Fig. 9.
Fig. 9.
It is an important “point” when burning shavings or sawdust with a blast, to keep the blower going without cessation, as there have been disastrous accidents caused by the flames going up the shutes, thence through the small dust tubes leading from the bin to the various machines.
Fig. 10.
Fig. 10.
In firing “shavings” by hand it is necessary to burn them from the top as otherwise the fire and heat are only produced when all the shavings are charred. To do this, provide a half-inch gas pipe, to be used as a light poker; light the shaving fire, and when nearly burned take the half-inch pipe and divide the burning shavings through the middle, banking them against the side-walls, as shown in Fig. 9. Now feed a pile of new shavings into the centre on the clean grate bars, as shown in Fig. 10, and close the furnace doors. The shavings will begin to burn from above, lighted from the two side fires, the air will pass through the bars into the shavings, where it will be heated and unite with the gas, making the combustion perfect, generating heat, and no smoke, and the fire will last much longer and require not half the labor in stoking.
locomotive furnace
This figure exhibits the interior of the furnace of a locomotive engine, which varies greatly from the furnace of either a land or marine boiler. This difference is largely caused by the method of applying the draught for the air supply; in the locomotive this is effected by conducting the exhaust steam through pipes from the cylinders to the smoke-box and allowing it to escape up the smoke stack from apertures called exhaust nozzles; the velocity of the steam produces a vacuum, by which the products of combustion are drawn into the smoke-box with great power and forced out of the smoke stack into the open air.
To prevent the too quick passage of the gases into the flues an appliance called a fire brick arch has been adopted and has proved very efficient. In order to be self supporting it is built in the form of an arch, supported by the two sides of the fire box which serve for abutments. The arch has been sometimes replaced by a hollow riveted arrangement called a water table designed to increase the fire surface of the boiler.
Firing a Locomotive.—No rules can possibly be given for firing a locomotive which would not be more misleading than helpful. This is owing to the great variations which exist in the circumstances of the use of the machine, as well as the differences which exist in the various types of the locomotive.
These variations may be alluded to, but not wholly described. 1. They consist of the sorts of fuel used in different sections of the country and frequently on different ends of the same railroad; hard coal, soft coal, and wood all require different management in the furnace. 2. The speed and weight of the train, the varying number of cars and frequency of stopping places, all influence the duties of the fireman and tax his skill. 3. The temperature of the air, whether cold or warm, dry weather or rain, and night time and day time each taxes the skill of the fireman.
Hence, to be an experienced fireman in one section of the country and under certain circumstances does not warrant the assurance of success under other conditions and in another location. The subject requires constant study and operation among not only “new men” but those longest in the service.
More than in any other case to be recalled, must the fireman of a locomotive depend upon the personal instruction of the engineer in charge of the locomotive.
Firing with Tan Bark.—Tan bark can be burned upon common grates and in the ordinary furnace by a mixture of bituminous screenings. One shovel full of screenings to four or five of bark will produce a more economical result than the tan bark separate, as the coal gives body to the fire and forms a hot clinker bed upon which the bark may rest without falling through the spaces in the grate bars, and with the coal, more air can be introduced to the furnace.
The above relates to common furnaces, but special fire boxes have been recently put into operation, fed by power appliances, which work admirably. The “point” principally to be noted as to the efficacy of tan bark as a fuel, is to the effect, that like peat, the drier it is the more valuable is it as a fuel.
The Process of Boiling.Let it be remembered that the boiling spoken of so often is really caused by the formation of the steam particles, and that without the boiling there can be but a very slight quantity of steam produced.
While pure water boils at 212°, if it is saturated with common salt, it boils only on attaining 224°, alum boils at 220°, sal ammoniac at 236°, acetate of soda at 256°, pure nitric acid boils at 248°, and pure sulphuric acid at 620°.
On the First Application of Heatto water small bubbles soon begin to form and rise to the surface; these consist of air, which all water contains dissolved in it. When it reaches the boiling point the bubbles that rise in it are principally steam.
In the case of a new plant, or where the boiler has some time been idle it is frequently advisable to build a small fire in the base of the chimney before starting the boiler fires. This will serve to heat the chimney and drive out any moisture that may have collected in the interior and will frequently prevent the disagreeable smoking that often follows the building of a fire in the furnace.
Always bear in mindthat the steam in the boilers and engines is pressing outward on the walls that confine it in every direction; and that the enormous forces you are handling, warn you to be careful.
When starting fires close the gauge cocks and safety valve as soon as steam begins to form.
Go slow.It is necessary to start all new boilers very slowly. The change from hot to cold is an immense one in its effects on the contraction and expansion of the boiler, the change of dimension by expansion is a force of the greatest magnitude and cannot be over-estimated. Leaks which start in boilers that were well made and perfectly tight can be attributed to this cause. Something must give if fires are driven on the start, and this entails trouble and expense that there is no occasion for. This custom applies to engines and steam pipes as well as to boilers. No one of any experience will open a stop valve and let a full head of live steam into a cold line of pipe or a cold engine.
To preserve the grate bars from excessive heat, when first firing a boiler, it is well to sprinkle a thin layer of coal upon the grates before putting in the shavings and wood for starting the fire. This practice tends greatly to prolong the life of the grate-bars.
The fuel should generally be dry when used. Hard coal, however, may be dampened a little to good advantage, as it is then less liable to crowd and will burn more freely.
Air, high temperature and sufficient time are the principal points in firing a steam boiler.
In first firing up make sure that the throttle valve is closed, in order that the steam first formed may not pass over into the engine cylinder and fill it with water of condensation. If the throttle valve leak steam it should be repaired at the first opportunity.
Keep all heating surfaces free from soot and ashes.
Radiant rays go in all directions, yet they act in the most efficient manner when striking a surface exactly at a right angle to their line of movement. The sides of a fire-box are for that reason not as efficient as the surface over the fire, and a flat surface over the fire is the best that can be had, so far as that fact alone is concerned.
When combustion is completed in a furnace, then the balance of the boiler beyond the bridge wall can be utilized for taking up heat from the gases. The most of this heat has to be absorbed by actual contact; thus by the tubes the gases are finally divided, allowing that necessary contact.
Combustion should be completed on the grates for the reason that it can be effected there at the highest temperature. When this is accomplished, the fullest benefit is had from radiant heat striking the bottom of the boiler—it is just there that the bulk of the work is done.
There must necessarily be some waste of heat by its passing up the chimney to maintain draft. It is well to have thegases, as they enter the chimney, as much below 600 deg. F. (down to near the temperature of the steam) as you can and yet maintain perfect combustion.
Every steam engine has certain well-defined sounds in action which we call noises, for want of a better term, and it is upon them and their continuance that an engineer depends for assurance that all is going well.
This remark also applies to the steam boiler, which has, so to speak, a language of its own, varying in volume from the slight whisper which announces a leaking joint to the thunder burst which terribly follows a destructive explosion. The hoarse note of the safety-valve is none the less significant because common.
The dampers and doors to the furnace and ash-pit should always be closed after the fire has been drawn, in order to keep the heat of the boiler as long as possible.
But the damper must never be entirely closed while there is fire on the grate, as explosions dangerous in their character might occur in the furnace from the accumulated gases.
Flues or tubes should often be swept, as soot, in addition to its liability to becoming charged with a corroding acid, is a non-conductor of heat, and the short time spent in cleaning them will be repaid by the saving of labor in keeping up steam. In an establishment where they used but half a ton of bituminous coal per day, the time of raising steam in the morning was fifty per cent. longer when the tubes were unswept for one week than when they were swept three times a week.
Smokewill not be seenif combustion is perfect. Good firing will abate most of the smoke.
Coals, at the highest furnace temperature, radiate much heat, whereas gases ignited at and beyond the bridge wall radiate comparatively little heat—it is a law in nature for a solid body highly heated to radiate heat to another solid body.
Dry and Cleanis the condition in which the boiler should be kept,i.e., dry outside and clean both inside and out.
To haul his furnace fire and open the safety valve before seeking his own safety or the preservation of property, is the duty of the fireman in the event of fire threatening to burn a whole establishment.
Many, now prominent, engineers have made their first reputation by remembering to do this at a critical time.
When Water is Pumpedinto the boiler or allowed to run in, some opening must be given for the escape of the contained air; usually the most convenient way is to open the upper gauge cock after the fire has been lighted until cloudy steam begins to escape.
In a summary of experiments made in England, it is stated that:—
“A moderately thick and hot fire with rapid draft uniformly gave the best results.
“Combustion of black smoke by additional air was a loss.
“In all experiments the highest result was always obtained when all the air was introduced through the fire bars.
“Difference in mode of firing only may produce a difference of 13 per cent. (in economy).”
The thickness of the fire under the boiler should be in accordance with the quality and size of the fuel. For hard coal the fire should be as thin as possible, from three to six inches deep; when soft coal is used, the fire should be thicker, from five to eight inches deep.
If it is required to burn coal dust without any change of grates, wetting the coal is of advantage; not that it increases its heat power, but because it keeps it from falling through the grates or going up the chimneys. The same is true of burning shavings; by watering they are held in the furnace, and the firing is done more easily and with better results.
Stirring the Fireshould be avoided as much as possible; firing should be performed evenly and regularly, a little at a time, as it causes waste fuel to disturb the combustion and by making the fuel fall through the grates into the ash pit; hence do not “clean” fires oftener than absolutely necessary.
The slower the velocity of the gases before they pass the damper, the more nearly can they be brought down to the temperature of the steam, hence with a high chimney and strong draft the dampers should be kept nearly closed, if the boiler capacity will permit it.
No arbitrary rule can be laid down for keeping fires thick or thin. Under some conditions a thin fire is the best, under others a thick fire gives best economy. This rule, however, governs either case: you must have so active a fire as to give strong radiant heat.
One of the highest aims of an expert fireman should be to keep the largest possible portion of his grate area in a condition to give great radiant heat the largest possible part of the day—using anthracite coal by firing light, quick and often, not covering all of the incandescent coals. Using bituminous coal, hand firing, by coking itvery nearthe dead plate, allowing some air to go through openings in the door, and by pushing toward the bridge wall only live coals—when slicing, to open the door only far enough to work the bar; this is done with great skill in some cases.
Regulating the Draft.—This should be done so as to admitthe exact quantity of airinto the furnace, neither too much nor too little. It should be remembered that fuel cannot be burned without air and if too much air is admitted it cools the furnace and checks combustion. It is a good plan to decrease the draft when firing or cleaning out, by partly closing the damper or shutting off the air usually admitted from below the grates; this is to have just draft enough to prevent the flame from rushing out when the door is opened.
By luminous flameis generally meant that which burns with a bright yellow to white color. All flame under a boiler is not luminous, sometimes the whole or a part of it will be red or blue. The more luminous the flame, that is to say, the nearer white it is, the better combustion.
To determine the temperature of a furnace Firethe following table is of use. The colors are to be observed and the corresponding degrees of heat will be approximately as follows:
Faint red960° F.Bright red1,300° F.Cherry red1,600° F.Dull orange2,000° F.Bright orange2,100° F.White heat2,400° F.Brilliant white heat2,700° F.
That is to say, when the furnace is at a “white heat” the heat equals 2,400 degrees Fahrenheit, etc.
Another method of finding the furnace heat is by submitting a small portion of a particular metal to the heat.
Tin melts at442° F.Lead „ „617° F.Zinc „ „700° F.nearly.Antimony melts at810 to 1,150° F.Silver melts at1,832 to 1,873° F.Cast Iron melts at2,000° F.nearly.Steel „ „2,500° F.„Wrought Iron melts at2,700° F.„Hammered Iron melts at2,900° F.„
The causes are—dirty water, trying to evaporate more water than the size and construction of the boiler is intended for, taking the steam too low down, insufficient steam room, imperfect construction of boiler, too small a steam pipe and sometimes it is produced by carrying the water line too high.
Too little attention is paid to boilers with regard to their evaporating power. Where the boiler is large enough for the water to circulate, and there is surface enough to give off the steam, foaming never occurs.
As the particles of the steam have to escape to the surface of the water in the boiler, unless that is in proportion to the amount of steam to be generated, it will be delivered with such violence that the water will be mixed with it, and cause foaming.
For violent ebullition a plate hung over the hole where the steam enters the dome from the boiler, is a good thing, and prevents a rush of water by breaking it, when the throttle is opened suddenly.
In cases of very violent foaming it is imperative to check the draft and cover the fires.
The steam pipe may be carried through the flange six inches into the dome—which will prevent the water from entering the pipes by following the sides of the dome as it does.
A similar case of priming of the boilers of the U. S. Steamer Galena was stopped by removing some of the tubes under the smoke stack and substituting bolts.
Clean water, plenty of surface, plenty of steam room, large steam pipes, boilers large enough to generate steam without forcing the fires, are all that is required to prevent foaming.
A high pressure insures tranquillity at the surface, and the steam itself being more dense it comes away in a more compact form, and the ebullition at the surface is no greater than at a lower pressure. When a boiler foams it is best usually to close the throttle to check the flow, and that keeps up the pressure and lessens the sudden delivery.
Too many flues in a boiler obstruct the passage of the steam from the lower part of the boiler on its way to the surface—this is a fault in construction.
An engineer who had been troubled with priming, finally removed 36 of the tubes in the centre of the boiler, so as to centralize the heating effect of the fire, thereby increasing the rapidity of ebullition at the centre, while reducing it at the circumference. The effect of the change was very marked. The priming disappeared at once. The water line became nearly constant, the extreme variation being reduced to two inches.
Which is another way of repeating what has already been said.
1.Don’tempty the boiler when the brick work is hot.2.Don’tpump cold water into a hot boiler.3.Don’tallow filth of any kind to accumulate around the boiler or boiler room.4.Don’tleave your shovel or any other tool out of its appointed place when not in use.5.Don’tfail to keep all the bright work about the boiler neat and “shiny.”6.Don’tforget that negligence causes great loss and danger.7.Don’tfail to be alert and ready-minded and ready-headed about the boiler and furnace.8.Don’tread newspapers when on duty.9.Don’tfire up too quickly.10.Don’tlet any water or dampness come on the outside of your boiler.11.Don’tlet any dampness get into the boiler and pipe coverings.12.Don’tfail to see that you have plenty of water in the boiler in the morning.13.Don’tfail to keep the water at the same height in the boiler all day.14.Don’tlet any one talk to you when firing.15.Don’tallow water to remain on the floor about the boiler.16.Don’tfail to blow off steam once or twice per day according as the water is more or less pure.17.Don’tfail to close the blow-off cock, when blowing off, when the water in the boiler has sunk to one and a half inches.18.Don’t fail, while cleaning the boiler, to examine and clean all cocks, valves and pipes and look to all joints and packings.19.Don’tcommence cleaning the boiler until it has had time to cool.20.Don’tforget daily to see that the safety-valve moves freely and is tight.21.Don’tfail to clean the boiler inside frequently and carefully.22.Don’tfail to notice that the steam gauge is in order.23.Don’tfail to keep an eye out for leaks and have them repaired immediately, no matter how small.24.Don’tfail to empty the boiler every week or two and re-fill it with fresh water.25.Don’tlet any air into the furnace, except what goes through the grate-bars, or the smoke burners, so called, by which the air is highly heated.26.Don’tincrease the load on the safety-valve beyond the pressure allowed by the inspector.27.Don’tfail to open the doors of the furnace and start the pump when the pressure is increased beyond the amount allowed,but28.Don’tfail to draw the fireswhen there is dangerfrom the water having fallen too low.29.Don’tfail to check the fire—if too hot to draw, do it with fresh coal, damp ashes, clinkers or soil;and30.Don’tfail to open the doors of the furnace and close the ash-pit doors at the time the fire is checked—and31.Don’tdecrease the steam pressure by feeding in water or suddenly blowing off steam,and32.Don’ttouch the safety-valve, even if it be opened or closed,and33.Don’tchange the feed apparatus if it is working, or the throttle-valve be open; let them both remain as they are for a short time,and34.Don’tfail to change them very cautiously and slowly when you close them, and35.Don’tfail to be very cool and brave while resolute in observing these last seven “Don’ts.”36.Don’tfail to keep yourself neat and tidy.37.Don’tfail to be polite as well as neat and brave.38.Don’tfail to keep the tubes clear and free from soot and ashes.39.Don’tlet too many ashes gather in the ash-pit.40.Don’tdisturb the fire when it is burning good nor stir it up too often.41.Don’tbe afraid to get instruction from books and engineering papers.42.Don’tfail to make an honest self-examination as to points upon which you may be ignorant, and really need to know in order to properly attend to your duties.43.Don’tallow too much smoke to issue from the top of the chimney if the cause lies within your power to prevent it.44.Don’tthink that after working at firing and its kindred duties for a year or two thatthe whole subjectof engineering has been learned.45.Don’tforget that one of the best helps in getting forward is the possession of a vigorous and well-balanced mind and body—this covers temperance and kindred virtues and a willingness to acquire and impart knowledge.46.Don’tforget to have your steam-gauge tested at least once in three months.47.Don’tuse a wire or metallic rod as a handle to a swab in cleaning the glass tube of a water-gauge for the glass may suddenly fly to pieces when in use within a short time afterwards.48.Don’tforget that steam pumps require as much attention as a steam engine.49.Don’trun a steam pump piston, unless in an emergency, at a speed exceeding 80 to 100 feet per minute.50.Don’tdo anything without a good reason for it about the engine or boiler, but when you are obliged to do anything, do it thoroughly and as quickly as possible.51.Don’tforget to sprinkle a thin layer of coal on the grates before lighting the shavings and wood in the morning. This practice preserves the grate bars.52.Don’ttake the cap off a bearing and remove the upper brass simply to see if things are working well; if there is any trouble it will soon give you notice, and, besides, you never can replace the brass in exactly its former position, so that you may find that the bearing will heat soon afterwards, owing to your own uncalled-for interference.53.Don’tput sulphur on a hot bearing, unless you intend to ruin the brasses.54.Don’tuse washed waste that has a harsh feel, as the chemicals used in cleansing it have not been thoroughly removed.55.Don’t, in case of an extensive fire, involving the whole business, rush off without drawing the fires, and raising andpropping openthe safety valve of the boiler.56.Don’tfail to preserve your health, for “a sound mind in a sound body” is beyond a money valuation.57.Don’tfail to remember that engineers and firemen are in control of the great underlying force of modern civilization; hence, to do nothing to lower the dignity of the profession.58.Don’tforget that in the care and management of the steam boiler the first thing required is an unceasing watchfulness—watch-care.59.Don’tforget that an intemperate, reckless or indifferent man has no business in the place of trust of a steam boiler attendant.60.Don’tallow even a day to pass without adding one or more facts to your knowledge of engineering in some of its branches.
1.Don’tempty the boiler when the brick work is hot.
2.Don’tpump cold water into a hot boiler.
3.Don’tallow filth of any kind to accumulate around the boiler or boiler room.
4.Don’tleave your shovel or any other tool out of its appointed place when not in use.
5.Don’tfail to keep all the bright work about the boiler neat and “shiny.”
6.Don’tforget that negligence causes great loss and danger.
7.Don’tfail to be alert and ready-minded and ready-headed about the boiler and furnace.
8.Don’tread newspapers when on duty.
9.Don’tfire up too quickly.
10.Don’tlet any water or dampness come on the outside of your boiler.
11.Don’tlet any dampness get into the boiler and pipe coverings.
12.Don’tfail to see that you have plenty of water in the boiler in the morning.
13.Don’tfail to keep the water at the same height in the boiler all day.
14.Don’tlet any one talk to you when firing.
15.Don’tallow water to remain on the floor about the boiler.
16.Don’tfail to blow off steam once or twice per day according as the water is more or less pure.
17.Don’tfail to close the blow-off cock, when blowing off, when the water in the boiler has sunk to one and a half inches.
18.Don’t fail, while cleaning the boiler, to examine and clean all cocks, valves and pipes and look to all joints and packings.
19.Don’tcommence cleaning the boiler until it has had time to cool.
20.Don’tforget daily to see that the safety-valve moves freely and is tight.
21.Don’tfail to clean the boiler inside frequently and carefully.
22.Don’tfail to notice that the steam gauge is in order.
23.Don’tfail to keep an eye out for leaks and have them repaired immediately, no matter how small.
24.Don’tfail to empty the boiler every week or two and re-fill it with fresh water.
25.Don’tlet any air into the furnace, except what goes through the grate-bars, or the smoke burners, so called, by which the air is highly heated.
26.Don’tincrease the load on the safety-valve beyond the pressure allowed by the inspector.
27.Don’tfail to open the doors of the furnace and start the pump when the pressure is increased beyond the amount allowed,but
28.Don’tfail to draw the fireswhen there is dangerfrom the water having fallen too low.
29.Don’tfail to check the fire—if too hot to draw, do it with fresh coal, damp ashes, clinkers or soil;and
30.Don’tfail to open the doors of the furnace and close the ash-pit doors at the time the fire is checked—and
31.Don’tdecrease the steam pressure by feeding in water or suddenly blowing off steam,and
32.Don’ttouch the safety-valve, even if it be opened or closed,and
33.Don’tchange the feed apparatus if it is working, or the throttle-valve be open; let them both remain as they are for a short time,and
34.Don’tfail to change them very cautiously and slowly when you close them, and
35.Don’tfail to be very cool and brave while resolute in observing these last seven “Don’ts.”
36.Don’tfail to keep yourself neat and tidy.
37.Don’tfail to be polite as well as neat and brave.
38.Don’tfail to keep the tubes clear and free from soot and ashes.
39.Don’tlet too many ashes gather in the ash-pit.
40.Don’tdisturb the fire when it is burning good nor stir it up too often.
41.Don’tbe afraid to get instruction from books and engineering papers.
42.Don’tfail to make an honest self-examination as to points upon which you may be ignorant, and really need to know in order to properly attend to your duties.
43.Don’tallow too much smoke to issue from the top of the chimney if the cause lies within your power to prevent it.
44.Don’tthink that after working at firing and its kindred duties for a year or two thatthe whole subjectof engineering has been learned.
45.Don’tforget that one of the best helps in getting forward is the possession of a vigorous and well-balanced mind and body—this covers temperance and kindred virtues and a willingness to acquire and impart knowledge.
46.Don’tforget to have your steam-gauge tested at least once in three months.
47.Don’tuse a wire or metallic rod as a handle to a swab in cleaning the glass tube of a water-gauge for the glass may suddenly fly to pieces when in use within a short time afterwards.
48.Don’tforget that steam pumps require as much attention as a steam engine.
49.Don’trun a steam pump piston, unless in an emergency, at a speed exceeding 80 to 100 feet per minute.
50.Don’tdo anything without a good reason for it about the engine or boiler, but when you are obliged to do anything, do it thoroughly and as quickly as possible.
51.Don’tforget to sprinkle a thin layer of coal on the grates before lighting the shavings and wood in the morning. This practice preserves the grate bars.
52.Don’ttake the cap off a bearing and remove the upper brass simply to see if things are working well; if there is any trouble it will soon give you notice, and, besides, you never can replace the brass in exactly its former position, so that you may find that the bearing will heat soon afterwards, owing to your own uncalled-for interference.
53.Don’tput sulphur on a hot bearing, unless you intend to ruin the brasses.
54.Don’tuse washed waste that has a harsh feel, as the chemicals used in cleansing it have not been thoroughly removed.
55.Don’t, in case of an extensive fire, involving the whole business, rush off without drawing the fires, and raising andpropping openthe safety valve of the boiler.
56.Don’tfail to preserve your health, for “a sound mind in a sound body” is beyond a money valuation.
57.Don’tfail to remember that engineers and firemen are in control of the great underlying force of modern civilization; hence, to do nothing to lower the dignity of the profession.
58.Don’tforget that in the care and management of the steam boiler the first thing required is an unceasing watchfulness—watch-care.
59.Don’tforget that an intemperate, reckless or indifferent man has no business in the place of trust of a steam boiler attendant.
60.Don’tallow even a day to pass without adding one or more facts to your knowledge of engineering in some of its branches.
In the examinations held by duly appointed officers to determine the fitness of candidates for receiving an engineer’s license the principal stress is laid upon the applicant’s knowledge of the parts and true proportions of the various designs of steam boilers, and his experience in managing them.
In fact, if there were no boilers there would be no examinations, as the laws are framed, certificates issued and steam boiler inspection companies formed to assure the public safety in life, limb and property, from the dangers arising from so-called mysterious boiler explosions.
Hence an almost undue proportion of engineers’ examinations are devoted to the steam boiler, its management and construction. But the subject is worthy of the best and most thoughtful attention. Every year adds to the number of steam boilers in use. With the expanding area and growth of population, the number of steam plants are multiplied and in turn each new steam boiler demands a careful attendant.
There is this difference between the boiler and the engine. When the latter is delivered from the shop and set up, it does its work with an almost unvarying uniformity, while the boiler is a constant care. It is admitted that the engine has reached a much greater state of perfection and does its duty with very much more reliability than the boiler.
Even when vigilant precautions are observed, from the moment a steam boiler is constructed until it is finally destroyed there are numerous insidious agents perpetually at work which tend to weaken it. There is nothing from which the iron can draw sustenance to replace its losses. The atmosphere without and the air within the boiler, the water as itenters through the feed-pipe and containing mineral and organic substances, steam into which the water is converted, the sediment which is precipitated by boiling the water, the fire and the sulphurous and other acids of the fuel, are all natural enemies of the iron; they sap its strength, not only while the boiler is at work and undergoing constant strain, but in the morning before fire is started, and at noon, night, Sundays, and other holidays it is preyed upon by these and other corroding agents.
These are the reasons which impress the true engineer with a constant solicitude regarding the daily and even momentary action of the steam generator.
The Steam Boiler in its simplest form was simply a closed vessel partly filled with water and which was heated by a fire box, but as steam plants are divided into two principal parts, the engine and the boiler, so the latter is divided again into the furnace and boiler, each of which is essential to the other. The furnace contains the fuel to be burnt, the boiler contains the water to be evaporated.
There must be a steam space to hold the steam when generated; heating surface to transmit the heat from the burning fuel to the water; a chimney or other apparatus to cause a draught to the furnace and to carry away the products of combustion; and various fittings for supplying the boiler with water, for carrying away the steam when formed to the engine in which it is used; for allowing steam to escape into the open air when it forms faster than it can be used; for ascertaining the quantity of water in the boiler, for ascertaining the pressure of the steam, etc., all of which, together with the engine and its appliances is calledA STEAM PLANT.
The forms in which steam generators are built are numerous, but may be divided into three classes, viz: stationary, locomotive and marine boilers, which terms designate the uses for which they are intended; in this work we have to deal mainly with the first-named, although a description with illustration is given of each type or form.
To illustrate the operations of a steam generator, we give the details of an appliance, which may be compared to the letter A of the alphabet, or the figure 1 of the numerals, so simple is it.
Fig. 11, is an elevation of boiler,fig. 12a vertical section through its axis, andfig. 13a horizontal section through the furnace bars.
Fig. 11.
Fig. 11.
Fig. 12.
Fig. 12.
The type of steam generator here exhibited is what is known as a vertical tubular boiler. The outside casing or shell is cylindrical in shape, and is composed of iron or steel plates riveted together. The top, which is likewise composed of the same plates is slightly dome-shaped, except at the center, which is away in order to receive the chimneya, which is round in shape and formed of thin wrought iron plates. The interior is shown in vertical section infig. 12. It consists of a furnace chamber,b, which contains the fire. The furnace is formed like the shell of the boiler of wrought iron or steel plates by flanging and riveting. The bottom is occupied by the grating, on which rests the incandescent fuel. The grating consists ofa number of cast-iron bars,d(fig. 12), and shown in plan infig. 13, placed so as to have interstices between them like the grate of an ordinary fireplace. The bottom of the furnace is firmly secured to the outside shell of the boiler in the manner shown infig. 12. The top covering platecc, is perforated with a number of circular holes of from one and a half to three inches diameter, according to the size of the boiler. Into each of these holes is fixed a vertical tube made of brass, wrought iron, or steel, shown atfff(fig. 12). These tubes pass through similar holes, at their top ends in the plateg, which latter is firmly riveted to the outside shell of the boiler. The tubes are also firmly attached to the two plates,cc,g. They serve to convey the flame, smoke, and hot air from the fire to the smoke box,h, and the chimney,a, and at the same time their sides provide ample heating surface to allow the heat contained in the products of combustion to escape into the water. The fresh fuel is thrown on the grating when required through the fire door, A (fig. 11). The ashes, cinders, etc., fall between the fire bars into the ash pit, B (fig. 12). The water is contained in the space between the shell of the boiler, the furnace chamber, and the tubes. It is kept at or about the level,ww(fig. 12), the space above this part being reserved for the steam as it rises. The heat, of course, escapes into the water, through the sides and top plate of the furnace, and through the sides of the tubes. The steam which, as it rises from the boiling water, ascends into the space aboveww, is thence led away by the steam pipe to the engine. Unless consumed quickly enough by the engine, the steam would accumulate too much within the boiler, and its pressure would rise to a dangerous point. To provide against this contingency the steam is enabled to escape when it rises above a certain pressure through the safety-valve, which is shown in sketch on the top of the boiler infig. 11. The details of the construction of safety-valves will be found fully described in another section of this work, which is devoted exclusively to the consideration of boiler fittings. In the same chapters will be found full descriptions of the various fittings and accessories of boilers, such as the water and pressure gauges, the apparatus for feeding the boiler with water, for producing the requisite draught of air to maintain the combustion, and also the particulars of the construction of the boilers themselves and their furnaces.
Fig. 13.
Fig. 13.
After the first crude forms, such as that used in connection with the Baranca and Newcomen engine, and numerous others, the steam boiler which came into very general use wasthe plain cylinder boiler. An illustration is given of this in figures14and15.
It consists of a cylinder A, formed of iron plate with hemispherical ends B. B. set horizontally in brick work C. The lower part of this cylinder contains the water, the upper part the steam. The furnace D is outside the cylinder, being beneath one end; it consists simply of grate barse eset in the brick work at a convenient distance below the bottom of the boiler.
Fig. 14.
Fig. 14.
Fig. 15.
Fig. 15.
The sides and front of the furnace are walls of brick work, which, being continued upwards support the end of the cylinder. The fuel is thrown on the bars through the door which is set in the front brick work. The air enters between the grate bars from below. The portion below the bars is called the ash pit. The flame and hot gases, when formed, first strike on the bottom of the boiler, and are then carried forward by the draft, to the so-called bridge wallo, which is a projecting piece of brick work which contracts the area of the fluenand forces allthe products of combustion to keep close to the bottom of the boiler.
Thence the gases pass along the fluen, and return part one side of the cylinder in the fluem(fig. 15) and back again by the other side fluemto the far end of the boiler, whence they escape up the chimney. This latter is provided with a door or damperp, which can be closed or opened at will, so as to regulate the draught.
This boiler has been in use for nearly one hundred years, but has two great defects. The first is that the area of heating surface, that is the parts of the boiler in contact with the flames, is too small in proportion to the bulk of the boiler; the second is, that if the water contains solid matter in solution, as nearly all the water does to a greater or less extent, this matter becomes deposited on the bottom of the boiler just where the greatest evaporation takes place. The deposit, being a non-conductor, prevents the heat of the fuel from reaching the water in sufficient quantities, thus rendering the heating surface inefficient; and further, by preventing the heat from escaping to the water, it causes the plates to become unduly heated, and quickly burnt out.
There is another defect belonging to this system of boiler to which many engineers attach great importance, viz.: that the temperature in each of the three fluesn,m,m´is very different, and consequently that the metal of which the shell of the boiler is composed expands very unequally in each of the flues, and cracks are very likely to take place when the effects of the changes of temperature are most felt. It will be noted that the flames and gases in this earliest type of steam boiler make three turns before reaching the chimney, and as these boilers were made frequently as much as 40 feet long it gave the extreme length of 120 feet to the heat products.
The Cornish Boileris the next form in time and excellence. This is illustrated in figures16and17.
It consists also of a cylindrical shellA, with flat ends as exhibited in cuts. The furnace, however, instead of being situated underneath the front end of the shell, is enclosed in it in a second cylinderB, having usually a diameter a little greater than half that of the boiler shell. The arrangement of the grate and bridge is evident from the diagram. After passing the bridge wall the heat products travel along through the internal cylinderB, till they reach the back end of the boiler; then return to the front again, by the two side fluesm,m´, and thence back again to the chimney by the bottom of fluen.
In this form of boiler the heating surface exceeds that of the last described by an amount equal to the area of the internal flues, while the internal capacity is diminished by its cubic contents; hence for boilers of equal external dimensions, the ratio of heating surface to mass of water to be heated, is greatly increased. Boilers of this sort can, however, never be made of as small diameters as the plain cylindrical sort, on account of the necessity of finding room inside, below the water level, for the furnace and flue.
Fig. 16.
Fig. 16.
Fig. 17.
Fig. 17.
The disadvantage, too, of the deposits mentioned in the plain cylinder is, to a great extent got over in the Cornish boiler, for thebottom, where the deposit chiefly takes place, is the coolest instead of being the hottest part of the heating surface.
But the disadvantage of unequal expansion also exists in this type of boiler, as the internal flue in the Cornish system is the hottest portion of the boiler, and consequently undergoes a greater lengthways expansion than the flues. The result is to bulge out the ends, and when the boiler is out of use, the flue returns to its regular size, and thus has a tendency to work loose from the ends to which it is riveted and if the ends are too rigid to move, a very serious strain comes on the points of the flue.
Even while in use the flue of a Cornish boiler is liable to undergo great changes in temperature, according to the state of the fire; when this latter is very low, or when fresh fuel has been thrown on, the temperature is a minimum and reaches a maximum again when the fresh fuel commences to burn fiercely. This constant expansion and contraction is found in practice to also so weaken the tube that it frequently collapses or is pressed together, resulting in great disaster.
This led to the production and adoption of the—
Lancashire Boiler, contrived to remedy this inconvenience and also to attain a more perfect combustion, the arrangement of the furnaces of which is shown infig. 19 and fig. 20.
It will be observed that there are two internal furnaces instead of one, as in the Cornish type. These furnaces are sometimes each continued as a separate flue to the other end of the boiler as shown in the cuts; but as a rule they emerge into one internal flue. They are supposed to be fired alternately, and the smoke and unburned gases issuing from the fresh fuel are ignited in the flue by the hot air proceeding from the other furnace, the fuel in which is in a state of incandescence. Thus all violent changes in the temperature are avoided, and the waste of fuel due to unburned gases is avoided, if the firing is properly conducted.