Steam boiler

“ A steam boiler or other vessel under severe internal strains had best be spherical if equality to resistance is particularly desired. Usually a cylindrical shape is much more convenient, and no other is given to simple steam boilers or to the tubes of water-tube or fire-tube boilers. Tubes comparatively narrow, are readily manufactured without seam, so that they may be quite safe though thin; large boilers of plates riveted together, must be built of thick metal. ”

“ In order to satisfy the requirements which have newly arisen in connection with navigation, locomotion, small motors and apparatus which need for their working an intermittent supply of steam, it became necessary to modify the construction of steam boilers, to augment their heating surface, to diminish their residue of water, and to gradually construct so-called inexplosible apparatus, of which the Belleville boiler forms one of the most characteristic prototypes. ”

“ You will find further on that all check valves are arranged to prevent any flowing out from the boiler, but nothing to prevent water flowing in. Such an occurrence will do no harm but the knowing how it was done may prevent your giving yourself away. A good authority on steam boilers, says: "All explosions come either from poor material, poor workmanship, too high pressure, or a too low gauge of water." Now to protect yourself from the first two causes, buy your engine from some factory having a reputation for doing good work and for using good material. ”

“ No well-informed proprietor suffers his boiler to be without one; but it is not a cure for carelessness. It is only a window for the vigilant eye to look through, not the eye itself. Steam-boilers will have to be constructed so that when the subsidence of the water fails to check itself by enlarging the supply, it shall, before the point of danger is reached, infallibly check the combustion, let off the steam, and blow a whistle or ring a bell, which the proprietor may, if he pleases, regard as the official death-knell of the careless engineer. ”

“ The demand for light weight and economy of space following the beginning of steam navigation and the invention of the locomotive required boilers designed and constructed to withstand heavier pressures and forced the adoption of the cylindrical form of boiler. There are in use to-day many examples of every step in the development of steam boilers from the first plain cylindrical boiler to the most modern type of multi-tubular locomotive boiler, which stands as the highest type of fire-tube boiler construction. ”

“ The selection of steam boilers is a matter to which the most careful thought and attention may be well given. Within the last twenty years, radical changes have taken place in the methods and appliances for the generation and distribution of power. These changes have been made largely in the prime movers, both as to type and size, and are best illustrated by the changes in central station power-plant practice. It is hardly within the scope of this work to treat of power-plant design and the discussion will be limited to a consideration of the boiler end of the power plant. ”

“ The bearing that the type of boiler has on the efficiency to be expected can only be realized from a study of the foregoing chapters.
Capacity—Capacity, as already defined, is the ability of a definite amount of boiler-heating surface to generate steam. Boilers are ordinarily purchased under a manufacturer’s specification, which rates a boiler at a nominal rated horse power, usually based on 10 square feet of heating surface per horse power. Such a builders’ rating is absolutely arbitrary and implies nothing as to the limiting amount of water that this amount of heating surface will evaporate. ”

“ Then, again, a boiler that might furnish a sufficient quantity of steam to work a certain type of engine up to 40 horse power without forcing the fire might, with another style of engine, in order to generate the same power and perform the same duty, require to be forced beyond the limits of safety or economy. Therefore, considering the varying conditions under which all steam boilers are placed, there is no such a thing as any reliable standard rule for calculating the horse power of boilers, but only an approximate one at the best. ”

“ In line with the development of the steam engine must be considered the efforts to economize fuel. These may be divided into the following classes: Increased steam generating surface in boiler construction; surface condensers for exhaust steam; devices for promoting the combustion of fuel and burning the smoke, and feed water heaters. Even before the Nineteenth Century Smeaton devised the cylindrical boiler traversed by a flue, but the multitubular steam boiler of to-day represents a very important Nineteenth Century adjunct to the steam engine. ”

“ The kinetic heat-energy developed in the furnace of the steam-boiler is partly transmitted through the metallic walls which inclose the steam and water within the boiler, there to evaporate water, and to assume that form of energy which exists in steam confined under pressure, and is partly carried away into the atmosphere in the discharged gaseous products of combustion, serving, however, a useful purpose, en route, by producing the draught needed to keep up combustion. ”

“ Having thus briefly looked at the advantages of circulation of water in steam boilers, let us see what are the best means of securing it under the most efficient conditions We have seen in our kettle that one essential point was that the currents should be kept from interfering with each other. If we could look into an ordinary return tubular boiler when steaming, we should see a curious commotion of currents rushing hither and thither, and shifting continually as one or the other contending force gained a momentary mastery. ”

“ Babcock’s lecture, is so necessary in the requirements of the perfect steam boiler. In boilers using water-leg construction, there is danger of defective circulation, leaks are common, and unsuspected corrosion may be going on in portions of the boiler that cannot be inspected. Stresses due to unequal expansion of the metal cannot be well avoided but they may be minimized by maintaining at the same temperature all pressure parts of the boiler. ”

“ 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. ”

“ They have been mentioned several times incidentally in the course of these sections, but, [135] with the exception of the small tin boiler for the oscillating engine first described, particulars of their construction have been omitted. It is not an easy task making a steam boiler, and in most cases it will be found cheapest in the end to purchase ready made.
The materials most generally used are brass and copper; sometimes iron, or what amounts to the same thing, tin-plate, is employed. Brass or copper, from the ease with which they can be manipulated, are the best for a beginner to work on. ”

“ Steam pressure may be guarded against by a safety valve; but as this may become set fast in a little time, it would be far better to avoid all chance of steam accumulation by such an arrangement as that shown in Fig. 55, where the circulating boiler is placed within an open-topped boiler behind the kitchen fire, and only receives its heat through the hot water surrounding it, and therefore cannot itself become sufficiently hot to generate steam. ”

“ The danger to the public arising from their use requires that the care and management of high-pressure steam boilers shall be in hands of careful, experienced and naturally ingenious men, hence it is on the affairs of the Boiler Room that the first tests are made, as to the worthiness of an aspirant for an engineer’s license, hence, too, the success of many firemen in obtaining the preference over engine-builders or school graduates, in the line of promotion as steam engineers. ”

“ When the steam is shut up very tightly, as it is in the boiler of a steam-engine, it has very great power, and the more steam there is thus shut up the greater is the power. Men are sometimes careless about this, and get so much steam made in the boiler that it bursts. This is just as the roasted chestnut is burst by the steam and heated air that are in it. The boiler bears the pressure of the steam as long as it can. ”

“ 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. ”

“ Steam is a good servant, but a terrible master. It must be kept under strict control. However strong a boiler may be, it will burst if the steam pressure in it be raised to a certain point; and some device must therefore be fitted on it which will give the steam free egress before that point is reached. A device of this kind is called a safety-valve. It usually blows off at less than half the greatest pressure that the boiler has been proved by experiment to be capable of withstanding. ”

“ 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. ”

“ The heating-surface is sometimes made of so great an area that the temperature of the escaping gases is too low to give good chimney-draught, and a “mechanical draught” is resorted to, revolving “fan-blowers” being ordinarily used for its production. It is most economical to adopt this method. The steam-boiler is generally constructed of iron—sometimes, but rarely, of cast-iron, although “steel,” where not hard enough to harden or temper, is better in consequence of its greater strength and homogeneousness of structure, and its better conductivity. ”

“ It is advisable, therefore, to give a sufficient calorimeter in all low pressure boilers, and a sufficient height of chimney to enable the chimney to operate without a steam jet; but it is useful to know that a steam jet is a resource in the case of a defective boiler, or where the boiler has to be urged beyond its power. ”

“ The boiler is carelessly left to get nearly empty, and the fire therefore makes it very hot. Then, when more water is let into it, a great deal of steam is made all at once. This exerts its expansive force with such violence that the boiler gives way. You can understand how this is if you see a little water dropped upon red-hot iron. A great cloud of steam arises, spreading itself in the air, and you can see that if this were pent up it would make a strong pressure in trying to get free. ”

“ In locomotive steam-engine boilers, the principal object is to generate steam with the greatest rapidity; hence the boiler consists of two parts—viz., a square box containing the fire, and around which a thin stratum of water circulates, whilst the draught for the fire rushes through a number of copper tubes placed in the second or cylindrical part of the boiler. By the use of these tubes an immense surface of water is exposed to the action of the fire, and the steam is not only generated with amazing rapidity, but is also maintained at a very high pressure. ”

“ Tubes are always “lap-welded” in the process of rolling them. Small tubes were first used in the United States, about 1785. In portable, locomotive, and marine steam-boilers, the fire must be built within the boiler itself, instead of (as in the above described stationary boilers) in a furnace of brickwork exterior to the boiler. The flame and gases from the furnace or fire-box in these kinds of boiler are never led through brick passages en route to the chimney, as often in the preceding case, but are invariably conducted through flues or tubes, or both, to the smoke-stack. ”

“ So long as factories, public and private buildings are erected, so long will warming and ventilation keep progress with steam engineering and remain a part of the general mechanical science required of the supervisory and practical engineer.
In what is called the system of open circulation, a supply main conveys the steam to the radiating surfaces, whence a return main conducts the condensed water either into an open tank for feeding the boiler, or into a drain to run to waste, the boiler being fed from some other source ”

“ The substantial advantages of the cylindrical boiler are: reliability, simplicity; it is well made and generally understood; it can use salty or dirty water, and it will stand hard usage without serious loss or injury. The disadvantages are: great weight; steam cannot be gotten up or taken off quickly; it does not readily adjust itself to change of output; and heavy forced draught cannot be used.
The advantages of the water-tube boiler are: lightness; adaptability to high pressure; rapidity of raising steam or taking it off ”