Class Lecture
Steam Engine
Applied Thermodynamics & Heat Engines
MOAZ HASSAN (Lecturer)
Mechanical Engineering Department
University of Lahore
MOAZ HASSAN
Applied Thermodynamics & Heat Engines
Thermodynamics –II
Class Lecture
Where Steam Engine/Turbine Stands
o Steam Engine is an External Combustion Engine, used to Produce
Electricity.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Steam Engine
o In a modern piston steam engine, steam from the boiler enters a
thick-walled metal chamber called a steam chest.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Construction
o Steam Engine are Double Acting i.e. It consists of a cylinder in
which a double-acting piston operates. Double-acting means that
both back and front faces of the piston are arranged to be working
faces.
o Thus, when one piston face is working, the other piston face is
exhausting.
o On the return stroke, what was the exhausting face now becomes
the working face, and what was the working face now becomes
the exhausting face.
o In this way there are two working strokes per revolution.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Construction
o The external end of the piston rod is connected to a Crosshead. This
crosshead reciprocates in guides and supports the piston rod end. The
small end of the connecting-rod is connected to the crosshead. The
reciprocating motion of the piston is thus transmitted, via the crosshead,
to the Connecting-Rod.
o
The big end of the connecting-rod is connected to the crank which turns
on the crankshaft that runs in the main bearings. A Flywheel is fitted to
the crankshaft.
o Control of the steam to and from the cylinder is by means of a Valve
which is assembled in the valve chest mounted on the side of the
cylinder. These are also called Piston Valves.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Construction
o A common way of operating this valve is to use an eccentric mounted on
the crankshaft. The reciprocating motion of the eccentric is transmitted
to the valve by a valve operating link called the Eccentric Rod.
o In order to reduced condensation loss, the cylinder is often surrounded
by a Steam Jacket. High-pressure steam from the main is fed to this
steam jacket, which thus helps to maintain a high general cylinder
temperature and cuts down working steam condensation loss.
Condensate from the jacket is drained through a steam trap.
o Note the Governor fitted to the end of the crankshaft. The governor is
connected via a linkage up to the governor valve. The governor serves to
maintain nearly constant engine speed at all loads up to full load.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Working Principle
o The Steam Engine Valve
o In a modern piston steam engine, steam from the boiler enters a thick-walled
metal chamber called a steam chest.
o There are three holes, or ports. The centre port opens into a pipe, the steam
outlet or exhaust; the other two ports, one on either side of the centre port,
open into another thick-walled metal chamber, the cylinder.
o This is the D-slide valve, which always covers the steam outlet port and
alternately covers one of the two intake ports.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Working Principle
o Working Method
o Steam from the boiler rushing through one of the intake ports strikes one
side of the piston and forces it towards the opposite end of the cylinder. The
movement of the piston is called a stroke.
o As the piston moves, the D-slide also moves so that, when the piston
reaches the end of its stroke, the port through which the steam entered the
cylinder is closed.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Working Principle
o Working Method (Continued….)
o At the same moment, the D-slide valve opens the other port so that
incoming steam is guided to the other side of the piston.
o Steam striking this side pushes the piston back towards the opposite end of
the cylinder.
o As the D-slide valve allows steam to enter alternately one intake port and
then the other, this valve also keeps open a channel to the steam outlet,
where the steam that has just finished pushing the piston escapes.
o There the steam is cooled and condenses into water, which may be sent back
to the boiler where it is again heated into steam. Exhaust pressure of Steam
is also called Back Pressure.
o The process of steam entering and escape of steam first from one side of the
piston and then the other, goes on continually as long as steam enters the
chest at pressure that is high enough.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Hypothetical Steam Engine Indicator Diagram
o The hypothetical indicator diagram for a single cycle of a steam engine
is shown in Fig. 11.4. Clearance has been neglected.
o The cycle operations is
o 1—2 At point I some steam is admitted to the cylinder; this continues until
point 2 is reached, where the steam is cut off. Point 1 is point of admission.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Hypothetical Steam Engine Indicator Diagram
o The Cycle Operations (Continued…..)
o Now the ratio V3/V2 is often written r. called the expansion ratio or the
number of expansions. Thus
o The mean effective pressure of the cycle can now be obtained as,
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Diagram Factor
o The Actual Steam Engine Diagram is different from theoretical Diagram
due to losses. Comparison of Actual & theoretical Diagram is follow,
o In Steam Engine Clearance Vol. is 1/10-1/50.
o For max. Pressure build up, Steam Admission
is opened before the Piston reaches IDC.
o At Admission, Pressure is low to Steam
Losses Main & throttling valve.
o Continuously Steam Loss through the Valve
causes the Damage like grooves on Valve’s
edges, is called Wire Drawing.
o Upon admission, the working steam will meet working surfaces which are at a
lower temperature. thus some steam will condense on these surfaces. There is no
corresponding increase in work output for this increased steam supply, so the
thermal efficiency tends to be reduced.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Diagram Factor
o Comparison of Actual & theoretical Diagram (Continued….)
o Due to the lower admission pressure, the expansion line will be lower than
the theoretical expansion line due to condensation on the cylinder wall.
o During Exhaust stroke , Actual Pressure is slightly higher in theoretical
diagram. Because high temperature & pressure may stay there due to
condensation.
o At Compression, some steam is locked in cylinder (called Cushion Steam)
to reduce Steam Consumption.
o Actual Area is Smaller as compare to theoretical Area of Diagram. So
o Diagram Factor = K =Theoretical Diagram Area/Actual Diagram Area
Similarly,
o Diagram Factor = K =Theoretical Work Done/Actual Work Done
o Diagram Factor = K =Theoretical Mean Eff. Prr./Actual Mean Eff. Prr.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Steam Engine Governing
o A steam engine must be capable of taking up or shedding load
o If load is increased then more work must be done in the cylinder.
o If load is decreased then less work must be done in the cylinder.
o So there are two ways as follow,
o A) Throttle governing
o control the engine output by varying the intake
pressure to the engine.
o The throttling of the steam is usually taken care
of using a valve controlled by a governor on the
engine.
o Due to the constant cut-off, large quantities of
steam are used, as much at low loads as at high
loads
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Steam Engine Governing
o Cut-off governing
o Method of controlling the output of the
engine by varying the cut-off. The intake
steam pressure now remains constant.
o No Steam Consumption. It is more efficient.
o Throttle governing and Willan’s line
o Linear relationship exists between the steam
consumption and the indicated power Output.
o The indicated power is zero at B. so B
represents loss due to condensation, leakage.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Indicator Diagram for Double-Acting
o Now when thc head end is carrying out its working stroke, for the same
piston direction, the crank end is exhausting. Also, when the head end is
exhausting, the crank end is carrying out its working stroke
o Work done per revolution is the sum of the diagram areas taken
separately.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
The Compound Steam Engine
o In Single Cylinder following Reasons Problems can occur:
o To accommodate high Pressure & large Volume Expansion, heavier
reciprocating masses required that result in unbalancing.
o A heavy flywheel will be required for smooth Operation.
o Higher temperature difference result in Condensation Loss.
o Multi-cylinder Engine is preferred over Single Cylinder Engine because
o The steam partially expanded in high-pressure cylinder then moves to next
cylinder. Cylinder with high-pressure steam need not have such a large
volume, so it can have lighter construction.
o Steam Exhausted from 1st Cylinder has larger volume & low Pressure. So
larger Cylinder Volume is required but with Lighter Construction.
o In each cylinder there will be a lower overall temperature range, so
condensation loss is reduced.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
The Compound Steam Engine
o Multi-cylinder Engine over Single Cylinder (Cont…)
o In two or more cylinders with Lighter Construction & a suitable crank
arrangement. it is possible to obtain better balancing than with a single
cylinder. These result in a smoother torque output.
o Note :
o A receiver is fitted between cylinders to hold the steam until the following
cylinder is ready for it.
o As the strokes of the cylinders of a compound engine are generally made
equal .To accommodate the larger steam volumes, the diameters of the
following cylinders are made larger.
o Double-Expansion Engine have two Cylinder, 1st Cylinder is High Pressure
& 2nd cylinder is Low Pressure Cylinder.
o Triple-Expansion Engine have three cylinder, cylinders are called the High-
pressure, Intermediate-pressure and Low-pressure cylinders
MOAZ HASSAN
Thermodynamics –II
Class Lecture
The Compound Steam Engine
o Hypothetical Diagram for Compound Engine
o High Pressure cylinder diagram is 1276
o Low Pressure cylinder diagram is 76345
o Expansion is Hyperbolic i.e. PV=C
o Area 4-5 is larger than 6-7 to
accommodate the Steam.
o HP cylinder volume is equal to cut-off
volume of Low pressure cylinder.
o
In Low Pressure Cylinder, Steam releases earlier
i.e. at point 3 instead of point 4’. Because
Cylinder Volume is more but it does not
compensate for complete Expansion/Work.
o
Two Diagrams fitted together, but they are quite
different.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
The Compound Steam Engine
o Equal Initial Piston Loads
o In double acting, for two Cylinder,
Net Pressure = P1 – P7
(For High Pressure Cylinder)
Net Pressure = P7 – P5
(For Low Pressure Cylinder)
Force = Pressure * Area
For Equal Initial Piston Loads,
FHP = FLP
Then
(P1 – P7)AHP = (P7 – P5)ALP
Where, AHP = Area of High Pressure Piston & ALP = Area of Low Pressure Piston
Stroke L is same for both cylinders.
So
(P1 – P7)AHP L = (P7 – P5)ALP L
And
(P1 – P7)VHP = (P7 – P5)VLP
.: A L=V= Volume
o It is necessary condition for Equal Initial Piston Loads.
MOAZ HASSAN
Thermodynamics –II
Class Lecture
The Compound Steam Engine
o Equal Work/Cylinder (Equal Work is done by both cylinder)
o For High Pressure Cylinder
Work Done = Area1267 = Area 1-2 + Area 2-6 – Area 6-7
= P2V2 + P2V2 ln(V6/V2) – P6V6
= P2V2 + P2V2 ln(V6/V2) – P2V2
.: P2V2 = P6V6 (PV=C)
= P2V2 ln(V6/V2)
= P2V2 lnr
.: r = Expansion Ratio = (V6/V2)
o For Low Pressure Cylinder
Work Done = P6V6 + P6V6 ln(V4/V6) – P4V4
= P6V6 (1 + lnR) – P4V4
.: R = Expansion Ratio = (V4/V6)
o For Equal Work
P2V2 lnr = P6V6 (1 + lnR) – P4V4
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Why Steam Engine?
o Steam Engine is preferred over IC Engine, Because
o Higher Power Output & Higher Efficiency (Double Acting)
o Low Noise
o Low Atmospheric pollution (No CO)
o Fuel flexibility (Natural gas Coal etc)
o Note : It is mostly used in Railways
MOAZ HASSAN
Thermodynamics –II
Class Lecture
Thank You !
MOAZ HASSAN
Applied Thermodynamics & Heat Engines
Thermodynamics –II