Part 26

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