Part 27

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 × O Pounds 8: Air per Pound of Column 1. 4.32 × O Pounds 9: Gaseous Product per Pound of Column 1 + 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 × 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 | |_______________|_____|_____|_____|_____|______|