Objectives Psychometrics Psychometrics of AHU Psychrometric Chart Need

Objectives Psychometrics • Psychometrics of AHU

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%

Calculation for W • PV = m. RT (IGL) • What do we know about R ratio? R = gas constant • P = Pw + Pa 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 = ∫cpad. T, hw = ∫cpwd. T • So, hda = cp, dat which assumes a reference state of 0 °F or 0 °C – Tables A 4 • Note different reference • hw = cpwt + hg 0 • h = cp, dat + W(cpwt + hg 0) Or you can use: • h = cpt + W∙hg 0, 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

HVAC Systems • Unlimited number • You can make your system • We will study several most common applied in practice • http: //www. ce. utexas. edu/prof/Novoselac/classes/ARE 389 H/Handouts. html

Air conditioning

Single-Zone Systems • Zone – any space with a thermostat