Reducing the turbine outlet pressure means reducing the condenser pressure. The average temperature decreases by decreasing the pressure. However, the average temperature must be always greater than the cooling water temperature in the condenser.

Hello Daniela Florez Martinez

First of all, I have attached a T-S diagram of a Rankine Cycle for two different condenser inlet pressure or turbine outlet pressure. The processes 3-4 & 3-4' are the expansion processes happening in the turbine. The processes 4-1 & 4-1' are the heat rejection processes in the condenser. From the figure, you see that the temperature of the working fluid going into the condenser is low for the process 3-4' when compared to the process 3-4. In terms of pressure, the pressure of the working fluid at 4' is less than the pressure at 4. So, reducing the condenser pressure reduces the heat rejection in the condenser. I also agree with Giuma Fellah's comment that there is a limit on the condenser pressure. If the pressure (temperature) is below the pressure (temperature) of the coolant, there will be a heat addition to the working fluid rather than heat rejection.

According to your other options,

i. Increasing the boiler pressure (point 2'): The boiler is linked to the heat addition process. Increasing the boiler pressures reduces the external heat added to the system.

ii. Increasing the turbine inlet pressure (point 3): It allows for the increase in the amount of external work extracted from the turbine.

iii. Increasing the condenser pressure (point 4): This condition results in the reduction of work extracted from the turbine.

Hope this answer helps!

Good evening @ Daniela,

I don't speak Spanish, so I'm forced to reply in English. In a power station heat is added to raise steam. In a conventional situation, the coal/oil/gas is combusted in a boiler to turn water into steam. In a nuclear power station, a circulating fluid (high pressure water in a PWR, gas in an AGR) goes through the reactor and gets hot. It then passes through a heat exchanger where it raises steam. In all the above designs, this steam is taken to a steam turbine which extracts its energy by lowering the steam pressure and temperature: the cooled steam, usually slightly wet, goes to a condenser. Here the steam is condensed to water and the water is pumped back to the boiler. Most power stations operate on a modified version of the above on an idealised Rankine cycle. The Rankine cycle, like the Carnot cycle, has maximum efficiency when the flow to the turbine is at maximum temperature and leaves at minimum temperature. However steam/water mixture means that you are handling two phase fluid in the case of the condenser hence if you remember you chemistry, F+P=C+2, and for much of the cycle, defining one parameter (e.g. pressure) defines all the other parameters (e,g. temperature, enthalpy, etc.).

Now to answer your questions. Increasing the boiler pressure and/or temperature (most boilers produce superheated steam, hence single phase) gives more energy to the steam and this higher energy can be extracted by the turbine. The outlet of the boiler goes straight to the turbine so this answers (a) and (b). In the condenser we are turning the exit steam from the turbine back into water and we know from our cycle theory that low temperature is our aim thus for our two phase situation, low pressure is our aim. Thus step (c) is wrong. You need as low a pressure in the condenser as you can realistically achieve. In many power stations the turbine sits directly above the condenser, thus reducing the condenser pressure means that you reduce the turbine outlet pressure which implies that (d) is correct. You really need to understand thermodynamic cycle theory. One book in English is "Engineering Thermodynamics, Work and Heat Transfer" by GFC Rogers and YR Mayhew (Longmans, UK). A bit dated now, but very good.

Hope that the above is not too long. Regards, Stephen.