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Objectives Learn more about Psychrometrics
Describe equations for psychometric processes Plot processes on psychrometric chart Use protractor to calculate SHR
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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
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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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Fog formation above rivers
Solution is in Psychometric chart
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Sensible heating
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Dehumidification by Cooling
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Real Dehumidification Process
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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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AHU
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Single-Zone Systems Zone – any space with a thermostat
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Air conditioning
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Recirculation
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Reheat
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Evaporative Cooler
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Air-to-air heat exchanger
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Summary Show HVAC processes on a psychrometric chart
Draw condition lines Define sensible heat ratio Draw HVAC processes for real buildings Single zone Multiple zone Skipping Chapter 9
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