Chapter 2

Before establishing, with Mr. Witz, a comparison of the two systems in pressure, steam or gas, let us state in a few words in what the latter consists, the steam engine and the boiler that supplies it being so well known that no description is necessary.

The Dowson gas generator does not differ essentially from the numerous generators devised during recent years for the manufacture of gaseous combustibles, the use of which is so often convenient. The motor that it supplies is the most powerful single cylinder one that has hitherto been constructed. It is of 100 indicated h.p., and its normal angular velocity is 100 revolutions per minute. On trial it has yielded 112 indicated h.p., and 76.8 effective h.p., corresponding to an organic rendering of 69 per cent. This motor, elaborated by Messrs. Delamare-Bouteville & Malandin, of Rouen, operates by compression and in four periods, according to the Beau de Rochas cycle. We give the aspect of it in Fig. 3. In the first period the mixture of air and gas is sucked in, in the second it is compressed, in the third it is ignited, and in the fourth the products of combustion are expelled.

Ignition is effected electrically by a series of sparks playing between two platinum points in the slide valve, and this permits of regulating the instant of ignition through the edges of the orifices. The angular velocity is regulated by a Watt's governor, which secures an isochronism of the motion independently of the charge.

The setting in motion of so powerful an engine is effected very easily by means of an arrangement that permits of introducing into the cylinder, while the piston is in the center of the stroke, a mixture of air and gas whose pressure is sufficient at the arrival to expel the inert products. After this the ignition takes place, and the explosion is sufficient to set the motor in motion.

The trials made by Mr. Witz with the motor represented in Fig. 3 gave the following results, deduced from an experiment of 68 hours. The figures relate to one effective horse power, measured with the brake upon the shaft of the motor.

Consumption of anthracite. 516 grammes. " " coke. 96 " Consumption of water for the injection of steam. 0.487 liters. Consumption of water for cooling the cylinder. 50.0 " Consumption of oil for lubricating the cylinder. 3.74 grammes. Consumption of grease. 0.45 " Consumption of gas reduced to 0° C. and to 760 mm. 2,370 liters.

This last figure will appear very high, but the fact must not be lost sight of that it is a question of poor gas, the net cost of which varies between one and two centimes per cubic meter, and the calorific power of which is but 1,487 heat units per cubic meter of constant volume, and supposing the steam condensed. This combustion of 612 grammes of combustible per effective horse hour is remarkable, and fully shows what may be expected of the gas motor supplied by a gas generator in putting to profit certain improvements that will hereafter be possible, such, for example, as the lightening of the movable parts of the motor, the bettering of its organic rendering (now quite feeble), the use of better oils, the reduction of the consumption of water, the superheating of the steam injected into the gas generator, etc.

A well constructed steam engine, carefully kept in repair and as much improved as it is possible to make it, would certainly consume twice as much coal to produce the same quantity of effective work, say at least 1,200 grammes per horse hour. But, as has been objected with reason, it does not suffice to compare the figures as to the consumption of fuel in order to institute a serious comparison between the steam engine and the motor using poor gas.

The gas generator requires the use of English anthracite, while a steam boiler is heated with any kind of coal. The prices of unity of weight are therefore very different. Moreover, the gas motor necessitates an immense amount of water for the washing of the gas and the cooling of the cylinder, through circulation in the jacket. It is well to keep this fact in view. On another hand, the lubrification of the cylinders requires a profusion of oil whose flashing point must be at a very high temperature, else it would burn at every explosion and fill the cylinder with coom. Such oil is very costly.

Does not the expenditure of oil in large motors largely offset the saving in coal? And then, gas motors are sold at high prices, as are gas generators, and this installation necessarily requires the addition of a large gasometer, scrubbers, etc. The wear of these apparatus is rapid, and if we take into account the interest and amortization of the capital engaged, we shall find that the use of steam is still more economical. The obstruction caused by bulky apparatus is another inconvenience, upon which it is unnecessary to dwell. In a word, the question is a very complex one. We look at but one side of it in occupying ourselves only with the coal consumed, and we shall certainly expose those who allowed themselves to be influenced by the seductive figures of consumption to bitter disappointment.

To answer such objections Mr. Aimé Witz has established a complete parallel between the two systems, in which he looks at the question from a theoretical and practical and scientific and financial point of view. Considered as a transformation apparatus, a steam motor burning good Cardiff coal in a Galloway boiler with feed water heaters will consume (with a good condensing engine utilizing an expansion of a sixth) from 1,100 to 1,250 grammes of coal per effective horse hour, which corresponds to a rough coefficient of utilization of 9.7 per cent. A gas generator supplying a gas motor burning Swansea anthracite and Noeux coke, medium quality, will consume 516 grammes of anthracite and 90 of coke to produce 2,370 liters of gas giving 1,487 heat units per cubic meter. Of the 3,524 heat units furnished to the motor by the 2,370 liters of gas, the motor will convert 18 per cent. into disposable mechanical work.

With the boiler, the gross rendering of the whole is 7 per cent. With the gas generator it reaches 12.7 per cent. From a theoretical point of view the advantage therefore rests with the gas generator and gas motor. In order to compare the net cost of the units of work, from an industrial point of view, it is necessary to form estimates of installation, costs of keeping in repair, interest and amortization.

Figs. 1 and 2 represent, on the same scale, the installations necessary in each of these systems. The legends indicate the names of the different apparatus in each installation. The following table shows that, as regards the surface occupied, the advantage is again with the gas generator and gas motor:

Steam Engine. Gas Motor. Surface covered. 85 sq. m. 72 sq. m. Surface exposed. 33 " 43 " --- --- Total surface. 118 " 115 "

The estimates of installation formed by Mr. Witz set forth the expense relative to the capital engaged exactly at the same figure of 32,000 francs for a motive power of 75 effective horses. The expenses of keeping in repair, interest, etc., summed up, show that the cost per day of 10 hours is 47.9 francs for the steam engine and 39.6 for the gas motor, say a saving of 8.3 francs per day, or about 2,500 francs for a year of 300 days' work.

The gas motor, therefore, effects a great saving, while at the same time occupying less space, consuming less water and operating just as well.

With Mr. Witz we cheerfully admit all the advantages that he so clearly establishes with his perfect competency in such matters, but there still remain two points upon which we wish to be enlightened. Are not the starting up, the operation and the keeping in repair of a gas generator actually more complicated and more delicate than the same elements of a steam engine? Does not the poor gas manufactured in a gas generator present, from a hygienic point of view, danger sufficiently great to proscribe the use of such apparatus in many circumstances?

Such are the points upon which we should like to be enlightened before unreservedly sharing Mr. Witz's enthusiasm, which, however, is justified, economically speaking, by the magnificent results of the experiments made by the learned engineer.--_La Nature_.

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IMPROVED PNEUMATIC HAMMER.

We publish illustrations of a Thwaites suspension pneumatic power ½ cwt. hammer of a new design, for planishing pipes and plates, for which we are indebted to _Engineering_. As indicated in the perspective view (Fig. 1) the mechanism is supported at the center of a cross girder resting on two cast iron square pillars, box section, each bolted down to the foundations by four 1¼ in. diameter bolts. The measurements of these columns and girders are given in Figs. 2 and 3, the former an elevation of the hammer and the latter a plan, partly in section, of the cross girder, while Fig. 4 is a cross section showing the arrangements for operating the hammer. In the center is a cast iron guide for working the ram, the guide being extended on two sides to receive the disk crank journals, 2 in. in diameter by 3½ in. long. The disk cranks are connected to a hollow steel ram by a connecting rod. The ram is divided inside into two compartments, each having a phosphor bronze air piston. These are connected together by a steel piston rod, the top air piston forming a connection for the small end of the connecting rod. The outside diameter of the ram is 3¾ in., and the diameter of the air pistons 2¾ in. and 2-7/8 in. respectively. Cottered into the bottom of the ram is a steel pallet holder with a dovetail, so that the pallet can be renewed or exchanged for one of another shape when required. Keyed on to the crankshaft is a flanged pulley 10 in. in diameter by 3¼ in. between flanges. There is also an overhead countershaft with strap shifting arrangement. At the side of one of the columns a hand lever and quadrant are provided, as shown in the perspective view and in Fig. 2, for working an arrangement for tightening the belt when the machine is working. To this arrangement is connected a powerful brake which stops the machine in a few revolutions. It will be seen that the brake is applied as the belt is slackened for stopping the machine. For planishing pipes or tubes a long wrought iron mandrel is provided mounted on two cast iron carriages, each having four flanged wheels for running on rails. The hammer is arranged so that tubes 4 feet in diameter can be worked for planishing plates. A pallet is fastened on the top of one of the mandrel carriages, Figs. 5 to 8 showing the details of the carriages. The general dimensions are: Distance between pillars, 6 feet; height under girder, 5 feet; height from ground to top of mandrel, 4 feet 1¾ in.; and length of stroke, 5 in. This machine is capable of delivering 500 blows per minute. The constructors are Messrs. Thwaites Brothers, Limited, Bradford, Yorkshire.

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SIBLEY COLLEGE LECTURES.--1890-91.

BY THE CORNELL UNIVERSITY NON-RESIDENT LECTURERS IN MECHANICAL ENGINEERING.

COMPRESSED AIR PRODUCTION.

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.

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:

TABLE 2.--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 --------+-----------+-----------+----------+------------+-------------