HW1 Errors Equation for Ra number in handouts Equation for Nu in book

Objectives Learn about Psychometrics Psychometric chart Equations

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

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

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

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

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

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)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Examples What is enthalpy of air in the classroom right now? Condensation on windows when taking a shower How 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% 73 for example 1

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

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

Adiabatic mixing Governing equation External heat

Sensible heating

Dehumidification by Cooling

Real Dehumidification Process

Transport of saturated air tsurface < tdp Condensation Mold in a duct

Humidification hw Specific enthalpy of water added to system hg Specific enthalpy of saturated water vapor

Summary Describe psychrometric quantities Given any two psychrometric quantities, calculate any other quantity Use Tables A4 or psychrometric charts to look up psychrometric quantities Calculate psychrometric quantities at non-standard conditions