How Do You Calculate Enthalpy Using Steam Tables and Dryness Fraction?

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To calculate the enthalpy of steam at 0.4 MPa with a dryness fraction of 0.7, the total enthalpy is derived from the enthalpy of dry saturated steam and the enthalpy of water. The dry saturated steam enthalpy (hg) is 2,739 kJ/kg, leading to an enthalpy of 1,917.3 kJ for 0.7 kg of steam. The enthalpy of the remaining 0.3 kg of water is calculated as 181.44 kJ. The total heat for the wet steam is then 2,098.74 kJ/kg. The discussion also clarifies the formula for enthalpy, emphasizing the role of the dryness fraction in calculations.
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Homework Statement


Find the enthalpy of steam at 0.4 MPa and 0.7 dryness


Homework Equations





The Attempt at a Solution


At a pressure of 0.4MPa dry saturated steam has a total enthalpy of hg = 2,739 kj/kg.

∴ 0.7 kg of steam has an enthalpy of (0.7 x 2,739) = 1917.3 kj

The total heat of 0.3kg of water is 0.3hf (at 0.4MPa), so 0.3 x 604.8 = 181.44 kj

∴ Total heat for wet steam = 181.44 + 1917.3 kj/kg
= 2,098.74 kj/kg


This is my first time using steam tables so am struggling to know if I am gripping it or not. Any help you can offer in order for me to answer this question is VERY much appreciated.

Regards
 
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Yes, your answer is correct.

Your method is correct but it might take a little longer to do if you are in an exam. (the same formula with less lines would be)

h = hf+ xhfg= hf+x(hg-hf)

(Your steam table might have values for hfg sometimes)


Formula is the same for specific entropy and specific volume in that

s=sf+xsfg

v = vf+xvfg
 
Thank you rock.freak667
 
Hi rock.freak667,

Sorry, can you just clarify how my calculations would fit into that equation?

I can follow it through but I am unsure what your X stands for? Can you clarify.

hf + x (hg - hf)

∴ 604.8 + x (2739-604.8)

∴ 604.8 + x (2134.2)

Can you clarify for me?

thanks a lot
 
sperrya said:
Hi rock.freak667,

Sorry, can you just clarify how my calculations would fit into that equation?

I can follow it through but I am unsure what your X stands for? Can you clarify.

hf + x (hg - hf)

∴ 604.8 + x (2739-604.8)

∴ 604.8 + x (2134.2)

Can you clarify for me?

thanks a lot

x = dryness = 70% = 0.7.
 
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