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The Enthalpy Chart

The Enthalpy Chart. Presented to CBE 317 Sept – 2003 Dick Hawrelak. A Typical Enthalpy Chart. The Liquid Line. The Vapor Line. The Latent Heat Line. Enthalpy Chart Zones. Equation of State.

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The Enthalpy Chart

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  1. The Enthalpy Chart Presented to CBE 317 Sept – 2003 Dick Hawrelak

  2. A Typical Enthalpy Chart

  3. The Liquid Line

  4. The Vapor Line

  5. The Latent Heat Line

  6. Enthalpy Chart Zones

  7. Equation of State • At low pressure (14.7 psia, 101.3 kPa) the Ideal Gas Law applies where PV = (z)(n)(R)(T) and Z = 1.0 • At high pressure, the Gas Law is non-ideal and Z is less than 1.0 • Vapor Density = (MW)(P) / [(Z)(R)(T)] • More mistakes are made with vapor density than any other physical property.

  8. Super-Critical Region • Above the Pc and the Tc which design equations apply? • Given temperature and pressure above the criticals, a compressor vendor will treat the data as a dense vapor and use vapor correlations. • Given the same data, a pump vendor may treat the data as a light liquid and use liquid correlations (Dow Cochin Pipeline system). Pump power required will be much lower than compressor power required.

  9. Just below the Pc and the Tc • This is a critical zone for distillation because the latent heat approaches zero as the Pc and Tc are approached. • Hence, Vapor flow = (BTU / hr) / (LH) becomes very high. • Design equations at conditions near the criticals are very complicated and many errors are made in this region.

  10. Low Pressures • Vacuum condition below 14.7 psia. • Low vapor densities mean high vapor flows by (cf / hr) = (lb / hr) / (lb / cf). • Compared with high pressure, pressure drop calculations in the vacuum zone have very little margin for error. • Hence, equipment such as exchangers and distillation towers and lines can be severely under-sized.

  11. Constant Entropy Line

  12. A Centrifugal Compressor

  13. A Refrigeration PFS

  14. Refrigeration on Enthalpy Chart

  15. Flashing

  16. Flashing Around E-3

  17. Solve the % Flash

  18. De-Superheating a Vapor

  19. Condensing the Vapor

  20. Sub-Cooling The Liquid

  21. Steam Expansion To Generate Power

  22. Steam Expansion

  23. Which Route To Get From A to B?

  24. Summary • The Enthalpy Chart is one of the most useful tools for solving chemical engineering problems. • Learn to communicate with it as shown in this demonstration. If done properly, you’ll make fewer mistakes in your design work. • An internet web site that allows you to draw enthalpy charts from a large chemical database can be found at: • http://www.questconsult.com/~jrm/enthpres.html

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