ASHRAE Student Chapter Meeting: Learn about HVAC Design and Building Environmental Systems

1. Student Chapter Meeting Thursday, Sept. 3rd7pm ECJ 4.304 Interested in HVAC design, MEP, or building environmental systems? Come learn about all of the opportunities and resources ASHRAE has to offer! Including: networking, scholarships, social events, and much more. Free Food!

2. Objectives Learn about Psychometrics • Psychometric chart • Equations

3. 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

4. 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

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

6. 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

7. 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

9. 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)

10. Ref: Tao and Janis (2001)

11. Ref: Tao and Janis (2001)

12. Ref: Tao and Janis (2001)

13. Ref: Tao and Janis (2001)

14. 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%

16. 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

17. 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

18. Adiabatic mixing • Governing equation External heat

19. Sensible heating

20. Dehumidification by Cooling

21. Real Dehumidification Process

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

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

24. 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