Writing Tech Brief By HackerNoon

The actual transfer rates in superheaters are affected by so many conditions that it has not so far been possible to evolve any formula of practical value.

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The rate at which heat is transmitted from a hot gas to a cooler metal surface over which the gas is flowing has been the subject of a great deal of investigation both from the experimental and theoretical side. A more or less complete explanation of this process is necessary for a detailed analysis of the performance of steam boilers. Such information at the present is almost entirely lacking and for this reason a boiler, as a physical piece of apparatus, is not as well understood as it might be. This, however, has had little effect in its practical development and it is hardly possible that a more complete understanding of the phenomena discussed will have any radical effect on the present design.

The amount of heat that is transferred across any surface is usually expressed as a product, of which one factor is the slope or linear rate of change in temperature and the other is the amount of heat transferred per unit’s difference in temperature in unit’s length. In Fourier’s analytical theory of the conduction of heat, this second factor is taken as a constant and is called the “conductivity” of the substance. Following this practice, the amount of heat absorbed by any surface from a hot gas is usually expressed as a product of the difference in temperature between the gas and the absorbing surface into a factor which is commonly designated the “transfer rate”. There has been considerable looseness in the writings of even the best authors as to the way in which the gas temperature difference is to be measured. If the gas varies in temperature across the section of the channel through which it is assumed to flow, and most of them seem to consider that this would be the case, there are two mean gas temperatures, one the mean of the actual temperatures at any time across the section, and the other the mean temperature of the entire volume of the gas passing such a section in any given time. Since the velocity of flow will of a certainty vary across the section, this second mean temperature, which is one tacitly assumed in most instances, may vary materially from the first. The two mean temperatures are only approximately equal when the actual temperature measured across the section is very nearly a constant. In what follows it will be assumed that the mean temperature measured in the second way is referred to. In English units the temperature difference is expressed in Fahrenheit degrees and the transfer rate in B. t. u.’s per hour per square foot of surface. Pecla, who seems to have been one of the first to consider this subject analytically, assumed that the transfer rate was constant and independent both of the temperature differences and the velocity of the gas over the surface. Rankine, on the other hand, assumed that the transfer rate, while independent of the velocity of the gas, was proportional to the temperature difference, and expressed the total amount of heat absorbed as proportional to the square of the difference in temperature. Neither of these assumptions has any warrant in either theory or experiment and they are only valuable in so far as their use determine formulae that fit experimental results. Of the two, Rankine’s assumption seems to lead to formulae that more nearly represent actual conditions. It has been quite fully developed by William Kent in his “Steam Boiler Economy”. Professor Osborne Reynolds, in a short paper reprinted in Volume I of his “Scientific Papers”, suggests that the transfer rate is proportional to the product of the density and velocity of the gas and it is to be assumed that he had in mind the mean velocity, density and temperature over the section of the channel through which the gas was assumed to flow. Contrary to [Pg 324]prevalent opinion, Professor Reynolds gave neither a valid experimental nor a theoretical explanation of his formula and the attempts that have been made since its first publication to establish it on any theoretical basis can hardly be considered of scientific value. Nevertheless, Reynolds’ suggestion was really the starting point of the scientific investigation of this subject and while his formula cannot in any sense be held as completely expressing the facts, it is undoubtedly correct to a first approximation for small temperature differences if the additive constant, which in his paper he assumed as negligible, is given a value. [83]

4,294.—Apparatus for Pitching Barrels.—Louis Schulze, Baltimore, Md. Patent No. 106,964, dated August 30, 1870.

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4,287.—Treating Fruits to Dry, Saccharify, and Preserve Them.—Charles Alden, Newburg, assignor of part interest to Alden Fruit Preserving Company, New York city. Patent No. 100,835, dated March 5, 1870; reissue No. 4,011, dated June 7, 1870.

4,288.—Device for Securing Pulleys to Shaft.—John H. Buckman (assignor to himself and Peter W. Reinshagen), Cincinnati, Ohio. Patent No. 98,144, dated December 21, 1839.

4,289.—Shawl Strap.—George Crouch, Westport, Conn. Patent No. 82,606, dated September 29, 1868.

4,290.—Atmospheric Dental Plate.—Nehemiah T. Folsom, Laconia, N. H. Patent No. 60,871, dated January 1, 1867.

4,291.—Pessary.—William R. Gardner, Leonardsville, N. Y. Patent No. 105,191, dated July 12, 1870.

4,292.—Division A.—Skate.—James L. Plimpton, New York city. Patent No. 37,305, dated January 6, 1863; reissue No. 3,906, dated April 5, 1870.

4,293.—Division B.—Skate.—James L. Plimpton, New York city. Patent No. 37,305, dated January 6, 1863; reissue No. 3,906, dated April 5, 1870.

Napier deduced an approximate formula for the outflow of steam into the atmosphere which checks closely with the figures just given.

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Various formulae for the flow of steam through pipes have been advanced, all having their basis upon Bernoulli’s theorem of the flow of water through circular pipes with the proper modifications made for the variation in constants between steam and water. The loss of energy due to friction in a pipe is given by Unwin (based upon Weisbach) as

where E is the energy loss in foot pounds due to the friction of W units of weight of steam passing with a velocity of v feet per second through a pipe d feet in diameter and L feet long; g represents the acceleration due to gravity (32.2) and f the coefficient of friction.

Numerous values have been given for this coefficient of friction, f , which, from experiment, apparently varies with both the diameter of pipe and the velocity of the passing steam. There is no authentic data on the rate of this variation with velocity and, as in all experiments, the effect of change of velocity has seemed less than the unavoidable errors of observation, the coefficient is assumed to vary only with the size of the pipe.

Unwin established a relation for this coefficient for steam at a velocity of 100 feet per second,

Hello Hackers! Welcome to the winner announcement of the Zero Knowledge Proof Writing Contest by Rareskills and HackerNoon.

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Zero-Knowledge Proofs: It's Like Magic, but I'll Explain It by @windley.

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Unlocking the Magic of Zero-Knowledge Proofs: Disintegration of Philosophy and Science (Part 1) by @damocles.

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All surfaces should be painted before the covering is applied. Canvas is ordinarily placed over the covering, held in place by wrought-iron or brass bands.

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In the design of a steam plant, the piping system should receive the most careful consideration. Aside from the constructive details, good practice in which is fairly well established, the important factors are the size of the piping to be employed and the methods utilized in avoiding difficulties from the presence in the system of water of condensation and the means employed toward reducing radiation losses.

Engineering opinion varies considerably on the question of material of pipes and fittings for different classes of work, and the following is offered simply as a suggestion of what constitutes good representative practice.

All pipe should be of wrought iron or soft steel. Pipe at present is made in “standard”, “extra strong”[76] and “double extra strong” weights. Until recently, a fourth weight approximately 10 per cent lighter than standard and known as “Merchants” was built but the use of this pipe has largely gone out of practice. Pipe sizes, unless otherwise stated, are given in terms of nominal internal diameter. Table 62 gives the dimensions and some general data on standard and extra strong wrought-iron pipe.

388.—Iron Rails and Bars, and Modes of Manufacturing the same.—Eldridge Wheeler, Philadelphia, Pa. February 15, 1871.

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350.—Breech-loading Fire-arms.—Eli Whitney, New Haven, Conn. February 10, 1871.

352.—Governor.—Stilliman B. Allen, ——, Mass. February 10, 1871.

357.—Windmill.—A. P. Brown, New York city. February 11, 1871.

332.—Furniture Casters.—F. A. Gardner and H. S. Turrell, Danbury Conn. February 8, 1871.

339.—Wire Fabrics for Mattresses.—Samuel Rogers, New York city. February 9, 1871.

340.—Screw Propeller Canal Boats.—Thomas Main, Pierpoint, N. Y. February 9, 1871.

362.—Flyer for Spinning Machinery.—Thomas Mayor and Geo. Chatterton, Providence, R. I. February 14, 1871.

373.—Telegraphic Apparatus and Detectors.—W. B. Watkins, Jersey City, N. J. February 14, 1871.

381.—Steam and other Safety Valves.—Walter Dawson Scranton, Pa. February 15, 1871.

388.—Iron Rails and Bars, and Modes of Manufacturing the same.—Eldridge Wheeler, Philadelphia, Pa. February 15, 1871.

The metal used was lead and antimony, seven to one.

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[We present herewith a series of inquiries embracing a variety of topics of greater or less general interest. The questions are simple, it is true, but we prefer to elicit practical answers from our readers, and hope to be able to make this column of inquiries and answers a popular and useful feature of the paper.]

1.—Emery Wheels.—Can I make emery wheels similar to those used in a foot lathe, that will answer for sharpening fine tools, such as gouges, rounds, and hollows, and if so, how shall I proceed?—F. W.

2.—Boiler Furnace.—I have two boilers, twenty-four feet long and four feet in diameter each, with five ten-inch flues. The fire passes under the boiler, and enters the flues at the back end, passes through the flues, and enters the smoke stack at the front end. I use hard pine wood for fuel. Will some of your many readers give me the best way of constructing the flue under the boiler, from the end of the grate bars to where it enters the flues at the back end, and also state the proper distance from the back wall to the end of the boiler?—N. H.

3.—Medal Casts.—I have some medals which I should like to copy. Having tried several times, and failed, I thought that I would ask advice through your query columns. I do not know of what the medals are manufactured. They are, I suppose, made to imitate bronze. I have tried casting them in plaster of Paris molds, but have had very poor success, as the surface of the medals was covered with small holes. The metal used was lead and antimony, seven to one. I should like to know, if there be any metal that I can cast them of, and bring out the bronze color afterwards, or if there be any metal that I can cast them of, and afterwards color by some solution. Also, of what should I make my molds?—J. E. M.

After the walls have become thoroughly heated, coal may be fired and the boiler placed in service.

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A consideration of the losses in boiler efficiency, due to the effects of excess air, clearly indicates the necessity of maintaining the brick setting of a boiler tight and free from air leaks. In view of the temperatures to which certain portions of such a setting are subjected, the material to be used in its construction must be of the best procurable.

Boiler settings to-day consist almost universally of brickwork—two kinds being used, namely, red brick and fire brick.

The red brick should only be used in such portions of the setting as are well protected from the heat. In such location, their service is not so severe as that of fire brick and ordinarily, if such red brick are sound, hard, well burned and uniform, they will serve their purpose.

The fire brick should be selected with the greatest care, as it is this portion of the setting that has to endure the high temperatures now developed in boiler practice. To a great extent, the life of a boiler setting is dependent upon the quality of the fire brick used and the care exercised in its laying.

The best fire brick are manufactured from the fire clays of Pennsylvania. South and west from this locality the quality of fire clay becomes poorer as the distance increases, some of the southern fire clays containing a considerable percentage of iron oxide.

Until very recently, the important characteristic on which to base a judgment of the suitability of fire brick for use in connection with boiler settings has been considered the melting point, or the temperature at which the brick will liquify and run. Experience has shown, however, that this point is only important within certain limits and that the real basis on which to judge material of this description is, from the boiler man’s standpoint, the quality of plasticity under a given load. This tendency of a brick to become plastic occurs at a temperature much below the melting point and to a degree that may cause the brick to become deformed under the stress to which it is subjected. The allowable plastic or softening temperature will naturally be relative and dependent upon the stress to be endured.

This invention relates to a new and useful compound for lubricating railroad car axle journals, and other journal bearings.

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Self-Acting Shackle and Car Brake.—Lyman Alphonzo Russell, Shrewsbury, Vt.—This invention relates to improvements in self-acting shackles and car brakes, and consists in an improved connection of the brakes with the shackle, for automatic operation, whereby the connection may be readily so adjusted that the brakes will not be set in action as when required to back up the train.

Feed Bags for Horses.—W. A. Hough, South Butler, N. Y.—This invention relates to a new and useful improvement in feed bags for horses, and consists in making the bag self-supplying, by means of one or more reservoirs, the discharge orifices of which reservoirs are closed by a valve or valves.

Truss.—Adam Hinoult, Montgomery, N. Y.—This invention has for its object to furnish an improved truss, which shall be so constructed as to yield freely to the various movements of the body of the wearer, while holding the rupture securely in place.

Special care should be taken to prevent air, steam or water leaks into the boiler or onto the pressure parts to obviate danger of corrosion.

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The general subject of boiler room practice may be considered from two aspects. The first is that of the broad plant economy, with a suggestion as to the methods to be followed in securing the best economical results with the apparatus at hand and procurable. The second deals rather with specific recommendations which should be followed in plant practice, recommendations leading not only to economy but also to safety and continuity of service. Such recommendations are dictated from an understanding of the nature of steam generating apparatus and its operation, as covered previously in this book.

It has already been pointed out that the attention given in recent years to steam generating practice has come with a realization of the wide difference existing between the results being obtained in every-day operation and those theoretically possible. The amount of such attention and regulation given to the steam generating end of a power plant, however, is comparatively small in relation to that given to the balance of the plant, but it may be safely stated that it is here that there is the greatest assurance of a return for the attention given.

In the endeavor to increase boiler room efficiency, it is of the utmost importance that a standard basis be set by which average results are to be judged. With the theoretical efficiency obtainable varying so widely, this standard cannot be placed at the highest efficiency that has been obtained regardless of operating conditions. It is better set at the best obtainable results for each individual plant under its conditions of installation and daily operation.

With an individual standard so set, present practice can only be improved by a systematic effort to approach this standard. The degree with which operating results will approximate such a standard will be found to be directly proportional to the amount of intelligent supervision given the operation. For such supervision to be given, it is necessary to have not only a full realization of what the plant can do under the best operating conditions but also a full and complete knowledge of what it is doing under all of the different conditions that may arise. What the plant is doing should be made a matter of continuous record so arranged that the results may be directly compared for any period or set of conditions, and where such results vary from the standard set, steps must be taken immediately to remedy the causes of such failings. Such a record is an important check in the losses in the plant.