In this article we will discuss about:- 1. Meaning 2. Steam Engine Parts and Their Function 3. Working 4. Indicator Diagram 5. Indicated Power (I.P.) 6. Efficiencies 7. Steam Consumption 8. Governing.
Contents:
- Meaning of Steam Engine
- Steam Engine Parts and Their Function
- Working of a Steam Engine
- Indicator Diagram of Steam Engines
- Indicated Power (I.P.) of Steam Engine
- Efficiencies of Steam Engine
- Steam Consumption by Engine
- Governing of Steam Engines
1. Meaning of Steam Engine:
Steam engine is a prime mover. It utilises steam as a working medium. The heat energy in steam engine is converted into the mechanical work and this conversion is done in the cylinder of the steam engine.
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Steam engine was first developed by James Watt (1736-1819) and was in continuous use upto 1930. Still, also we can see some of the steam engines in railway locomotives. But nowadays they are getting replaced by Diesel or Electric Locomotives.
Generally it has been stated that the steam engine owes little to science, than science owes to the steam engine. Nowadays steam turbines and IC engines have largely replaced to steam engines because of their higher power outputs, higher efficiencies and smaller bulk.
2. Steam Engine Parts and Their Function:
i. Frame:
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It is made of cast iron. It support the moving parts; and gives rigidity to various members, It rests upon the foundation.
ii. Cylinder:
It is made of cast iron and is bolted to the frame. In small engines it is cast integral with the frame. It forms a chamber in which a piston moves to and fro due to the action of steam. One end of the cylinder is closed by means of a cover. This end is known as cover end. The other end is known as crank end. It carries a cover in which a stuffing box is provided. The piston rod passes through the stuffing box.
iii. Steam Chest:
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It is cast integral with the cylinder. The high pressure steam is first admitted to the steam chest. The steam chest accommodates the D slide valve. On one end it carries a stuffing box through which valve rod comes out. It is closed from top by a cover. The steam chest is connected to the cylinder by means of passage made in the casting ports, through which the steam passes from the steam chest to the cylinder.
iv. Piston:
It is made of cast iron. The steam under pressure acts on the face of the piston and creates force. Piston transmits this force to the main shaft via piston rod, and connecting rod.
v. Piston Rings:
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They are made of cast iron. The piston is provided with grooves on its circumference. The piston rings are fitted into these grooves. The piston rings prevent leakage of steam from one side of piston to the other sides. This is known as leakage past piston.
vi. Piston Rod:
It is made of mild steel. It connects the piston to the cross head.
vii. Stuffing Box and Gland:
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This is fitted on the crank end cover of the cylinder at a point where piston rod passes through the cylinder cover. It prevents leakage of steam from the cylinder to the atmosphere. A similar stuffing box is also fitted on the steam chest where valve rod passes through the steam chest side.
viii. Cross Head:
This moves between the guides. It guides the motion of piston and prevents the bending of piston rod. The cross head is essential for double acting steam engines. For single acting steam engine cross head is not needed.
ix. Connecting Rod:
It is made of forged steel. One end of it is connected to the piston rod by a gudgeon pin and the other end is connected to the crank by crank pin. It converts reciprocating motion of the piston (and of cross head) into rotary motion of the crank.
x. Crank Shaft:
It is made of steel. It is supported on the main bearings, but is free to rotate in the bearings. The crank webs are keyed or pressed on to it. The crank webs may be made integral with the shaft also. In that case the entire shaft is made as a forging. The crank shaft carries the eccentric and flywheel.
xi. Eccentric:
It converts rotary motion of the crank shaft into the reciprocating motion of the slide valve.
xii. Valve Rod and Eccentric Rod:
Both of them are made of mild steel. The valve rod is connected at one end to the valve and at the other to the valve guide. The valve guide guides the valve rod and prevents it from bending. The eccentric rod connects the valve rod and the eccentric. It converts rotary motion of eccentric into reciprocating motion of valve guide and therefore of valve itself.
xiii. Slide Valve:
It is placed in the steam chest. The steam chest is divided from the cylinder by a wall. The valve has ports through which steam is admitted to the engine cylinder. The slide valve controls the amount of steam admitted by closing and opening the steam ports in predetermined sequence.
In double action steam engine there are two ports which are used alternately as admission and exhaust port.
xiv. Fly Wheel:
It is made of cast iron and has heavy mass and therefore large inertia. The pressure on the piston face during a stroke is not uniform. The torque produced also is not uniform. To reduce the fluctuations in the torque a fly wheel is used. It absorbs energy from the engine when it is in excess. Stores it in the form of inertia and gives it back when it is deficit.
3. Working of a Steam Engine:
We describe here the working of a simple, vertical, double acting, non-condensing, D slide valve type, steam engine.
The function of a D slide valve is to control the flow of steam and to connect the steam ports to the admission side and exhaust side alternately. It is a hollow rectangular box open at its lower face. It has projections on its lower face and the surfaces are machined flat. The valve is made to slide over the machined face of the cylinder.
The cylinder wall has two rectangular steam ports and an exhaust port which opens to atmosphere (or to the condenser in a condensing type steam engine).
Figure 18.2 (a) shows the beginning of a stroke (on cover side). The steam chest is full of high pressure steam. The valve has just opened the steam port and the high pressure steam is admitted to the top side of the piston. The steam pressure acting on the piston pushes the piston down.
At the same time the cranker port on crank end side is just uncovered and the steam on the crank end side of piston goes out of the cylinder via exhaust port.
As the piston moves downwards the D slide valve completes its stroke and comes back to close the admission port [Fig.18.2 (b)]. This is known as cut off point. The supply of steam is cut off and rest of the stroke is completed by the expansion of steam. The pressure drops, volume increases and expanding steam does work. When expansion continuous till the end of the stroke, just before the end of the stroke the valve uncovers the cover end steam port and connects it to the exhaust port. The pressure of steam inside the cylinder suddenly drops. This is known as point of release.
The crank end steam port also is closed and steam in the cylinder on the crank end side of piston gets compressed between the piston and cylinder cover, thus providing a cushioning effect which slows down piston speed gradually.
The steam port on crank end side now opens up to the steam chest and fresh high pressure steam is admitted on the crank end side. The piston now starts its upward stroke. The steam on cover end side now is exhausted.
Thus steam admitted on either side of piston alternately keeps the piston continuously in reciprocating motion which is converted to rotary motion by means of cross head, connecting rod and crank.
4. Indicator Diagram of Steam Engines:
Theoretical or Hypothetical Indicator Diagram and MEP:
An indicator diagram is a plot of steam pressure in the cylinder corresponding to a steam volume during the cycle of operation. The indicator diagram for a single cycle of steam engine (Neglecting clearance) is shown in Fig. 18.3.
Assumptions made for the construction of theoretical indicator diagram:
1. Clearance volume is neglected.
2. Steam is admitted at constant pressure
3. Closing of port at cut off point 2 is instantaneous. So we get a well-defined point 2.
4. Expansion of steam is hyperbolic
5. We get a well-defined point 3 as opening of exhaust port is instantaneous.
6. Steam will be exhausted at constant pressure through the return stroke.
7. 5-1 is constant volume of steam addition
During starting in case of stationary steam engines, the flywheel will be rotated manually, so that the marking on the flywheel coincides with the pointer. Then the position of D-slide valve with respect to the ports will be adjusted (as shown in Fig. 18.3) so that it uncovers the head end port.
As shown in Fig. 18.3 the cycles of operations are as follows:
i. Process 1-2:
At point 1 the steam is admitted and it continuous to do so until the point 2 is reached, where the steam is cut off. Point 1 is called the point of admission Point 2 is called the point of cut off.
ii. Process 2-3:
After cut off, steam locked in the cylinder, expands hyperbolic ally from 2-3.
iii. Process 3-4:
Just before the piston reaches to Bottom dead center, the D-slide valve will uncover the head end port. Now the pressure of steam, from above the piston will be released at constant volume along 3-4.
At the same time steam will be entering from below the piston through crank end port. Note that till the end of downward stroke, this steam gives cushioning effect and after the piston reaches Bottom dead center its upward stroke starts and the steam entering from below will force the piston to move upwards.
iv. Process 4-5:
The steam from above the piston will be exhausted along this. Work done/cycle will be given by the area 1-2-3-4-5-1 on the pV-diagram.
Actual Indicator Diagram and Diagram Factor:
Figure 18.5 shows the actual indicator diagram (showing continuous line) and the dotted line diagram shows the theoretical.
The difference between the diagrams is on account of certain practical factors given below, which were not considered in arriving at the theoretical diagram:
1. Point A:
From the Fig. 18.5 it can be seen that point A is below ‘1’ that is the steam pressure drops considerably between the boiler and engine cylinder. This is due to the condensation of steam in the pipeline connecting between the boiler and engine (or wire drawing through valve).
2. Points. A-B:
There is a gradual pressure drops that is P ≠ C, before the cutoff point. This is mainly because the steam entering the cylinder, comes in contact with the cold cylinder surfaces and condenses. The cylinder walls can be kept hot to avoid excessive condensation by passing live steam through the cylinder jackets.
3. Points:
5-Corner at B is rounded off because (when the piston comes to some intermediate position the D- slide valve will return retrains its path and in the reverse direction and closes the head end port) closing of admission port is not instantaneous.
4. Points:
B-C-It is not hyperbolic due to condensation of steam in the cylinder.
5. Points:
C-D-Rounded off because exhaust does not open instantaneously.
6. Points:
D-E-Vat exhaust pressure is slightly above the condenser pressure or back pressure as the steam has to be forced out of the cylinder.
7. Points:
EF-The exhaust port closes at E and the extrapped steam compresses to point, F. At F admission port opens and the extrapped steam or cushion steam mixes with the admission steam and reaches the initial starting point, A, and the cycle continues.
It will be seen from Fig. 18.5 that the area of actual Indicator diagram is less than the area of theoretical Indicator diagram. The ratio between the areas of these two diagrams is known as Diagram factor and is denoted by K i.e.,
5. Indicated Power (I.P.) of Steam Engine:
The total power developed by the engine is commonly known as Indicated Power (I.P). It is called as indicated power because an instrument known as indicator is used to measure it.
If, Pm1 = Actual MEP in N/m2 for head end side
Pm1 = Actual MEP in N/m2 for crank end side
L = Length of stroke in m.
A1 = Area in m2 on head end side
A2 = Area in rn2 on crank end side
N = Number of power or working stroke/min.
Then force on the piston = Pm1 x A1
Newtons for head end side and work done/working stroke for head end side
= Force x Displacement
= (Pm1 x A1 x L) Nm or J
and work done/min for head end side = work done per working stroke x Number of strokes/minute.
The total power developed by the engine will not be available for doing useful work, but some of the power will be lost in overcoming the frictional resistance and is termed as frictional power (FP.) The power available for doing useful work is termed as Brake Power (BP) or shaft power.
The BP of the engine can be determined by a brake of some kind applied to the brake pulley of the engine. The arrangement for the determination 1 of BP is termed as dynamometer. The various types of dynamometers are Rope brake, prony brake, hydraulic dynamometers, etc.
6. Efficiencies of Steam Engine:
7. Steam Consumption by Engine:
Steam Consumption/hr:
Amount of steam consumed by the steam engine in kg/hr.
Specific Steam Consumption:
It is the steam consumption per unit of power produced.
When SSC is based on kW (Indicated) then it is known as SSC/indicated kWh and when SSC is based on kW (brake) then it is known as SSC/brake kWh.
Cylinder Condensation and the Method for Reducing:
When the steam is admitted and allowed to expand in the engine cylinder some of the steam condenses, when it comes in contact with the colder cylinder surface. This reduces the power to be developed.
So, the methods adopted to reduce condensation are:
1. Using superheated steam.
2. The pipe line connecting between boiler and steam engine should be Lagged by asbestos in order to reduce, steam condensation in it.
3. Jacketing the cylinder with hot steam from the boiler, which reduces condensation of steam in the cylinder.
4. Using of compound steam engines, which reduces temperature range/cylinder.
5. Using increased engine speeds.
8. Governing of Steam Engines:
The function of a governor is to maintain the speed of the engine within the pre-specified limits, depending upon the load on the engine. In other words, the engine should be able to adjust its power developed according to the load on the engine.
There are two methods used for maintaining the constant speed of the engine by the action of the governor:
(i) Throttle Governing,
(ii) Cut-off Governing.
(i) Throttle Governing:
In this method of governing the point of cut-off is kept constant and the steam is throttled from boiler pressure. P1 to a lower pressure according to the reduction in load by partial closure of throttle valve under the control of a governor. The decreasing pressure of steam during admission process reduces the work developed by the engine.
(ii) Cut-off Governing:
In this method of governing, the steam inlet pressure P1 is kept constant and the work developed by the engine is varied by alternating the point of cut-off with the help of a slide valve operating under the control of a centrifugal governor.
Comparison between Throttle and Cut-off Governing:
In case of cut-off governing since the admission pressure is corresponding to the boiler pressure and only the volume/ mass of steam supplied is varied, it efficiency is not affected. But, in case of throttle governing since the admission pressure is lowered, it lowers the thermal efficiency. Also, the steam consumption is lower in case of cut-off governing compared to throttle governing.
Willan’s Line:
In case of throttle governing, if we plot a graph between the steam consumption in kg/hr taken on y-axis against the indicated power (LP) taken on X-axis, we get a straight line variation as shown in Fig. 18.9. This particular relationship was first observed by Mr. Willan and after his name this straight line relationship between steam consumption (kg/ hr) and indicator power is called the Willan’s line.
Energy Balance (Heat Balance Sheet):
This gives an account of total heat energy supplied in, per minute or per second and its utilization as under,
