Introduction to Food Engineering Psychrometrics Introduction to Food Engineering

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

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

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

Properties of Dry Air Specific Heat (Cpa) Enthalpy (heat content) 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

Properties of Dry Air Dry Bulb Temperature Indicated by sensor

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

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

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

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)

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

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

X = mole fraction

Relative Humidity Relative to maximum amount of moisture at dry bulb temperature

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

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

Specific Volume Volume of 1 kg dry air + water vapor

Adiabatic Saturation of Air Evaporation of water by sensible heat of entering air

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

The Psychrometric Chart

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

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

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

Mixing of Air Inverse proportion

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)

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