Objectives Air mixing and humidifying Discuss equations for calculation of moist air properties

Thermodynamics properties of most air are also in Tabel A4 E (textbook)

Real Dehumidification Process

Adiabatic mixing Governing equation External heat

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)

Alternate calculation for W PV = mRT What do we know about R ratio? P = Pw + Pa Then you can connect RH and W: R = gas constant P = pressure V = volume T = absolute temperature W = humidity ratio Subscripts: w is water vapor, a is dry air, and s indicates saturated

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 More details are in Chapter 7 of the textbook

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

Systems: Single-Zone Air Systems Zone – any space with a thermostat

Air conditioning 12

Recirculation 13

Reheat 14

Evaporative Cooler 15