TURBINES •STEAM TURBINES

•GAS TURBINES

•WATER TURBINES

PRIME MOVERS

Prime movers are self moving devices which convert the available natural soured of energy to mechanical energy to drive other machines.

• Steam Turbines

• Gas turbines

• IC engines

Steam Turbine

• It is type of Thermal prime mover.

• Heat energy of steam is converted to mechanical energy.

• They are used in thermal power plants for driving the electric generators, textile & sugar industries.

• This equipment is adiabatic since they are un-cooled.

Classification of Steam Turbines

BASIS TYPES

TYPE OF EXPANSION •IMPULSE TURBINE •REACTION TURBINE •COMBINATION TURBINE

NUMBER OF STAGES •SINGLE STAGE TURBINE •MULTISTAGE TURBINE

TYPE OF STEAM FLOW •AXIAL FLOW TURBINE •RADIAL FLOW TURBINE

STEAM PRESSURE •LOW PRESURE TURBINES •MEDIUM PRESSURE TURBINES •HIGH PRESSURE TURBINES •MIXED PRESSURE TURBINES

EXIT PRESSURE •CONDENSING TURBINES •NON – CONDENSING TURBINES

Expansion of steam in nozzle High velocity jet of steam is produced by

expanding the high pressure steam in convergent-divergent nozzle as shown in fig. below. Due to pressure expansion enthalpy of steam is reduced, as there is no external work and heat transfer in the nozzle –”the loss in enthalpy should be equal to gain in velocity of the steam.”

Impulse & Reaction turbines

Impulse Turbine Reaction Turbine

Steam expands from high pressure to low pressure in the nozzle before entering to the blades.

High pressure steam continuously expands successively in both fixed & moving blades.

Steam expansion does not occur while passing in moving blades due to symmetrical profile of blades.

Due to asymmetrical profile steam expansion occurs when it flows between moving & fixing blades.

Steam pressure is same at both ends of the blade.

Steam pressure is different at both ends of the blade.

Pressure drop is high ,hence higher speeds can be obtained.

Pressure drop is low ,hence lesser speeds can be obtained.

Occupies less space per unit power Occupies more space per unit power

Used for small power generation Used for high power generation

Compounding is needed due to high rotor speeds

Since speeds are moderate compounding is not needed

Delaval’s Turbine

The working of the De Laval turbine is as follows: The steam is blown through stationary divergent nozzles where it is allowed to expand to the pressure of the exhaust chamber. Each particle of steam, which moves very rapidly, strikes against a concave vane or plate which projects from the drum like a spoke. This causes the wheel to move rapidly. The outer end of the buckets are covered by a ring which prevents the centrifugal escape of the steam. The nozzles vary in number and can be closed independently of each other, so that the number In use may be made to suit conditions of running. As the material composing the turbine machine limits the speed at which it can safely be run, it is necessary to have some form of reducing gear in the transmission. The smaller types of De Laval turbines run at about 30,000 R. P. M., and are geared down to about 3000. The larger sizes run at about 10,000 R. P. M. under gear. Even with all the disadvantages of gearing, the turbine is used extensively in units ranging from 1 1/2 to 200 H. P. Its principal parts are the shaft, drum, cylindrical case inside of which the drum revolves, vanes on the drum and cylindrical part, balance pistons.

Parson’s Turbine

This is example for reaction turbine. It consists of wheel with casing which gradually increase with the cross section. This is done to allow steam for expansion. A ring of blades attached to the wheel are called moving blades & they are shaped in a direction such that they provide a nozzle effect. The fixed blades sizes also increases with the direction of the steam flow. The steam enters & passes alternatively on fixed & moving blades allowing the steam to expand gradually, as a result reaction force is set up which helps in turbines rotation.

Compounding of 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.

The steam produced in the boiler has got 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. Moreover at such high speeds the centrifugal forces are immense, which can damage the structure. Hence, compounding is needed

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 (Curtis Impulse Turbine)

Velocity Compounding (Curtis Impulse Turbine)

• The velocity compounded Impulse turbine was first proposed by C G Curtis to solve the problem of single stage Impulse turbine for use of high pressure and temperature steam.

• The rings of moving blades are separated by rings of fixed blades. The moving blades are keyed to the turbine shaft and the fixed blades are fixed to the casing. The high pressure steam coming from the boiler is expanded in the nozzle first. The Nozzle converts the pressure energy of the steam into kinetic energy. It is interesting to note that the total enthalpy drop and hence the pressure drop occurs in the nozzle. Hence, the pressure thereafter remains constant.

• 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 losses 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 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.

• A schematic diagram of the Curtis stage impulse turbine, with two rings of moving blades one ring of fixed blades is shown in figure . The figure also shows the changes in the pressure and the absolute steam velocity as it passes through the stages.

where, Pi = pressure of steam at inlet Vi = velocity of steam at inlet

Po = pressure of steam at outlet Vo = velocity of steam at outlet

• In the above figure there are two rings of moving blades separated by a single of ring of fixed blades. 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 (Rateau's Impulse Turbine)

Pressure Compounding (Rateau's Impulse Turbine)

The pressure compounded Impulse turbine is also called as Rateau turbine, after its inventor.

• 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.

• This is then passed over the set of moving blades. As the steam flows over the moving blades nearly all its velocity is absorbed. However, the pressure remains constant during this process. After this it is passed into the nozzle ring and is again partially expanded. Then it is fed into the next set of moving blades, and this process is repeated until the condenser pressure is reached.

Pressure-Velocity compounding (Curtis & Moore Impulse Turbine)

(Combined Impulse Turbine)

Pressure-Velocity compounding (Curtis & Moore Impulse Turbine)

(Combined 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.

Advantages of steam turbines over other prime movers

• Thermal efficiency is high & more durable. • Heavy foundation is not needed. • Used for driving high speed machines such as

generators, gas compressors, etc. • Propelling force is directly applied on the rotating

element. • Used for constant speed operations like textile

industries. • They are used in thermal power plants as they can take

up sudden overloads with negligible loss in efficiency. • Can generate powers ranging from few KW to 1000KW.

Steam turbine Construction

1 – steam pipeline

2 – inlet control valve

3 – nozzle chamber

4 – nozzle-box

5 – outlet

6 – stator

7 – blade carrier

8 – casing

9 – rotor disc

10 – rotor

11 – journal bearing

13 – thrust bearing

14 – generator rotor

15 – coupling

16 – labyrinth packing

19 – steam bleeding (extraction)

21 – bearing pedestal

22 – safety governor

23 – main oil pump

24 – centrifugal governor

25 – turning gear

29 – control stage impulse blading

Construction of steam turbines

Construction of steam turbines

Introduction to Gas Turbines • The first gas turbine was designed and manufactured in

England by Stolze in 1872. In United States, Charles G. Curtis was the first to patent and develop a gas turbine in 1914. World war – 1 and world war – 2 have given a great boost for the development of the gas turbine. Frank whittle was a first scientist patented the design of the gas turbine aircraft engine. Gas turbine is used in wide range of applications like, aircraft, industrial, ship and power generation plants.

• In the gas turbine plant, the atmospheric air is drawn and is compressed to a high pressure, the fuel is injected to the compressed air, the fuel burns and the energy is released, the energy is utilized to rotate a turbine. The heat transfer to the working fluid may be through direct contact or through indirect heating without any change in the composition of the working fluid.

Gas Turbines Gas Turbine is also a prime mover. A jet of burnt gases

& air is made to flow through the blades of turbines which are interconnected to the rotor. The shape of the blades are similar to that of steam turbine.

Applications • Generating electricity. • Steel , oil & chemical industries. • Rockets , missiles , ship propulsions Classification • Constant pressure open cycle gas turbine • Constant pressure closed cycle gas turbine

Open Cycle Gas Turbine

Open Cycle Gas Turbine

• Combustion chamber is connected between the compressor & turbine as shown in sketch.

• Compressor, generator & turbine are coupled co-axially. • Atmospheric air is drawn into compressor to increase its

pressure. The compressed air is delivered to combustion chamber where fuel ( kerosene, coal, coal-gas, gasoline) is injected into it at constant pressure. Heat is obtained by burning the fuel at constant pressure.

• The products of combustion that are obtained at high temperature & pressure are made to flow into the turbine. There the elements expands & drives the turbines.

• Then the resultant gases are let out to the atmosphere through exhaust.

Closed Cycle Gas Turbine

Closed Cycle Gas Turbine

• Heat exchanger is connected between the compressor & turbine as shown in sketch.

• Compressor, generator & turbine are coupled co-axially. • Here atmospheric air or stable gas like nitrogen, CO2, helium

are used. • Atmospheric air is drawn into compressor for increasing its

pressure. This air is passed to heat exchanger for preheating at constant pressure. This results in the gas to be in high pressure & high temperature state.

• This high pressure gas is allowed to expand in turbine which in- turn rotates the blades of turbine & thus energy can be derived from it.

• The resultant exhaust gases(low pressure & temperature) are passed through the heat exchanger (cooling) where they can be reused & continuous cycle is obtained.

Differences b/w open & closed cycle gas turbines

Closed cycle Open cycle

Working substance is continuously re-circulated.

Working substance is continuously replaced in every cycle.

Exhaust gases are reused. Exhaust gases are let into atmosphere

Any fluid is used as working substance

Working substance comprises of air & fuel mixture.

No loss of working substance Fresh air is drawn in every cycle.

Large amounts of cooling water is needed

No requirement of cooling.

Thermal efficiency is high. Thermal efficiency is lesser.

Smaller compressor needed Big compressor needed

Maintenance cost is high Maintenance cost is low

Water Turbines

• Water turbines were developed in the nineteenth century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. They harness a clean and renewable energy source.

• A hydraulic turbine is a machine, which converts pressure energy in to mechanical energy. It uses the kinetic energy end potential energy of water and sets the rotor in motion by the dynamic action of water flowing from high head.

Classification of Hydraulic Turbines The classification of water turbines are as follows

1. According to the type of energy at inlet a) Impulse turbine. b) Reaction turbine.

2. According to the direction of flow of water through the runner a) Tangential flow. b) Radial inward flow. c) Radial outward flow. c) Axial flow. d) Mixed flow.

3. According to the head under which turbine works a) High head, Impulse turbine Ex: pelton wheel. b) Medium head, reaction turbine Ex: Francis turbine. c) Low head, reaction turbine: Ex: kaplan turbine. 4. According to the specific speed of the turbine a) Low specific speed turbine, impulse turbine. Ex: pelton wheel. b) Medium specific speed, reaction turbine. Ex: Francis turbine. c) High specific speed, reaction turbine. Ex: Kaplan turbine.

5. According to the position of the shaft a) Horizontal shaft. b) Vertical shaft.

Impulse water turbines In impulse turbine a high-velocity jet of water

hits a series of specially shaped cups on the runner. Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which reverse the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy. Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.

Pelton Wheel

Pelton wheel, a type of impulse turbine, named after L. A. Pelton who invented it in 1880. Water passes through nozzles and strikes cups arranged on the periphery of a runner, or wheel, which causes the runner to rotate, producing mechanical energy. The runner is fixed on a shaft, and the rotational motion of the turbine is transmitted by the shaft to a generator. Pelton turbines are suited to high head, low flow applications; they are used in storage power stations (dams) with downward gradients above 300 meters

Reaction Water Turbine Reaction turbine needs a low head &

high flow rate. Only some part of the pressure energy of water is converted into kinetic energy. First the water passes through the guide blades & then to the moving blades which are mounted on the turbine wheel. The function of guide blades is to deflect or guide the water into the moving blades. The water leaving the moving blades are at low pressure. This difference in pressure at entry & exit of moving blades sets up the turbine wheel to rotate in opposite direction.

Kaplan turbine

Kaplan turbine The Kaplan turbine is an inward flow reaction turbine,

which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. The design combines radial and axial features. The inlet is a scroll-shaped tube that wraps around the turbine's wicket gate (guide vanes). Water is directed tangentially, through the guide vanes, and spirals on to a propeller shaped runner, causing it to spin. The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy. The turbine does not need to be at the lowest point of water flow, as long as the draft tube remains full of water. Variable geometry of the guide vanes and turbine blades allow efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.

Francis turbine

Francis turbine • It is a reaction turbine working under medium head handling

medium quantity of water. Francis turbines can either be volute-cased or open-flume machines. The spiral casing is tapered to distribute water uniformly around the entire perimeter of the runner and the guide vanes feed the water into the runner at the correct angle. Thus, water possessing pressure and kinetic energy enters the runner vanes in the radial direction and leaves in the axial direction. The runner blades are profiled in a complex manner and direct the water so that it exits axially from center of the runner. In doing so the water imparts most of its pressure energy to the runner before leaving the turbine via a draft tube.

• The Francis turbine is generally fitted with adjustable guide vanes. These regulate the water flow as it enters the runner and are usually linked to a governing system which matches flow to turbine loading in the same way as a spear valve or deflector plate in a Pelton turbine. When the flow is reduced the efficiency of the turbine falls away.