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Steam Enthalpy Maximum?


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#1 2bsss

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Posted 08 December 2011 - 02:02 AM

I originally posted this in the technical forum (mechanical) since it isn't necessarily PE exam related but I'll post it here anyway. Looking at steam tables for saturated steam, it looks like the enthalpy (H) reaches a maximum at about 450 psia (as does the internal energy, U) and decreases as pressure/temperature increases at saturation after that. Thermodynamically, why is this and what does it mean?

#2 CbusPaul

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Posted 08 December 2011 - 01:14 PM

The enthalpy of what? the saturated liquid, saturated vapor? You will begin undergoing phase transitions which can be seen on a Mollier diagram of ln P vs. H. The enthalpy of the saturated liquid will continue to increase with increasing temp and pressure above 450 psia but the enthalpy of saturated vapor will decrease. I would review the phase transition lines on a Mollier diagram to get a more thorough understanding.

#3 2bsss

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Posted 10 December 2011 - 08:36 PM

The enthalpy of the saturated vapor reaches a maximum at about 450 psia. I am trying to understand why this is. How does saturated steam (vapor) at 1000 psia have less enthalpy than saturated steam at 450 psia?

#4 CbusPaul

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Posted 12 December 2011 - 03:37 PM

Does it have to do with the VLE properties of steam as it continues to be heated beyond the temp that corresponds to the enthalpy you're reading at 450 psia?




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