Objectives Continue with Psychometrics Psychometric chart Equations Plot processes in AHU and buildings on psychrometric chart

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

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)

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 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

AHU

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

Evaporative Cooling (air washing)

Condition Line for a Space

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

Reading assignment Chapters 7 & 8