By Basanagouda Shivalli Dept. of Mechanical Engineering BVBCET, Hubli Chapter 4 Prime Movers

STEAM Turbines Classification 1.Impulse Turbine 2.Reaction Turbine

Impulse Steam turbine Impulse Steam turbine In this type of turbine, the steam is initially expanded in a nozzle from high pressure to low pressure. The high velocity jet of steam coming out of the nozzle is made to glide over a curved vane, called blade.

The jet of steam gliding over the blade gets deflected very nearly in the circumferential direction. This causes the particles of steam to suffer a change in the direction of motion, which gives rise to a change of momentum and therefore a force, which will be centrifugal in nature. The particles of steam exert centrifugal pressures all along their path on the curved surface of the blades. The resultant of all these centrifugal forces acting on the entire curved surface of the blade causes it to move.

When a number of such blades are fitted on the circumference of a revolving wheel, called rotor, they will be moved by the action of the steam, and they in turn sets the rotor in continuous rotation. The rotation of the rotor makes all the blades fitted on the rim to get exposed to the action of the steam jet in succession.

In the impulse turbines the steam is expanded from its high initial pressure to a lower pressure before it is delivered to the moving blades on the rotor. The pressure of the steam over the blades will be at a lower pressure. However, the velocity of the steam continuously decreases as it glides over the blades owing to the conversion of kinetic energy into mechanical energy of rotation. Thus in the impulse turbines the mechanical power is produced by the combined action of the resultant of the centrifugal pressures due the change of momentum and the effect of change of velocity of the steam as it glides over the blades.

Although there is no direct impulsive action on the moving blade that is causing the turbine rotor to rotate, but the impelling action of the jet of steam on the blades drives the rotor to rotate in the same direction of the propelling force, this type of turbine is called impulse turbine. The examples of impulse turbines are De Laval Turbine, Curtis turbine, Zoelly turbine, Rateau turbine.

Pressure Velocity Changes in Impulse Turbine  The lower portion shows the nozzle and the blade, and the top portion shows the variation of pressure and velocity of the steam as it flows through the nozzle and over the blades.  Since the expansion of the steam takes place in the nozzle, the pressure drop is represented by the curve AB.  As there is no change in the pressure of the steam that is passing over the blade, this flow is represented by the horizontal line BC.  Since the velocity of the steam in the nozzle increases due to the expansion of the steam, the increase in the velocity of the steam is represented by the curve VLQ.  As the blades absorb the kinetic energy of the steam as it flows over it, the velocity decreases. This is represented by the curve QR.

Reaction Turbine In this type of turbine the high pressure steam does not initially expand in the nozzle as in the case of impulse turbine, but instead directly passes onto the moving blades, whose shapes are designed in such a way that the steam flowing between the blades will be subjected to the nozzle effect. Hence the pressure of the steam drops continuously as it flows over the blades causing simultaneous increase in the velocity of the steam.

The increase in the velocity of the steam flowing over the blades develops a force within itself which enables it to move further, consequently there will be a backward reaction to the force causing the motion of the jet. Thus the reaction force acting on the blades constitutes a fraction of the propelling force driving the turbine rotor. In addition to this reaction force, there is also the centrifugal force exerted by the steam due to the change in the momentum because of the change in the direction of the steam passing over the blades. This reduces the velocity of the steam. Thus the net force acting on the moving blades of a reaction turbine is the vector sum of the centrifugal and the reaction forces. This type of turbine is called reaction turbine.

Pressure Velocity Changes in Reaction Turbine Both the fixed and moving blades are designed in the shape of the nozzles. Therefore the expansion of the steam takes place both in the fixed and moving blades. The fixed blade ring between the moving blade rotors enables to deflect and guide the steam to enter from one row of moving blades to the next row. The high pressure steam passing in the first row of fixed blades undergoes a small drop in pressure causing the increase in the velocity of the steam. It then enters the first row of moving blades where it suffers further drop in pressure and the velocity energy is converted into the mechanical energy of rotation of the rotor. Thus the velocity of the steam decreases. This continues in the further rows of moving and fixed blades till the pressure of the steam is almost completely reduced.

Compounding of steam turbines Compounding of steam turbines is the method in which energy from the steam is extracted in a number of stages rather than a single stage in a turbine. A compounded steam turbine has multiple stages i.e. it has more than one set of nozzles and rotors, in series, keyed to the shaft or fixed to the casing, so that either the steam pressure or the jet velocity is absorbed by the turbine in number of stages.nozzlesrotors

Why is it required?  The steam produced in the boiler has very high enthalpy. In all turbines the blade velocity is directly proportional to the velocity of the steam passing over the blade.  Now, if the entire energy of the steam is extracted in one stage, i.e. if the steam is expanded from the boiler pressure to the condenser pressure in a single stage, then its velocity will be very high.  Hence the velocity of the rotor (to which the blades are keyed) can reach to about 30,000 rpm, which is pretty high for practical uses because of very high vibration. Moreover at such high speeds the centrifugal forces are immense, which can damage the structure.  Hence, compounding is needed. The high velocity which is used for impulse turbine just strikes on single ring of rotor that cause wastage of steam ranges 10% to 12%. To overcome the wastage of steam compounding of steam turbine is used.

Types of compounding In an Impulse steam turbine compounding can be achieved in the following three ways: - 1. Velocity compounding 2. Pressure compounding 3. Pressure-Velocity Compounding In a Reaction turbine compounding can be achieved only by Pressure compounding.

Velocity compounding of Impulse Turbine

 This high velocity steam is directed on to the first set (ring) of moving blades.  As the steam flows over the blades, due the shape of the blades, it imparts some of its momentum to the blades and loses some velocity.  Only a part of the high kinetic energy is absorbed by these blades. The remainder is exhausted on to the next ring of fixed blade.  The function of the fixed blades is to redirect the steam leaving from the first ring of moving blades to the second ring of moving blades.  There is no change in the velocity of the steam as it passes through the fixed blades.  The steam then enters the next ring of moving blades; this process is repeated until practically all the energy of the steam has been absorbed. As discussed earlier the entire pressure drop occurs in the nozzle, and there are no subsequent pressure losses in any of the following stages. Velocity drop occurs in the moving blades and not in fixed blades.

Pressure compounding of Impulse Turbine

 It consists of alternate rings of nozzles and turbine blades. The nozzles are fitted to the casing and the blades are keyed to the turbine shaft.  In this type of compounding the steam is expanded in a number of stages, instead of just one (nozzle) in the velocity compounding.  It is done by the fixed blades which act as nozzles. The steam expands equally in all rows of fixed blade.  The steam coming from the boiler is fed to the first set of fixed blades i.e. the nozzle ring.  The steam is partially expanded in the nozzle ring. Hence, there is a partial decrease in pressure of the incoming steam.  This leads to an increase in the velocity of the steam. Therefore the pressure decreases and velocity increases partially in the nozzle.

Pressure-Velocity compounded Impulse Turbine

 It is a combination of the above two types of compounding.  The total pressure drop of the steam is divided into a number of stages. Each stage consists of rings of fixed and moving blades.  Each set of rings of moving blades is separated by a single ring of fixed blades.  In each stage there is one ring of fixed blades and 3-4 rings of moving blades.  Each stage acts as a velocity compounded impulse turbine.  The fixed blades act as nozzles. The steam coming from the boiler is passed to the first ring of fixed blades, where it gets partially expanded.  The pressure partially decreases and the velocity rises correspondingly.  The velocity is absorbed by the following rings of moving blades until it reaches the next ring of fixed blades and the whole process is repeated once again.

Pressure compounding of Reaction Turbine

 As explained earlier a reaction turbine is one which there is pressure and velocity loss in the moving blades.  The moving blades have a converging steam nozzle. Hence when the steam passes over the fixed blades, it expands with decrease in steam pressure and increase in kinetic energy.  This type of turbine has a number of rings of moving blades attached to the rotor and an equal number of fixed blades attached to the casing.  In this type of turbine the pressure drops take place in a number of stages.  The steam passes over a series of alternate fixed and moving blades.  The fixed blades act as nozzles i.e. they change the direction of the steam and also expand it.  Then steam is passed on the moving blades, which further expand the steam and also absorb its velocity.

Difference between Reaction turbine and Impulse turbine

IC Engines

Introduction Internal-combustion engine, one in which combustion of the fuel takes place in a confined space, producing expanding gases that are used directly to provide mechanical power.

Classification of IC Engines Classification Based on -No. of Cylinders. -Position of Cylinders. -Type of Coolant used. -Type of Fuel used. -Mode of Ignition. -No. of Strokes. -Type of Thermodynamic Cycle. CCCFISTCCCFIST

Based on No. of Cylinders. 1.Single Cylindered engine. 2.Multi Cylindered engine.

Based on Position of Cylinders. 1.Horizontal Engines. 2.Vertical Engines. 3.Inclined Engines. 4.V engines. 5.Opposed type Cylinder Engines. 6.Radial Engines.

Based on Type of Coolant used. 1.Air Cooled Engine. 2.Water Cooled Engine. 3.Oil Cooled Engine.

Based on Type of Fuel used. 1.Petrol Engine. 2.Diesel Engine. 3.Gas Engine. 4.Bi-fuel Engine.

Based on Mode of Ignition. 1.Spark Ignition Engine. 2.Compression Ignition Engine. Based on No of Strokes. 1.4-Stroke Engine. 2.2-Stroke Engine.

Based on Thermodynamic Cycle. 1.Otto Cycle Engine. 2.Diesel Cycle Engine. 3.Dual Combustion Cycle Engine.

Parts Of IC Engine.

Four-stroke Engine A four-stroke engine is an Internal combustion engine in which the piston completes four separate strokes which comprise a single thermodynamic cycle. A stroke refers to the full travel of the piston along the cylinder, in either direction.

Working Principle of 4-stroke Diesel Engine The Diesel engines work on the principle of theoretical diesel cycle, also known as constant pressure heat addition cycle.

Working Principle of 4-stroke Diesel Engine

The Petrol engines work on the principle of theoretical Otto cycle, also known as constant volume cycle. Working Principle of 4-stroke Petrol Engine

1.INTAKE: this stroke of the piston begins at top dead center. The piston descends from the top of the cylinder to the bottom of the cylinder, increasing the volume of the cylinder. A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port.

2. COMPRESSION: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air or fuel-air mixture into the cylinder head.

3. POWER: this is the start of the second revolution of the cycle. While the piston is close to Top Dead Centre, the compressed air–fuel mixture in a gasoline engine is ignited, by a spark plug in gasoline engines, or which ignites due to the heat generated by compression in a diesel engine. The resulting pressure from the combustion of the compressed fuel- air mixture forces the piston back down toward bottom dead centre.

4. EXHAUST: during the exhaust stroke, the piston once again returns to top dead centre while the exhaust valve is open. This action expels the spent fuel-air mixture through the exhaust valve(s).

Two-stroke Engine  Two-stroke internal combustion engines are more simple mechanically than four-stroke engines, but more complex in thermodynamic and aerodynamic processes.  In a two-stroke engine, the four "cycles" of internal combustion engine theory (intake, compression, ignition, exhaust) occur in one revolution, while in a four-stroke engine it occurs in two complete revolutions.  In a two-stroke engine, more than one function occurs at any given time during the engine's operation.

Working of 2-stroke Diesel Engine

 Intake begins when the piston is near the bottom dead center. Air is admitted to the cylinder through ports in the cylinder wall (there are no intake valves).  All two-stroke Diesel engines require artificial aspiration to operate, and will either use a mechanically-driven blower or a hybrid turbo-supercharger to charge the cylinder with air.  In the early phase of intake, the air charge is also used to force out any remaining combustion gases from the preceding power stroke, a process referred to as scavenging.

Working of 2-stroke Diesel Engine  As the piston rises, the intake charge of air is compressed. Near top dead center, fuel is injected, resulting in combustion due to the extremely high pressure and heat created by compression, which drives the piston downward.  As the piston moves downward in the cylinder it will reach a point where the exhaust port is opened to expel the high-pressure combustion gasses.  However, most current two-stroke diesel engines use top-mounted poppet valves and uniflow scavenging. Continued downward movement of the piston will expose the air intake ports in the cylinder wall, and the cycle will start again.

Working of 2-stroke Petrol Engine

Intake. The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston through the reed valve.

Compression. The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston). Working of 2-stroke Petrol Engine

Power. At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward. Working of 2-stroke Petrol Engine

Exhaust/Transfer: Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Working of 2-stroke Petrol Engine

Comparison between 4-stroke and 2-stroke I.C engines

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Comparison between 4-stroke and 2-stroke I.C engines L

Comparison between 4-stroke and 2-stroke I.C engines 14 15

Comparison between Petrol and Diesel engines

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