Usually good for air n 2 o 2 h 2 he ar ne at low

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Usually good for air, N 2 , O 2 , H 2 , He, Ar, Ne at low pressures and high temperatures (less than 1% error). Works well for water vapor below 10 kPa irrespective of temperature (less than 0.1% error). Fails close to critical point and saturated vapor line. Definitely not applicable for water vapor in steam power plants at very high pressures

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9/22/2010 ME200 Therm I Lecture 13, Prof. Mongia 11 0 50 100 150 200 250 300 350 400 1 10 100 1000 10000 100000 Saturation Temperature, o C Pressure, kPa Saturated Steam: Temperature vs. pressure
9/22/2010 ME200 Therm I Lecture 13, Prof. Mongia 12 0 20 40 60 80 100 120 140 160 180 1 10 100 1000 10000 100000 Specific volume, m 3 /kg Pressure, kPa Saturated Steam: Specific volume vs. pressure

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9/22/2010 13 0% 10% 20% 30% 40% 50% 60% 70% 80% 1 10 100 1000 10000 100000 Error=1-pv/RT Pressure, kPa Saturated Steam: Check Ideal Gas Law Applicability * *Source: Cengel & Boles, 2008
9/22/2010 ME200 Therm I Lecture 13, Prof. Mongia 14 Ideal-gas approximation of steam: Calculated error bands given in Figure from Cengel & Boles, 2008

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9/22/2010 15 Compressibility Factor What do you mean by low pressure and high temperature? Compressibility factor quantifies the deviation of a pure substance from ideal gas behavior at a given temperature and pressure For a real gas, Z > 1 or Z < 1 For Z very close to unity, we can usually assume ideal gas behavior except that Z can equal 1 for very high pressures in some cases
Compressibility Factor Plot for Hydrogen 9/22/2010 16 ME200 Therm I Lecture 13, Prof. Mongia

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9/22/2010 17 ME200 Therm I Lecture 13, Prof. Mongia
9/22/2010 ME200 Therm I Lecture 13, Prof. Mongia 18

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19 Different gases behave differently at a given pressure and temperature; but they behave similarly at pressure and temperature normalized by the critical pressure and temperature Reduced pressure and temperature: p R = reduced pressure; T R = reduced temperature p c = critical pressure; T c = critical temperature Principle of Corresponding States : Compressibility factor is approximately the same at same p R and T R for all fluids.
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