Mohammad Faisal Haider Lecturer Department of Mechanical Engineering Bangladesh University of Engineering and Technology
Steam Turbine 2
Vapor Power Cycle
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Steam Turbine A steam turbine is prime mover which uses steam as its working fluid It operates by performing two functions : a part or whole of the pressure energy of steam is transformed into kinetic energy by means of expansion through suitable passages such as nozzles the kinetic energy and the remaining portion of the pressure energy of steam, if any, are converted into mechanical work with the help of moving blades fitted on the wheel 6
Steam Turbine Steam turbines are steady flow devices where steam enters thru nozzles, expands to lower pressure and in doing so develops high velocity i.e. high kinetic energy. Part of this kinetic energy of the jet can be used in the same manner as water jet is used in water wheel. This kind of turbine is called impulse turbine. 7
Turbine Detail Main components Nozzles Blades or buckets Wheel or rotor Casing or cylinder Diaphragms Glands The blades, also called moving blades are fitted over the circumference of the wheel which is again mounted over a shaft. The wheel is covered with a casing. The nozzles and fixed blades are fitted with casing 8
Nozzle A nozzle is a steady flow device and is nothing but a passage of varying cross section for the flow of steam in order to increase its velocity by expansion with degrease of pressure. Its main function is to convert the available enthalpy intokineticenergybyproducingajetofsteamata high velocity. The section of a nozzle may be round, square, or rectangular. They are used in impulse turbines and fitted with the casing or with diaphragms. 9
Nozzle In designing an ideal nozzle for constant entropy expansion, the cross sectional area at any point n may be computed by use of continuity equation A n = m v n /V n When the flow is adiabatic and frictionless, the entropy of the steam of any point in the nozzle is equal the initial entropy. It is necessary to choose the throat area so that the desired amount of steam may flow with the desired pressure drop. At the beginning i of the flow in the nozzle, the steam velocity increase is rapid, although the corresponding volume increases at a lesser rate. m is const and V/v=m /A 10
Nozzle It is apparent that A must decrease until the flow has reached that section where the rate of increase in volume is equal to the rate of increase of in velocity. Here V/v is maximum and A is minimum. In divergent section, divergence angle is approximately 6 degrees from the centerline. The length of the nozzle is not critical and may be proportional on the basis of throat area by the relation Length of the nozzle from throat to exit, L = (15 A 0 ) Efficiency does no depend on the shape of the cross section 11
Nozzle Specific volume, υ υ, V, A Inlet Nozzle Length Exit 12
Flow characteristic of a Nozzle Citi Critical pressure for wet steam, P c =P 2 /P 1 = 0.58 For super heated steam, P c=0.54 For air and other gas, P c =0.53 Flow rate 0 1 Pressure e ratio, P 2 2/ /P 1 13
Blades Turbine blades also called buckets may be classified according to its shape as impulse blades and reaction blades. The blades of both the groups may be of moving type or of stationary type. Moving blades are fixed on the rim of the wheel or rotor and stationary or guide or fixed blades ate fitted with ih the casing. 14
Wheel and Diaphragms A turbine wheel in its simplest form is like a flat disc mounted on a shaft. It is also called the disc. Moving blades are fitted over the rim of the wheel in the form of a ring. The rotor consists of several discs. The diaphragm is in the form of a disc which h is fitted inside the cylinder. It serves the purpose of separating walls between the different stages of the turbine and carries nozzles and fixed blades. It must be strong enough to withstand the high temperature and the pressure difference of working fluid. 15
Glands Glands are required to prevent (a) the leakage of working fluid from the cylinder to the outside if itspressure isabove theatmosphere, (b) the leakage of air from outside to the cylinder if the inside pressure is less than the atmosphere, (c) the leakage of working fluid from one stage to the other. Glands are fitted in the place where the shaft enters into the cylinder and the passage between the rotor and the diaphragms. 16
Classification Depending on the types of blades and methods of energy transfer from fluid to rotor wheel, turbines may be of two types: Impulse Turbine Reaction Turbine 17
Impulse and Reaction Principles 18
Impulse Turbine In an impulse turbine, the steam taken from the boiler first comes to the steam chest and then it passes through nozzles and impacts on the moving blades. Due to the impulse of steam over the moving blades, the wheel rotates and so the power is available from the shaft. As the steam expands through the nozzle, the velocity and the volume of steam are increased with decrease in pressure 19
Pi Principle i of Impulse Turbine 20
Impulse Turbine 21
Impulse Turbine If all the pressure is dropped in one stage, the rotor speed is very large 10,000 to 30,000. If ideal condition can be provided, i.e. all kinetic energy can be converted to the rotor, then speed may be 20,000 to 40,000 rpm. Such high speed is required to be reduced by gearing of under proportion. So more than one staging is done in impulse turbine namely either pressure staging (Rateau staging) or velocity staging (Curtis staging). 22
Pressure Staging (Rateau) In the nozzles, only a small pressure drop is provided giving limited increase of kinetic energy Rotating rows of blades and fixed rows of blades are being keyed to the shaft hf in series 23
Pressure Staging (Rateau) The rotating blades have the typical symmetrical shape of impulse turbine. The fixed blades will not only change the direction of steam, but will increase the speed also. So, they are in fact nozzles. Since pressure gradually goes down, volume will increase and therefore the blade height has to be increased towards the low pressure side. 24
Velocity Staging (Curtis) Like a single stage impulse turbine, velocity compounding allows a larger pressure drop in one set of nozzles. According to the velocity with which the steam issues from the nozzle, two or more rings of moving blades separated by rings of fixed blades are keyed in series on the common shaft. 25
Velocity Staging (Curtis) A fall in velocity occurs every time when the steam passes over rings of moving blades. Since the rows of blades are connected to the same shaft, there is no loss of power. 26
Reaction Turbine Constructed t of rows of fixed and moving blades Fixed blades act as nozzles Moving blades move as a result of change of momentum of steam andalsoasaresultof expansion. 27
Reaction Turbine In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. 28
Typical Rankine Cycle T s diagram of a typical Rankine cycle operating between pressures of 0.06bar and 50bar 29
Typical Rankine Cycle There are four processes in the Rankine cycle, these states are identified by number in the diagram to the right. Process 1 2: The working fluid is pumped from low to high h pressure, as the fluid is a liquid at this stage the pump requires little input energy. Process 2 3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor. The input energy required can be easily calculated using mollier diagram or h s chart or enthalpy entropy chart 30
Typical Rankine Cycle Process 3 4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur. The output in this process can be easily calculated l using the Enthalpy entropy chart Process 4 1: The wet vapor then enters a condenser where it is condensed at a constant pressure to become a saturated liquid. 31
Rankine ecycle with Superheateat The compression by the pump and the expansion in the turbine are not isentropic. This somewhat increases the power required by the pump and decreases the power generated by the turbine. In particular the efficiency of the steam turbine will be limited by water droplet formation. As the water condenses, water droplets hit the turbine blades at high speed causing pitting and erosion, gradually decreasing the life of turbine blades and efficiency of the turbine. 32
Rankine Cycle with Rh Reheat In this variation, two turbines work in series. The first accepts vapor from the boiler at high pressure. After the vapor has passed through the first turbine, it re enters e sthe boiler and is reheated before passing through a second, lower pressure turbine. 33
Rankine Cycle with Regeneration In regenerative Rankine cycle, after emerging from the condenser the working fluid is heated by steam tapped from the hot portion of the cycle. On the edaga diagram shown,,the fluid at 2 is mixed with the fluid at 4 (both at the same pressure) to end up with ih the saturated liquid at 7. This is called "direct contact heating". 34
Rankine Cycle with Regeneration The Regenerative Rankine cycle (with minor variants) is commonly used in real power stations. Another variation is where 'bleed steam' from between turbine stages is sent to feedwater heaters to preheat the water on its way from the condenser to the boiler. These heaters do not mix the input steam and condensate, function as an ordinary tubular heat exchanger, and are named "closed feedwater heaters". The regenerative features here effectively raise the nominal cycle heat input temperature, by reducing the addition of heat from the boiler/fuel source at the relatively low feedwater temperatures that would exist without regenerative feedwater heating. This improves the efficiency of the cycle, as more of the heat flow into the cycle occurs at higher temperature. 35