Steam Turbine Power, Eficiency and Steam Quality

[AndreChE]

Hi to you all

I'm facing some questions on steam quality and Mollier Diagram direct reading.

I want to calculate a steam turbine eficiency. The turbine is feed with HPI (High pressure) steam (105bar,468ºC). There is one extraction to HPII steam (43,5bar,364ºC) and the rest is condensed to Vacuum (0.112bara, 50,2ºC).

Both HPI and HPII are superheated so the enthalpies are easy to read but the condensed steam is not 100% dry (saturated) since at 0.112bara the saturation temperature is 48.07ºC and the steam is at 50.2ºC. My question is the steam quality calculation. How should I do it or read it on the Mollier diagram? I just want to calculate the enthalpy of the condensed steam (liquid and vapour). That's why I need the steam quality.

Thanks,
AndreChE

 

[25362]

If at the given pressure of 0.112 bar the saturation temperature is 48.07^oC and the actual operating temperature is 50.2^oC, you are saying the steam is superheated. To be in the wet steam range either the pressure or the temperature readings, or both, may be wrong. Can you confirm ?

 

[AndreChE]

Ok, you are right. Above the saturation temperature, it is always superheated, of course.

But I can't read the steam quality on the Mollier Diagram for example at 45 or 40ºC

 

[kmpillai]

Steam quality is only applicable if it is in between the saturated liquid and saturated vapour states. As per your data the steam is in the slightly in the super heated region.
Enthalpy of steam before condensation = Saturated steam enthalpy at 0.112 bara + Cp*Degree of superheat

where Degree of super heat is
T - Tsat = (50.2 - 48.07)
Cp = 1.87 kJ/kg K is for around 50 deg C water vapour

Heat removed from the steam in the condenser (if there is no sub cooling) = Latent heat of the steam at 0.112 bara + Cp*Degree of superheat
Now you can calculate the enthalpy of the condensed steam.

Regards,
KMPillai

Regards,
KMPillai

 

[25362]

I assume that because of the unavoidable presence of noncondensables the pressure in the surface condenser would always be a bit higher than the VP of the condensate leaving the condenser. True ?