Hand-book for steam engineers and owners of steam engines, being a practical guide to the selection and care of steam machinery

Front matter

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"© STATES OF AMERICA.

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Contents

CHAPTER. PAGE.

I. Fuel and Combustion , 7

II. Heat and Steam 14

Chap. II. Heat And Steam

The steam engine is a machine for the conversion of heat into power in motion. The heat is generated by the combustion of fuel ; the transmission is accomplished through the agency of steam ; the power is made available and brought under control by means of the engine.

The effect of heat upon water is to vaporize it, if there be intensity enough, the heat will under proper conditions cause water to boil ; the vapor produced by boiling is called steam, and steam under pressure is a product which is the end and aim of that portion of the steam engine known as the boiler and furnace.

The steam engine then is to be considered as a form of heat engine ; of which the furnace, boiler, and the engine itself are to be regarded as separate portions of the same mechanism.

The conditions demanded upon economic grounds to secure the highest efficiency in the steam engine are :

1. A proper construction of the furnace so as to secure the

perfect combustion of fuel.

2. The heat generated in the furnace must be transferred to the

water in the boiler without loss.

3. The circulation in the boiler must be so complete that the

heat from the furnace may be quickly and thoroughly diffused throughout the whole body of water.

4. The construction of an engine that w T ill use the steam with-

out loss of heat, except so much as may be necessary to perform work required of the engine.

5. The recovery of heat from exhaust steam.

6. The absence of friction and back pressure in the working of

the engine.

LOSS BY BAD FIRING. 15

It is superfluous to say that these conditions are not fulfilled in any engine of the present day. At best the combustion of fuel is only approximately perfect, the losses being due to several causes among which are, — unburned fuel falling through the spaces in the grates and mingling with the ashes. This, with some kinds of coal, and improper firing, amounts to a large percentage of the furnace waste.

It is not possible w r ith any present method of setting boilers to transfer all the heat of the furnace to the water in the boiler ; nor can there be for the reason that, the temperature of the escaping gases must not be lower than that of the steam in the boilers, or direct loss will result in the radiation of heat from the tubes or flues in the boiler, by thus reheating the gases to the steam temperature.

If the steam pressure is 80 lbs. per square inch above the atmosphere, the corresponding temperature due to this pressure is 324° Fahr. The temperature of the escaping gases ought not, therefore, to be less than 350° Fahr. where they leave the boiler flues or tubes to pass off into the chimney.

If the temperature of the furnace be taken at 2000° Fahr. and the escaping gases at 400° Fahr. it w T ill be seen that one-fifth of the heat generated in the furnace is passing off without performing work. This is a very great loss, and these figures understate rather than correctly give the loss from this one source. . Efforts have been made to utilize the temperature of these waste gases by making them heat feed water by means of coils, or by that particular disposition of pipes and connections known a& an economizer. Others have turned it into account by making it heat the air supplied the fuel on the grates. Any heat so reclaimed is money saved, provided it does not cost more to get it than it is worth in coal to generate a similar quantity of heat. It is doubtful whether the loss in this particular direction can be brought below 20 per cent, of the fuel burned, at least, by any method of saving now known. : •

The loss by bad firing and by a bad construction of furnace is often a large one. It has been demonstrated experimentally that ' 20 to 30 per cent of fuel can be saved by a proper construction and operating of the furnace. The direct causes of loss are, too low temperature of furnace for properly burning fuels, especially such as are rich in hydro-carbon gases; or, by the admission of too much cold air over or back of the tire ; or by the admission of too little air under the lire so that carbonic oxide gas is generated instead of carbonic acid gas, the former being a product of incomplete, the latter the product of complete combustion.

The relative heating powers of fuel burned, resulting in the production of either of these two gases being as follows :

Heat units.

1 pound of carbon burned to carbonic acid 14,500

1 pound of carbon burned to carbonic oxide 4,500

Units of heat lost by burning to carbonic oxide 10,000

It will be seen that here is an enormous source of loss and all that is required to prevent it is a proper construction of furnace.

Smoke is a nuisance which ought to be prohibited by stringent legislation.

There is no good reason for its polluting presence in the atmosphere, defiling everything with which it comes in contact. Smoke regarded as a source of direct loss is greatly overestimated, the fact is, the actual amount of coal lost to produce smoke is very trifling.

The presence of smoke indicates a low temperature of furnace or combustion chamber ; if the temperature were sufficiently high and the furnace, properly constructed, smoke could not be generated. The prevention of smoke is easily accomplished, and with it a more economical combustion of hydro carbon fuels.

Radiation. — A considerable loss of heat occurs by radiation from the furnace walls ; this may be prevented in part by making the walls hollow with an air space between. If a force blast is used the air may be admitted at the back end of the boiler-setting and by passing through between the walls will become heated and if conveyed into the ash pit at a high temperature will greatly assist combustion and thus tend to a higher economy.

Air required.— In regard to the quantity of air required, it will vary somewhat with the fuel used, but in general 12 pounds of

MEASUREMENT CF HEAT. 17

air are sufficient to completely burn one pound of coal ; practically however, 15 to 25 pounds are furnished, being largely in excess of that which the lire can use, and must pass off with the gases as a w T aste product.

This surplus air enters cold and leaves the furnace heated to the same temperature as that of the legitimate and proper products of combustion, and thus directly operates to the lowering of the furnace temperature.

Measurement of Heat. — A heat unit is that quantity of heat necessary to raise the temperature of one pound of water one degree, from 39° to 40° Fahr., this being the temperature of the greatest density of water. A thermal unit, heat unit, or unit of heat, all mean the same thing.

Experiments have been made to determine the mechanical equivalent of a heat unit, and it is found to be equal to 772 pounds raised one foot high. This is sometimes called "Joule's equivalent," after Dr. Joule of England, it is also known as the dynamic value of a heat unit.

Knowing the number of heat units in a pound of coal qnables us to calculate the amount of work it should perform. Let us suppose a pound of coal to be burned to carbonic acid gas and to <levelope during its combustion 14,000 heat units, then: —

That is to say: if one pound of coal were burned under the above conditions it would have a capacity for doing work represented by the lifting of eleven millions of pounds one foot high against the action of gravity. Suppose this to be done in one hour then w r e should expect to get from one pound of coal an equivalent of 5.56 H. P. It is well known that only a very small fraction of such equivalent is secured in the very best modern practice. The question is where does this heat go, and why is it so small a portion of it is actually utilized?

The losses may be accounted for in several ways and perhaps as Allows :

The heat wasted iu the chimney 25 ]>er cent.

Through bad firing If.

Heat accounted for by the engine (not indicated) lft ki

Heat lost by exhaust steam 55

This is about 2 pounds of coal per hour per indicated horse power, which is regarded as a very high attainment and is seldom reached in ordinary cut off engines.

It requires good coal, good firing, and an economical engine to get an indicated horse power from two pounds of coal burned per hour. As coal varies in quality it is a better plan to deduct the ashes and other incombustible matter and take the net combustible as a basis of comparison.

The best coal when properly burned is capable of evaporating 15 pounds of water from and at a temperature of 212° Fahr. The common evaporation is about half that amount, and with the best improved furnaces, and careful management it is seldom that 10 pounds of water is exceeded and is to be regarded as a high rate of evaporation. In experimental tests 12 pounds have been reported, but it is doubtful whether there is any steam boiler and furnace which is constantly yielding any such results.

Circulation of water in a boiler is a very important feature to secure the highest evaporative results. Other things being equal the boiler which affords the best circulation of water will be found to be the most economical in service. Circulation is greatly hindered in some boilers by having too many tubes ; in others, by introducing in the water space of the boiler too many stays and making the water spaces too narrow.

To secure the highest economy there must be thorough circulation from below upwards, in the boiler. There is no doubt that a great deal of heat is lost because the construction is such as to hinder a free flow of water around the tubes and sides of the boiler.

The construction of an engine that will use steam without loss

of heat, except so much as may be necessary to perform work required of it, is a physical impossibility. Among the sources of loss in an engine are : — radiation, condensation of steam in un jacketed cylinders, and the enormous loss of heat occasioned by exhausting the steam into the atmosphere.

Radiation is usually classed among the minor losses in a steam engine. There is a considerable loss of heat caused by radiation

COVERING BOILERS AND PIPES. 19

from steam boilers and pipes exposed to the atmosphere, and not protected by a suitable covering.

Much of this heat may be saved by employing a non-conducting material as a covering which, though not preventing all radiation, will save enough heat to make its application economical. It is well known that some bodies conduct and radiate heat less rapidly than others, but it must not be understood that the absolute value of such a covering is inversely proportioned to the conducting power of the material employed, because in its application the outer surface is enlarged and the radiation will be going on less actively at any given point, but the enlarged surface exposed reduces somewhat the apparent gain.

Covering Boilers and Pipes. — Locomotive boilers have a casing of wood around the boilers and covered with sheet iron, this makes a very good and low priced covering, and is also well adapted to portable engines.

Owing to the competition and consequent low price at which the latter engines are offered they are seldom covered in this way.

Stationery boilers generally have the side walls carried up to a height of a few inches above the top of the boiler and this space is filled in with dry ashes. There are now in the market several proprietary articles which have been found quite efficient in preventing loss by radiation, among these are air space coverings, asbestos, and others of which the composition is not made known.

Steam Jackets. — This term is commonly used to designate a steam tight casing around an engine cylinder, and which it is intended shall be kept filled with steam, that the inner cylinder shall be maintained at a temperature equal to or higher than that of the steam before expansion begins. The particular object in jacketing steam cylinders is to prevent the condensation of steam within the cylinder during any portion of the stroke.

"When steam is admitted into an ordinary cylinder, the temperature of the cylinder itself is lower than that of the steam, consequently the steam gives up to the cylinder so much of its heat as may be necessary to equalize the temperature. This occasions loss of pressure, which means loss of heat, therefore, the steam has less capacity to do work ; the usual expression for this is " waste of energy." If the cylinder is surrounded by a body of steam at full boiler pressure, its temperature will be higher than that of the inflowing steam and no such condensation will take place and there will be nothing within the cylinder but dry steam throughout the stroke. Condensation is going on just the same however, but it is transferred from the cylinder where the pressure is variable, to the steam jacket, where the pressure is constant.

However satisfactory the theory of the steam jacket may be, it has not always been satisfactory in its practical workings. No doubt much of this is due to defective design, and to incompetent handling after erection. It is doubtful whether it is of any special advantage in ordinary high pressure engines cutting off not earlier than | to \ stroke. When a large engine is working with high grades of expansion, say \ or \ cut off, then a steam jacket may be of advantage.

One thing is certain with respect to steam jackets, and that is, the water of condensation must not be allowed to collect in the jacket, and that is about the only duty devolving upon the engineer. If this is not carefully attended to, an efficient steam jacket may be made not only inoperative, but result in a greater waste through enormous condensation, than would result if the engine had no jacket at all.

It must not be supposed that the benefits occurring or likely to occur from the use of a steam jacket are confined to large engines ; the reason why they are seldom applied to small engines is on account of the increased cost of manufacture, and the change required? in the patterns for cylinders to be so fitted.

Condensation in Steam Cylinders. — This is a source of waste in all steam engines in which no special provision has been made to prevent it. The two methods usually employed in its prevention are either superheating the steam, or by the use of a steam jacket around the cylinder.

To entirely prevent condensation would require that the cylinder and piston be made of a non-conducting and a non-absorbing material, this would prevent any waste of fuel by con-

SUPERHEATED STEAM. 21

duction and radiation. Condensation occurs through another cause, and which cannot be thus remedied ; that is, the condensation of steam due to the performance of work in the cylinders, but this can hardly be reckoned as a loss or waste of fuel.

So far as choice of material for a steam cylinder goes, we are practically confined to cast-iron, which is a good conducting material and absorbs heat rapidly during the first part of the stroke and as the temperature falls within the cylinder gives it out rapidly during the latter part of the stroke, if it is a cut-off engine or one in which the steam is used expansively ; the loss continuing during the whole of the return stroke of the piston, and which reaches the maximum if the engine be a condensing one ; the loss of heat being less if the steam be exhausted into the atmosphere w T ith the usual two or three pounds back pressure.

The use of superheated steam has not been employed in high pressure steam engines, except in rare instances and under circumstances which cannot ordinarily be trusted, or safely confided to the care of an ordinary workmen.

The objections to a steam superheating apparatus are principally the excessive wear and tear to which it is subjected and which makes it very expensive to operate, together with the danger of accident likely to result by having a high steam pressure within an over-heated pipe or chamber, and the mischief the steam itself is likely to occasion the valves and piston by destroying the lubricating qualities of the oil between the wearing surfaces.

Exhaust Steam. — The recovery of heat from exhaust steam, and its utilization without interfering with the performance of the engine, is as yet an unsolved problem. In a non-condensing engine more than half the heat generated by the combustion of fuel in the furnace is carried aw T ay in the exhaust steam as it escapes into the atmosphere. The reduction of pressure during the entire stroke of the piston is favorable to the abstraction of heat from the cylinder during the period of exhaust, and results in greater or less loss of heat, depending on the time occupied.

This may be lessened somewhat by the early closing of the exhaust valve and thus compressing the steam up to, as near as possible, the pressure in the steam chest. A high piston speed and rapid rate of revolution will very materiallv contribute to economy of heat by lessening the quantity abstracted by the exhaust.

Chap. III. Selection Of A Boiler

The selection of a boiler for a particular service will naturally suggest the following questions : —

1. What kind of a boiler shall it be ?

2. Of what material shall it be made ?

3. What size shall it be in order to furnish a certain power? In reply to the first question it is to be expected there will

be wide differences of opinion varying with the locality, usage, and service for which it is intended.

One of the first things to be taken into account in the selection of a boiler is the quality of water to be used in it for generating steam. If the water is pure, then it makes little difference what kind of boiler be selected so far as incrustation affects selection. If the water is hard and will deposit scale upon evaporation then a boiler should be selected w T hich w r ill admit of thorough inspection and removal of any deposit formed within it.

For hard water, the ordinary two flue boiler will be found a good one, as it is favorable to a thorough circulation of water, and permits easy access to all parts of it for examination and cleaning. It docs not, however, present the extent of heating surface for a given space that tubular boilers offer ; but with hard water the boiler is quite as economical if kept in good condition.

The difficulty with tubular boilers when used in connection with hard water is that the tubes will in a short time become coated w r ith scale; this prevents the transmission of heat not only, but impairs the circulation of the water around them. Both of these are opposed to economy in the fact that it requires more coal to generate a given weight of steam in the first case ; and second by reason of deficient circulation, the plates over the fire are likely to become overheated and burnt and so become dangerous ; thus directly contributing to accident or disaster.

The matter of circulation in boilers is one which should have careful attention in making a selection. There is little trouble in this regard with any of the ordinary types of boilers so long as they are clean and new, and properly proportioned. Nor is there likely to be .any difficulty thereafter if the water is soft and clean. Circulation is often seriously impaired by putting in too many tubes in a boiler, the effect of which is to so fill up the space that the heated particles of water forcing their way upwards from below meet with so much resistance that they can hardly overcome it, and the result is that a boiler does not furnish from one-fourth toone-half as much steam for a given weight of fuel as it should,, from this very cause.

Boilers intended for use in distant localities where the facilitiesfor repairs are meager or entirely wanting, and fuel low priced, should be of the simplest description. Cylinder boilers or two flue boilers will perhaps be found most suitable. These are largely used by coal miners, blast furnaces, saw mills, and other branches of industry which must of necessity be removed from the larger towns and engineering work shops.

In selecting a boiler for a planing mill or any other place in which the fuel is of similar description and the firing irregular, there should be large water capacity in the boiler that it may act as a reservoir of power in much the same way that a fly wheel acts as a regulator for a steam engine. It is a common notion among wood-workers that firing with shavings or light fuel is "' easy on the boiler." There is abundant reason to doubt this. The suddenness and the rapidity with which an intense fire is kindled in the furnace, filling all the furnace space and the tubes with flame, and with an intense heat which envelopes all within the limits of draft opening, continuing thus for a few minutes only, and as suddenly going out, can hardly be regarded as the ideal furnace. Yet there are thousands of just such furnaces at work, and it is

HORIZONTAL TUBULAR BOILERS. 2o

altogether probable that little or no change will be made in them by this class of manufacturers, at least in the near future.

In regard to the selection of a boiler for this service, we are brought back again to the question of hard or soft water. The decision should be largely influenced by this, but whatever type of boiler is selected there should be a surplus of boiler power of at least 20 per cent, that is if a 50 horse power boiler is needed to do the work, put in one of 60 horse-power, this will prevent the fluctuations of speed in the engine which are sure to follow a reduction of boiler pressure.

This increase in boiler power ought not to be simply that of tube surface, but should also include extra water space. The reserve power of a boiler is in the water heated up to a temperature corresponding to the steam pressure, when this pressure is lowered the water then gives off steam corresponding to the lower pressure, the more water the more steam, and in this w T ay the water in the boiler stores up heat when over fired, to give it off again when the fire is low, and so acts a regulator of pressure, a thing that extra

tube surface cannot do.

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This kind of firing is apt to induce priming, and for this reason a boiler should be selected having a large water surface. Horizontal boilers are, in general, to be preferred over vertical ones for planing mills because of the larger water surface exposed in proportion to the heating surface. If a tubular boiler is selected, the water line above the tubes should be not higher than two-thirds the diameter of the boiler measured from the bottom, and the boiler should be made having the upper edge of the top row of tubes at least three inches below this, there should also be a clear space up through the center of the boiler of sufficient width to insure a perfect circulation of water.

Horizontal tubular boilers are to be recommended when pure soft water is used. They combine at once the qualities of great strength without excessive bracing, large heating surface, high evaporative capacity without liability to priming, and are convenient of access for external and internal examination when set in the furnace.

Boilers of this class are apt to contain too many tubes, and are thus rendered less efficient than if a smaller number were used ior the same diameter of shell.

The tubes should be not less than three inches and are seldom more than four inches in diameter, when secured to the heads by expanding. The tubes may be three inches for all diameters of shell up to 48 inches; for diameters of shell ranging from 44 to GO inches, three and a half inch tubes may be used ; and for diameters of shell from 48 to 72 inches the tubes may be four inches in diameter.

In regard to length, 3 inch tubes may be used up to 12 to 14 ft; 3£ inch tubes from 12 to 16 feet long, and 4-inch tubes from 14 to . 18 feet long.

The space between the tubes should be at least one-third their diameter, and there should be ample space between the nearest tube and the shell for circulation of water up the sides of the boiler ; a center space should also be allowed in boilers of large diameter, say in all boilers over 48 inches in diameter.

Flue boilers are not regarded as being quite so economical as tubular boilers. In a '• competitive test " they would probably be " figured out" on the score of economy, yet with all their apparent w r ant of economy they are not to be despised ; on the contrary, there are circumstances or conditions in which they are to be strongly recommended. Chief among the the advantages claimed for this style of boiler may be mentioned the fact that it affords every possible facility for examination and cleaning. In this respect it is second only to the cylinder boiler.

Boilers of this kind should be fitted with two man heads ; one at the back end above and the other at the front end underneath the flues.

The principal danger connected with this kind of a boiler is the liability of the flues to collapse, and for this reason the boilers should not be made excessively long, nor of very large diameter of flues. The commonest diameters for two flue boilers are from 40 to 44 inches ; the flues are usually one-third the diameter of the shell ; the lengths do not as a general thing vary much from six

SELECTION OF A BOILER. 27

times the diameter of the shell. These may be taken as fair average proportions.

Five Flue Boilers.— Boilers are also made with several large flues, perhaps the five flue boiler is the oftenest met with ; the arrangement being that of three large flues above and two smaller ones below. This is not a very desirable form of construction, and lias little to recommend it over the two flue boiler.

Instead of a five flue boiler, it is perhaps preferable to take the same diameter of shell and insert as many six inch lap-welded flues as it will contain allowing three inches water space between the flues and the side of the shell.

Boilers of this kind may be made from 16 to 20 feet long and will be found to be a good style of boiler.

Fire box boilers, or locomotive boilers as they are commonly called are best adapted for small powers and with a fuel which deposits but little soot in the tubes. This kind of boiler is supplied with portable or agricultural engines and is very well adapted for that particular service.

In canvassing the desirability of this kind of a boiler for stationary use, we must again refer to the kind of water to be used in it. If the water is soft and clean there is then no particular objection to a boiler of this construction being used for small powers; if the water is hard and will form scale it ought not to be chosen, but a flue boiler selected instead.

Tertical boilers are used in great numbers for small engines, heating, etc. They have the merit of being compact and low priced. A common defect in the construction of this kind of boiler is that too many tubes are put in the head in the fire box, thereby preventing a proper circulation of water between them. This defect m construction induces priming with all its attendant annoyances and dangers.

This style of boiler is not suited to hard water, but pure soft water only. If the water is hard an upright boiler with a single flue is better if not safer than the tubular boiler. These boilers

28 HAND HOOK FOR STEAM ENGINEERS.

should be provided with hand holes above the crown sheet and around the bottom of the water legs ; at least three at each place mentioned.

Boiler Material. — In regard to the material of which a boiler shall be made there is but the simple choice between iron and steel.

If iron should be selected, it should be of a grade not lower than C. IT. No. 1, and capable of withstanding a tensile strain of at least 40,000 pounds per square inch of section lengthwise of the grain of the metal. It should also show a contraction of area at the point of fracture of at least twelve per cent. This is the lowest grade of iron which should enter into boiler construction.