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Psychrometrics

Psychrometrics. Introduction to Food Engineering. Psychrometrics. Thermodynamic properties of gas-vapor mixtures (air-water). Properties of Dry Air. Composition N 2 78 %, O 2 20.9 % Standard dry air MW = 28.9645 Gas constant for dry air R a = 287.055 m 3 .Pa/kg.K.

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Psychrometrics

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  1. Psychrometrics Introduction to Food Engineering

  2. Psychrometrics • Thermodynamic properties of gas-vapor mixtures (air-water)

  3. Properties of Dry Air • Composition • N2 78 %, O2 20.9 % • Standard dry air MW = 28.9645 • Gas constant for dry air Ra = 287.055 m3.Pa/kg.K

  4. Properties of Dry Air • Specific Volume Ra = gas constant TA = absolute temperature (K) Pa = partial pressure of dry air

  5. Properties of Dry Air • Specific Heat (Cpa) • At 1 atm (101.325 kPa), T = - 40 – 60 C • Average value 1.005 kJ/kg.K • Enthalpy (heat content) • Reference 1 atm, 0 C

  6. Properties of Dry Air • Dry Bulb Temperature • Indicated by sensor

  7. Properties of Water Vapor • Moist air = dry air + water vapor • Vapor in the air is superheated steam at low pressure & temperature • Moist air is clear or foggy • MW of water = 18.01534 • Gas constant for water vapor • Rw = 461.52 m3Pa/kg.K

  8. Properties of Water Vapor • Specific Volume of Water Vapor • Below 66 C vapor follows ideal gas law

  9. Properties of Water Vapor • Specific Heat of Water Vapor • Within –71 to 184 C Cpw = 1.88 kJ/kg.K • Enthalpy of water vapor Ta = dry bulb temp

  10. Properties of Air-Vapor Mixtures • Gibbs-Dalton Law • Up to 3 atm air-water mixtures follow perfect gas laws PB = barometric (total pressure) of moist air (kPa)

  11. Dew-Point Temperature • Dew-Point Temperature Water vapors in air = steam at low pressure • Saturation temperature = dew point • Obtained from steam table at partial pressure exerted by water vapor • Below this temp, condensation of moisture

  12. Humidity Ratio • Humidity Ratio (Moisture Content) or specific humidity, W (Kg water / ky dry air)

  13. X = mole fraction

  14. Relative Humidity • Relative to maximum amount of moisture at dry bulb temperature

  15. Relative Humidity • Density Density of water vapor/density of saturated vapor at dry bulb temp of air

  16. Humid Heat • Of air-water vapor mixture • Heat required to raise temp of 1 kg dry air + water vapor by 1 K (kJ/kg dry air . K) W = humidity ratio

  17. Specific Volume • Volume of 1 kg dry air + water vapor

  18. Adiabatic Saturation of Air • Evaporation of water by sensible heat of entering air

  19. Wet-Bulb Temperature • Psychrometric wet bulb temperature • Movement of air • Thermodynamic wet bulb temperature

  20. The Psychrometric Chart

  21. Example • An air-vapor mixture is at 60 C dry bulb temp and 35 C wet bulb. Determine relative humidity, humidity ratio, specific volume, enthalpy and dew-point temp. • RH = 20 %, W = 0.026 kg/kg • Enthalpy = 129 kJ/kg dry air • Specific volume = 0.98 m3/kg dry air • Dew-point temp = 29 C

  22. Use of psychrometric chart to evaluate complex air-conditioning processes • Heating or Cooling of Air • Humidity ratio constant

  23. Example • Calculate the rate of thermal energy required to heat 10 m3/s of outside air at 30 C dry bulb temp and 80 % RH to a dry bulb temp of 80 C Mass flow rate = vol flow rate/ specific vol

  24. Mixing of Air Inverse proportion

  25. Drying • Adiabatic saturation process • Heat of evaporation is supplied only by drying air • Dry bulb temp decreases, enthalpy constant ie. constant wet bulb • Humidity ratio increases (gain moisture)

  26. Example • Heated air at 50 C, 10 % RH is used to dry rice. Air exits under saturated condition. Determine amount of water removed per kg of dry air. • W1 = 0.0078 kg/kg • Follow constant enthalpy line • W2 = 0.019 kg/kg • Moisture removed = 0.0112 kg/kg

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