Chapter VII: Part 7

The above estimates are on the basis of a thoroughly dry saturated steam or steam just at the point of being superheated or containing a few degrees of superheat. If the saturated steam is moist, the saving due to superheat is more and ordinarily the gain in economy due to superheated steam, for equal boiler efficiencies, as compared with commercially dry steam is, say, 5 per cent for each 100 degrees of superheat. Aside from this gain, as already stated, superheated steam prevents erosion of the turbine buckets that would be caused by water in the steam, and for the reasons enumerated it is standard practice to use superheated steam for turbine work. The less economical the steam motor, the more the gain due to superheated steam, and where there are a number of auxiliaries that are run with superheated steam, the percentage of gain will be greater than the figures given above, which are the minimum and are for the most economical type of large steam turbines.

An example from actual practice will perhaps best illustrate and emphasize the foregoing facts. In October 1909, a series of comparable tests were conducted by The Babcock & Wilcox Co. on the steam yacht "Idalia" to determine the steam consumption both with saturated and superheated steam of the main engine on that yacht, including as well the feed pump, circulating pump and air pump. These tests are more representative than are most tests of like character in that the saving in the steam consumption of the auxiliaries, which were much more wasteful than the main engine, formed an important factor. A résumé of these tests was published in the Journal of the Society of Naval Engineers, November 1909.

The main engines of the "Idalia" are four cylinder, triple expansion, 11-1/2 × 19 inches by 22-11/16 × 18 inches stroke. Steam is supplied by a Babcock & Wilcox marine boiler having 2500 square feet of boiler heating surface, 340 square feet of superheating surface and 65 square feet of grate surface.

The auxiliaries consist of a feed pump 6 × 4 × 6 inches, an independent air pump 6 × 12 × 8 inches, and a centrifugal pump driven by a reciprocating engine 5-7/16 × 5 inches. Under ordinary operating conditions the superheat existing is about 100 degrees Fahrenheit.

Tests were made with various degrees of superheat, the amount being varied by by-passing the gases and in the tests with the lower amounts of superheat by passing a portion of the steam from the boiler to the steam main without passing it through the superheater. Steam temperature readings were taken at the engine throttle. In the tests with saturated steam, the superheater was completely cut out of the system. Careful calorimeter measurements were taken, showing that the saturated steam delivered to the superheater was dry.

The weight of steam used was determined from the weight of the condensed steam discharge from the surface condenser, the water being pumped from the hot well into a tank mounted on platform scales. The same indicators, thermometers and gauges were used in all the tests, so that the results are directly comparable. The indicators used were of the outside spring type so that there was no effect of the temperature of the steam. All tests were of sufficient duration to show a uniformity of results by hours. A summary of the results secured is given in Table 26, which shows the water rate per indicated horse power and the heat consumption. The latter figures are computed on the basis of the heat imparted to the steam above the actual temperature of the feed water and, as stated, these are the results that are directly comparable.

TABLE 26

RESULTS OF "IDALIA" TESTS _______________________________________________________________________ | | | | | | | |Date 1909 |Oct. 11|Oct. 14|Oct. 14|Oct. 12|Oct. 13| |_______________________________|_______|_______|_______|_______|_______| |Degrees of superheat Fahrenheit| 0 | 57 | 88 | 96 | 105 | |Pressures, pounds per} Throttle| 190 | 196 | 201 | 198 | 203 | |square inch above } First | | | | | | |Atmospheric Pressure } Receiver| 68.4 | 66.0 | 64.3 | 61.9 | 63.0 | | } Second | | | | | | | } Receiver| 9.7 | 9.2 | 8.7 | 7.8 | 8.4 | |Vacuum, inches | 25.5 | 25.9 | 25.9 | 25.4 | 25.2 | |Temperature, Degrees Fahrenheit| | | | | | | } Feed | 201 | 206 | 205 | 202 | 200 | | } Hot Well | 116 | 109.5 | 115 | 111.5 | 111 | | | | | | | | |Revolutions per minute | | | | | | | {Air Pump | 57 | 56 | 53 | 54 | 45 | | {Circulating Pump| 196 | 198 | 196 | 198 | 197 | | {Main Engine | 194.3 | 191.5 | 195.1 | 191.5 | 193.1 | |Indicated Horse Power, | | | | | | | Main Engine | 512.3 | 495.2 | 521.1 | 498.3 | 502.2 | |Water per hour, total pounds |9397 |8430 |8234 |7902 |7790 | |Water per indicated | | | | | | | Horse Power, pounds | 18.3 | 17.0 | 15.8 | 15.8 | 15.5 | |B. t. u. per minute per | | | | | | | indicated Horse Power | 314 | 300 | 284 | 286 | 283 | |Per cent Saving of Steam | ... | 7.1 | 13.7 | 13.7 | 15.3 | |Percent Saving of Fuel | | | | | | | (computed) | ... | 4.4 | 9.5 | 8.9 | 9.9 | |_______________________________|_______|_______|_______|_______|_______|

The table shows that the saving in steam consumption with 105 degrees of superheat was 15.3 per cent and in heat consumption about 10 per cent. This may be safely stated to be a conservative representation of the saving that may be accomplished by the use of superheated steam in a plant as a whole, where superheated steam is furnished not only to the main engine but also to the auxiliaries. The figures may be taken as conservative for the reason that in addition to the saving as shown in the table, there would be in an ordinary plant a saving much greater than is generally realized in the drips, where the loss with saturated steam is greatly in excess of that with superheated steam.

The most conclusive and most practical evidence that a saving is possible through the use of superheated steam is in the fact that in the largest and most economical plants it is used almost without exception. Regardless of any such evidence, however, there is a deep rooted conviction in the minds of certain engineers that the use of superheated steam will involve operating difficulties which, with additional first cost, will more than offset any fuel saving. There are, of course, conditions under which the installation of superheaters would in no way be advisable. With a poorly designed superheater, no gain would result. In general, it may be stated that in a new plant, properly designed, with a boiler and superheater which will have an efficiency at least as high as a boiler without a superheater, a gain is certain.

Such a gain is dependent upon the class of engine and the power plant equipment in general. In determining the advisability of making a superheater installation, all of the factors entering into each individual case should be considered and balanced, with a view to determining the saving in relation to cost, maintenance, depreciation etc.

In highly economical plants, where the water consumption for an indicated horse power is low, the gain will be less than would result from the use of superheated steam in less economical plants where the water consumption is higher. It is impossible to make an accurate statement as to the saving possible but, broadly, it may vary from 3 to 5 per cent for 100 degrees of superheat in the large and economical plants using turbines or steam engines, in which there is a large ratio of expansion, to from 10 to 25 per cent for 100 degrees of superheat for the less economical steam motors.

Though a properly designed superheater will tend to raise rather than to decrease the boiler efficiency, it does not follow that all superheaters are efficient, for if the gases in passing over the superheater do not follow the path they would ordinarily take in passing over the boiler heating surface, a loss may result. This is noticeably true where part of the gases are passed over the superheater and are allowed to pass over only a part or in some cases none of the boiler heating surface.

With moderate degrees of superheat, from 100 to 200 degrees, where the piping is properly installed, there will be no greater operating difficulties than with saturated steam. Engine and turbine builders guarantee satisfactory operation with superheated steam. With high degrees of superheat, say, over 250 degrees, apparatus of a special nature must be used and it is questionable whether the additional care and liability to operating difficulties will offset any fuel saving accomplished. It is well established, however, that the operating difficulties, with the degrees of superheat to which this article is limited, have been entirely overcome.

The use of cast-iron fittings with superheated steam has been widely discussed. It is an undoubted fact that while in some instances superheated steam has caused deterioration of such fittings, in others cast-iron fittings have been used with 150 degrees of superheat without the least difficulty. The quality of the cast iron used in such fittings has doubtless a large bearing on the life of such fittings for this service. The difficulties that have been encountered are an increase in the size of the fittings and eventually a deterioration great enough to lead to serious breakage, the development of cracks, and when flanges are drawn up too tightly, the breaking of a flange from the body of the fitting. The latter difficulty is undoubtedly due, in certain instances, to the form of flange in which the strain of the connecting bolts tended to distort the metal.

The Babcock & Wilcox Co. have used steel castings in superheated steam work over a long period and experience has shown that this metal is suitable for the service. There seems to be a general tendency toward the use of steel fittings. In European practice, until recently, cast iron was used with apparently satisfactory results. The claim of European engineers was to the effect that their cast iron was of better quality than that found in this country and thus explained the results secured. Recently, however, certain difficulties have been encountered with such fittings and European engineers are leaning toward the use of steel for this work.

The degree of superheat produced by a superheater placed within the boiler setting will vary according to the class of fuel used, the form of furnace, the condition of the fire and the rate at which the boiler is being operated. This is necessarily true of any superheater swept by the main body of the products of combustion and is a fact that should be appreciated by the prospective user of superheated steam. With a properly designed superheater, however, such fluctuations would not be excessive, provided the boilers are properly operated. As a matter of fact the point to be guarded against in the use of superheated steam is that a maximum should not be exceeded. While, as stated, there may be a considerable fluctuation in the temperature of the steam as delivered from individual superheaters, where there are a number of boilers on a line the temperature of the combined flow of steam in the main will be found to be practically a constant, resulting from the offsetting of various furnace conditions of one boiler by another.

PROPERTIES OF AIR

Pure air is a mechanical mixture of oxygen and nitrogen. While different authorities give slightly varying values for the proportion of oxygen and nitrogen contained, the generally accepted values are:

By volume, oxygen 20.91 per cent, nitrogen 79.09 per cent.
By weight, oxygen 23.15 per cent, nitrogen 76.85 per cent.

Air in nature always contains other constituents in varying amounts, such as dust, carbon dioxide, ozone and water vapor.

Being perfectly elastic, the density or weight per unit of volume decreases in geometric progression with the altitude. This fact has a direct bearing in the proportioning of furnaces, flues and stacks at high altitudes, as will be shown later in the discussion of these subjects. The atmospheric pressures corresponding to various altitudes are given in Table 12.

The weight and volume of air depend upon the pressure and the temperature, as expressed by the formula:

Pv = 53.33 T (9)

Where P = the absolute pressure in pounds per square foot,
v = the volume in cubic feet of one pound of air,
T = the absolute temperature of the air in degrees Fahrenheit,
53.33 = a constant for air derived from the ratio of pressure, volume
and temperature of a perfect gas.

The weight of one cubic foot of air will obviously be the reciprocal of its volume, that is, 1/v pounds.

TABLE 27

VOLUME AND WEIGHT OF AIR AT ATMOSPHERIC PRESSURE AT VARIOUS TEMPERATURES _______________________________________ | | | | | | Volume | | | Temperature | One Pound | Weight One | | Degrees | in | Cubic Foot | | Fahrenheit | Cubic Feet | in Pounds | |_____________|____________|____________| | | | | | 32 | 12.390 | .080710 | | 50 | 12.843 | .077863 | | 55 | 12.969 | .077107 | | 60 | 13.095 | .076365 | | 65 | 13.221 | .075637 | | 70 | 13.347 | .074923 | | 75 | 13.473 | .074223 | | 80 | 13.599 | .073535 | | 85 | 13.725 | .072860 | | 90 | 13.851 | .072197 | | 95 | 13.977 | .071546 | | 100 | 14.103 | .070907 | | 110 | 14.355 | .069662 | | 120 | 14.607 | .068460 | | 130 | 14.859 | .067299 | | 140 | 15.111 | .066177 | | 150 | 15.363 | .065092 | | 160 | 15.615 | .064041 | | 170 | 15.867 | .063024 | | 180 | 16.119 | .062039 | | 190 | 16.371 | .061084 | | 200 | 16.623 | .060158 | | 210 | 16.875 | .059259 | | 212 | 16.925 | .059084 | | 220 | 17.127 | .058388 | | 230 | 17.379 | .057541 | | 240 | 17.631 | .056718 | | 250 | 17.883 | .055919 | | 260 | 18.135 | .055142 | | 270 | 18.387 | .054386 | | 280 | 18.639 | .053651 | | 290 | 18.891 | .052935 | | 300 | 19.143 | .052238 | | 320 | 19.647 | .050898 | | 340 | 20.151 | .049625 | | 360 | 20.655 | .048414 | | 380 | 21.159 | .047261 | | 400 | 21.663 | .046162 | | 425 | 22.293 | .044857 | | 450 | 22.923 | .043624 | | 475 | 23.554 | .042456 | | 500 | 24.184 | .041350 | | 525 | 24.814 | .040300 | | 550 | 25.444 | .039302 | | 575 | 26.074 | .038352 | | 600 | 26.704 | .037448 | | 650 | 27.964 | .035760 | | 700 | 29.224 | .034219 | | 750 | 30.484 | .032804 | | 800 | 31.744 | .031502 | | 850 | 33.004 | .030299 | |_____________|____________|____________|

Example: Required the volume of air in cubic feet under 60.3 pounds gauge pressure per square inch at 115 degrees Fahrenheit.

P = 144 (14.7 + 60.3) = 10,800.

T = 115 + 460 = 575 degrees.

53.33 × 575
Hence v = ----------- = 2.84 cubic feet, and
10,800

1 1
Weight per cubic foot = - = ---- = 0.352 pounds.
v 2.84

Table 27 gives the weights and volumes of air under atmospheric pressure at varying temperatures.

Formula (9) holds good for other gases with the change in the value of the constant as follows:

For oxygen 48.24, nitrogen 54.97, hydrogen 765.71.

The specific heat of air at constant pressure varies with its temperature. A number of determinations of this value have been made and certain of those ordinarily accepted as most authentic are given in Table 28.

TABLE 28

SPECIFIC HEAT OF AIR AT CONSTANT PRESSURE AND VARIOUS TEMPERATURES ______________________________________________________________ | | | | | Temperature Range | | | |_________________________|_______________|____________________| | | | | | | Degrees | Degrees | Specific Heat | Authority | | Centigrade | Fahrenheit | | | |____________|____________|_______________|____________________| | | | | | | -30- 10 | -22- 50 | 0.2377 | Regnault | | 0-100 | 32- 212 | 0.2374 | Regnault | | 0-200 | 32- 392 | 0.2375 | Regnault | | 20-440 | 68- 824 | 0.2366 | Holborn and Curtis | | 20-630 | 68-1166 | 0.2429 | Holborn and Curtis | | 20-800 | 68-1472 | 0.2430 | Holborn and Curtis | | 0-200 | 32- 392 | 0.2389 | Wiedemann | |____________|____________|_______________|____________________|

This value is of particular importance in waste heat work and it is regrettable that there is such a variation in the different experiments. Mallard and Le Chatelier determined values considerably higher than any given in Table 28. All things considered in view of the discrepancy of the values given, there appears to be as much ground for the use of a constant value for the specific heat of air at any temperature as for a variable value. Where this value is used throughout this book, it has been taken as 0.24.

Air may carry a considerable quantity of water vapor, which is frequently 3 per cent of the total weight. This fact is of importance in problems relating to heating drying and the compressing of air. Table 29 gives the amount of vapor required to saturate air at different temperatures, its weight, expansive force, etc., and contains sufficient information for solving practically all problems of this sort that may arise.

TABLE 29

WEIGHTS OF AIR, VAPOR OF WATER, AND SATURATED MIXTURES OF AIR AND VAPOR
AT DIFFERENT TEMPERATURES,
UNDER THE ORDINARY ATMOSPHERIC PRESSURE OF 29.921 INCHES OF MERCURY

Column Headings: 1: Temperature Degrees Fahrenheit 2: Volume of Dry Air at Different Temperatures, the Volume at 32 Degrees being 1.000 3: Weight of Cubic Foot of Dry Air at the Different Temperatures Pounds 4: Elastic Force of Vapor in Inches of Mercury (Regnault) 5: Elastic Force of the Air in the Mixture of Air and Vapor in Inches of Mercury 6: Weight of the Air in Pounds 7: Weight of the Vapor in Pounds 8: Total Weight of Mixture in Pounds 9: Weight of Vapor Mixed with One Pound of Air, in Pounds 10: Weight of Dry Air Mixed with One Pound of Vapor, in Pounds 11: Cubic Feet of Vapor from One Pound of Water at its own Pressure in Column 4 ____________________________________________________________________________ | | | | | | | | | | | | Mixtures of Air Saturated | | | | | | | with Vapor | | |___|_____|_____|______|______________________________________________|______| | | | | | |Weight of Cubic Foot | | | | | | | | | | of the Mixture of | | | | | | | | | | Air and Vapor | | | | | | | | | |_____________________| | | | | | | | | | | | | | | | | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | |___|_____|_____|______|______|_____|_______|_______|________|________|______| | | | | | | | | | | | | | 0| .935|.0864| .044|29.877|.0863|.000079|.086379| .00092|1092.4 | | | 12| .960|.0842| .074|29.849|.0840|.000130|.084130| .00155| 646.1 | | | 22| .980|.0824| .118|29.803|.0821|.000202|.082302| .00245| 406.4 | | | 32|1.000|.0807| .181|29.740|.0802|.000304|.080504| .00379| 263.81 |3289 | | 42|1.020|.0791| .267|29.654|.0784|.000440|.078840| .00561| 178.18 |2252 | | | | | | | | | | | | | | 52|1.041|.0776| .388|29.533|.0766|.000627|.077227| .00810| 122.17 |1595 | | 62|1.061|.0761| .556|29.365|.0747|.000881|.075581| .01179| 84.79 |1135 | | 72|1.082|.0747| .785|29.136|.0727|.001221|.073921| .01680| 59.54 | 819 | | 82|1.102|.0733| 1.092|28.829|.0706|.001667|.072267| .02361| 42.35 | 600 | | 92|1.122|.0720| 1.501|28.420|.0684|.002250|.070717| .03289| 30.40 | 444 | | | | | | | | | | | | | |102|1.143|.0707| 2.036|27.885|.0659|.002997|.068897| .04547| 21.98 | 334 | |112|1.163|.0694| 2.731|27.190|.0631|.003946|.067046| .06253| 15.99 | 253 | |122|1.184|.0682| 3.621|26.300|.0599|.005142|.065042| .08584| 11.65 | 194 | |132|1.204|.0671| 4.752|25.169|.0564|.006639|.063039| .11771| 8.49 | 151 | |142|1.224|.0660| 6.165|23.756|.0524|.008473|.060873| .16170| 6.18 | 118 | | | | | | | | | | | | | |152|1.245|.0649| 7.930|21.991|.0477|.010716|.058416| .22465| 4.45 | 93.3| |162|1.265|.0638|10.099|19.822|.0423|.013415|.055715| .31713| 3.15 | 74.5| |172|1.285|.0628|12.758|17.163|.0360|.016682|.052682| .46338| 2.16 | 59.2| |182|1.306|.0618|15.960|13.961|.0288|.020536|.049336| .71300| 1.402| 48.6| |192|1.326|.0609|19.828|10.093|.0205|.025142|.045642| 1.22643| .815| 39.8| | | | | | | | | | | | | |202|1.347|.0600|24.450| 5.471|.0109|.030545|.041445| 2.80230| .357| 32.7| |212|1.367|.0591|29.921| 0.000|.0000|.036820|.036820|Infinite| .000| 27.1| |___|_____|_____|______|______|_____|_______|_______|________|________|______|

Column 5 = barometer pressure of 29.921, minus the proportion of this due to vapor pressure from column 4.

COMBUSTION

Combustion may be defined as the rapid chemical combination of oxygen with carbon, hydrogen and sulphur, accompanied by the diffusion of heat and light. That portion of the substance thus combined with the oxygen is called combustible. As used in steam engineering practice, however, the term combustible is applied to that portion of the fuel which is dry and free from ash, thus including both oxygen and nitrogen which may be constituents of the fuel, though not in the true sense of the term combustible.

Combustion is perfect when the combustible unites with the greatest possible amount of oxygen, as when one atom of carbon unites with two atoms of oxygen to form carbon dioxide, CO_{2}. The combustion is imperfect when complete oxidation of the combustible does not occur, or where the combustible does not unite with the maximum amount of oxygen, as when one atom of carbon unites with one atom of oxygen to form carbon monoxide, CO, which may be further burned to carbon dioxide.

Kindling Point--Before a combustible can unite with oxygen and combustion takes place, its temperature must first be raised to the ignition or kindling point, and a sufficient time must be allowed for the completion of the combustion before the temperature of the gases is lowered below that point. Table 30, by Stromeyer, gives the approximate kindling temperatures of different fuels.

TABLE 30

KINDLING TEMPERATURE OF VARIOUS FUELS

____________________________________ | | | | | Degrees | | | Fahrenheit | |_________________|__________________| | | | | Lignite Dust | 300 | | Dried Peat | 435 | | Sulphur | 470 | | Anthracite Dust | 570 | | Coal | 600 | | Coke | Red Heat | | Anthracite | Red Heat, 750 | | Carbon Monoxide | Red Heat, 1211 | | Hydrogen | 1030 or 1290 | |_________________|__________________|

Combustibles--The principal combustibles in coal and other fuels are carbon, hydrogen and sulphur, occurring in varying proportions and combinations.

Carbon is by far the most abundant as is indicated in the chapters on fuels.

Hydrogen in a free state occurs in small quantities in some fuels, but is usually found in combination with carbon, in the form of hydrocarbons. The density of hydrogen is 0.0696 (Air = 1) and its weight per cubic foot, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.005621 pounds.

Sulphur is found in most coals and some oils. It is usually present in combined form, either as sulphide of iron or sulphate of lime; in the latter form it has no heat value. Its presence in fuel is objectionable because of its tendency to aid in the formation of clinkers, and the gases from its combustion, when in the presence of moisture, may cause corrosion.

Nitrogen is drawn into the furnace with the air. Its density is 0.9673 (Air = 1); its weight, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.07829 pounds per cubic foot; each pound of air at atmospheric pressure contains 0.7685 pounds of nitrogen, and one pound of nitrogen is contained in 1.301 pounds of air.

Nitrogen performs no useful office in combustion and passes through the furnace without change. It dilutes the air, absorbs heat, reduces the temperature of the products of combustion, and is the chief source of heat losses in furnaces.

Calorific Value--Each combustible element of gas will combine with oxygen in certain definite proportions and will generate a definite amount of heat, measured in B. t. u. This definite amount of heat per pound liberated by perfect combustion is termed the calorific value of that substance. Table 31, gives certain data on the reactions and results of combustion for elementary combustibles and several compounds.

TABLE 31

OXYGEN AND AIR REQUIRED FOR COMBUSTION

AT 32 DEGREES AND 29.92 INCHES

Column headings:

1: Oxidizable Substance or Combustible 2: Chemical Symbol 3: Atomic or Combining Weight 4: Chemical Reaction 5: Product of Combustion 6: Oxygen per Pound of Column 1 Pounds 7: Nitrogen per Pound of Column 1. 3.32[23] × O Pounds 8: Air per Pound of Column 1. 4.32[24] × O Pounds 9: Gaseous Product per Pound of Column 1[25] + Column 8 Pounds 10: Heat Value per Pound of Column 1 B. t. u. 11: Volumes of Column 1 Entering Combination Volume 12: Volumes of Oxygen Combining with Column 11 Volume 13: Volumes of Product Formed Volume 14: Volume per Pound of Column 1 in Gaseous Form Cubic Feet 15: Volume of Oxygen per Pound of Column 1 Cubic Feet 16: Volume of Products of Combustion per Pound of Column 1 Cubic Feet 17: Volume of Nitrogen per Pound of Column 1 3.782[26] × Column 15 Cubic Feet 18: Volume of Gas per pound of Column 1 = Column 10 ÷ Column 17 Cubic Feet

BY WEIGHT ________________________________________________________________________ | | | | | | | | 1 | 2 | 3 | 4 | 5 | 6 | |________________|_______|____|________________|_________________|_______| | | | | | | | | Carbon | C | 12 | C+2O = CO_{2} | Carbon Dioxide | 2.667 | | Carbon | C | 12 | C+O = CO | Carbon Monoxide | 1.333 | | Carbon Monoxide| CO | 28 | CO+O = CO_{2} | Carbon Dioxide | .571 | | Hydrogen | H | 1 | 2H+O = H_{2}O | Water | 8 | | | | / CH_{4}+4O = | Carbon Dioxide \ | | Methane | CH_{4}| 16 | | | 4 | | | | \ CO_{2}+2H_{2}O | and Water / | | Sulphur | S | 32 | S+2O = SO_{2} | Sulphur Dioxide | 1 | |________________|_______|____|________________|_________________|_______|

________________________________________________________ | | | | | | | | 1 | 2 | 7 | 8 | 9 | 10 | |________________|_______|_______|_______|_______|_______| | | | | | | | | Carbon | C | 8.85 | 11.52 | 12.52 | 14600 | | Carbon | C | 4.43 | 5.76 | 6.76 | 4450 | | Carbon Monoxide| CO | 1.90 | 2.47 | 3.47 | 10150 | | Hydrogen | H | 26.56 | 34.56 | 35.56 | 62000 | | | | | | | | | Methane | CH_{4}| 13.28 | 17.28 | 18.28 | 23550 | | | | | | | | | Sulphur | S | 3.32 | 4.32 | 5.32 | 4050 | |________________|_______|_______|_______|_______|_______|

BY VOLUME

________________________________________________________________ | | | | | | | | 1 | 2 | 11 | 12 | 13 | 14 | |_________________|________|______|____|________________|________| | | | | | | | | Carbon | C | 1C | 2 | 2CO_{2} | 14.95 | | Carbon | C | 1C | 1 | 2CO | 14.95 | | Carbon Monoxide | CO | 2CO | 1 | 2CO_{2} | 12.80 | | Hydrogen | H | 2H | 1 | 2H_{2}O | 179.32 | | Methane | CH_{4} | 1C4H | 4 | 1CO_{2} 2H_{2}O| 22.41 | | Sulphur | S | 1S | 2 | 1SO_{2} | 5.60 | |_________________|________|______|____|________________|________|

_____________________________________________________________ | | | | | | | | 1 | 2 | 15 | 16 | 17 | 18 | |_________________|________|_______|________|________|________| | | | | | | | | Carbon | C | 29.89 | 29.89 | 112.98 | 142.87 | | Carbon | C | 14.95 | 29.89 | 56.49 | 86.38 | | Carbon Monoxide | CO | 6.40 | 12.80 | 24.20 | 37.00 | | Hydrogen | H | 89.66 | 179.32 | 339.09 | 518.41 | | Methane | CH_{4} | 44.83 | 67.34 | 169.55 | 236.89 | | Sulphur | S | 11.21 | 11.21 | 42.39 | 53.60 | |_________________|________|_______|________|________|________|

It will be seen from this table that a pound of carbon will unite with 2-2/3 pounds of oxygen to form carbon dioxide, and will evolve 14,600 B. t. u. As an intermediate step, a pound of carbon may unite with 1-1/3 pounds of oxygen to form carbon monoxide and evolve 4450 B. t. u., but in its further conversion to CO_{2} it would unite with an additional 1-1/3 times its weight of oxygen and evolve the remaining 10,150 B. t. u. When a pound of CO burns to CO_{2}, however, only 4350 B. t. u. are evolved since the pound of CO contains but 3/7 pound carbon.

Air Required for Combustion--It has already been shown that each combustible element in fuel will unite with a definite amount of oxygen. With the ultimate analysis of the fuel known, in connection with Table 31, the theoretical amount of air required for combustion may be readily calculated.

Let the ultimate analysis be as follows:

_Per Cent_ Carbon 74.79 Hydrogen 4.98 Oxygen 6.42 Nitrogen 1.20 Sulphur 3.24 Water 1.55 Ash 7.82 ------ 100.00

When complete combustion takes place, as already pointed out, the carbon in the fuel unites with a definite amount of oxygen to form CO_{2}. The hydrogen, either in a free or combined state, will unite with oxygen to form water vapor, H_{2}O. Not all of the hydrogen shown in a fuel analysis, however, is available for the production of heat, as a portion of it is already united with the oxygen shown by the analysis in the form of water, H_{2}O. Since the atomic weights of H and O are respectively 1 and 16, the weight of the combined hydrogen will be 1/8 of the weight of the oxygen, and the hydrogen available for combustion will be H - 1/8 O. In complete combustion of the sulphur, sulphur dioxide SO_{2} is formed, which in solution in water forms sulphuric acid.

Expressed numerically, the theoretical amount of air for the above analysis is as follows:

0.7479 C × 2-2/3 = 1.9944 O needed
( 0.0642 )
( 0.0498 - -------) H × 8 = 0.3262 O needed
( 8 )
0.0324 S × 1 = 0.0324 O needed
------
Total 2.3530 O needed

One pound of oxygen is contained in 4.32 pounds of air.

The total air needed per pound of coal, therefore, will be 2.353 × 4.32 = 10.165.

The weight of combustible per pound of fuel is .7479 + .0418[27] + .0324 + .012 = .83 pounds, and the air theoretically required per pound of combustible is 10.165 ÷ .83 = 12.2 pounds.

The above is equivalent to computing the theoretical amount of air required per pound of fuel by the formula:

( O)
Weight per pound = 11.52 C + 34.56 (H - -) + 4.32 S (10)
( 8)

where C, H, O and S are proportional parts by weight of carbon, hydrogen, oxygen and sulphur by ultimate analysis.

In practice it is impossible to obtain perfect combustion with the theoretical amount of air, and an excess may be required, amounting to sometimes double the theoretical supply, depending upon the nature of the fuel to be burned and the method of burning it. The reason for this is that it is impossible to bring each particle of oxygen in the air into intimate contact with the particles in the fuel that are to be oxidized, due not only to the dilution of the oxygen in the air by nitrogen, but because of such factors as the irregular thickness of the fire, the varying resistance to the passage of the air through the fire in separate parts on account of ash, clinker, etc. Where the difficulties of drawing air uniformly through a fuel bed are eliminated, as in the case of burning oil fuel or gas, the air supply may be materially less than would be required for coal. Experiment has shown that coal will usually require 50 per cent more than the theoretical net calculated amount of air, or about 18 pounds per pound of fuel either under natural or forced draft, though this amount may vary widely with the type of furnace, the nature of the coal, and the method of firing. If less than this amount of air is supplied, the carbon burns to monoxide instead of dioxide and its full heat value is not developed.

An excess of air is also a source of waste, as the products of combustion will be diluted and carry off an excessive amount of heat in the chimney gases, or the air will so lower the temperature of the furnace gases as to delay the combustion to an extent that will cause carbon monoxide to pass off unburned from the furnace. A sufficient amount of carbon monoxide in the gases may cause the action known as secondary combustion, by igniting or mingling with air after leaving the furnace or in the flues or stack. Such secondary combustion which takes place either within the setting after leaving the furnace or in the flues or stack always leads to a loss of efficiency and, in some instances, leads to overheating of the flues and stack.

Table 32 gives the theoretical amount of air required for various fuels calculated from formula (10) assuming the analyses of the fuels given in the table.

The process of combustion of different fuels and the effect of variation in the air supply for their combustion is treated in detail in the chapters dealing with the various fuels.

TABLE 32

CALCULATED THEORETICAL AMOUNT OF AIR
REQUIRED PER POUND OF VARIOUS FUELS

____________________________________________________________ | |Weight of Constituents in One |Air Required| | Fuel |Pound Dry Fuel |per Pound | | |______________________________|of Fuel | | | Carbon | Hydrogen| Oxygen |Pounds | | | Per Cent| Per Cent| Per Cent | | |________________|_________|_________|__________|____________| |Coke | 94.0 | . | . | 10.8 | |Anthracite Coal | 91.5 | 3.5 | 2.6 | 11.7 | |Bituminous Coal | 87.0 | 5.0 | 4.0 | 11.6 | |Lignite | 70.0 | 5.0 | 20.0 | 8.9 | |Wood | 50.0 | 6.0 | 43.5 | 6.0 | |Oil | 85.0 | 13.0 | 1.0 | 14.3 | |________________|_________|_________|__________|____________|

ANALYSIS OF FLUE GASES

The object of a flue gas analysis is the determination of the completeness of the combustion of the carbon in the fuel, and the amount and distribution of the heat losses due to incomplete combustion. The quantities actually determined by an analysis are the relative proportions by volume, of carbon dioxide (CO_{2}), oxygen (O), and carbon monoxide (CO), the determinations being made in this order.

The variations of the percentages of these gases in an analysis is best illustrated in the consideration of the complete combustion of pure carbon, a pound of which requires 2.67 pounds of oxygen,[28] or 32 cubic feet at 60 degrees Fahrenheit. The gaseous product of such combustion will occupy, when cooled, the same volume as the oxygen, namely, 32 cubic feet. The air supplied for the combustion is made up of 20.91 per cent oxygen and 79.09 per cent nitrogen by volume. The carbon united with the oxygen in the form of carbon dioxide will have the same volume as the oxygen in the air originally supplied. The volume of the nitrogen when cooled will be the same as in the air supplied, as it undergoes no change. Hence for complete combustion of one pound of carbon, where no excess of air is supplied, an analysis of the products of combustion will show the following percentages by volume:

_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 20.91
Oxygen 0 = 0.00
Nitrogen 121 = 79.09
--- ------
Air required for one pound Carbon 153 = 100.00

For 50 per cent excess air the volume will be as follows:

153 × 1½ = 229.5 cubic feet of air per pound of carbon.

_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 13.91 }
Oxygen 16 = 7.00 } = 20.91 per cent
Nitrogen 181.5 = 79.09
----- ------
229.5 = 100.00

For 100 per cent excess air the volume will be as follows:

153 × 2 = 306 cubic feet of air per pound of carbon.

_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 10.45 }
Oxygen 32 = 10.45 } = 20.91 per cent
Nitrogen 242 = 79.09
--- ------
306 = 100.00

In each case the volume of oxygen which combines with the carbon is equal to (cubic feet of air × 20.91 per cent)--32 cubic feet.

It will be seen that no matter what the excess of air supplied, the actual amount of carbon dioxide per pound of carbon remains the same, while the percentage by volume decreases as the excess of air increases. The actual volume of oxygen and the percentage by volume increases with the excess of air, and the percentage of oxygen is, therefore, an indication of the amount of excess air. In each case the sum of the percentages of CO_{2} and O is the same, 20.9. Although the volume of nitrogen increases with the excess of air, its percentage by volume remains the same as it undergoes no change while combustion takes place; its percentage for any amount of air excess, therefore, will be the same after combustion as before, if cooled to the same temperature. It must be borne in mind that the above conditions hold only for the perfect combustion of a pound of pure carbon.

Carbon monoxide (CO) produced by the imperfect combustion of carbon, will occupy twice the volume of the oxygen entering into its composition and will increase the volume of the flue gases over that of the air supplied for combustion in the proportion of

100 + ½ the per cent CO
1 to -----------------------
100

When pure carbon is the fuel, the sum of the percentages by volume of carbon dioxide, oxygen and one-half of the carbon monoxide, must be in the same ratio to the nitrogen in the flue gases as is the oxygen to the nitrogen in the air supplied, that is, 20.91 to 79.09. When burning coal, however, the percentage of nitrogen is obtained by subtracting the sum of the percentages by volume of the other gases from 100. Thus if an analysis shows 12.5 per cent CO_{2}, 6.5 per cent O, and 0.6 per cent CO, the percentage of nitrogen which ordinarily is the only other constituent of the gas which need be considered, is found as follows:

100 - (12.5 + 6.5 + 0.6) = 80.4 per cent.

The action of the hydrogen in the volatile constituents of the fuel is to increase the apparent percentage of the nitrogen in the flue gases. This is due to the fact that the water vapor formed by the combustion of the hydrogen will condense at a temperature at which the analysis is made, while the nitrogen which accompanied the oxygen with which the hydrogen originally combined maintains its gaseous form and passes into the sampling apparatus with the other gases. For this reason coals containing high percentages of volatile matter will produce a larger quantity of water vapor, and thus increase the apparent percentage of nitrogen.

Air Required and Supplied--When the ultimate analysis of a fuel is known, the air required for complete combustion with no excess can be found as shown in the chapter on combustion, or from the following approximate formula:

Pounds of air required per pound of fuel =

(C O S)
34.56 (- + (H - -) + -)[29] (11)
(3 8 8)

where C, H and O equal the percentage by weight of carbon, hydrogen and oxygen in the fuel divided by 100.

When the flue gas analysis is known, the total, amount of air supplied is:

Pounds of air supplied per pound of fuel =

N
3.036 (-----------) × C[30] (12)
CO_{2} + CO

where N, CO_{2} and CO are the percentages by volume of nitrogen, carbon dioxide and carbon monoxide in the flue gases, and C the percentage by weight of carbon which is burned from the fuel and passes up the stack as flue gas. This percentage of C which is burned must be distinguished from the percentage of C as found by an ultimate analysis of the fuel. To find the percentage of C which is burned, deduct from the total percentage of carbon as found in the ultimate analysis, the percentage of unconsumed carbon found in the ash. This latter quantity is the difference between the percentage of ash found by an analysis and that as determined by a boiler test. It is usually assumed that the entire combustible element in the ash is carbon, which assumption is practically correct. Thus if the ash in a boiler test were 16 per cent and by an analysis contained 25 per cent of carbon, the percentage of unconsumed carbon would be 16 × .25 = 4 per cent of the total coal burned. If the coal contained by ultimate analysis 80 per cent of carbon the percentage burned, and of which the products of combustion pass up the chimney would be 80 - 4 = 76 per cent, which is the correct figure to use in calculating the total amount of air supplied by formula (12).

The weight of flue gases resulting from the combustion of a pound of dry coal will be the sum of the weights of the air per pound of coal and the combustible per pound of coal, the latter being equal to one minus the percentage of ash as found in the boiler test. The weight of flue gases per pound of dry fuel may, however, be computed directly from the analyses, as shown later, and the direct computation is that ordinarily used.

The ratio of the air actually supplied per pound of fuel to that theoretically required to burn it is:

N
3.036(---------)×C
CO_{2}+CO
------------------ (13)
C O
34.56(- + H - -)
3 8

in which the letters have the same significance as in formulae (11) and (12).

The ratio of the air supplied per pound of combustible to the amount theoretically required is:

N ------------------ (14) N - 3.782(O - ½CO)

which is derived as follows:

The N in the flue gas is the content of nitrogen in the whole amount of air supplied. The oxygen in the flue gas is that contained in the air supplied and which was not utilized in combustion. This oxygen was accompanied by 3.782 times its volume of nitrogen. The total amount of excess oxygen in the flue gases is (O - ½CO); hence N - 3.782(O - ½CO) represents the nitrogen content in the air actually required for combustion and N ÷ (N - 3.782[O - ½CO]) is the ratio of the air supplied to that required. This ratio minus one will be the proportion of excess air.

The heat lost in the flue gases is L = 0.24 W (T - t) (15)

Where L = B. t. u. lost per pound of fuel,
W = weight of flue gases in pounds per pound of dry coal,
T = temperature of flue gases,
t = temperature of atmosphere,
0.24 = specific heat of the flue gases.

The weight of flue gases, W, per pound of carbon can be computed directly from the flue gas analysis from the formula:

11 CO_{2} + 8 O + 7 (CO + N) ---------------------------- (16) 3 (CO_{2} + CO)

where CO_{2}, O, CO, and N are the percentages by volume as determined by the flue gas analysis of carbon dioxide, oxygen, carbon monoxide and nitrogen.

The weight of flue gas per pound of dry coal will be the weight determined by this formula multiplied by the percentage of carbon in the coal from an ultimate analysis.

[Graph: Temperature of Escaping Gases--Deg. Fahr. against Heat carried away by Chimney Gases--In B.t.u. per pound of Carbon burned.[31]

Fig. 20. Loss Due to Heat Carried Away by Chimney Gases for Varying Percentages of Carbon Dioxide. Based on Boiler Room Temperature = 80 Degrees Fahrenheit. Nitrogen in Flue Gas = 80.5 Per Cent. Carbon Monoxide in Flue Gas = 0. Per Cent]

Fig. 20 represents graphically the loss due to heat carried away by dry chimney gases for varying percentages of CO_{2}, and different temperatures of exit gases.

The heat lost, due to the fact that the carbon in the fuel is not completely burned and carbon monoxide is present in the flue gases, in B. t. u. per pound of fuel burned is:

( CO )
L' = 10,150 × (-----------) (17)
(CO + CO_{2})

where, as before, CO and CO_{2} are the percentages by volume in the flue gases and C is the proportion by weight of carbon which is burned and passes up the stack.

Fig. 21 represents graphically the loss due to such carbon in the fuel as is not completely burned but escapes up the stack in the form of carbon monoxide.

[Graph: Loss in B.T.U. per Pound of Carbon Burned[32] against Per Cent CO_{2} in Flue Gas

Fig. 21. Loss Due to Unconsumed Carbon Contained in the CO in the Flue Gases]

Apparatus for Flue Gas Analysis--The Orsat apparatus, illustrated in Fig. 22, is generally used for analyzing flue gases. The burette A is graduated in cubic centimeters up to 100, and is surrounded by a water jacket to prevent any change in temperature from affecting the density of the gas being analyzed.

For accurate work it is advisable to use four pipettes, B, C, D, E, the first containing a solution of caustic potash for the absorption of carbon dioxide, the second an alkaline solution of pyrogallol for the absorption of oxygen, and the remaining two an acid solution of cuprous chloride for absorbing the carbon monoxide. Each pipette contains a number of glass tubes, to which some of the solution clings, thus facilitating the absorption of the gas. In the pipettes D and E, copper wire is placed in these tubes to re-energize the solution as it becomes weakened. The rear half of each pipette is fitted with a rubber bag, one of which is shown at K, to protect the solution from the action of the air. The solution in each pipette should be drawn up to the mark on the capillary tube.

The gas is drawn into the burette through the U-tube H, which is filled with spun glass, or similar material, to clean the gas. To discharge any air or gas in the apparatus, the cock G is opened to the air and the bottle F is raised until the water in the burette reaches the 100 cubic centimeters mark. The cock G is then turned so as to close the air opening and allow gas to be drawn through H, the bottle F being lowered for this purpose. The gas is drawn into the burette to a point below the zero mark, the cock G then being opened to the air and the excess gas expelled until the level of the water in F and in A are at the zero mark. This operation is necessary in order to obtain the zero reading at atmospheric pressure.

The apparatus should be carefully tested for leakage as well as all connections leading thereto. Simple tests can be made; for example: If after the cock G is closed, the bottle F is placed on top of the frame for a short time and again brought to the zero mark, the level of the water in A is above the zero mark, a leak is indicated.

Before taking a final sample for analysis, the burette A should be filled with gas and emptied once or twice, to make sure that all the apparatus is filled with the new gas. The cock G is then closed and the cock I in the pipette B is opened and the gas driven over into B by raising the bottle F. The gas is drawn back into A by lowering F and when the solution in B has reached the mark in the capillary tube, the cock I is closed and a reading is taken on the burette, the level of the water in the bottle F being brought to the same level as the water in A. The operation is repeated until a constant reading is obtained, the number of cubic centimeters being the percentage of CO_{2} in the flue gases.

The gas is then driven over into the pipette C and a similar operation is carried out. The difference between the resulting reading and the first reading gives the percentage of oxygen in the flue gases.

The next operation is to drive the gas into the pipette D, the gas being given a final wash in E, and then passed into the pipette C to neutralize any hydrochloric acid fumes which may have been given off by the cuprous chloride solution, which, especially if it be old, may give off such fumes, thus increasing the volume of the gases and making the reading on the burette less than the true amount.

The process must be carried out in the order named, as the pyrogallol solution will also absorb carbon dioxide, while the cuprous chloride solution will also absorb oxygen.

As the pressure of the gases in the flue is less than the atmospheric pressure, they will not of themselves flow through the pipe connecting the flue to the apparatus. The gas may be drawn into the pipe in the way already described for filling the apparatus, but this is a tedious method. For rapid work a rubber bulb aspirator connected to the air outlet of the cock G will enable a new supply of gas to be drawn into the pipe, the apparatus then being filled as already described. Another form of aspirator draws the gas from the flue in a constant stream, thus insuring a fresh supply for each sample.

The analysis made by the Orsat apparatus is volumetric; if the analysis by weight is required, it can be found from the volumetric analysis as follows:

Multiply the percentages by volume by either the densities or the molecular weight of each gas, and divide the products by the sum of all the products; the quotients will be the percentages by weight. For most work sufficient accuracy is secured by using the even values of the molecular weights.

The even values of the molecular weights of the gases appearing in an analysis by an Orsat are:

Carbon Dioxide 44 Carbon Monoxide 28 Oxygen 32 Nitrogen 28

Table 33 indicates the method of converting a volumetric flue gas analysis into an analysis by weight.

TABLE 33

CONVERSION OF A FLUE GAS ANALYSIS BY VOLUME TO ONE BY WEIGHT

Column Headings:

A: Analysis by Volume Per Cent B: Molecular Weight C: Volume times Molecular Weight D: Analysis by Weight Per Cent _____________________________________________________________________ | | | | | | | Gas | A | B | C | D | |________________________|_______|___________|________|_______________| | | | | | | | | | | | | | | | | | 536.8 | | Carbon Dioxide CO_{2} | 12.2 | 12+(2×16) | 536.8 | ------ = 17.7 | | | | | | 3022.8 | | | | | | | | | | | | 11.2 | | Carbon Monoxide CO | .4 | 12+16 | 11.2 | ------ = .4 | | | | | | 3022.8 | | | | | | | | | | | | 220.8 | | Oxygen O | 6.9 | 2×16 | 220.8 | ------ = 7.3 | | | | | | 3022.8 | | | | | | | | | | | | 2254.0 | | Nitrogen N | 80.5 | 2×14 | 2254.0 | ------ = 74.6 | | | | | | 3022.8 | |________________________|_______|___________|________|_______________| | | | | | | | Total | 100.0 | | 3022.8 | 100.0 | |________________________|_______|___________|________|_______________|

Application of Formulae and Rules--Pocahontas coal is burned in the furnace, a partial ultimate analysis being:

_Per Cent_ Carbon 82.1 Hydrogen 4.25 Oxygen 2.6 Sulphur 1.6 Ash 6.0 B. t. u., per pound dry 14500

The flue gas analysis shows:

_Per Cent_

CO_{2} 10.7 O 9.0 CO 0.0 N (by difference) 80.3

Determine: The flue gas analysis by weight (see Table 33), the amount of air required for perfect combustion, the actual weight of air per pound of fuel, the weight of flue gas per pound of coal, the heat lost in the chimney gases if the temperature of these is 500 degrees Fahrenheit, and the ratio of the air supplied to that theoretically required.

Solution: The theoretical weight of air required for perfect combustion, per pound of fuel, from formula (11) will be,