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PSYCHROMETRY AND HVAC

PSYCHROMETRY AND HVAC. Psychrometry (hygrometry): science of moist air HVAC : Heating, ventilating, and air-conditioning. I am teaching Engineering Thermodynamics to a class of 75 undergraduate students. I plan to go through these slides in one 90-minute lecture.

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PSYCHROMETRY AND HVAC

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  1. PSYCHROMETRY AND HVAC Psychrometry (hygrometry): science of moist air HVAC: Heating, ventilating, and air-conditioning • I am teaching Engineering Thermodynamics to a class of 75 undergraduate students. • I plan to go through these slides in one 90-minute lecture. • Zhigang Suo, Harvard University https://www.youtube.com/playlist?list=PL2E9_TlQKsGQ4PbJ0KJEvIFxjVUCw7s30 Watch this tutorial on psychrometry

  2. Plan • Psychrometric properties • Measuring humidity • Psychrometric chart • HVAC

  3. Dry air

  4. Moist air a mixture of many gases and water vapor, modeled as an ideal-gas mixture Dry air is far from the dome Water vapor is near the dome Partial pressure of dry air, Pa Partial pressure of water vapor, Pv Total (barometric) pressure

  5. Humidity ratioabsolute humidity, specific humidity

  6. Relative humidity • Obtain the saturated pressure Pg(T) from the steam table. • Dry air. Relative humidity = 0% • Air saturated with water. Relative humidity = 100% • Human comfort. Relative humidity = 40-60%

  7. Enthalpy Specific enthalpy of dry air: Specific enthalpy of water vapor: Enthalpy of moist air: Specific enthalpy in terms of (P,T,w):

  8. Plan • Psychrometric properties • Measuring humidity • Psychrometric chart • HVAC

  9. Dew-point liquid gas http://www.climate.be/textbook/glossary_s.xml

  10. Foggy morning At sunset, the temperature is 20 C and the relative humidity is 50%. In the morning, how much must the temperature have dropped for fog to form? Assume that the partial pressure of water remains fixed. sunset dew point

  11. Frost point

  12. Adiabatic saturation temperature P2 An experiment to measure humidity Conservation of dry air: Conservation of water: Conservation of energy: saturated air at outlet: Algebra:

  13. Wet-bulb temperature. Dry-bulb temperature Sling psychrometer For air–water vapor mixtures at atmospheric pressure, Twb is approximately equal to the adiabatic saturation temperature.

  14. Hygrometerhumidity sensorsFirst inventor: Johann Heinrich Lambert (1755)Humidity affects everything. Everything is a hygrometer.Today’s opportunity: The Internet of things. • Bimaterial strip • Hair-tension hygrometer • Wet-bulb and dry-bulb • Dew-point hygrometer • Capacitor • Resistor • Thermal conductivity • Weight https://en.wikipedia.org/wiki/Hygrometer

  15. Plan • Psychrometric properties • Measuring humidity • Psychrometric chart • HVAC

  16. Psychrometric propertiesName all states of moist air by 3 variables: P,T,w • P, (barometric, or total) pressure • Pa, dry-air pressure • Pv, vapor pressure (partial pressure of water) • T, (dry-bulb) temperature • Tdp, dew-point temperature • Twb, wet-bulb temperature • ma, mass of dry air • mv, mass of water vapor • w, specific humidity (absolute humidity, humidity ratio) • f, relative humidity (RH) • v, specific volume • h, specific enthalpy

  17. Name all states of moist air by 3 variables: P,T,w Volume: Specific volume in terms of (P,T,w): Specific humidity: Partial pressure of water in terms of (P,T,w): Relative humidity: Relative humidity in terms of (P,T,w):

  18. Since Chinese invented paper, humans have spent disproportional amount of time on one particular number: 2

  19. Psychrometric chart Henry Ford: “You can have Model T in any color, so long as it’s black.” You can draw figure in any dimensions, so long as it’s two-dimensional. Fix P = 1 atm

  20. Psychrometric chart Name all states of moist air using 3 variables: P,T,w P = 1 atm w Tdb

  21. https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter10b.htmlhttps://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter10b.html

  22. Foggy morning At sunset, the temperature is 20 C and the relative humidity is 50%. In the morning, how much must the temperature have dropped for fog to form? Assume that the partial pressure of water remains fixed. dew point sunset

  23. Plan • Psychrometric properties • Measuring humidity • Psychrometric chart • HVAC

  24. Engineering a state of happiness

  25. Lee Kuan YewThe founder of Singapore died on 23 March 2015, aged 91 Among a number of 20th-century luminaries asked by the Wall Street Journal in 1999 to pick the most influential invention of the millennium, he alone shunned the printing press, electricity, the internal combustion engine and the internet and chose the air-conditioner. He explained that, before air-con, people living in the tropics were at a disadvantage because the heat and humidity damaged the quality of their work. http://www.economist.com/news/asia/leekuanyew

  26. Singapore

  27. Boston

  28. Denver

  29. HVAC processesapproach the comfort zone by changing temperature and humidity • Hardware: duct, fan, heater, cooler, source and sink of water, flow of air • Psychrometric chart • Steady-flow, control-volume analysis.

  30. Heating  =constant, but f drops Conservation of dry air: Conservation of water: Conservation of energy:

  31. Heating and humidifying Conservation of dry air: Conservation of water: Conservation of energy:

  32. Cooling and dehumidifying dry air: water: energy:

  33. Evaporative Cooling h = constant dry air: water: energy:

  34. Senator Rubio, There is no clash of civilization. Drink more water and study psychrometry! Evaporative cooling Desert: T = 50 C, RH = 10% What is temperature of the cloth? cloth desert

  35. Adiabatic mixing Rule of mixture Conservation of dry air: Conservation of water: Conservation of energy:

  36. Coal power stationcoverts coal to electricity

  37. Cooling tower The moist air in the tower has a high water-vapor content, and is lighter than the outside air. The light air in the tower rises, and the heavier outside air fills the vacant space, creating an airflow from the bottom of the tower to the top. Conservation of dry air: Conservation of water: Conservation of energy:

  38. Summary • Name all states of moist air by 3 psychrometric properties: (total) pressure P, (dry-bulb) temperature T, and humidity ratio w. • Hygrometer • Psychrometric chart: Fix P, and represent all other psychrometric properties on a plane by lines of constant values. • Comfort zone (T = 22-27 C, RH = 40%-60%). • Reach comfort zone by HVAC processes.

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