TOPICS ON MECHANICS

GASES AND VAPOURS

The most fluid state in which matter exists is known as gas which has neither independent shape nor volume but tends to expand indefinitely.

The 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.

In 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.

Though 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.

The 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,

All 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.

Several laws that affect gases :

1) Boyle's law states that the volume of a gas varies inversely as the pressure upon it, if the temperature is constant.

2) Charles's law states that the volume of a given quantity of gas at constant pressure is proportional to the absolute temperature.

3) 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.

4) 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 is gaseous.

5) Graham's law states that the rates at which different gases diffuse are inversely proportional to the square roots of their respective densities.

6) 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.

STEAM

Steam 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.

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.

Superheated steam is steam, the temperature of which is raised to the required degree as, for instance, by passing it through red-hot tubes.

At 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.

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.

 

STEAM ENGINES

Steam engines are those in which heat energy is converted into mechanical energy by using steam as working element.

The motive power of a steam engine depends on elasticity and expansion of steam or on its rapid condensation.

Steam engines are broadly classified as condensing and noncondensing.

Condensing 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.

Steam engines belong to two types: reciprocating and rotary.

The 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.

The 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.

Reciprocating 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.

Rotary 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.

Here 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 again.

Steam 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.

The 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 possible temperature.

 

BOILERS

A boiler 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.

Boilers 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.

Most of them are. cylindrical for many practical reasons, although the sphere is the strongest shape to bear the internal pressure produced by steam.

Boilers 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.

Stationary 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.

Marine 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.

Locomotive 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.

These boilers are usually fitted with a dome to supply an extra amount of pure, dry steam.

In 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.

A sectional boiler is one in which the parts are made and shaped in sections and placed in suitable supporting frames and brickwork settings.

In an internally-fired boiler. the water is conveyed in tubes and heated from the furnace or from hot gases surrounding the tubes.

An express boiler is a water-tube boiler with comparatively small light tubes designed for compactness and rapid generation of steam.

A 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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