Steam, Its Generation and Use Part 17

Such a gain is dependent upon the cla.s.s 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 pa.s.sing over the superheater do not follow the path they would ordinarily take in pa.s.sing over the boiler heating surface, a loss may result. This is noticeably true where part of the gases are pa.s.sed over the superheater and are allowed to pa.s.s 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 f.l.a.n.g.es are drawn up too tightly, the breaking of a f.l.a.n.g.e from the body of the fitting. The latter difficulty is undoubtedly due, in certain instances, to the form of f.l.a.n.g.e in which the strain of the connecting bolts tended to distort the metal.

The Babc.o.c.k & Wilc.o.x 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 cla.s.s 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.

[Ill.u.s.tration: 8400 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers and Superheaters at the Butler Street Plant of the Georgia Railway and Power Co., Atlanta, Ga. This Company Operates a Total of 15,200 Horse Power of Babc.o.c.k & Wilc.o.x Boilers]

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 const.i.tuents 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 alt.i.tude. This fact has a direct bearing in the proportioning of furnaces, flues and stacks at high alt.i.tudes, as will be shown later in the discussion of these subjects. The atmospheric pressures corresponding to various alt.i.tudes 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 quant.i.ty 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 const.i.tuents 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 | | c.o.ke | Red Heat | | Anthracite | Red Heat, 750 | | Carbon Monoxide | Red Heat, 1211 | | Hydrogen | 1030 or 1290 | |_________________|__________________|

Combustibles--The princ.i.p.al combustibles in coal and other fuels are carbon, hydrogen and sulphur, occurring in varying proportions and combinations.