 Let us now take up the discussion of steam turbines. This illustration shows supercritical rack and cycle with two stage reheat. The boiler pressure at state one is 300 bar 650 degree Celsius and the condenser pressure in this case is 0.1 bar which is usually the case in most steam power installations. It can be seen that there is one reheat between two and three and another one between four and five. You must be familiar with this cycle and you must have done calculations involving this cycle in your applied thermodynamics course. The most important or most significant aspect of the cycle insofar as steam turbines are concerned is the expansion ratio that the steam goes through from state one all the way up to say state six. Notice that the steam is expanded from a pressure of 300 bar to a pressure of 0.1 bar which means the expansion ratio in this case is 300 divided by 0.1 which is 3000. Now this is two orders of magnitude higher than the value of expansion ratio 40 that we saw for land-based gas turbine power plants. In fact the pressure ratio of 40 in the case of a land-based gas turbine power plant is the pressure ratio at which most modern plants operate. So that is the highest pressure ratio that you will see and so there the expansion ratio for the turbine is 40 whereas here the expansion ratio is 3000. In fact in the case of ultra supercritical Rankin cycle the higher high pressure or the highest pressure is 400 bars or so. So in such cases the expansion ratio is even higher of the order of 4000 or so. It may be recalled that in the case of land-based gas turbine power unit the turbine had only about a dozen or so stages whereas in the case of an installation like this the steam turbine as you can see has about three or four times that many stages. So here we have the high pressure turbine which corresponds to the expansion from a state one to state two or state two as shown here and this represents the intermediate pressure turbine which is three to four and this represents the low pressure turbine and in addition that could also be additional turbines as for the requirements in the installation. So you can see the number of stages here is significantly higher when compared to land-based gas turbine power plants. Again notice that it would not be practical or feasible to expand the steam in a single turbine from the highest pressure of 300 bar to the condenser pressure of 0.1 bar. So with that in mind that the expansion itself is now split into three stages as we see here HP stage and intermediate pressure stage and a low pressure stage. Even within each stage the expansion of the turbine usually is quite challenging because the pressure ratios can be quite high even in the stages that we see here. So special strategies are required to deal with the expansion in these stages. In addition to the differences that we just mentioned between steam turbines and gas turbines other points are also of interest. For instance in the case of the gas turbine the highest temperature in the cycle is around 1700 Kelvin whereas here the highest temperature is around 802 900 Kelvin or so. Although the highest temperature is less the highest pressure is significantly higher and since the duty cycle for such steam turbine power plants are close to 100% metallurgical considerations play as much of importance in the case of steam turbines as in as they did in the case of gas turbines. Now both land-based gas turbine power generating units as well as steam turbines usually operate at rpms of 3000 which corresponds to 50 hertz or an rpm of 3600 which corresponds to 60 hertz because that is determined by the supply frequency. The aero turbines that we saw or gas turbine units used for aircraft engines run at higher rpms they are in fact the high pressure turbine there runs at 10000 rpm or so. So these are some of the important distinctions and similarities between steam turbines and the turbine portion of a gas turbine power plant. So what we will look at now is how we handle or how the expansion in each one of the stage which itself has a very high expansion ratio. How such a situation is handled in practical installations. Several strategies are required in order to deal with this effectively or several strategies have been utilized in order to deal with this effectively. Let us see what they are. The first one is called velocity compounding. The term compounding refers to a stepwise change or incremental change of whichever is being compounded. So in this case velocity compounding refers to the fact that velocity change takes place in the turbine in an incremental or stepwise manner. Let us see what this means. So the pressure drop across each turbine normally takes place in this case takes place in a single nozzle preceding the turbine blade. So keep in mind that we are talking about strategies for any one of these turbine individual turbines. So when we say pressure drop it is pressure drop across the either the hp turbine or the ip turbine or the lp turbine not the pressure drop from here to here. So the entire pressure drop across the turbine takes place in a single nozzle as you can see here. So the variation of pressure here goes from the boiler pressure or steam chest pressure to the either condenser pressure or the pressure at the end of expansion in that particular turbine. So in this case the pressure drop entire pressure drop is converted to velocity and the resulting kinetic energy is then converted to work across many rotor blade passages. So the velocity initially increases to a maximum value across the nozzle and then it decreases in a stepwise fashion in each one of the rotor blade. Note that there is a certain drop here in the first rotor. It is almost a constant in the next stator passage because the stator only redirects the flow then there is again another drop in the next set of rotors and so on until it reached the final exit velocity. And as the velocity decreases you can see here that the steam also I am sorry the steam expands and consequently the blade passages also increase in size. So the cross sectional area also increases correspondically. Since the pressure remains constant across the rotor this is an impulse stage and notice that in this case the stator blades merely serve to change the flow direction. There is no pressure drop in the stator blades also and there is no velocity change in the stator blade passage also. So the stator blades merely serve to change the direction. So this is an impulse machine and because the velocity decreases in a stepwise manner this is called velocity compounding. The next design strategy that we see here is called the pressure compounding and here as the name suggests the pressure change from the inlet to the exit takes place in steps rather than in one single go as we saw here. So accordingly we see now multiple nozzles there is a set of nozzles here set of nozzles here set of nozzles here and so on. So the pressure drop across the entire turbine now takes place in a stepwise manner. So there is an initial decrease here in the first set of nozzles then it remains constant across the rotor suggesting that the rotor is an impulse rotor. Then there is another stepwise decrease in the next fixed nozzles again constant across the rotor and so on until we reach the exit pressure. Now you can also see that associated with each pressure decrease there is a corresponding increase in velocity and that the velocity remains constant in the the velocity decreases in the moving blade passage where the kinetic energy is converted to work and then again it increases as a result of expansion in the nozzle and again it decreases in the rotor as a result of conversion to work and so on until we reach the terminal velocity. Notice that here the increase in height of the blades is not quite as much as what we saw in the previous case because the decrease in pressure is gradual but after some point after or after some number of stages there is a considerable increase in the cross sectional area to accommodate the continuously expanding steam and we notice that here so the cross sectional area increases here and the rotor passage area also increases here. So the important points are that the pressure drop rather than taking place in one single step takes place in many steps here like this and the velocity increases in the nozzle passage then decreases in the rotor passage and repeats itself like this until we reach the exit velocity. Since the pressure decrease takes place in steps this is called pressure compounding. The next strategy that is also used is pressure and velocity compounding. So what is done here is to have velocity change also stepwise and pressure change also takes place in a stepwise manner that is what is done here. Let us see how this is done. So basically what is done here is we go with this type of an expansion but instead of having one nozzle we have maybe two or three no more than that not as many as what we are seeing here but the pressure drop takes place across two or three nozzle sets and then in between the nozzle sets the velocity is compounded. So let us take a look. So as we can see from here we have one set of nozzle here another set of nozzle here so instead of the pressure drop taking place across one nozzle it now takes place across a finite number of nozzles so hence this is pressure compounding. Notice that the pressure decreases in a stepwise manner from this value to this value in the first set of nozzles remains constant until the steam reaches the second set of nozzles at which point it again drops in a stepwise manner to a certain value because we can also have additional sets of nozzles who have further steps in this profile. Now in between the nozzles we can see that we are utilizing the velocity compounding strategy because as a result of the drop in this first nozzle the velocity increases then the velocity decreases in the first set of rotor blade where the kinetic energy is converted to work then it decreases again in the second set of rotor blade then it remains constant again increases after the drop in the second set of nozzles and again it decreases in a stepwise manner across the rotors as we reach the final exhaust velocity. So that is why this strategy is called velocity and pressure compounding pressure compounding because the pressure drop takes place in a finite number of steps not in as many steps as this but finite number of steps nevertheless and velocity compounding because the velocity changes in a stepwise fashion as the pressure remains constant across the rotor blade so this is also an impulse machine and the stator blades here merely serve to change the direction of the fluid. Notice that the velocity at the end of the nozzle or velocity anywhere in the turbine is the highest in this case because the entire pressure drop is converted to kinetic energy. So correspondingly the rpm of this design simple velocity compounding will be very very high. In fact it can be unacceptably high in this case. In the case of pressure compounding since the total pressure drop occurs across several rows of nozzles as we can see here the velocity never reaches as high a value as it does in the case of velocity compounding. So consequently the shaft speed and the rpm in the case of pressure compounding are considerably less than what they are in the case of velocity compounding. This is always better because the physical stresses are going to be less in this case than before. In this case now that pressure and velocity compounding also gives velocity maximum velocity values which are probably somewhere in between velocity pure velocity compounding or pure pressure compounding. Now the last variation in this is the reaction turbine. So here the pressure decrease is continuous from inlet to outlet. So the pressure decreases in the stator blade passage and the pressure decreases in the rotor blade passage also and it decreases continuously. So the degree of reaction here is non-zero and the velocity as we can see increases in the stator as the stator acts like a nozzle and then it decreases as the kinetic energy is converted to work in the rotor blade passage. Notice that the rotor blade passage also tries to or causes an increase in the relative velocity component. So because the pressure decreases in the rotor there is an increase in velocity component and the rotor blade passage acts like a nozzle but the overall kinetic energy still decreases because we are extracting work from the rotor. So the velocity increases in the stator blade passage decreases in the rotor blade passage again increases in the next stator blade passage due to the pressure drop decreases again in the next rotor blade blade passage as the kinetic energy is converted to work and this goes on continuously as we can see here. So the RPM of this design probably will be the lowest or as we can expect the RPM of this design to be the lowest among all the four designs that we looked at because the pressure drop is continuous and it's seen in both the stator and the rotor. Okay so this is a reaction design. So these are the four strategies that are used to handle the very high expansion ratios that are seen in steam turbines. Of course one would be correct in saying that this is no different from the turbine part of the gas turbine that we saw earlier and that would be true. So this resembles the turbine portion of the gas turbine very much and in fact they are identical in that sense but notice the number of stages that we see here is far more than the 10 stages or a dozen stages that we saw in the case of a gas turbine.