How It Works · Part 12

The most useful form of household filter is one which can be attached to a tap connected with the main. Such a filter is usually made of porcelain or biscuit china. The Berkefeld filter has an outer case of iron, and an interior hollow "candle" of porcelain from which a tube passes through the lid of the filter to a storage tank for the filtered water. The water from the main enters the outer case, and percolates through the porcelain walls to the internal cavity and thence flows away through the delivery pipe.

Whatever be the type of filter used it must be cleansed at proper intervals. A foul filter is very dangerous to those who drink the water from it. It has been proved by tests that, so far from purifying the water, an inefficient and contaminated filter passes out water much more highly charged with microbes than it was before it entered. We must not therefore think that, because water has been filtered, it is necessarily safe. The reverse is only too often the case.

GAS TRAPS.

Dangerous microbes can be breathed as well as drunk into the human system. Every communication between house and drains should be most carefully "trapped." The principle of a gas trap between, say, a kitchen sink and the drain to carry off the water is given in Fig. 186. Enough water always remains in the bend to rise above the level of the elbow, effectually keeping back any gas that there may be in the pipe beyond the bend.

WATER-ENGINES.

Before the invention of the steam-engine human industries were largely dependent on the motive power of the wind and running water. But when the infant nursed by Watt and Stephenson had grown into a giant, both of these natural agents were deposed from the important position they once held. Windmills in a state of decay crown many of our hilltops, and the water-wheel which formerly brought wealth to the miller now rots in its mountings at the end of the dam. Except for pumping and moving boats and ships, wind-power finds its occupation gone. It is too uncertain in quantity and quality to find a place in modern economics. Water-power, on the other hand, has received a fresh lease of life through the invention of machinery so scientifically designed as to use much more of the water's energy than was possible with the old-fashioned wheel.

The turbine, of which we have already spoken in our third chapter, is now the favourite hydraulic engine. Some water-turbines work on much the same principle as the Parsons steam-turbine; others resemble the De Laval. Among the latter the Pelton wheel takes the first place. By the courtesy of the manufacturers we are able to give some interesting details and illustrations of this device.

The wheel, which may be of any diameter from six inches to ten feet, has buckets set at regular intervals round the circumference, sticking outwards. Each bucket, as will be gathered from our illustration of an enormous 5,000 h.p. wheel (Fig. 187), is composed of two cups. A nozzle is so arranged as to direct water on the buckets just as they reach the lowest point of a revolution (see Fig. 188). The water strikes the bucket on the partition between the two cups, which turns it right and left round the inside of the cups. The change of direction transfers the energy of the water to the wheel.

The speed of the wheel may be automatically regulated by a deflecting nozzle (Fig. 189), which has a ball and socket joint to permit of its being raised or lowered by a centrifugal governor, thus throwing the stream on or off the buckets. The power of the wheel is consequently increased or diminished to meet the change of load, and a constant speed is maintained. When it is necessary to waste as little water as possible, a concentric tapered needle may be fitted inside the nozzle. When the nozzle is in its highest position the needle tip is withdrawn; as the nozzle sinks the needle protrudes, gradually decreasing the discharge area of the nozzle.

Pelton wheels are designed to run at all speeds and to use water of any pressure. At Manitou, Colorado, is an installation of three wheels operated by water which leaves the nozzle at the enormous pressure of 935 lbs. per square inch. It is interesting to note that jets of very high-pressure water offer astonishing resistance to any attempt to deflect their course. A three-inch jet of 500-lb. water cannot be cut through by a blow from a crowbar.

In order to get sufficient pressure for working hydraulic machinery in mines, factories, etc., water is often led for many miles in flumes, or artificial channels, along the sides of valleys from the source of supply to the point at which it is to be used. By the time that point is reached the difference between the gradients of the flume and of the valley bottom has produced a difference in height of some hundreds of feet.

The full-page illustration on p. 380 affords a striking testimony to the wonderful progress made in engineering practice during the last fifty years. The huge water-wheel which forms the bulk of the picture is that at Laxey, in the Isle of Man. It is 72-1/2 feet in diameter, and is supposed to develop 150 horse-power, which is transmitted several hundreds of feet by means of wooden rods supported at regular intervals. The power thus transmitted operates a system of pumps in a lead mine, raising 250 gallons of water per minute, to an elevation of 1,200 feet. The driving water is brought some distance to the wheel in an underground conduit, and is carried up the masonry tower by pressure, flowing over the top into the buckets on the circumference of the wheel.

The little cut in the upper corner represents a Pelton wheel drawn on the same scale, which, given an equal supply of water at the same pressure, would develop the same power as the Laxey monster. By the side of the giant the other appears a mere toy.

THE CREAM SEPARATOR.

In 1864 Denmark went to war with Germany, and emerged from the short struggle shorn of the provinces of Lauenburg, Holstein, and Schleswig. The loss of the two last, the fairest and most fertile districts of the kingdom, was indeed grievous. The Danish king now ruled only over a land consisting largely of moor, marsh, and dunes, apparently worthless for any purpose. But the Danes, with admirable courage, entered upon a second struggle, this time with nature. They made roads and railways, dug irrigation ditches, and planted forest trees; and so gradually turned large tracts of what had been useless country into valuable possessions. Agriculture being much depressed, owing to the low price of corn, they next gave their attention to the improvement of dairy farming. Labour-saving machinery of all kinds was introduced, none more important than the device for separating the fatty from the watery constituents of milk. It would not be too much to say that the separator is largely responsible for the present prosperity of Denmark.

How does it work? asks the reader. Centrifugal force[37] is the governing principle. To explain its application we append a sectional illustration (Fig. 191) of Messrs. Burmeister and Wain's hand-power separator, which may be taken as generally representative of this class of machines. Inside a circular casing is a cylindrical bowl, D, mounted on a shaft which can be revolved 5,000 times a minute by means of the cog-wheels and the screw thread chased on it near the bottom extremity. Milk flows from the reservoir R (supported on a stout arm) through tap A into a little distributer on the top of the separator, and from it drops into the central tube C of the bowl. Falling to the bottom, it is flung outwards by centrifugal force, finds an escape upwards through the holes a a, and climbs up the perforated grid e, the surface of which is a series of pyramidical excrescences, and finally reaches the inner surface of the drum proper. The velocity of rotation is so tremendous that the heavier portions of the milk--that is, the watery--crowd towards the point furthest from the centre, and keep the lighter fatty elements away from contact with the sides of the drum. In the diagram the water is represented by small circles, the cream by small crosses.

As more milk enters the drum it forces upwards what is already there. The cap of the drum has an inner jacket, F, which at the bottom all but touches the side of the drum. The distance between them is the merest slit; but the cream is deflected up outside F into space E, and escapes through a hole one-sixteenth of an inch in diameter perforating the plate G. The cream is flung into space K and trickles out of spout B, while the water flies into space H and trickles away through spout A.

THE "HYDRO.,"

used in laundries for wringing clothes by centrifugal force, has a solid outer casing and an inner perforated cylindrical cage, revolved at high speed by a vertical shaft. The wet clothes are placed in the cage, and the machine is started. The water escapes through the perforations and runs down the side of the casing to a drain. After a few minutes the clothes are dry enough for ironing. So great is the centrifugal force that they are consolidated against the sides of the cage, and care is needed in their removal.

[35] Inventor of the lathe slide-rest.

[36] Living germs; some varieties the cause of disease.

[37] That is, centre-fleeing force. Water dropped on a spinning top rushes towards the circumference and is shot off at right angles to a line drawn from the point of parting to the centre of the top.

Chapter XIX.

HEATING AND LIGHTING.

The hot-water supply--The tank system--The cylinder system--How a lamp works--Gas and gasworks--Automatic stoking--A gas governor--The gas meter--Incandescent gas lighting.

HOT-WATER SUPPLY.

A well-equipped house is nowadays expected to contain efficient apparatus for supplying plenty of hot water at all hours of the day. There is little romance about the kitchen boiler and the pipes which the plumber and his satellites have sometimes to inspect and put right, but the methods of securing a proper circulation of hot water through the house are sufficiently important and interesting to be noticed in these pages.

In houses of moderate size the kitchen range does the heating. The two systems of storing and distributing the heated water most commonly used are--(1) The tank system; (2) the cylinder system.

THE TANK SYSTEM

is shown diagrammatically in Fig. 192. The boiler is situated at the back of the range, and when a "damper" is drawn the fire and hot gases pass under it to a flue leading to the chimney. The almost boiling water rises to the top of the boiler and thence finds its way up the flow pipe into the hot-water tank A, displacing the somewhat colder water there, which descends through the return pipe to the bottom of the boiler.

Water is drawn off from the flow pipe. This pipe projects some distance through the bottom of A, so that the hottest portion of the contents may be drawn off first. A tank situated in the roof, and fed from the main by a ball-cock valve, communicates with A through the siphon pipe S. The bend in this pipe prevents the ascent of hot water, which cannot sink through water colder than itself. From the top of A an expansion pipe is led up and turned over the cold-water tank to discharge any steam which may be generated in the boiler.

A hot-water radiator for warming the house may be connected to the flow and return pipes as shown. Since it opens a "short circuit" for the circulation, the water in the tank above will not be so well heated while it is in action. If cocks are fitted to the radiator pipes, the amount of heat thus deflected can be governed.

A disadvantage of the tank system is that the tank, if placed high enough to supply all flows, is sometimes so far from the boiler that the water loses much of its heat in the course of circulation. Also, if for any reason the cold water fails, tank A may be entirely emptied, circulation cease, and the water in the boiler and pipes boil away rapidly.

THE CYLINDER SYSTEM

(Fig. 193) is open to neither of these objections. Instead of a rectangular tank up aloft, we now have a large copper cylinder situated in the kitchen near the range. The flow and return pipes are continuous, and the cold supply enters the bottom of the cylinder through a pipe with a siphon bend in it. As before, water is drawn off from the flow pipe, and a radiator may be put in the circuit. Since there is no draw-off point below the top of the cylinder, even if the cold supply fails the cylinder will remain full, and the failure will be discovered long before there is any danger of the water in it boiling away.

Boiler explosions are due to obstructions in the pipes. If the expansion pipe and the cold-water supply pipe freeze, there is danger of a slight accumulation of steam; and if one of the circulation pipes is also blocked, steam must generate until "something has to go,"[38] which is naturally the boiler. Assuming that the pipes are quite full to the points of obstruction, the fracture would result from the expansion of the water. Steam cannot generate unless there be a space above the water. But the expanding water has stored up the heat which would have raised steam, and the moment expansion begins after fracture this energy is suddenly let loose. Steam forms instantaneously, augmenting the effects of the explosion. From this it will be gathered that all pipes should be properly protected against frost; especially near the roof.

Another cause of disaster is the furring up of the pipes with the lime deposited by hard water when heated. When hard water is used, the pipes will sooner or later be blocked near the boiler; and as the deposit is too hard to be scraped away, periodical renewals are unavoidable.

HOW A LAMP WORKS.

From heating we turn to lighting, and first to the ordinary paraffin lamp. The two chief things to notice about this are the wick and the chimney. The wick, being made of closely-woven cotton, draws up the oil by what is known as capillary attraction. If you dip the ends of two glass tubes, one half an inch, the other one-eighth of an inch in diameter, into a vessel of water, you will notice that the water rises higher in the smaller tube. Or get two clean glass plates and lay them face to face, touching at one end, but kept slightly apart at the other by some small object. If they are partly submerged perpendicularly, the water will rise between the plates--furthest on the side at which the two plates touch, and less and less as the other edge is approached. The tendency of liquids to rise through porous bodies is a phenomenon for which we cannot account.

Mineral oil contains a large proportion of carbon and hydrogen; it is therefore termed hydro-carbon. When oil reaches the top of a lighted wick, the liquid is heated until it turns into gas. The carbon and hydrogen unite with the oxygen of the air. Some particles of the carbon apparently do not combine at once, and as they pass through the fiery zone of the flame are heated to such a temperature as to become highly luminous. It is to produce these light-rays that we use a lamp, and to burn our oil efficiently we must supply the flame with plenty of oxygen, with more than it could naturally obtain. So we surround it with a transparent chimney of special glass. The air inside the chimney is heated, and rises; fresh air rushes in at the bottom, and is also heated and replaced. As the air passes through, the flame seizes on the oxygen. If the wick is turned up until the flame becomes smoky and flares, the point has been passed at which the induced chimney draught can supply sufficient oxygen to combine with the carbon of the vapour, and the "free" carbon escapes as smoke.

The blower-plate used to draw up a fire (Fig. 194) performs exactly the same function as the lamp chimney, but on a larger scale. The plate prevents air passing straight up the chimney over the coals, and compels it to find a way through the fire itself to replace the heated air rising up the chimney.

GAS AND GASWORKS.

A lamp is an apparatus for converting hydro-carbon mineral oil into gas and burning it efficiently. The gas-jet burns gases produced by driving off hydro-carbon vapours from coal in apparatus specially designed for the purpose. Gas-making is now, in spite of the competition of electric lighting, so important an industry that we shall do well to glance at the processes which it includes. Coal gas may be produced on a very small scale as follows:--Fill a tin canister (the joints of which have been made by folding the metal, not by soldering) with coal, clap on the lid, and place it, lid downwards, in a bright fire, after punching a hole in the bottom. Vapour soon begins to issue from the hole. This is probably at first only steam, due to the coal being more or less damp. But if a lighted match be presently applied the vapour takes fire, showing that coal gas proper is coming off. The flame lasts for a long time. When it dies the canister may be removed and the contents examined. Most of the carbon remains in the form of coke. It is bulk for bulk much lighter than coal, for the hydrogen, oxygen, and other gases, and some of the carbon have been driven off by the heat. The coke itself burns if placed in a fire, but without any smoke, such as issues from coal.

Our home-made gas yields a smoky and unsatisfactory flame, owing to the presence of certain impurities--ammonia, tar, sulphuretted hydrogen, and carbon bisulphide. A gas factory must be equipped with means of getting rid of these objectionable constituents. Turning to Fig. 195, which displays very diagrammatically the main features of a gas plant, we observe at the extreme right the retorts, which correspond to our canister. These are usually long fire-brick tubes of D-section, the flat side at the bottom. Under each is a furnace, the flames of which play on the bottom, sides, and inner end of the retort. The outer end projecting beyond the brickwork seating has an iron air-tight door for filling the retort through, immediately behind which rises an iron exit pipe, A, for the gases. Tar, which vaporizes at high temperatures, but liquefies at ordinary atmospheric heat, must first be got rid of. This is effected by passing the gas through the hydraulic main, a tubular vessel half full of water running the whole length of the retorts. The end of pipe A dips below the surface of the water, which condenses most of the tar and steam. The partly-purified gas now passes through pipe B to the condensers, a series of inverted U-pipes standing on an iron chest with vertical cross divisions between the mouths of each U. These divisions dip into water, so that the gas has to pass up one leg of a U, down the other, up the first leg of the second pipe, and so on, till all traces of the tar and other liquid constituents have condensed on the inside of the pipe, from which they drop into the tank below.

The next stage is the passage of the scrubber, filled with coke over which water perpetually flows. The ammonia gas is here absorbed. There still remain the sulphuretted hydrogen and the carbon bisulphide, both of which are extremely offensive to the nostrils. Slaked lime, laid on trays in an air-tight compartment called the lime purifier, absorbs most of the sulphurous elements of these; and the coal gas is then fit for use. On leaving the purifiers it flows into the gasometer, or gasholder, the huge cake-like form of which is a very familiar object in the environs of towns. The gasometer is a cylindrical box with a domed top, but no bottom, built of riveted steel plates. It stands in a circular tank of water, so that it may rise and fall without any escape of gas. The levity of the gas, in conjunction with weights attached to the ends of chains working over pulleys on the framework surrounding the holder, suffices to raise the holder.

Some gasometers have an enormous capacity. The record is at present held by that built for the South Metropolitan Gas Co., London, by Messrs. Clayton & Son of Leeds. This monster (of which we append an illustration, Fig. 196) is 300 feet in diameter and 180 feet high. When fully extended it holds 12,158,600 cubic feet of gas. Owing to its immense size, it is built on the telescopic principle in six "lifts," of 30 feet deep each. The sides of each lift, or ring, except the topmost, have a section shaped somewhat like the letter N. Two of the members form a deep, narrow cup to hold water, in which the "dip" member of the ring above it rises and falls.

AUTOMATIC STOKING.

The labour of feeding the retorts with coal and removing the coke is exceedingly severe. In the illustration on p. 400 (made from a very fine photograph taken by Mr. F. Marsh of Clifton) we see a man engaged in "drawing" the retorts through the iron doors at their outer ends. Automatic machinery is now used in large gasworks for both operations. One of the most ingenious stokers is the De Brouwer, shown at work in Fig. 198. The machine is suspended from an overhead trolley running on rails along the face of the retorts. Coal falls into a funnel at the top of the telescopic pipe P from hoppers in the story above, which have openings, H H, controlled by shutters. The coal as it falls is caught by a rubber belt working round part of the circumference of the large wheel W and a number of pulleys, and is shot into the mouth of the retort. The operator is seen pulling the handle which opens the shutter of the hopper above the feed-tube, and switching on the 4 h.p. electric motor which drives the belt and moves the machine about. One of these feeders will charge a retort 20 feet long in twenty-two seconds.

A GAS GOVERNOR.

Some readers may have noticed that late at night a gas-jet, which a few hours before burned with a somewhat feeble flame when the tap was turned fully on, now becomes more and more vigorous, and finally may flare up with a hissing sound. This is because many of the burners fed by the main supplying the house have been turned off, and consequently there is a greater amount of gas available for the jets still burning, which therefore feel an increased pressure. As a matter of fact, the pressure of gas in the main is constantly varying, owing partly to the irregularity of the delivery from the gasometer, and partly to the fact that the number of burners in action is not the same for many minutes together. It must also be remembered that houses near the gasometer end of the main will receive their gas at a higher pressure than those at the other end. The gas stored in the holders may be wanted for use in the street lamps a few yards away, or for other lamps several miles distant. It is therefore evident that if there be just enough pressure to give a good supply to the nearest lamp, there will be too little a short distance beyond it, and none at all at the extreme point; so that it is necessary to put on enough pressure to overcome the friction on all these miles of pipe, and give just enough gas at the extreme end. It follows that at all intermediate points the pressure is excessive. Gas of the average quality is burned to the greatest advantage, as regards its light-giving properties, when its pressure is equal to that of a column of water half an inch high, or about 1/50 lb. to the square inch. With less it gives a smoky, flickering light, and with more the combustion is also imperfect.

Every house supply should therefore be fitted with a gas governor, to keep the pressure constant. A governor frequently used, the Stott, is shown in section in Fig. 199. Gas enters from the main on the right, and passes into a circular elbow, D, which has top and bottom apertures closed by the valves V V. Attached to the valve shaft is a large inverted cup of metal, the tip of which is immersed in mercury. The pressure at which the governor is to act is determined by the weights W, with which the valve spindle is loaded at the top. As soon as this pressure is exceeded, the gas in C C lifts the metal cup, and V V are pressed against their seats, so cutting off the supply. Gas cannot escape from C C, as it has not sufficient pressure to force its way through the mercury under the lip of the cup. Immediately the pressure in C C falls, owing to some of the gas being used up, the valves open and admit more gas. When the fluctuations of pressure are slight, the valves never close completely, but merely throttle the supply until the pressure beyond them falls to its proper level--that is, they pass just as much gas as the burners in use can consume at the pressure arranged for.

Governors of much larger size, but working on much the same principle, are fitted to the mains at the point where they leave the gasometers. They are not, however, sensitive to local fluctuations in the pipes, hence the necessity for separate governors in the house between the meter and the burners.

THE GAS-METER

commonly used in houses acts on the principle shown in Fig. 200. The air-tight casing is divided by horizontal and vertical divisions into three gas-chambers, B, C, and D. Gas enters at A, and passes to the valve chamber B. The slide-valves of this allow it to pass into C and D, and also into the two circular leather bellows E, F, which are attached to the central division G, but are quite independent of one another.

We will suppose that in the illustration the valves are admitting gas to chamber C and bellows F. The pressure in C presses the circular head of E towards the division G, expelling the contents of the bellows through an outlet pipe (not shown) to the burners in operation within the house. Simultaneously the inflation of F forces the gas in chamber D also through the outlet. The head-plates of the bellows are attached to rods and levers (not shown) working the slide-valves in B. As soon as E is fully in, and F fully expanded, the valves begin to open and put the inlet pipe in communication with D and E, and allow the contents of F and C to escape to the outlet. The movements of the valve mechanism operate a train of counting wheels, visible through a glass window in the side of the case. As the bellows have a definite capacity, every stroke that they give means that a certain volume of gas has been ejected either from them or from the chambers in which they move: this is registered by the counter. The apparatus practically has two double-action cylinders (of which the bellows ends are the pistons) working on the same principle as the steam-cylinder (Fig. 21). The valves have three ports--the central, or exhaust, leading to the outlet, the outer ones from the inlet. The bellows are fed through channels in the division G.

INCANDESCENT GAS LIGHTING.

The introduction of the electric arc lamp and the incandescent glow-lamp seemed at one time to spell the doom of gas as an illuminating agent. But the appearance in 1886 of the Welsbach incandescent mantle for gas-burners opened a prosperous era in the history of gas lighting.

The luminosity of a gas flame depends on the number of carbon particles liberated within it, and the temperature to which these particles can be heated as they pass through the intensely hot outside zone of the flame. By enriching the gas in carbon more light is yielded, up to a certain point, with a flame of a given temperature. To increase the heat of the flame various devices were tried before the introduction of the incandescent mantle, but they were found to be too short-lived to have any commercial value. Inventors therefore sought for methods by which the emission of light could be obtained from coal gas independently of the incandescence of the carbon particles in the flame itself; and step by step it was discovered that gas could be better employed merely as a heating agent, to raise to incandescence substances having a higher emissivity of light than carbon.

Dr. Auer von Welsbach found that the substances most suitable for incandescent mantles were the oxides of certain rare metals, thorium, and cerium. The mantle is made by dipping a cylinder of cotton net into a solution of nitrate of thorium and cerium, containing 99 per cent. of the former and 1 per cent. of the latter metal. When the fibres are sufficiently soaked, the mantle is withdrawn, squeezed, and placed on a mould to dry. It is next held over a Bunsen gas flame and the cotton is burned away, while the nitrates are converted into oxides. The mantle is now ready for use, but very brittle. So it has to undergo a further dipping, in a solution of gun-cotton and alcohol, to render it tough enough for packing. When it is required for use, it is suspended over the burner by an asbestos thread woven across the top, a light is applied to the bottom, and the collodion burned off, leaving nothing but the heat-resisting oxides.

The burner used with a mantle is constructed on the Bunsen principle. The gas is mixed, as it emerges from the jet, with sufficient air to render its combustion perfect. All the carbon is burned, and the flame, though almost invisible, is intensely hot. The mantle oxides convert the heat energy of the flame into light energy. This is proved not only by the intense whiteness of the mantle, but by the fact that the heat issuing from the chimney of the burner is not nearly so great when the mantle is in position as when it is absent.