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The Enthalpy Chart

Learn to use an enthalpy chart effectively to solve chemical engineering problems, avoiding common mistakes and optimizing design work. Discover the application of the chart in different pressure zones and equipment sizing considerations.

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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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