2PsychrometricsIt involves determination of thermodynamic properties of gas-vapor mixtures.The most common applications are associated with the air-water vapor system.
3Properties of dry air composition % by volume Nitrogen Oxygen Argon Composition of aircomposition% by volumeNitrogenOxygenArgonCarbon dioxideNeonHeliumOthers (methane, sulfurdioxide, hydrogen, krypton และ xenon)
4Properties of dry air Molecular weight = 28.9645 Gas constant for dry air = m3Pa/kg.KSpecific volume V’a =Specific heat (average) = kJ/kg.KEnthalpy = Ha = (Ta-To) kJ/kgDry bulb temperature = temperature indicated by unmodified sensor
5Properties of water vapor Molecular weight =Gas constant for dry air = m3Pa/kg.KSpecific volume V’w =Specific heat (average) = 1.88 kJ/kg.KEnthalpy = Hw = (Ta-To) kJ/kg
6Properties of air-vapor mixtures Gibbs-Dalton LawpB = pa + pwwhere pB = total pressurepa = partial pressure of dry airpw = partial pressure of water vapor
7Dew-point temperature The water vapor in the air will be saturated when air is at a temperature equal to the saturation temperature corresponding to the partial pressure exerted by the water vapor.This temperature is called dew-point temperature.
8Humidity ratio/ moisture content (W) PV = mRT (ideal gas law)P = Pw + PaR = gas constantP = total pressureV = volumeT = absolute temperatureW = humidity ratioSubscripts: w is water vapor, a is dry air
9Relative Humidity Φ = xw/xw,s = Pw/Pws = w/ws Function of T Easy to measure and useful in some contexts, but often need to know temperature as wellx = mole fractionP = pressureμ = degree of saturationW = humidity ratio
10Humid heatIt is amount of heat (kJ) required to raise the temperature of 1 kg dry air plus water vapor present by 1 K.Cs = W
11Specific volumeIt is the volume (m3) of 1 kg dry air plus water vapor in the air.V’m =
12Wet bulb temperature pw = where pw = partial pressure of water vapor pB = total pressure = barometric pressurepwb = saturation pressure of water vaporat wet bulb temp.Ta = dry bulb temp.Tw = wet bulb temp.
13ExampleFind dew-point temperature, humidity ratio, humid volume, and relative humidity of air having a dry bulb temperature of 40C and a wet bulb temperature of 30C.
14Adiabatic saturation process Phenomenon of adiabatic saturation of air is applicable to convective drying of food materials.Adiabatic conditionWell insulated chamber: no heat gain and lossAir is allowed to contact a large surface area of waterPart of sensible heat of entering air is transformed into latent heat
15Adiabatic saturation process Process of evaporation water into the air results in saturation by converting part of sensible heat of the entering air into latent heat
18Psychrometric Chart Need two quantities for a state point Can get all other quantities from a state pointCan do all calculations without a chartOften require iterationMany “digital” psychrometric charts availableCan make your own
19Temperature Absolute Temperature Dry-bulb temperature Wet-bulb temperatureDew-point temperature