In this article we will discuss about the steam engine:- 1. Classification of Steam Engine 2. Parts of a Simple Steam Engine 3. Function of Steam Engine Parts 4. Working 5. Function of Steam Engine Governors 6. Function of Flywheel 7. Purpose of Engine Trials in Steam Engine. [with diagram]
Contents:
- Classification of Steam Engine
- Parts of a Simple Steam Engine
- Function of Steam Engine Parts
- Working of a Steam Engine
- Function of Steam Engine Governors
- Function of Flywheel in Steam Engine
- Purpose of Engine Trials in Steam Engine
1. Classification of Steam Engine:
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The steam engine utilizes steam as the working medium to convert heat energy into mechanical energy. The energy contained in steam can be transformed into mechanical energy either in rotary or reciprocating types of machines.
Steam engines may be classified according to the following considerations:
(i) By Position of the Axis of the Cylinder- Vertical, horizontal or inclined.
(ii) By the Action of Steam on the Piston- Single acting or double acting. In a single acting steam engine, steam acts on one side of the piston only and the other side of the piston is open to atmosphere. In a double acting engine steam is alternately admitted on both sides.
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(iii) By the Number of Cylinders in which the Steam Expands- Simple or compound. In a simple steam engine, total expansion of steam takes place in one cylinder while in a compound steam engine total expansion takes place in more than one cylinder.
(iv) By the Method of Exhausting Steam- Condensing or non-condensing. In a condensing engine, the steam is exhausted into a closed vessel known as a condenser usually under low pressure. In a non-condensing engine, it is exhausted into the atmosphere. The latter method is wasteful.
(v) By the Valve Gears Used- Slide valve either of D-type or piston type, Corliss valve or poppet valve.
(vi) By the Speed of Rotation- Low, medium and high.
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(vii) By the Applications- Stationary, locomotive, marine, etc.
2. Parts of a Simple Steam Engine:
The parts of a steam engine can be grouped in the following manner:
(I) Stationary Parts:
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Frame, cylinder, steam chest, cylinder cover, steam chest cover, main bearings, guides and stuffing boxes.
(II) Moving Parts:
Piston, piston rod, crosshead, connecting rod with its end bearings, crank, crankshaft and flywheel. These are moved by the action of steam on the piston.
(III) Valve Gears:
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It controls the distribution of steam and consists of eccentric, eccentric rod, valve rod, valves and governor.
3. Function of Steam Engine Parts:
i. Frame:
It is generally made of cast iron and supports the moving parts in proper relative positions and gives rigidity to the various members. It rests upon the foundations.
ii. Cylinder:
It is made of cast iron and is bolted to the frame at one end. In small engines it is integral with the frame. It forms a chamber in which the piston moves to and for due to action of steam. One end of the cylinder is closed by means of a separate cover and the end is known as cover end of the cylinder. The other end known as the crank end carries the stuffing box through which the piston rod passes.
iii. Steam Chest:
It is always cast integral with the cylinder and is closed by a cover known as steam chest cover which may have a circular or rectangular shape depending upon the kind of valve used.
iv. Main Bearings:
These are attached to the frame at ends opposite to the cylinder and support the crankshaft. The part of the shaft which turns in bearings in known as the journal.
v. Piston:
It is generally made of cast iron. It transmits force due to steam pressure acting on it to the crosshead through piston rod.
vi. Piston Rings:
They are usually made of cast iron and are located in circumferential grooves cut on the piston. Their function is to prevent the leakage of steam past the piston.
vii. Piston Rod:
It is placed at the point where the piston rod passes through the cylinder cover. It prevents the leakage of steam from the cylinder to the atmosphere.
viii. Stuffing Box:
It is placed at the point where the piston rod passes through the cylinder cover. It prevents the leakage of steam from the cylinder to the atmosphere.
ix. Crosshead:
It moves between guides which prevents the piston rod from getting bent.
x. Connecting Rod:
It is made of steel and is supported by but is free to rotate in main bearings. The crank is keyed or pressed on it. Sometimes it is integral with the shaft. It carries a flywheel and an eccentric.
xi. Valve:
It is placed in the steam chest, which is connected to or may be integral with the cylinder body. The steam chest is connected to cylinder by ports or passages made in the casting. The valve reciprocates on its seat by the eccentric so located on the crankshaft relative to crank so as to admit or exhaust steam to and from the cylinder at proper time. The eccentric is made of cast iron and its sheave may be in two parts.
xii. Valve Rod and Eccentric Rod:
Both of them are made of mild steel. The valve rod is connected to the valve and the valve rod guide prevents the bending of the valve rod. The eccentric rod connects the valve rod and the eccentric. It converts the rotary motion of the eccentric into reciprocating motion which is transmitted to the valve through the valve rod.
xiii. Flywheel:
It has a heavy rim and absorbs energy when the supply is greater than the demand and gives it out when the demand is greater than the supply. It thus equalizes the energy distribution and prevents rapid fluctuations during a revolution. Due to its heavy mass, it helps the mechanism to overcome dead centre positions.
xiv. Governor:
It maintains the speed of the engine very nearly constant by controlling either the quantity or pressure of the steam supplied to the engine.
4. Working of a Steam Engine:
The working of a simple, horizontal, double acting, noncondensing, D-slide valve type engine is described here. It may be best understood by first considering what takes place on one side of the piston.
Steam at a high pressure enters the steam chest and presses the valve tightly against its seat. Just before the piston has reached the end of the stroke, the D-slide valve moves so as to open the port and admit steam in the cylinder. The pressure of the steam pushes the piston, thus performing the forward stroke.
The motion of the piston moves the crank, crankshaft and the eccentric; the movement of the latter leading to the further opening of the port. Steam continues to enter, pushing the piston till the eccentric has moved the valve to open port and return to close it.
The cut-off takes place after this and steam supply to the cylinder from the ports is stopped by the valves. The steam in the cylinder at the cut-off takes place after this and steam supply to the cylinder from the ports is stopped by the valves. The steam in the cylinder at the cut-off expands upto the point of release.
During expansion the volume of steam increases and the pressure falls; thereby the steam performs work. Before the end of the forward stroke the valve opens the same port to exhaust side and the release of steam takes place. The exhaust steam pressure falls to the back pressure which is slightly higher than the atmospheric pressure in the case of non-condensing engines.
For condensing engine the back pressure will be slightly higher than condenser pressure. Shortly after release the piston reverses its motion performing the return stroke. The steam is exhausted during the return stroke. When the piston nearly reaches its end of the return stroke, the valve closes the port to exhaust, thus stopping the flow of steam. The steam entrapped between the piston and cylinder head acts as a cushion.
A similar cycle is repeated for the other end of the cylinder in case of double acting steam engines. When admission and expansion occur for the cover end side, exhaust and compression occur for the crank end side.
5. Function of
Steam Engine Governors:
The function of a governor is to control the speed of the engine irrespective of the power it develops. In steam engines the governor maintains a constant speed of rotation either by changing the point of cut-off or by varying the initial pressure of steam supplied.
Governors which control the speed of the engine by increasing or reducing the steam pressure in the steam chest according to the load conditions, are called throttling governors. This type of governor is used mainly on engines of the plain D-slide valve type.
There is, in this type, a loss in available energy and this loss leads to reduce thermal efficiency. The governor operates a throttle valve placed in the main steam pipe, just before the pipe enters the steam chest.
One type of throttle governor, generally called the fly-ball governor, is shown in fig. 9-13. The valve is of the balanced type i.e. the resultant force due to pressure acting on it is zero. The valve is attached to a valve stem, which passes through a sleeve and it bears against the upper ends of the governor arms.
The valve steam and balls are driven from the engine shaft by a belt which is connected to the pulley and which in turn runs the bevel gears. The arms supporting the governor are pivoted to the upper parts of the sleeve and revolve with it. An adjusting screw is provided to change the speed of the engine in a particular range.
6. Function of
Flywheel in Steam Engine:
The flywheel is usually made of cast iron and consists of a rim, a hub and arms that connect the hub to the rim. The hub fits over the shaft to which it is attached by a key. The rim, hub and arms may be made in one piece or they may be cast in sections which are then bolted together.
The function of a flywheel is to reduce the range of variation of speed and to carry the engine over the dead centres. The dead centres occur when the crank and connecting rod are in a straight line at either end of the stroke, at which time the steam acting on the piston is unable to turn the crank. The flywheel may also be employed as a driving pulley. The flywheels store energy when it is surplus and releases the energy when it is deficient.
To carry out is function successfully, the flywheel should be heavy and must have a high mass moment of inertia.
7. Purpose of Engine Trials in Steam Engine:
The engine trials are performed for testing the performance of engine under different operating conditions. The thermal, mechanical and overall efficiencies of a given engine can be deduced theoretically; but it is necessary to check up the theoretical deductions by trials under actual working conditions before putting an engine in the market.
The testing of engine is carried out when the manufactures have to guarantee the specified efficiency. The testing may be carried out under specified condition of pressure, temperature etc. In both the cases the trials are carried out for various loads.
The comparison of actual results with the theoretical values and the analysis of the causes of deviation result in improvement in design.
For assessment of performance of engine the trial must enable us to obtain the following:
(i) Power developed – indicated as well as brake power
(ii) Rate of supply of energy
(iii) Energy balance
(iv) Efficiency ratio of the engine compared to that using the corresponding standard cycle.
(i) Power Developed:
This is measured either:
(a) By an indicator
(b) By a brake.
The indicator can be used for reciprocating engines alone and gives data for indicated power. For steam turbine the indicated power can be calculated from the value of the brake power by adding the losses which have to be obtained from separate tests.
The brake power of the engine can be measured by a brake test.
(ii) Energy Supply:
To obtain the heat supply to the engine it is necessary to measure –
(a) The amount of substance going to the engine during the test period and
(b) The heat carried by each unit of substance.
The former is obtained by condensing the steam used in the case of steam engines. This assumes absence of leakage. After natively, it may be obtained with the help of steam meters installed in supply steam pipes.
For steam engines, only the working conditions of steam need be observed, the corresponding heat content per unit mass being obtained from steam tables.
(iii) Energy Balance:
The heat balance or the energy balance is obtained by observing and recording the distribution of energy.
In the case of steam engines, the energy carried away by the circulating water and by the condensate is calculated by recording the pressure and temperature of steam.
(iv) Efficiency Ratio:
From the observations, the indicated thermal efficiency, the fuel consumption rate or steam consumption per kWh, etc. may be deduced and is usually shown by graphs. The efficiency ratio is obtained by dividing the indicated thermal efficiency by the ideal efficiency.