Psychrometrics

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Use of psychrometric chart for analyzing air conditioning systems

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Psychrometrics

  1. 1. Psychrometrics & Air Conditioning Design Kalyan Chatterjea
  2. 2. Introduction to Comfort Air-conditioning
  3. 3. Introduction - Psychrometry <ul><li>The air conditioning processes are best understood by studying Psychrometry. </li></ul><ul><li>Psychrometry is the study of atmospheric air and its associated water vapour. </li></ul>A psychrometric chart presents physical and thermal properties of moist air in a graphical form.
  4. 4. Psychrometry – Dry Bulb Temp <ul><li>Dry-bulb temperature (DBT) is the commonly measured temperature from a thermometer. </li></ul>
  5. 5. Psychrometry – Wet Bulb Temp <ul><li>Wet Bulb Temperature (WBT) is determined when air is circulated past a wetted sensor tip. </li></ul><ul><li>Intersection of DBT and WBT provides the status point. </li></ul>
  6. 6. Psychrometry – Saturation Line <ul><li>Saturation line is a temperature condition at which water will begin to condense out of moist air. </li></ul><ul><li>It is also called the dew point temperature line. </li></ul>
  7. 7. Psychrometry – Relative Humidity <ul><li>Relative Humidity (RH) is a ratio between actual moisture content of the air compared with the moisture content of the air required for saturation at the same temperature. </li></ul><ul><li>RH is affects human comfort significantly. </li></ul>Sensible heat removal with increase in RH
  8. 8. Psychrometry – Specific Volume <ul><li>Specific Volume indicates the space occupied by air. </li></ul><ul><li>This is useful in converting volumetric flow to mass-flow rate calculation. </li></ul>
  9. 9. Psychrometry - Enthalpy <ul><li>Enthalpy is the energy content of dry air & moisture expressed in kJ/kg of dry air. </li></ul><ul><li>The information is useful is sizing up heating & cooling coils. </li></ul>
  10. 10. Psychrometry – Sensible Heat Processes <ul><li>Heating & Cooling process – addition or removal of sensible heat without change in absolute moisture content. </li></ul><ul><li>But RH changes as temperature changes. </li></ul>
  11. 11. Psychrometry - Dehumidification <ul><li>Dehumidification by cooling – in cooling coils temp of air reduces and the saturation point (dew point) is reached. </li></ul><ul><li>Further cooling results in reduction of absolute humidity. </li></ul>
  12. 12. Psychrometry – Comfort Zone <ul><li>Comfort Zone – statistical studies have been performed on large numbers of subjects of all ages, sexes and nationalities to arrive at a quantitative description of human comfort. </li></ul>
  13. 13. <ul><li>Comfort zone … comfort is a very subjective term! </li></ul><ul><li>Statistically, majority of human beings are comfortable when temp is between 18-22 0 C and RH is 40-65% </li></ul>Psychrometry – Comfort Zone
  14. 14. Psychrometry – Climate Classification <ul><li>Climate Classification shows the various approximate zones of temperature and humidity on the psychrometric chart. </li></ul>
  15. 15. Psychrometry – Heating at Sea <ul><li>Heating without humidification at sea . </li></ul>Sensible heat addition with decrease in RH…away from comfort zone
  16. 16. Psychrometry – Heating at Sea <ul><li>Heating with humidification at sea . </li></ul>Heat addition with humidification resulting in more comfortable RH
  17. 17. Heating Process in Detail <ul><li>Process showing sensible heat addition. </li></ul>
  18. 18. Heating Process in Detail <ul><li>Applying the SFEE </li></ul>h-values can be directly read from the psychrometric chart
  19. 19. <ul><li>Process showing sensible heat cooling, followed by </li></ul><ul><li>Further cooling, which leads to precipitation. </li></ul>Cooling & Dehumidification in Detail
  20. 20. Cooling & Dehumidification in Detail <ul><li>Applying the SFEE </li></ul>Mass flow balance equation for the water & steam In practice, water mass flow is very small compared to air mass flow. So a simpler equation could be used without undue error.
  21. 21. References <ul><li>Psychrometric Chart Download </li></ul><ul><ul><li>http://www.ent.ohiou.edu/~thermo/me328/quiz.info/Nautica_SI.pdf </li></ul></ul>
  22. 22. References
  23. 23. References <ul><li>Overview of Psychrometrics: </li></ul><ul><ul><li>http://www.handsdownsoftware.com/Overview_of_Psychrometrics.pdf </li></ul></ul>
  24. 24. References
  25. 25. References <ul><li>Coil Load Calculations: </li></ul><ul><ul><li>http://courses.washington.edu/me425/425%20Psychrometric%20Review%202007.pdf </li></ul></ul>
  26. 26. References
  27. 27. References <ul><li>Glossary: </li></ul><ul><ul><li>http://air.1800anytyme.com/info/air_conditioning_glossary.php </li></ul></ul>
  28. 28. References
  29. 29. References <ul><li>Standard: British Standards Institution QC100.B862 BS EN ISO 2006 </li></ul><ul><ul><li>Ship and Marine Technology - Air-conditioning & Ventilation of Accommodation Spaces - Design Conditions & Basis of Calculations </li></ul></ul>
  30. 30. References
  31. 31. References <ul><li>Thermodynamic Expert System </li></ul><ul><ul><li>http://energy.sdsu.edu/testcenter/testhome/index.html </li></ul></ul>TEST - a visual platform to analyze thermo-fluids problems, verify hand calculations, pursue what-if scenarios, visualize thermal systems, and engage in a life-long journey to master thermodynamics. Developed by Dr. S. Bhattacharjee, San Diego State University.
  32. 32. References
  33. 33. References <ul><li>ABS Guide for Crew Habitability on Off-shore Installations, 2002 </li></ul><ul><ul><li>&quot;habitability&quot; is defined as the acceptability of conditions of an offshore installation in terms of vibration, noise, indoor climate and lighting, as well as physical and spatial characteristics, according to prevailing research and standards for human efficiency and comfort </li></ul></ul>
  34. 34. References
  35. 35. References <ul><li>Simulation </li></ul><ul><ul><li>Hui, S. C. M. and K. P. Cheung, 1998. Application of building energy simulation to air-conditioning design, In Proc. of the Mainland-Hong Kong HVAC Seminar '98 , 23-25 March 1998, Beijing, pp. 12-20. </li></ul></ul><ul><ul><ul><li>http://www.arch.hku.hk/~cmhui/hvac98b.pdf </li></ul></ul></ul>
  36. 36. References
  37. 37. Thank You!

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