most fluid state in which matter exists is known as gas which has
neither independent shape nor volume but tends to expand indefinitely.
term gas was used at first by chemists as a synonym of air but afterwards
it was applied to all fluids that could not get liquefied by pressure
at any attainable temperature as oxygen, hydrogen, etc. in distinction
from steam, which becomes liquid on a reduction of temperature.
everyday language, the word gas is applied to the gaseous state
of a substance more known in that state, while vapour is used to
call the gaseous state of substance more known as liquid or solid.
there is no definite line of distinction between a vapour and a
gas, it is generally understood that a vapour is in such a condition
that it does not obey the laws describing the behaviour of true
gases under changes of temperature and pressure.
fact that a gas can be compressed to a small fraction of its original
volume tests that the molecules are widely separated. The diffusion
of a gas shows that the molecules are moving. The higher the temperature
of a gas the greater the speed of the molecules,
gases may be liquefied at a low temperature and under high pressure.
The pressure of a gas in a closed vessel is due to the impact of
the gases against the sides of the container.
laws that affect gases :
Boyle's law states that the volume of a gas varies inversely as
the pressure upon it, if the temperature is constant.
Charles's law states that the volume of a given quantity of gas
at constant pressure is proportional to the absolute temperature.
A third law may be deduced from the first two: The pressure exerted
by a given mass of gas varies directly as the absolute temperature
if the volume remains constant.
Gay-Lussac's law or the law of gaseous volumes, states that when
gases react, the volumes in which they do so, are in a simple numerical
relation to one another, and to the volume of, the product if that
Graham's law states that the rates at which different gases diffuse
are inversely proportional to the square roots of their respective
Avogadro's hypothesis establishes that equal volume of all gases
under the same conditions of temperature and pressure contains equal
numbers of molecules. As the hypothesis was in many cases verified
by experiment, it is sometimes described as Avogadro's law.
is water vapour at a temperature of 100° C or above. It
is a colourless, invisible gas. The white clouds which are
frequently called "steam" consist of droplets of
liquid water formed by the condensation of steam.
is used as a source of power, as a means of conveying heat
from one place to another and as a raw material in many chemical
operations as, for example, in the manufacture of hydrogen.
steam is steam, the temperature of which is raised to the
required degree as, for instance, by passing it through red-hot
normal pressure, steam condenses to liquid water at 100° C.
At increased pressure, however, it may be condensed even at higher
temperatures. Above 374° C steam cannot be condensed to liquid
water by any applied pressure, however great. Hence 374° C is
described as the critical temperature of steam.
power is usually applied by means of steam engines, but it is also
used in steam hammers and other devices. In a simple steam hammer,
the hammer block is raised by admitting steam under pressure into
a cylinder, the block being attached to the piston-rod. When the
steam escapes through an adjustable valve, the piston with its block
falls under the force of gravity. By manipulation of the valve,
the hammer descends as far and as quickly as wanted.
engines are those in which heat energy is converted into mechanical
energy by using steam as working element.
motive power of a steam engine depends on elasticity and expansion
of steam or on its rapid condensation.
engines are broadly classified as condensing and noncondensing.
engines are those in which the steam, after exhausting from the
cylinders is condensed in a separate condenser. On the other hand,
when steam engines exhaust steam directly into the air, we term
them noncondensing engines.
engines belong to two types: reciprocating and rotary.
common reciprocating steam engine consists essentially of a piston
driven in a closed cylinder by steam at a pressure initially higher
than the atmospheric one. This piston is connected by a piston-rod
to a crosshead and its reciprocating motion is transformed into
rotary movement by a connecting rod, crankpin, crank and crankshaft.
steam distribution is carried out by a slide valve or other automatic
valves and is regulated by a link motion or some other form of valve-gear.
engines are either vertical or horizontal according to the angle
at which the cylinders are placed, and single- or double- acting
whether the steam is admitted to only one side of the piston or
to each side alternately.
engines constitute a class by themselves known as turbines. In them
a blade, vane or the like moves in a circular path making the use
of connecting rod and crank unnecessary.
is how any simple steam engine works: steam enters the valve-chest
through a steam-pipe connected to the boiler. In the valve-chest,
the steam pressure keeps the slide valve pressed closely against
the valve face of the chest. In this face there are two ports: inlet
and outlet connected by channels to the cylinder top and the open
air respectively. As the piston begins to descend, the slide valve
operated by the eccentric on the crankshaft, also moves down and
uncovers the inlet port. Steam now enters the cylinder; the piston,
under its pressure, continues its downward movement as a power stroke.
The slide valve advances, rises again and covers the inlet port
as the piston reaches bottom dead centre. While it rises, the slide
valve again uncovers the port of the connecting channel but now
in such a way that the cylinder is connected to the outside air
instead of to the steam-chest, the expanded steam being thus exhausted.
The cycle of operations has come to an end to begin over and over
at high pressure flows from a boiler into a cylinder containing
a movable piston. The steam forces the piston back along the cylinder.
When a piston has moved a short distance, the steam supply is cut
off, but the steam which is already in the cylinder goes on expanding
and exerting a force on the piston until the latter gets to the
end of its stroke. That end of the cylinder which contains the steam
is connected to the exhaust port, through which the steam may escape,
and at the same time steam from the boiler is let in to the other
end of the cylinder to force the piston to its initial position.
other end of the cylinder is now connected to the exhaust port and
the cycle of changes is repeated over again.
If the steam leaves the engine at a given temperature, the efficiency
can be raised by increasing the temperature at which steam -is supplied
to it. It is, therefore, an advantage to use steam at the highest
is, in general, any closed liquid-containing vessel to which heat
is applied. It is also called steam generator as it transforms water
into steam. Boilers move large volumes of water using water pumps.
generally consist of metal shells (or bodies), headers and tubes
that form the container of the steam and water under pressure and,
in certain types, of the furnace and passages for the hot gases.
Some boilers have additional drums called superheaters.
of them are. cylindrical for many practical reasons, although the
sphere is the strongest shape to bear the internal pressure produced
are classified into stationary, locomotive and marine boilers. They
may be also classed into vertical and horizontal, but their main
parts are always the same.
boilers belong to three types: the Lancashire, the Cornish and the
multitubular boiler, the Cornish being the simplest of the three.
A Cornish boiler consists of a cylindrical shape that contains a
smaller tube with furnace, two side flues and a central one. On
the other hand , a Lancashire boiler has two internal furnaces and
two flues. The products of combustion pass from the fire grate up
to the end of the furnace tube and go back by the two side flues
going to the chimney by a central flue.
boilers are of a large diameter. They are made of double riveted
steel plate, and they usually have three furnaces built of corrugated
steel plates. Their combustion chambers are supported by iron stays.
boilers have horizontal fire tubes that extend from a rectangular
fire-box through the cylindrical
barrel of the locomotive to the smoke-box. The furnace is rectangular
and has a fire door with a deflector plate.
boilers are usually fitted with a dome to supply an extra amount
of pure, dry steam.
addition to the main parts of a boiler - the cylinder, tubes and
fire-box - there are some indispensable accessories such as the
pressure-gauge, water-gauge,. safety valve and feed-water pump or
injector. A superheater, though not quite necessary, is often included
to dry the steam thus avoiding the use of the separator.
sectional boiler is one in which the parts are made and shaped in
sections and placed in suitable supporting frames and brickwork
an internally-fired boiler. the water is conveyed in tubes and heated
from the furnace or from hot gases surrounding the tubes.
express boiler is a water-tube boiler with comparatively small light
tubes designed for compactness and rapid generation of steam.
type of water-tube boiler which is useful for the rapid provision
of steam as required where the service is intermittent is a flash-boiler.
It contains little or no water until it is fed by the force-feed
pumps. It generally has the shape of a coiled tube or series of
coils, capable of bearing great heat. Water is pumped into them
at one end and flashed as steam at the other. High pressure may
be kept and there is little danger of explosion, the amount of steam
within the boiler being very small at any moment.
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