How it Works Part 1

How it Works.

by Archibald Williams.

PREFACE.

How does it work? This question has been put to me so often by persons young and old that I have at last decided to answer it in such a manner that a much larger public than that with which I have personal acquaintance may be able to satisfy themselves as to the principles underlying many of the mechanisms met with in everyday life.

In order to include steam, electricity, optics, hydraulics, thermics, light, and a variety of detached mechanisms which cannot be cla.s.sified under any one of these heads, within the compa.s.s of about 450 pages, I have to be content with a comparatively brief treatment of each subject.

This brevity has in turn compelled me to deal with principles rather than with detailed descriptions of individual devices--though in several cases recognized types are examined. The reader will look in vain for accounts of the Yerkes telescope, of the latest thing in motor cars, and of the largest locomotive. But he will be put in the way of understanding the essential nature of _all_ telescopes, motors, and steam-engines so far as they are at present developed, which I think may be of greater ultimate profit to the uninitiated.

While careful to avoid puzzling the reader by the use of mysterious phraseology I consider that the parts of a machine should be given their technical names wherever possible. To prevent misconception, many of the diagrams accompanying the letterpress have words as well as letters written on them. This course also obviates the wearisome reference from text to diagram necessitated by the use of solitary letters or figures.

I may add, with regard to the diagrams of this book, that they are purposely somewhat unconventional, not being drawn to scale nor conforming to the canons of professional draughtsmanship. Where advisable, a part of a machine has been exaggerated to show its details.

As a rule solid black has been preferred to fine shading in sectional drawings, and all unnecessary lines are omitted. I would here acknowledge my indebtedness to my draughtsman, Mr. Frank Hodgson, for his care and industry in preparing the two hundred or more diagrams for which he was responsible.

Four organs of the body--the eye, the ear, the larynx, and the heart--are noticed in appropriate places. The eye is compared with the camera, the larynx with a reed pipe, the heart with a pump, while the ear fitly opens the chapter on acoustics. The reader who is unacquainted with physiology will thus be enabled to appreciate the better these marvellous devices, far more marvellous, by reason of their absolutely automatic action, than any creation of human hands.

A.W.

UPLANDS, STOKE POGES, BUCKS.

HOW IT WORKS.

Chapter I.

THE STEAM-ENGINE.

What is steam?--The mechanical energy of steam--The boiler--The circulation of water in a boiler--The enclosed furnace--The mult.i.tubular boiler--Fire-tube boilers--Other types of boilers--Aids to combustion--Boiler fittings--The safety-valve--The water-gauge--The steam-gauge--The water supply to a boiler.

WHAT IS STEAM?

If ice be heated above 32 Fahrenheit, its molecules lose their cohesion, and move freely round one another--the ice is turned into water. Heat water above 212 Fahrenheit, and the molecules exhibit a violent mutual repulsion, and, like dormant bees revived by spring sunshine, separate and dart to and fro. If confined in an air-tight vessel, the molecules have their flights curtailed, and beat more and more violently against their prison walls, so that every square inch of the vessel is subjected to a rising pressure. We may compare the action of the steam molecules to that of bullets fired from a machine-gun at a plate mounted on a spring. The faster the bullets came, the greater would be the continuous compression of the spring.

THE MECHANICAL ENERGY OF STEAM.

If steam is let into one end of a cylinder behind an air-tight but freely-moving piston, it will bombard the walls of the cylinder and the piston; and if the united push of the molecules on the one side of the latter is greater than the resistance on the other side opposing its motion, the piston must move. Having thus partly got their liberty, the molecules become less active, and do not rush about so vigorously. The pressure on the piston decreases as it moves. But if the piston were driven back to its original position against the force of the steam, the molecular activity--that is, pressure--would be restored. We are here a.s.suming that no heat has pa.s.sed through the cylinder or piston and been radiated into the air; for any loss of heat means loss of energy, since heat _is_ energy.

THE BOILER.

The combustion of fuel in a furnace causes the walls of the furnace to become _hot_, which means that the molecules of the substance forming the walls are thrown into violent agitation. If the walls are what are called "good conductors" of heat, they will transmit the agitation through them to any surrounding substance. In the case of the ordinary house stove this is the air, which itself is agitated, or grows warm. A steam-boiler has the furnace walls surrounded by water, and its function is to transmit molecular movement (heat, or energy) through the furnace plates to the water until the point is reached when steam generates. At atmospheric pressure--that is, if not confined in any way--steam would fill 1,610 times the s.p.a.ce which its molecules occupied in their watery formation. If we seal up the boiler so that no escape is possible for the steam molecules, their motion becomes more and more rapid, and _pressure_ is developed by their beating on the walls of the boiler.

There is theoretically no limit to which the pressure may be raised, provided that sufficient fuel-combustion energy is transmitted to the vaporizing water.

To raise steam in large quant.i.ties we must employ a fuel which develops great heat in proportion to its weight, is readily procured, and cheap.

Coal fulfils all these conditions. Of the 800 million tons mined annually throughout the world, 400 million tons are burnt in the furnaces of steam-boilers.

A good boiler must be--(1) Strong enough to withstand much higher pressures than that at which it is worked; (2) so designed as to burn its fuel to the greatest advantage.

Even in the best-designed boilers a large part of the combustion heat pa.s.ses through the chimney, while a further proportion is radiated from the boiler. Professor John Perry[1] considers that this waste amounts, under the best conditions at present obtainable, to eleven-twelfths of the whole. We have to burn a shillingsworth of coal to capture the energy stored in a pennyworth. Yet the steam-engine of to-day is three or four times as efficient as the engine of fifty years ago. This is due to radical improvements in the design of boilers and of the machinery which converts the heat energy of steam into mechanical motion.

CIRCULATION OF WATER IN A BOILER.

If you place a pot filled with water on an open fire, and watch it when it boils, you will notice that the water heaves up at the sides and plunges down at the centre. This is due to the water being heated most at the sides, and therefore being lightest there. The rising steam-bubbles also carry it up. On reaching the surface, the bubbles burst, the steam escapes, and the water loses some of its heat, and rushes down again to take the place of steam-laden water rising.

[Ill.u.s.tration: FIG. 1.]

[Ill.u.s.tration: FIG. 2.]

If the fire is very fierce, steam-bubbles may rise from all points at the bottom, and impede downward currents (Fig. 1). The pot then "boils over."

Fig. 2 shows a method of preventing this trouble. We lower into our pot a vessel of somewhat smaller diameter, with a hole in the bottom, arranged in such a manner as to leave a s.p.a.ce between it and the pot all round. The upward currents are then separated entirely from the downward, and the fire can be forced to a very much greater extent than before without the water boiling over. This very simple arrangement is the basis of many devices for producing free circulation of the water in steam-boilers.

We can easily follow out the process of development. In Fig. 3 we see a simple U-tube depending from a vessel of water. Heat is applied to the left leg, and a steady circulation at once commences. In order to increase the heating surface we can extend the heated leg into a long incline (Fig. 4), beneath which three lamps instead of only one are placed. The direction of the circulation is the same, but its rate is increased.

[Ill.u.s.tration: FIG. 3.]

A further improvement results from increasing the number of tubes (Fig.

5), keeping them all on the slant, so that the heated water and steam may rise freely.

THE ENCLOSED FURNACE.

[Ill.u.s.tration: FIG. 4.]

[Ill.u.s.tration: FIG. 5.]

Still, a lot of the heat gets away. In a steam-boiler the burning fuel is enclosed either by fire-brick or a "water-jacket," forming part of the boiler. A water-jacket signifies a double coating of metal plates with a s.p.a.ce between, which is filled with water (see Fig. 6). The fire is now enclosed much as it is in a kitchen range. But our boiler must not be so wasteful of the heat as is that useful household fixture. On their way to the funnel the flames and hot gases should act on a very large metal or other surface in contact with the water of the boiler, in order to give up a due proportion of their heat.

[Ill.u.s.tration: FIG. 6.--Diagrammatic sketch of a locomotive type of boiler. Water indicated by dotted lines. The arrows show the direction taken by the air and hot gases from the air-door to the funnel.]

THE MULt.i.tUBULAR BOILER.

[Ill.u.s.tration: FIG. 7.--The Babc.o.c.k and Wilc.o.x water-tube boiler. One side of the brick seating has been removed to show the arrangement of the water-tubes and furnace.]

To save room, boilers which have to make steam very quickly and at high pressures are largely composed of pipes. Such boilers we call mult.i.tubular. They are of two kinds--(1) _Water_-tube boilers; in which the water circulates through tubes exposed to the furnace heat. The Babc.o.c.k and Wilc.o.x boiler (Fig. 7) is typical of this variety. (2) _Fire_-tube boilers; in which the hot gases pa.s.s through tubes surrounded by water. The ordinary locomotive boiler (Fig. 6) ill.u.s.trates this form.

The Babc.o.c.k and Wilc.o.x boiler is widely used in mines, power stations, and, in a modified form, on shipboard. It consists of two main parts--(1) A drum, H, in the upper part of which the steam collects; (2) a group of pipes arranged on the principle ill.u.s.trated by Fig. 5. The boiler is seated on a rectangular frame of fire-bricks. At one end is the furnace door; at the other the exit to the chimney. From the furnace F the flames and hot gases rise round the upper end of the sloping tubes TT into the s.p.a.ce A, where they play upon the under surface of H before plunging downward again among the tubes into the s.p.a.ce B. Here the temperature is lower. The arrows indicate further journeys upwards into the s.p.a.ce C on the right of a fire-brick division, and past the down tubes SS into D, whence the hot gases find an escape into the chimney through the opening E. It will be noticed that the greatest heat is brought to bear on TT near their junction with UU, the "uptake" tubes; and that every succeeding pa.s.sage of the pipes brings the gradually cooling gases nearer to the "downtake" tubes SS.

The pipes TT are easily brushed and sc.r.a.ped after the removal of plugs from the "headers" into which the tube ends are expanded.

Other well-known water-tube boilers are the Yarrow, Belleville, Stirling, and Thorneycroft, all used for driving marine engines.

FIRE-TUBE BOILERS.