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Post by squaretaper LIT AF PE on Jun 27, 2022 15:25:08 GMT -5
Hey, first post in mechanical!
This is a plug for using the Mollier diagram first before diving into steam tables. Very helpful for power cycle questions and DEFINITELY helpful for anything with superheated steam.
Diving into the steam tables too quickly introduces too much risk for getting lost, referencing the wrong value under exam pressure, etc etc. Mollier first!
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Post by squaretaper LIT AF PE on Jul 1, 2022 16:24:44 GMT -5
Friendly reminder #1: STEAM IS NOT AN IDEAL GAS.
Calculations using the ideal gas law will not work. I mean...you'll get a number, but it'll be wrong. You have been warned!  |
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Post by squaretaper LIT AF PE on Jul 1, 2022 16:30:53 GMT -5
Friendly reminder #2 (for those who have been out of school for a while): the ideal gas law can only be used in ABSOLUTE terms (i.e. pressure in PSFA or °R if using USCS).
So be prepared to quickly and correctly convert to absolute and don't forget to check your units! Gotta bring the anal in dimensional analysis.
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Post by slaythepe on Jan 3, 2023 19:04:09 GMT -5
Friendly reminder #1: STEAM IS NOT AN IDEAL GAS.
Calculations using the ideal gas law will not work. I mean...you'll get a number, but it'll be wrong. You have been warned! I am Slay the PE and I approve this message  |
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Post by slaythepe on Jan 3, 2023 19:26:05 GMT -5
Friendly reminder #1: STEAM IS NOT AN IDEAL GAS.
Calculations using the ideal gas law will not work. I mean...you'll get a number, but it'll be wrong. You have been warned! ACKCHYUALLY... For low pressures (<5 psia or so) the ideal gas equation describes steam very well, with a gas constant of 85.78 ft-lbf/(lbm°R) and a constant specific heat of 0.445 Btu/(lbm°R). Modeling steam as an ideal gas is the foundation of the equations used to draw the psychrometric chart. Moist air is modeled as a mixture of two ideal gases: water vapor and dry air. This works because in the typical HVAC application, the partial pressure of water vapor is very low. HOWEVER: Having said all that, we're with you and discourage trying to solve any problems using the ideal gas equations instead of the tables. The region of the phase diagram in which the ideal gas equation is accurate is also covered by the tables, so there is no need to mess with state equations at all, be it ideal gas, Van derWaals, BWR, etc. |
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