By WM. L. SAUNDERS, C.E., of New York.
I cannot but realize as I stand before you that I would be very much
more at home were I in your midst. I feel but little older and so very
much less wise than when I sat in the class room an undergraduate of the
University of Pennsylvania, that I trust I may expect you to give me
this afternoon, not only your attention, but your sympathy.
The present situation is not without suggestions of my own experience. I
recall a lecture in the ordinary course, given by our professor of
mining, whose struggles with the English language were quite as
conspicuous as were our efforts to tell what he was driving at. He was
describing an ordinary windlass hoist used at the shaft of a mine. He
said "There is a windlass at de top of de shaft around which is coiled a
rope, on de two ends of which is fastened two er--er--_pans_, one of
which is a _bucket_ and de oder a _platform_." I mention this because I
shall ask you to attribute my shortcomings in this lecture, not so much
to my lack of familiarity with my native tongue, as to--well, because I
was not educated at Cornell University.
We all know what free air is. You who are privileged to live upon these
beautiful hills overlooking Ithaca and the lake, doubtless know more
about free air than we do who are choked in the dusty confines of New
York City. Compressed air is simply air under pressure. That pressure
may be an active one, as in the case of the piston of an air compressor;
or passive, as with the walls of a receiver or transmission pipe. It is
usual to define compressed air as air increased in density by pressure,
but we know that we may produce compressed air by heat alone. A simple
illustration of this is the pressure which will blow a cork from an
empty bottle when that bottle has been placed near the fire. Here we
have pressure, or compressed air, in the bottle produced by heat alone.
Having defined compressed air, we must next define heat; for in dealing
with compressed air, we are brought face to face with the complex laws
of Thermodynamics. We cannot produce compressed air without also
producing heat, and we cannot use compressed air as a power without
producing cold. Based on the material theory of heat, we would say that
when we take a certain volume of free air and compress it into a smaller
space, we get an increase in temperature because we have the heat of one
volume occupying less space, but no one at this date accepts the
material theory of heat. Your distinguished director, Professor
Thurston, in discussing "Steam and its Rivals," in the _Forum_, said:
"The science of Thermodynamics teaches that heat and mechanical energy
are only different phases of the same thing, the one being the motion of
molecules, and the other that of masses." This is the accepted theory of
heat. In other words, we do not believe that there is any such _thing_
as heat, but that what we call heat is only the sensible effect of
motion. In the cylinder of an air compressor the energy of the piston is
converted into molecular motion in the air and the result, or the
equivalent, is heat. A higher temperature means an increased speed of
vibration, and a lower temperature means that this speed of vibration is
reduced. If I hold an open cylinder in my left hand and a piston in my
right, and place the piston within the cylinder, I here have a confined
volume of air at the temperature and the pressure of this room. These
particles of air are in motion and produce heat and pressure in
proportion to that motion. Now if I press the piston to a point in the
center of the cylinder, that is, to one-half the stroke, I here decrease
the distance between the cylinder head and the piston just one-half,
hence each molecule of air strikes twice as many blows upon the piston
and head in traveling the same distance and the pressure is doubled. We
have also produced about 116 degrees of heat, because we have expended a
certain amount of work upon the air; the air has done no work in return,
but we have increased the energy of molecular vibration in the air and
the result is heat.
But what of this heat? What harm does it do? If I instantly release the
piston which I hold at one-half stroke it will return to its original
position, less only a little friction. I have, therefore, recovered all,
or nearly all, the power spent in compressing the air. I have simply
pressed a spring, and have let it recover. We see what a perfect spring
compressed air is. We see the possibility of expending one horse power
of energy upon air and getting almost exactly one horse power in return.
Such would be the case provided we used the compressed air power
_immediately and at the point where the compression takes place_. This
is never done, but the heat which has been boxed up[1] in the air is
lost by radiation, and we have lost power. Let us see to what extent
this takes place.
[Footnote 1: I use material terms because they add to simplicity of
expression and notwithstanding the fact that heat is vibration.]
Thirteen cubic feet of free air at normal temperature and barometric
pressure weigh about one pound. We have seen that 116 degrees of heat
have been liberated at half stroke. The gauge pressure at this point
reaches 24 pounds. According to Mariotte's law, "The temperature
remaining constant, the volume varies inversely as the pressure," we
should have 15 pounds gauge pressure. The difference, 9 pounds,
represents the effect of the heat of compression in increasing the
relative volume of the air.
[Illustration: FIG. 1.: CURVES OF COMPRESSION ILLUSTRATEDIN VOLUMES,
PRESSURES, AND TEMPERATURES.]
The specific heat of air under constant pressure being 0.238, we have
0.238 × 116 = 27.6 heat units produced by compressing one pound or
thirteen cubic feet of free air into one-half its volume. 27.6 × 772
(Joule's equivalent) = 21,307 foot pounds. We know that 33,000 foot
pounds is one horse power, and we see how easily about two-thirds of a
horse power in heat units may be produced and lost in compressing one
pound of air. I would mention here that exactly this same loss is
suffered when compressed air does work in an engine and is expanded down
to its original pressure. In other words, _the heat of compression and
the cold of expansion are in degree equal_.
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:
TABLE l.--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[1] 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
[Transcribers note 1: last digit illegible]
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: