CrazyEngineers Archive
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@Mdixon7795 • 11 Jul, 2010
Hi guys, I'm new to the forum and very glad I found it! I'm trying to clear up a few things on my understanding of vapor cycles, so if my questions sound like they don't make any sense.. please bare with me haha.

In reference to a system such as a steam power plant, I understand that the Rankine cycle is the most feasible cycle to consider. But, in looking in my text book, it's comparing the carnot vapor cycle to the rankine and I'm confused as to why the carnot cycle is constrained within the limits of the saturation curve. Obviously, thermal efficiency increases whenever you can increase the gap between Tmax and Tmin, but if Tmax is located outside saturation, as a superheated vapor.. you can't take this Tmax into consideration with the carnot vapor cycle, correct? In other words, I know I can calculate the thermal efficiency of a rev. carnot heat engine between a max temp and a min temp, but if these temperatures are outside of the saturation curve, can the carnot cycle still be applied? If the Carnot cycle IS only constrained within the saturation dome, why is this? Is it because isothermal conditions only exist within the saturation dome?

Again, I'm new to the forum and I appreciate all input. Also, if I have posted in the wrong area, please let me know and I will figure out how to move it.

@ISHAN TOPRE • 02 Mar, 2011 Well you can of course apply Carnot cycle.But it will be impossible for you to adjust the compressor every time.
@vignesh sp • 17 Mar, 2011 I cant understand could u please explain me.
@ISHAN TOPRE • 18 Mar, 2011 If you know about the Carnot cycle, then you will come to know that An ideal Carnot cycle is not possible if superheated steam is used because we cannot give the superheated steam at constant temperature. The water cannot be heated in a carnot cycle and the temperature increase should be obtained only in compression. While in a Rankine cycle we use superheated steam at constant pressure.

Also in Rankine cycle we have complete condensation of steam (i.e; water vapor) while in Carnot cycle we get partial condensation which is more of a problem

As a thumb rule always remember that the dryness fraction should not go below 0.85 else it will have adverse effect on rotor blades. 😀
@mech. royal • 23 Mar, 2011 in carnot there steam not come to its saturation temp.from turbine so we have to use compressor to pump the steam in boiler and it consume a lot of power and reduce its efficiency. but in rankine steam is taken to saturated temp.from turbine and we use pump in place of compressor , it consume less power as compare to compressor .... so we get more work output....
@ramrawat89 • 17 Aug, 2011 In case of Carnot Vapour Cycle, the process of isentropically compression of wet steam (mix. of saturated liquid + vapour) should take place in equilibrium condition i.e, there must be heat transfer from vapour to saturated water. But what actually happens ? ...During the compression process there is rise in temperature of water and vapour , as a result vapour gets superheated and eventually there is rise in entropy of the system. Hence the Process no longer remains Isentropic. So to achieve the isentropic compression of wet steam, we need to slow the process. But slowing the process cause the loss of heat to the surrounding which shows the process is no longer adiabatic.

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