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 = c v Δt Δh = c p Δt c p - c v = R M = molecular weight (g/mol, lbm/mol) P = pressure (Pa, psi) V = volume (m 3, ft 3 ) v = specific volume (m 3 /kg, ft 3 /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 = m x m y V = V x V y T = T x T y P = P x P y Assume air is an ideal gas -70 °C to 80 °C (-100 °F to 180 °F) P x V = m x R x ∙ T P y V = m y R y ∙ T What is ideal gas law for mixture? m = mass (g, lbm) P = pressure (Pa, psi) V = volume (m 3, ft 3 ) R = material specific gas constant T = absolute temperature (K, °R)

6. Mass-Weighted Averages Quality, x, is m g /(m f + m g ) Vapor mass fraction φ= v or h or s in expressions below φ = φ f + x φ fg φ = (1- x) φ f + x φ g s = entropy (J/K/kg, BTU/°R/lbm) m = mass (g, lbm) h = enthalpy (J/kg, Btu/lbm) v = specific volume (m 3 /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 = m w /m a Degree of saturation, µ = W/W s Humidity ratio is hard to measure, but very useful in calculations What are units? Is W a function of temperature? What about W s ? W s = humidity ratio at saturation m a = mass of dry air m w = mass of water vapor

9. Relative Humidity Φ = x w /x w,s = P w /P ws 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, t d Temperature at which condensation will form Under appropriate surface conditions Vapor is saturated Φ = ? W s (P, t d ) = 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 W s for calculations Subscripts: a = dry air, s = saturated air v = v a +µv as h = h a +µh as s = s a +µs as

14. 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)

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

16. Calculation of psychometric quantities For an ideal gas, h da = ∫c pa dT, h w = ∫c pw dT So, h da = c p,da t which assumes a reference state of 0 °F or 0 °C – Tables A4 Note different reference h w = c pw t + h g0 h = c p,da t + W(c pw t + h g0 ) Or you can use: h = c p t + W∙h g0, c p = c p,da + Wc pw c p = 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

17. Adiabatic mixing Governing equation External heat

18. Sensible heating

19. Dehumidification by Cooling

20. Real Dehumidification Process