Chapter 5

But the water piston fraternity promptly brings forward the question of speed. They say that, admitting that the cooling surfaces are equal, we have in one case _more time_ to absorb the heat than in the other. This is true, and here we come to an important class division in air compressing machinery--_high speed and short stroke_ as against _slow speed and long stroke_. Hydraulic piston compressors are subject to the laws that govern piston pumps, and are, therefore, limited to a piston speed of about 100 feet per minute. It is quite out of the question to run them at much higher speed than this without shock to the engine and fluctuations of air pressure due to agitation of the water piston. The quantity of heat produced, that is, the degree of temperature reached, depends entirely upon the conditions in the air itself, as to density, temperature and moisture, and is entirely independent of speed. We have seen that it is possible to lose 21.3 per cent. of work when compressing air to five atmospheres without any cooling arrangements. With the best compressors of the dry system one-half of this loss is saved by water jacket absorption, so that we are left with about 11 per cent., which the slow moving compressor seeks to erase. We are quite safe in saying that the element of _time alone_ in the stroke of an air compressor could not possibly effect a saving of more than half of this, or 5½ per cent. Now, in order to get this 5½ per cent. saving, we reduce the speed of an air-compressing engine from 350 feet per minute to 100 feet per minute. We must, therefore, in one case have a piston area _three and one-half_ times that of the other in order to get the _same capacity of air_, and in doing this we build an engine of enormous proportions with heavy moving parts. We load it down with a large mass of water, which it must move back and forth during its work, and thus we produce a percentage of friction loss alone equal to twice or even three times the 5½ per cent. heat loss which is responsible for all this expense in first cost and in maintenance, but which really is not saved after all unless water injection in the form of spray also forms a part of the system.

It is obvious that cost of construction and maintenance have much to do with the commercial value of an air compressor. The hydraulic piston machine not only costs a great deal more in proportion to the power it produces, but it costs more to maintain it, and it costs more to run it. It is not an uncommon thing to hear engineers speak of the hydraulic piston compressor as the "most economical" machine for the purpose, but that it is so "expensive" and takes up so much room, and requires such expensive foundations that, unless persons are "willing to spend so much money," they had better take the next best thing, a high speed machine. We hear of "magnificent air-compressing engines, the largest in the country," and pilgrimages are made to see these artificial wonders when, not unlike the old pyramids, they represent a pile of inert matter--a monument to moneyed kings.

The hydraulic piston compressor has one solitary advantage, and that is, it has no dead spaces. It was conceived at a time when dead spaces were very serious conditions--were positive specters! Valves and other mechanism connected with the cylinder of an air compressor were once of such crude construction that it was impossible to reduce the clearance spaces to a reasonable point, and, furthermore, the valves were heavy and so complicated that anything like a high speed would either break them or wear them out rapidly, or derange them so that leakages would occur. But we have now reduced inlet and discharge valves and all other moving parts connected with an air cylinder to a point of extreme simplicity. Clearance space is in some cases destroyed altogether by what is, as it were, an elastic air head which is brought into direct contact with the piston. All this reduces clearance to so small a point that it has no influence of any consequence. The moving parts are made extremely simple, even arriving at a point where inlet valves are opened and closed by their natural inertia. Mr. Sturgeon, of England, has applied a most ingenious and successful inlet valve, which is opened and closed by the friction of the air piston rod through the gland. We have, therefore, reached a point at which high speed is made possible.

Long-stroke air compressors are evidently objectionable on the basis of greater expense of construction. All the parts must be larger and heavier. The fly wheels are increased enormously in diameter and weight, and the strength of bearings must be enlarged in proportion. It is difficult to equalize power and resistance in air compressors with long strokes. The speed will be jerky, and when slow, the fly wheel rather retards than assists in the work of compression. This action tends to derange the parts and makes large bearings a necessity. The piston in a long-stroke compressor travels through considerable space before the pressure reaches a point where the discharge valve opens, and after reaching that point it has to go on still further against a prolonged uniform resistance. This makes rotative speed difficult. During the early part of the stroke, the energy of the steam piston must be stored up in the moving parts, to be given out when the steam pressure has been reduced through an early cut-off. With a short stroke and a large diameter of steam cylinder we are able to get steam economy or early cut-off and expansion without the complications of compounding.

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[Continued from SUPPLEMENT, No. 793, page 12677.]

THE POWER OF WATER, OR HYDRAULICS SIMPLIFIED.

By G.D. Hiscox.

CURRENT WHEELS FOR POWER AND RAISING WATER.

The natural flow of water in a current is probably one of the oldest and cheapest of the methods for obtaining power, or the lifting of water within moderate elevations, for a supply for irrigation and domestic purposes; and we propose, apart from the current wheel, to treat only of self-water-raising devices in this chapter.

Water wheels of various forms for this purpose have been used from time immemorial in Europe, Asia and Egypt, where the record gives examples of wheels of the noria class from 30 to 90 feet in diameter; the term _noria_ having been applied to water wheels carrying buckets for raising water; the Spanish _noria_ having buckets on an endless chain.

Records of a Chinese noria, of 30 feet diameter, made of bamboo, show a lifting capacity of 300 tons of water per day to a height of ¾ of the diameter of the wheel--velocity of current not stated.

For less quantity and greater elevation, these forms of wheel may have pumps attached to the shaft, by crank, that will give a fair duty for a high water supply.

For power purposes, as in the plain current wheel, Fig. 23, there are two principal factors in the problem of power--the velocity of the current and the area of the buckets or blades.

Their efficiency is very low, from 25 to 36 per cent., according to their lightness of make and form of buckets. A slightly curved plate iron bucket gives the highest efficiency, thus ( to the current, and an additional value may also be given by slightly shrouding the ends of the buckets.

The relative velocity of the periphery of the wheel to the velocity of the current should be 50 per cent. with curved blades for best effect.

The most useful and convenient sizes for power purposes are from 10 to 20 feet, and from 2 to 20 feet wide, although, as before stated, there is scarcely a limit under 100 feet diameter for special purposes.

In designing this class of wheels special attention should be given to the concentration and increase of the velocity of the current by wing dams or by the narrowing of shallow streams; always bearing in mind that any increase in the velocity of the current is economy in increased power, as well as in the size and cost of a wheel for a given power.

The blades in the smaller size wheels should be 1/4 of the radius in width, and for the larger sizes up to 20 feet, 1/5 to 1/6 of the radius in width and spaced equal to from 1/4 to 1/3 of the radius.

They should be completely submerged at the lowest point.

For obtaining the horse power of a current wheel, the formula is

Area of 1 blade × velocity of the current in ft. per sec. ---------------------------------------------------------- 400

× by the square of difference of velocities of current and wheel periphery = the horse power; or

A × V 2 ------ × (V - v) = h. p. 400

[TEX: \frac{A \times V}{400} \times (V - v)^2 = h. p.]

in which A equals the area of blade in square feet, V and v velocities of current and wheel periphery respectively, in feet per second. Thus, for example, a wheel 10 feet in diameter with blades 6 feet long and 1 foot in width, running in a stream of 5 feet per second--assuming the wheel to be giving as much power as will reduce its velocity to one half that of the stream--the figures will be

6' × 5' 2 ------- × 2.5 = 0.468 400

[TEX: \frac{6' \times 5'}{400} \times 2.5^2 = 0.468]

horse power of the wheel.

The total power of the stream due to the area of the blade equals the

Square of the velocity of the stream ------------------------------------ × Twice gravity (64.33)

volume of water in cubic feet per second × 62.5 (weight of 1 C') = the value or gross effect in pounds falling 1 foot per second. This sum divided by 550 = horse power. Thus, as per last example,

2 5 ------ × 30 × 62.5 64.33 ---------------------- = 1.32 the horse power of the current 550

[TEX: \frac{\frac{5^2}{64.33} \times 30 \times 62.5}{550} = 1.32 \text{ the horse power of the current}]

due to the area of the blades of the water wheel.

For the efficiency of this class of wheel, with slightly curved and thin blades, divide the horse power of the wheel by the horse power of the current area, equals the percentage of efficiency.

As in the last case,

0.468 / 1.32 = 0.35½

per cent. efficiency of the water wheel.

With higher velocities of stream and wheel the efficiency will be from 2 to 3 per cent. less, although the horse power will increase nearly with the increase in velocity of the current.

For details of application of various forms of current wheels for power purposes see illustrated description Yagn's and Roman's floating motors in SCIENTIFIC AMERICAN SUPPLEMENT, No. 463.

A very good example of a floating motor of the propeller class is Nossian's fluviatile motor, illustrated and described in SCIENTIFIC AMERICAN SUPPLEMENT, No. 656.

Fig. 24 represents a very complete floating motor, in which the floats are wedge shaped at the stem, for the purpose of increasing the current between them, the wheel being an ordinary current wheel, as shown in Fig. 23, with a curved shield or gate in front, which can be moved around the periphery of the wheel for the purpose of regulating its speed or stopping its motion by cutting off the stream from the buckets.

The float, rising and falling with the stream, is held in position by a braced frame swinging on anchorages within the mill on shore, and parallel with a swiveled shaft.

Tide wheels and tidal current wheels have been in use for more than 800 years, and were largely in use in Europe and the United States during the first half of the present century. No less than three were running in the immediate vicinity of New York, in 1840, for milling purposes.

Their day seems to be past, except in some special localities. We will also pass them, and illustrate some of the

SELF-ACTING WATER-RAISING DEVICES.

The tympanum derives its name from its similarity to a drum as made by the Romans, but its origin was Egyptian. It is a current wheel with frame like Fig. 23, to the outside of which a set of chambers or tubes are fixed, radiating spirally, so as to lead the water to the shaft as the wheel revolves, as shown in Fig. 25. It has a lift of a little less than half its diameter, and answers an excellent purpose for the irrigation of rice and cranberry fields, or on streams running through low lands in arid districts. It is still one of the Nile irrigating wheels.

The building of these wheels is within the scope of the carpenter and the tinsmith. A short wooden shaft made square or octagonal, as convenient, with gudgeons in the ends and arms of wood bolted across each of the sides of the shaft, or as shown in the cut, will form a frame work upon which a rim may be fastened, to which the blades and tubular buckets can be attached.

The directions in regard to the current wheel, Fig. 23, may be followed as to number and form of blades, which must be made in length and width proportional to the velocity of the stream and the quantity of water to be lifted by each tubular arm. The tubes may be made of galvanized sheet iron and attached to the outside of the wheel, as shown in Fig. 25.

THE NORIA OR BUCKET WHEEL.

This is a simple current wheel with pot buckets, rigid or swinging, arranged on the rim of the wheel, to carry up and discharge the water nearly at the top of the wheel, and through the long ages that it has been in use for irrigation, village water supply, and even for private establishments, has assumed a variety of forms in detail of construction ranging from the bamboo wheels of the Chinese to the light iron wheels of modern construction.

We illustrate the most simple of these forms in Figs. 26 and 27, in which the first is a series of boxes or chambers in the rim of the wheel with side openings in the forward part of the box as the wheel revolves, and a lip extending from the inner edge of the opening to direct the outflow into the trough.

Another form, Fig. 27, is arranged with swing buckets or pots, pivoted just above their centers, and with the catch trough so fixed as to tip the buckets at the highest point, thus giving this wheel the greatest possible advantage as to height of discharge for a given diameter.

The power value of these wheels for raising water is a matter of computation as nearly reliable as for other devices for the same purpose, when the velocity of the current is known at the point of contact with the blades.

The horse power of the wheel may be computed as for the current wheel, Fig. 23, and, as the horse power is equal to 33,000 pounds raised one foot high per minute, we may assume a construction of wheel that will allow of discharging at 8 feet above the stream; then 33,000 / 8 = 4,125 pounds of water discharged at 8 feet elevation per horse power per minute. As the net power of the wheel in the last example, for Fig. 23, was 0.468 of a horse power, then 4,125 × 0.468 = 1,930 pounds of water raised 8 ft. per minute by the size of bucket and velocity of current in that case. From this a deduction of 20 per cent. should be made for loss by spill and imperfect construction, so that 1,500 pounds or 176 gallons per minute would be the probable output--over 253,000 gallons per day; or, for irrigating purposes, equal to a rainfall of over 1¼ inches in depth on 50 acres in one week.

The proportion of capacity of the lifting buckets for such a wheel becomes of as great importance as its efficiency.

If the buckets are too large, the wheel will stall, and if too small, the wheel will not give its full duty.

For obtaining the approximate capacity of the lifting buckets, assuming the example as above computed, a 10 foot wheel with the velocity at periphery of 2½ feet per second is 150 feet per minute, or five revolutions per minute, nearly. Then 1,930 lb. per m. / 5 revolutions = 386 pounds water capacity for all of the buckets on the wheel.

If such a wheel is constructed with 16 blades and 16 buckets, one between each blade, then 386 / 16 = 24 pounds for each bucket, or 38 / 100 of a cubic foot.

The spill from this capacity of bucket being sufficient to compensate for the friction of the shaft journals.

The lifting buckets of the noria class, Figs. 26 and 27, can be made of positive dimensions to suit the computations as above; but those of the tympanum class, Fig. 25, should be made of dimensions to conform with the required capacity at the moment of leaving the water, as the water at this point flows into the arm.

(_To be continued_.)

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To remove paint and varnishes, which resist the action of strong lye, Dr. Stockmeier recommends a mixture of water of ammonia, two parts, and turpentine, one part; this applied to the surface to be cleaned will, after a few minutes' action, enable the paint to be removed by use of cotton waste or similar material.--(_Bayr. Gen. Ztg_.), Rundschau.

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ON GAS MOTORS.

M. Witz, says the _Gas World_, has been conducting a series of experiments on the Delamare-Deboutteville and Malindin gas engine, driven by Dowson gas, and in which the gas generator takes the place of the ordinary steam boiler. The engine was a one-cylinder motor in the establishment of Messrs. Matter & Co., Rouen. Its power was 100 horse indicated; the cylinder was 23 inches in diameter, the stroke 38 inches, and the normal speed 100 revolutions. The engine is of the Simplex type; the kindling is electric; the cycle of operations is fourfold, with powerful compression. The Dowson generator is 30 inches inside diameter and 76 inches in height from the bars to the top. Air is blown in by steam driven in under the hearth. There is a siphon, a coke scrubber 110 inches high, a sawdust purifier, and a gasholder of 750 cubic feet capacity, and a pipe to the engine 5.2 inches in diameter. The total area occupied by this apparatus is 140 square yards, of which two-thirds are built on. The anthracite employed was from Swansea, containing 5.4 per cent. of ash. The observations made with a string friction brake were continued for 68 hours, everything used being carefully weighed and measured. One day the machine was worked for 15¼ hours on end; the other days it was worked with an interval of half an hour every 12 hours to clear the hearth, poke the fire and lubricate the machine; and it was clearly established that with a big enough generator it would be quite possible to work continuously for several days.

The following were the data for a day of 24 hours, with an interval of half an hour: 8:55 P.M. one day to 8:55 P.M. the next, interval 8:30 to 9 A.M. Anthracite used, 18.4 cwt.; coke used, 3.42 cwt.; water used for steam injection, 217.3 gallons; water used in scrubber, 4,106 gallons; water used in cooling the cylinder, 20,000 gallons; oil used in cylinder, 14.84 pounds; grease, 1.8 pounds; revolutions of machine, 142,157, or 100.8 per minute; effective work, 75.86 French horse power, or 77.4 British; gas used, 6,742 cubic feet per hour, at 772 mm. pressure and 70.7° F., or 83.7 cubic feet per effective horse power; efficiency, 69 per cent.

Now, with regard to the comparison between the large gas motors and steam engines of the same size, M. Witz goes on to remark that the gas engine is by no means, as was formerly thought on high authority, necessarily restricted to the domain of smaller work and sizes. Even in early times it was seen that the gas engine belonged to a type in which there were possibilities of improvement greater than those available in the steam engine, because the difference of temperature between the working substance in its hotter and its cooler condition was greater than in the steam engine; and consumptions of 5,250 cubic feet per horse power per hour soon descended step by step as far as 2,060, while the power went up, past 4, 8 and 12, to 25 or 50 horse power; and in the exhibition of 1889 there were gas engines seen in which the explosion chamber had a diameter of as much as 23 inches.

But the price of coal gas seemed to be too high for use in these large engines, in which sizes steam is comparatively cheap; and so poorer gas, which, though possessing only about 28 per cent. of the heating power, is still cheaper in proportion than coal gas, when it is made on the spot, was introduced to tide over the difficulty. Difficulties have been successively overcome, with the result which we have just seen, namely, 1.37 pounds of anthracite per effective horse power, or about half the carbon which a steam engine of the same power of excellent design, and well kept up, would consume. A 50 horse simplex at Marseilles, in Barataud's flour mill, is said to have run for the last 2 years on 1.12 pounds of English anthracite per effective horse power; and thus M. Witz says his predictions of 10 years ago, that the gas producer would some day replace the boiler, are being verified in such a way as to surprise even himself.

But the objection is stated, and it is a serious one: the weight of fuel is not the only thing to be considered. The steam engine uses coal, the producer requires English anthracite, which is dearer; the gas motor uses a great deal of water and a great deal of oil, which cost money; and gas motors are dear, while gas producers and their adjuncts cost a tidy bit of money, and wear out pretty fast. Is not steam, after all, more economical in the long run? Besides, producers are bulky and take up a great deal of space; the weight of fuel is only one element in a complicated problem.

In order to study the grounds of this objection, M. Witz has instituted a comparison between the actual cost of large steam engines and that of gas motors of similar size.

Take a good Galloway or multitubular boiler; for 75 horse power effective the heating surface must be at least 74 square feet. Using good Cardiff coal, with 4 per cent. of ash, and a heating power of 15,660 Fahr. units; the steam raised will be 8 to 9 pounds per pound of coal, so that 9,400 to 10,577 Fahr. units are utilized in raising steam, or 68 to 76 per cent., which is an excellent result. Take an engine of 16 inch cylinder diameter, 40 inch stroke, and 66 revolutions, etc.; it will use 22.4 pounds of steam per horse power effective, which represents 2.47 to 2.8 pounds of coal under the boiler. These 10 pounds of steam carry 11,752 Fahr. units of heat, and produce work equal to 75 horse, or 1,143 Fahr. units of heat; which corresponds to an efficiency of 9.7 per cent. In a gas motor, on the other hand, we find the materials employed, as per the above data, to contain 8,958 Fahr. units of heat, and to make gaseous fuel in which 6,343 units are available; a return of 70.6 per cent, in the producer. The motor receives these 6,343, and converts 1,143 of them into work; an efficiency of 18 per cent. In order to be equivalent from the heat point of view, a steam engine ought to produce a horse power effective per 9.72 pounds of steam at 5 atmospheres; but no such steam engine exists.

M. Witz goes on with comparative estimates. For a Corliss engine and boiler, with chimney, etc., complete, and putting these up, he allows £1,280; for a Simplex gas motor and Dowson producer complete, including putting up, he allows £1,290, which he explains to be average actual prices; but these prices do not cover cost of transport, and M. Witz does not go into cost of masonry for buildings, apart from foundations, etc., for the apparatus and machinery.

As to water, the gas motor takes 215 cubic feet per horse power effective. A condensing steam engine uses five times as much.

The lubricating oil used at Rouen was a mixture of Russian oil at 430 fr. per ton, and Ferry and Heduit F.H. oil at 900 fr.; the average was 650 fr. per ton, or 2.8d. per pound. Wanner grease, at 6.4d. per pound, was used for the moving parts. A steam engine requires less oil for the cylinder, but the same quantity for the moving parts.