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.
[Illustration: FIG. 192.--The "tank" system of hot-water supply.]
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.
[Illustration: FIG. 193.--The "cylinder" system of hot-water supply.]
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.
[Illustration: FIG. 194.--Showing how a blower-plate draws up the
fire.]
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.
[Illustration: FIG. 195.--Sketch of the apparatus used in the
manufacture of coal gas.]
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.
[Illustration: FIG. 196.--The largest gasholder in the world: South
Metropolitan Gas Co., Greenwich Gas Works. Capacity, 12,158,600 cubic
feet.]
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.
[Illustration: FIG. 197.--Drawing retorts. (_Photo by F. Marsh._)]
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.
[Illustration: FIG. 198.--De Brouwer automatic retort charger.]
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.
[Illustration: FIG. 199.]
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.
[Illustration: FIG. 200.--Sketch of the bellows and chambers of a "dry"
gas meter.]
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.
The incandescent mantle is more extensively used every year. In Germany
90 per cent. of gas lighting is on the incandescent system, and in
England about 40 per cent. We may notice, as an interesting example of
the fluctuating fortunes of invention, that the once doomed gas-burner
has, thanks to Welsbach's mantle, in many instances replaced the
incandescent electric lamps that were to doom it.
[38] If, of course, there is no safety-valve in proper working order
included in the installation.