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

Download Objectives Describe psychrometric quantities Given any two psychrometric quantities, calculate any other quantity Use Tables A4 or psychrometric charts

Post on 17-Jan-2016

222 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • 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

Recommended

View more >