CHAPTER II
THE STEAM GENERATOR
The most widely known and utilized source of power is the steam engine.
Before its discovery wind and water were the only available means, except the muscular power of man, horses and other animals, which was used with the crudest sort of contrivances.
In primitive days men did not value their time, so they laboriously performed the work which machinery now does for us.
The steam engine, like everything else which man has devised, was a growth, and, singular as it may seem, the boiler, that vital part of the organism, was, really, the last to receive due consideration and improvement.
As the boiler is depended upon to produce the steam pressure, and since the pressure depends on the rapid and economical evaporation of water, the importance of the subject will be understood in treating of the steam engine.
Water as an Absorbent of Heat.--Water has the capacity to absorb a greater amount of heat than any other substance. A pewter pot, which melts at 500 degrees, will resist 2000 degrees of heat if it is filled with water, since the latter absorbs the heat so rapidly that the temperature of the metal is kept near the boiling point of water, which is 212 degrees.
Notwithstanding the great heat-absorbing qualities of water, a large portion of the heat of the fuel pa.s.ses through the flues and escapes from the stack. This fact has caused inventors to devise various forms of boilers, the object being to present as large an area of water as possible to the heat of the burning fuel. How that was accomplished we shall try to make plain.
Cla.s.sification of Boilers.--Numerous types of boilers have been devised, the object being, in all cases to evaporate the largest amount of water with the minimum quant.i.ty of fuel. All boilers may be put under two general heads, namely, those which contain a large quant.i.ty of water, and those which are intended to carry only a small charge.
In the first division the boilers are designed to carry a comparatively small pressure, and in the latter high pressures are available.
Mode of Applying Heat.--The most important thing to fully understand is the manner in which heat is applied to the boiler, and the different types which have been adapted to meet this requirement.
The Cylindrical Boiler.--The most primitive type of boiler is a plain cylindrical sh.e.l.l A, shown in Fig. 3, in which the furnace B is placed below, so that the surface of the water in contact with the fire area is exceedingly limited.
[Ill.u.s.tration: _Fig. 3. Primitive Boiler._]
In such a type of boiler it would be impossible for water to extract more than quarter the heat of the fuel. Usually it was much less. The next step was to make what is called a return tubular type in which the heat of the burning gases is conveyed to the rear end of the boiler, and then returned to the front end through tubes.
Fig. 4 shows this construction. The head of the sh.e.l.l holds the ends of a plurality of tubes, and the products of combustion pa.s.s through the conduit, below the boiler to the rear end, and are conducted upwardly to the tubes. As all the tubes are surrounded by water, it will absorb a large amount of the heat as the gases move through, and before pa.s.sing out of the stack.
[Ill.u.s.tration: _Fig. 4. Return Tubular Boiler._]
[Ill.u.s.tration: _Fig. 5. Cornish, or Scotch Boiler._]
The Cornish Boiler.--One of the most important inventions in the generation of steam was the Cornish boiler, which for many years was the recognized type for marine purposes. It had the advantage that a large amount of water could be carried and be subjected to heat at all times.
Aside from that it sought to avoid the great loss due to radiation.
It will be seen from an examination of Fig. 5 that the sh.e.l.l is made very large, and its length does not exceed its diametrical measurement.
Two, and sometimes three, fire tubes are placed within the sh.e.l.l, these tubes being secured to the heads. Surrounding these fire tubes, are numerous small tubes, through which the products of combustion pa.s.s after leaving the rear ends of the fire tubes.
In these boilers the tubes are the combustion chambers, and are provided with a grating for receiving the coal, and the rear ends of the tubes are provided with bridge walls, to arrest, in a measure, the free exit of the heated gases.
These boilers would be very efficient, if they could be made of sufficient length to permit the water to absorb the heat of the fuel, but it will be seen that it would be difficult to make them of very great length. If made too small diametrically the diameter of the fire boxes would be reduced to such an extent that there would not be sufficient grate surface.
It is obvious, however, that this form of boiler adds greatly to the area of the water surface contact, and in that particular is a great improvement.
[Ill.u.s.tration: _Fig. 6. Water Tube Boiler: End View._]
The Water Tube Boiler.--In the early days of the development of boilers, the universal practice was to have the products of combustion pa.s.s through the flues or the tubes. But quick generation of steam, and high pressures, necessitated a new type. This was accomplished by connecting an upper, or steam drum, with a lower, or water drum, by a plurality of small tubes, and causing the burning fuel to surround these tubes, so that the water, in pa.s.sing upwardly, would thus be subjected to the action of the fuel.
This form of boiler had two distinct advantages. First, an immense surface of water could be provided for; and, second, the water and steam drums could be made very small, diametrically, and thus permit of very high pressures.
In Fig. 6, which is designed to show a well known type of this structure, A A, represent the water drums and B, the steam drum. The water drums are separated from each other, so as to provide for the grate bars C, and each water drum is connected with the steam drum by a plurality of tubes D.
It will thus be seen that a fire box, or combustion chamber, is formed between the two sets of tubes D, and to retain the heat, or confine it as closely as possible to the tubes, a jacket E is placed around the entire structure.
The ends of the water and steam drums are connected by means of tubes F, shown in side view, Fig. 7, for the return or downward flow of the water. The diagrams are made as simple as possible, to show the princ.i.p.al features only. The structure ill.u.s.trated has been modified in many ways, princ.i.p.ally in simplifying the construction, and in providing means whereby the products of combustion may be brought into more intimate contact with the water during its pa.s.sage through the structure.
[Ill.u.s.tration: _Fig. 7. Water Tube Boiler: Side View._]
As heretofore stated, this type of boiler is designed to carry only a small quant.i.ty of water, so that it is necessary to have practically a constant inflow of feed water, and to economize in this respect the exhaust of the steam engine is used to initially heat up the water, and thus, in a measure, start the water well on its way to the evaporation point before it reaches the boiler.
Various Boiler Types.--The different uses have brought forth many kinds of boilers, in order to adapt them for some particular need. It would be needless to ill.u.s.trate them, but to show the diversity of structures, we may refer to some of them by their characteristics.
Compound Steam-Boiler.--This is a battery of boilers having their steam and water s.p.a.ces connected, and acting together to supply steam to a heating apparatus or a steam engine. These are also made by combining two or more boilers and using them as a feed water heater or a superheater, for facilitating the production of steam, or to be used for superheating steam.
The terms _feed water heater and super heater_ are explained in chapter III.
Locomotive Steam-Boiler.--This is a tubular boiler which has a contained furnace and ash pit, and in which the gases of combustion pa.s.s from the furnace directly into the horizontal interior tubes, and after pa.s.sing through the tubes are conveyed directly into the smoke box at the opposite ends of the tubes. The name is derived from the use of such boilers on locomotive engines, but it is typical in its application to all boilers having the construction described, and used for generating steam.
Vertical Steam-Boiler.--This is a form of construction in which the sh.e.l.l, or both the sh.e.l.l and the tubes, are vertical, and the tubes themselves may be used to convey the products of combustion, or serve as the means for conveying water through them, as in the well known water tube type.
This form of boiler is frequently used to good advantage where it is desired to utilize ground s.p.a.ce, and where there is sufficient head room. Properly constructed, it is economical as a steam generator.
From the foregoing it will be seen that the structural features of all boilers are so arranged as to provide for the exposure of the largest possible area of water to a heated surface so that the greatest amount of heat from the fuel may be absorbed.
CHAPTER III
STEAM ENGINES
The first steam engine was an exceedingly simple affair. It had neither eccentric, cylinder, crank, nor valves, and it did not depend upon the pressure of the steam acting against a piston to drive it back and forth, because it had no piston.
It is one of the remarkable things in the history and development of mechanism, that in this day of perfected steam engines, the inventors of our time should go back and utilize the principles employed in the first recorded steam engine, namely, the turbine. Instead of pressure exerting a force against a piston, as in the reciprocating engine, the steam acted by impacting against a moving surface, and by obtaining more or less reaction from air-resistance against a freely discharging steam jet or jets.
The original engine, so far as we have any knowledge, had but one moving part, namely, a vertical tubular stem, to which was attached a cross or a horizontal tube.
The Original Engine.--Figure 8 is a side view of the original engine.
The vertical stem A is pivoted to a frame B, and has a bore C which leads up to a cross tube D. The ends of the tube D are bent in opposite directions, as shown in the horizontal section, Fig. 9.
[Ill.u.s.tration: _Fig. 8. The Original Engine._]