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Introduction to Food Engineering
Psychrometrics Introduction to Food Engineering
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Psychrometrics Thermodynamic properties of gas-vapor mixtures (air-water)
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Properties of Dry Air Composition N2 78 %, O2 20.9 %
Standard dry air MW = Gas constant for dry air Ra = m3.Pa/kg.K
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Properties of Dry Air Specific Volume Ra = gas constant
TA = absolute temperature (K) Pa = partial pressure of dry air
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Properties of Dry Air Specific Heat (Cpa) Enthalpy (heat content)
At 1 atm ( kPa), T = - 40 – 60 C Average value kJ/kg.K Enthalpy (heat content) Reference 1 atm, 0 C
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Properties of Dry Air Dry Bulb Temperature Indicated by sensor
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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 = Gas constant for water vapor Rw = m3Pa/kg.K
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Properties of Water Vapor
Specific Volume of Water Vapor Below 66 C vapor follows ideal gas law
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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
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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)
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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
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Humidity Ratio Humidity Ratio (Moisture Content) or specific humidity, W (Kg water / ky dry air)
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X = mole fraction
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Relative Humidity Relative to maximum amount of moisture at dry bulb temperature
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Relative Humidity Density
Density of water vapor/density of saturated vapor at dry bulb temp of air
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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
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Specific Volume Volume of 1 kg dry air + water vapor
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Adiabatic Saturation of Air
Evaporation of water by sensible heat of entering air
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Wet-Bulb Temperature Psychrometric wet bulb temperature
Movement of air Thermodynamic wet bulb temperature
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The Psychrometric Chart
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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 = kg/kg Enthalpy = 129 kJ/kg dry air Specific volume = 0.98 m3/kg dry air Dew-point temp = 29 C
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Use of psychrometric chart
to evaluate complex air-conditioning processes Heating or Cooling of Air Humidity ratio constant
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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
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Mixing of Air Inverse proportion
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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)
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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 = kg/kg Follow constant enthalpy line W2 = kg/kg Moisture removed = kg/kg
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