Steam power cycle

[Saad85]

Hi ,

We have a steam power cycle ( Rankine) if we increase pressure of steam before entering a steam turbine what happened to the following:

1- cycle efficiency.

2- volumatiric flow rate of steam ( drcreased or increased)

3- mass flow rate ( decresed or increased )

4- load on condenser ( Decreased or increased)

thanks,

 

[Sheshtawy]

Excellent question, it can be kinda tricky but the T-S diagram will help immensely.

1. Cycle efficiency:
Assuming both the turbine inlet temperature and the condenser's pressure stays constant, the inlet enthalpy will decrease but so will the outlet enthalpy. In general, the work gain is more than the work loss (h3' - h4' > h3 - h4) and therfore the overall cycle efficiency increases. 

2. Condenser Load:
Assuming the same above parameters stay constant, the turbine outlet enthalpy decreases, the condenser's load will also decrease. The T-S diagram will help you visualize this.

3. Volumetric flow rate:
I want to say that the volumetric flow rate stays the same if the temperature stays constant as the velocity of the steam won't change (Q= A * v) but I'm not 100% sure.

4. Mass flow rate:
Assuming constant volumetric flow rate, at higher pressure and constant temperatures, the density of steam increases (specific volume decreases) and therefore the mass flow rate increases as well (mass flow rate = density * Q)

Finally, here's a handy tool that might help as well. Play around with the different parameters to see the effect on the cycle.
https://www5.eere.energy.gov/manufacturing/tech_deployment/amo_steam_tool/equipTurbine

 

[justin-hawaii]

I believe conservation of mass still applies to an ideal Rankine cycle, so your mass flow rate should remain constant.  

 

[Sheshtawy]

You know what, I believe it does too. In that case i think the volumetric flow rate will have to decrease to make up for the increase in density.

 

[Saad85]

Thanks all so the volumatiric flow rate will be decreased and mass flow rate will remain constant 

 

[Sheshtawy]

Yup!