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Objectives Psychometrics Psychometrics of AHU
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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)
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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
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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
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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
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Adiabatic mixing Governing equation External heat
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AHU
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Sensible heating
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Dehumidification by Cooling
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Real Dehumidification Process
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Transport of saturated air
tsurface < tdp Condensation Mold in a duct
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Humidification hw Specific enthalpy of water added to system
hg Specific enthalpy of saturated water vapor
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Evaporative Cooling (air washing)
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Condition Line for a Space
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HVAC Systems Unlimited number
You can make your system We will study several most common applied in practice
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Air conditioning
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Single-Zone Systems Zone – any space with a thermostat
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