADAPTED FROM C. R. WEYMOUTH'S TABLE (TRANS. A. S. M. E. VOL. 34)

+ + + + -+ Height in Feet Above Boiler Room Floor Diameter+ + + + -+ + Inches 80 90 100 120 140 160 + + + + + + + + 33 161 206 233 270 306 315 36 208 253 295 331 363 387 39 251 303 343 399 488 467 42 295 359 403 474 521 557 48 399 486 551 645 713 760 54 519 634 720 847 933 1000 60 657 800 913 1073 1193 1280 66 813 993 1133 1333 1480 1593 72 980 1206 1373 1620 1807 1940 84 1373 1587 1933 2293 2560 2767 96 1833 2260 2587 3087 3453 3740 108 2367 2920 3347 4000 4483 4867 120 3060 3660 4207 5040 5660 6160 + + + + + + + + + +

Figures represent nominal rated horse power. Sizes as given good for 50 per cent overloads.

Based on centrally located stacks, short direct flues and ordinary operating efficiencies.

Table 56 gives the sizes of stacks, and horse power which they will serve for oil fuel. This table is, in modified form, one calculated by Mr. C. R. Weymouth after an exhaustive study of data pertaining to the subject, and will ordinarily give satisfactory results.

Stacks for Blast Furnace Gas Work—For boilers burning blast furnace gas, as in the case of oil-fired boilers, stack sizes as suited for coal firing will have to be modified. The diameter of stacks for this work should be approximately the same as for coal-fired boilers. The volume of gases would be slightly greater than from a coal fire and would decrease the draft with a given stack, but such a decrease due to volume is about offset by an increase due to somewhat higher temperatures in the case of the blast furnace gases.

Records show that with this class of fuel 175 per cent of the rated capacity of a boiler can be developed with a draft at the boiler damper of from 0.75 inch to 1.0 inch, and it is well to limit the height of stacks to one which will give this draft as a maximum. A stack of proper diameter, 130 feet high above the ground, will produce such a draft and this height should ordinarily not be exceeded. Until recently the question of economy in boilers fired with blast furnace gas has not been considered, but, aside from the economical standpoint, excessive draft should be guarded against in order to lower the upkeep cost.

Stacks should be made of sufficient height to produce a draft that will develop the maximum capacity required, and this draft decreased proportionately for loads under the maximum by damper regulation. The amount of gas fed to a boiler for any given rating is a fixed quantity and if a draft in excess of that required for that particular rate of operation is supplied, economy is decreased and the wear and tear on the setting is materially increased. Excess air which is drawn in, either through or around the gas burners by an excessive draft, will decrease economy, as in any other class of work. Again, as in oil-fired practice, it is essential on the other hand that a suction be maintained within all parts of the setting, in this case not only to provide against setting deterioration but to protect the operators from leakage of gas which is disagreeable and may be dangerous. Aside from the intensity of the draft, a poor mixture of the gas and air or a "laneing" action may lead to secondary combustion with the possibility of dangerous explosions within the setting, may cause a pulsating action within the setting, may increase the exit temperatures to a point where there is danger of burning out damper boxes, and, in general, is hard on the setting. It is highly essential, therefore, that the furnace be properly constructed to meet the draft which will be available.

Stacks for Wood-fired Boilers—For boilers using wood as fuel, there is but little data upon which to base stack sizes. The loss of draft through the bed of fuel will vary over limits even wider than in the case of coal, for in this class of fuel the moisture may run from practically 0.0 per cent to over 60 per cent, and the methods of handling and firing are radically different for the different classes of wood (see chapter on Wood-burning Furnaces). As economy is ordinarily of little importance, high stack temperatures may be expected, and often unavoidably large quantities of excess air are supplied due to the method of firing. In general, it may be stated that for this class of fuel the diameter of stacks should be at least as great as for coal-fired boilers, while the height may be slightly decreased. It is far the best plan in designing a stack for boilers using wood fuel to consider each individual set of conditions that exist, rather than try to follow any general rule.

One factor not to be overlooked in stacks for wood burning is their location. The fine particles of this fuel are often carried unconsumed through the boiler, and where the stack is not on top of the boiler, these particles may accumulate in the base of the stack below the point at which the flue enters. Where there is any air leakage through the base of such a stack, this fuel may become ignited and the stack burned. Where there is a possibility of such action taking place, it is well to line the stack with fire brick for a portion of its height.

Draft Gauges—The ordinary form of draft gauge, Fig. 35, which consists of a U-tube, containing water, lacks sensitiveness in measuring such slight pressure differences as usually exist, and for that reason gauges which multiply the draft indications are more convenient and are much used.



An instrument which has given excellent results is one introduced by Mr. G. H. Barrus, which multiplies the ordinary indications as many times as desired. This is illustrated in Fig. 36, and consists of a U-tube made of one-half inch glass, surmounted by two larger tubes, or chambers, each having a diameter of 2-1/2 inches. Two different liquids which will not mix, and which are of different color, are used, usually alcohol colored red and a certain grade of lubricating oil. The movement of the line of demarcation is proportional to the difference in the areas of the chambers and the U-tube connecting them. The instrument is calibrated by comparison with the ordinary U-tube gauge.

In the Ellison form of gauge the lower portion of the ordinary U-tube has been replaced by a tube slightly inclined to the horizontal, as shown in Fig. 37. By this arrangement any vertical motion in the right-hand upright tube causes a very much greater travel of the liquid in the inclined tube, thus permitting extremely small variation in the intensity of the draft to be read with facility.



The gauge is first leveled by means of the small level attached to it, both legs being open to the atmosphere. The liquid is then adjusted until its meniscus rests at the zero point on the left. The right-hand leg is then connected to the source of draft by means of a piece of rubber tubing. Under these circumstances, a rise of level of one inch in the right-hand vertical tube causes the meniscus in the inclined tube to pass from the point 0 to 1.0. The scale is divided into tenths of an inch, and the sub-divisions are hundredths of an inch.