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Announcements

Announcements. Tonight ASHRAE Student Chapter Meeting. Objectives. Finish with review thermodynamics Learn about Psychrometrics . p-h diagram. Ideal gas law. Pv = RT or PV = nRT R is a constant for a given fluid For perfect gasses Δ u = c v Δ t Δ h = c p Δ t c p - c v = R.

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Announcements

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  1. Announcements • Tonight ASHRAE Student Chapter Meeting

  2. Objectives • Finish with review thermodynamics • Learn about Psychrometrics

  3. p-h diagram

  4. Ideal gas law • Pv = RT or PV = nRT • R is a constant for a given fluid • For perfect gasses • Δu = cvΔt • Δh = cpΔt • cp - cv= R M = molecular weight (g/mol, lbm/mol) P = pressure (Pa, psi) V = volume (m3, ft3) v = specific volume (m3/kg, ft3/lbm) T = absolute temperature (K, °R) t = temperature (C, °F) u = internal energy (J/kg, Btu, lbm) h = enthalpy (J/kg, Btu/lbm) n = number of moles (mol)

  5. Mixtures of Perfect Gasses • m = mx my • V = Vx Vy • T = Tx Ty • P = Px Py • Assume air is an ideal gas • -70 °C to 80 °C (-100 °F to 180 °F) PxV = mx Rx∙T PyV = my Ry∙T What is ideal gas law for mixture? m = mass (g, lbm) P = pressure (Pa, psi) V = volume (m3, ft3) R = material specific gas constant T = absolute temperature (K, °R)

  6. Mass-Weighted Averages • Quality, x, is mg/(mf + mg) • Vapor mass fraction • φ= v or h or s in expressions below • φ = φf + x φfg • φ = (1- x) φf + x φg s = entropy (J/K/kg, BTU/°R/lbm) m = mass (g, lbm) h = enthalpy (J/kg, Btu/lbm) v = specific volume (m3/kg) Subscripts f and g refer to saturated liquid and vapor states and fg is the difference between the two

  7. Properties of water • Water, water vapor (steam), ice • Properties of water and steam (pg 675 – 685) • Alternative - ASHRAE Fundamentals ch. 6

  8. Humidity Ratio, W • W = mw/ma • Degree of saturation, µ = W/Ws • Humidity ratio is hard to measure, but very useful in calculations • What are units? • Is W a function of temperature? What about Ws? Ws = humidity ratio at saturation ma = mass of dry air mw = mass of water vapor

  9. Relative Humidity • Φ = xw/xw,s = Pw/Pws • Function of T Easy to measure and useful in some contexts, but often need to know temperature as well x = mole fraction P = pressure μ = degree of saturation W = humidity ratio

  10. Dew-point temperature, td • Temperature at which condensation will form • Under appropriate surface conditions • Vapor is saturated • Φ = ? • Ws(P, td) = W

  11. Wet-bulb temperature, VBT (t*) • Temperature of wet surface or • 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 temperature • Note, distinct from that measured by a sling psychrometer • Section 9.5

  12. Tables for Moist Air (P = 1 atm) • Tables A.4 in your text • Ability to get Ws for calculations • Subscripts: • a = dry air, s = saturated air v = va+µvas h = ha+µhas s = sa+µsas

  13. Psychrometric Chart • Need two quantities for a state point • Can get all other quantities from a state point • Can do all calculations without a chart • Often require iteration • Many “digital” psychrometric charts available • Can make your own • Best source is ASHRAE fundamentals (Chapter 6) • Also in your text (back cover fold-out)

  14. Alternate calculation for W • PV = mRT (IGL) • What do we know about R ratio? • P = Pw + Pa R = gas constant P = pressure V = volume T = absolute temperature W = humidity ratio Subscripts: w is water vapor, a is dry air

  15. Calculation of psychometric quantities • For an ideal gas, • hda = ∫cpadT, hw = ∫cpwdT • So, hda = cp,dat which assumes a reference state of 0 °F or 0 °C – Tables A4 • Note different reference • hw = cpwt + hg0 • h = cp,dat + W(cpwt + hg0) Or you can use: • h = cpt + W∙hg0, cp = cp,da + Wcpw cp = specific heat h = enthalpy T = absolute temperature t = temperature W = humidity ratio Subscripts: w is water vapor, a is dry air, g is saturated water vapor

  16. Adiabatic mixing • Governing equation External heat

  17. Sensible heating

  18. Dehumidification by Cooling

  19. Real Dehumidification Process

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