This section is from "Scientific American Supplement". Also available from Amazon: Scientific American Reference Book.

Experiments made by M. Regnault and others on the influence of heat on pressures and volumes of gases have enabled us to fix the absolute zero of temperature as -461 degrees Fahrenheit. This point, 461 degrees below zero, is the theoretical point at which a volume of air is reduced to nothing. The volume of air at different temperatures is in proportion to the absolute temperature, and on this basis Box gives us the following table:

OF THE VOLUME AND WEIGHT OF DRY AIR AT DIFFERENT TEMPERATURES UNDER A CONSTANT ATMOSPHERIC PRESSURE OF 29.92 INCHES OF MERCURY IN THE BAROMETER (ONE ATMOSPHERE), THE VOLUME AT 32° FAHRENHEIT BEING 1.

Temperature Volume in Weight of a in degrees. cubic feet. cubic foot in lb. 32 1.000 0.0807 42 1.020 0.0791 52 1.041 0.0776 62 1.061 0.0761 72 1.082 0.0747 82 1.102 0.0733 92 1.122 0.0720 102 1.143 0.0707 112 1.163 0.0694 122 1.184 0.0682 132 1.204 0.0671 142 1.224 0.0660 152 1.245 0.0649 162 1.265 0.0638 172 1.285 0.0628 182 1.306 0.0618 192 1.326 0.0609 202 1.347 0.0600 212 1.367 0.0591 230 1.404 0.0575 250 1.444 0.0559 275 1.495 0.0540 300 1.546 0.0522 325 1.597 0.0506 350 1.648 0.0490 375 1.689 0.0477 400 1.750 0.0461 450 1.852 0.0436 500 1.954 0.0413 550 2.056 0.0384 600 2.15[2] 0.0376 650 2.260 0.0357 700 2.362 0.0338 750 2.464 0.0328 800 2.566 0.0315 850 2.668 0.0303 900 2.770 0.0292 950 2.872 0.0281 1,000 2.974 0.0268 1,100 3.177 0.0254 1,200 3.381 0.0239 1,300 3.585 0.0225 1,400 3.789 0.0213 1,500 3.993 0.0202 1,600 4.197 0.0192 1,700 4.401 0.0183 1,800 4.605 0.0175 1,900 4.809 0.0168 2,000 5.012 0.0161 2,100 5.216 0.0155 2,200 5.420 0.0149 2,300 5.624 0.0142 2,400 5.828 0.0138 2,500 6.032 0.0133 2,600 6.236 0.0130 2,700 6.440 0.0125 2,800 6.644 0.0121 2,900 6.847 0.0118 3,000 7.051 0.0114 3,100 7.255 0.0111 3,200 7.459 0.0108

The effect of this heat of compression in increasing the volume, and the heat produced at different stages of compression, are shown by the following table:

HEAT PRODUCED BY COMPRESSION OF AIR.

--------+-----------------------+----------+------------+------------- | Pressure. | | | Atmo- +-----------+-----------+ Volume |Temperature | Total spheres.|Pounds per |Pounds per | in Cubic | of the Air | Increase of |Square Inch|Square Inch| Feet. | throughout | Temperature. | above a |above the | |the Process.| Degrees. | Vacuum. |Atmosphere | | Degrees. | | |(Gauge | | | | |Pressure). | | | --------+-----------+-----------+----------+------------+------------- 1.00 | 14.70 | 0.00 | 1.0000 | 60.0 | 00.0 1.10 | 16.17 | 1.47 | 0.9346 | 74.6 | 14.6 1.25 | 18.37 | 3.67 | 0.8536 | 94.8 | 34.8 1.50 | 22.05 | 7.35 | 0.7501 | 124.9 | 64.9 1.75 | 25.81 | 11.11 | 0.6724 | 151.6 | 91.6 2.00 | 29.40 | 14.70 | 0.6117 | 175.8 | 115.8 2.50 | 36.70 | 22.00 | 0.5221 | 218.3 | 158.3 3.00 | 44.10 | 29.40 | 0.4588 | 255.1 | 195.1 3.50 | 51.40 | 36.70 | 0.4113 | 287.8 | 227.8 4.00 | 58.80 | 44.10 | 0.3741 | 317.4 | 257.4 5.00 | 73.50 | 58.80 | 0.3194 | 369.4 | 309.4 6.00 | 88.20 | 73.50 | 0.2806 | 414.5 | 354.5 7.00 | 102.90 | 88.20 | 0.2516 | 454.5 | 394.5 8.00 | 117.60 | 102.90 | 0.2288 | 490.6 | 430.6 9.00 | 132.30 | 117.60 | 0.2105 | 523.7 | 463.4 10.00 | 147.00 | 132.30 | 0.1953 | 554.0 | 494.0 15.00 | 220.50 | 205.80 | 0.1465 | 681.0 | 621.0 20.00 | 294.00 | 279.30 | 0.1195 | 781.0 | 721.0 25.00 | 367.50 | 352.80 | 0.1020 | 864.0 | 804.0 --------+-----------+-----------+----------+------------+-------------

A cubic foot of free air at a pressure of one atmosphere (equal to 14.7 pounds above a vacuum) at a temperature of 60 degrees, when compressed to twenty-five atmospheres, will register 367.5 pounds above a vacuum (352.8 pounds gauge pressure), will occupy a volume of 0.1020 cubic foot, will have a temperature of 864 degrees, and the total increase of temperature is 804 degrees.

The thermal results of air compression and expansion are shown by the accompanying diagram.

The horizontal and vertical lines are the measures of volumes, pressures and temperatures. The figures at the top indicate pressures in atmospheres above a vacuum, the corresponding figures at the bottom denote pressures by the gauge. At the right are volumes from one to one-tenth. At the left are degrees of temperatures from zero to 1,000 Fahrenheit. The two curves which begin at the upper left hand corner and extend to the lower right are the lines of compression or expansion.

The upper one being the Adiabatic curve, or that which represents the pressure at any point on the stroke with the heat developed by compression remaining in the air; the lower is the Isothermal, or the pressure curve uninfluenced by heat. The three curves which begin at the lower left hand corner and rise to the right are heat curves and represent the increase of temperature corresponding with different pressures and volumes, assuming in one case that the temperature of the air before admission to the compressor is zero, in another sixty degrees, and in another one hundred degrees.

Beginning with the adiabatic curve, we find that for one volume of air when compressed without cooling the curve intersects the first vertical line at a point between 0.6 and 0.7 volume, the gauge pressure being 14.7 pounds. If we assume that this air was admitted to the compressor at a temperature of zero, it will reach about 100 degrees when the gauge pressure is 14.7 pounds. We find this by following down the first line intersected by the adiabatic curve to the point where the zero heat curve intersects this same line, the reading being given in figures to the left immediately opposite. If the air had been admitted to the compressor at 60 degrees, it would register about 176 degrees at 14.7 pounds gauge pressure. If the air were 100 degrees before compression, it would go up to about 230 degrees at this pressure. Following this adiabatic curve until it intersects line No. 5, representing a pressure of five atmospheres above a vacuum (58.8 lb. gauge pressure), we see that the total increase of temperature on the zero heat curve is about 270 degrees, for the 60 degree curve it is about 370 degrees, and for the 100 degree curve it is about 435 degrees.

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