Sibley College Lectures. - 1890-91. By The Cornell University Non-Resident Lecturers In Mechanical Engineering.

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.

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.



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.