# objectives describe psychrometric quantities given any two psychrometric quantities, calculate any...

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ObjectivesDescribe psychrometric quantitiesGiven any two psychrometric quantities, calculate any other quantityUse Tables A4 or psychrometric charts to look up psychrometric quantitiesCalculate psychrometric quantities at non-standard conditionsEvaluate psychrometric processes (8.1-8.4)

Humidity Ratio, WW = mw/maDegree of saturation, = W/WsHumidity ratio is hard to measure, but very useful in calculationsWhat are units?Is W a function of temperature? What about Ws?Ws = humidity ratio at saturationma = mass of dry airmw = mass of water vapor

Relative Humidity = xw/xw,s = Pw/PwsFunction of T

Easy to measure and useful in some contexts, but often need to know temperature as wellx = mole fractionP = pressure = degree of saturationW = humidity ratio

Dew-point temperature, tdTemperature at which condensation will formUnder appropriate surface conditionsVapor is saturated = ?Ws(P, td) = W

Wet-bulb temperature, t*Temperature at which water, by evaporating into the air, will bring air to saturation adiabatically* superscript is designation that variable is evaluated at the wet-bulb temperatureNote, distinct from that measured by a sling psychrometerSection 9.5

Tables for Moist Air (P = 1 atm)Tables A.4 in your textAbility to get Ws for calculationsSubscripts:a = dry air, s = saturated airv = va+vash = ha+hass = sa+sas

Psychrometric ChartNeed two quantities for a state pointCan get all other quantities from a state pointCan do all calculations without a chartOften require iterationMany digital psychrometric charts availableCan make your ownBest source is ASHRAE fundamentals (Chapter 6)Also in your text (back cover fold-out)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

ExamplesWhat is enthalpy of air in the classroom right now?Condensation on windows when taking a showerHow cold does it have to be outside for condensation to form on windows?Assumption is that windows are the same temperature as outside air 80 F, RH = 80%

Alternate calculation for WPV = mRT (IGL)

What do we know about R ratio?P = Pw + PaR = gas constantP = pressureV = volumeT = absolute temperatureW = humidity ratio

Subscripts: w is water vapor, a is dry air

A diversionFor an ideal gas,ha = cpadT, hw = cpwdTSo, ha = cpat which assumes a reference state of 0 F or 0 C Tables A4Note different referencehw = cpwt + hg0h = cpat + W(cpwt + hg0)h = cpt + Whg0, cp = cpa + Wcpw

cp = specific heath = enthalpyT = absolute temperaturet = temperature W = humidity ratio

Subscripts: w is water vapor, a is dry air, g is saturated water vapor

Approximating Psychrometric Processeshs = cpt, hL = hwW=(cpwt + hg0) WWhere do these come from?h = h2 h1, t = t2 t1, W = W2 W1, t = (t1 + t2)/2, W = (W1 + W2)/2h = hs + hL

Major resulths = cpt , hL = hwW

SummaryDescribe psychrometric quantitiesGiven any two psychrometric quantities, calculate any other quantityUse Tables A4 or psychrometric charts to look up psychrometric quantitiesCalculate psychrometric quantities at non-standard conditions

Psychrometric ProcessesAdiabatic mixing of two streamsSensible heating and coolingDehumidification by coolingHumidificationEvaporative cooling

Adiabatic mixingGoverning equation

Sensible heating

Dehumidification by Cooling

73 for example 1

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