Lecture Objectives Learn about airconditioning psychrometric by looking
Lecture Objectives: • Learn about air-conditioning (psychrometric) • by looking into swimming pool example
Heat pump example A recreation center Swimming pool Condenser Heat Pump Evaporator Ice Rink 30 o. F 85 -89 o. F Rejects heat energy Consumes heat energy Electric power for a compressor
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) • Or electronic version at: http: //www. handsdownsoftware. com/
Connection between W and Pw (W humidity ratio – Pw water partial pressure) • PV = m. RT (Ideal Gas Law ) • 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 = ∫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
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
Summary • Describe psychrometric quantities • Given any two psychrometric quantities, calculate any other quantity • Use Tables A 4 or psychrometric charts to look up psychrometric quantities • Calculate psychrometric quantities at nonstandard conditions
HW Assignment Textbook (Thermal Environmental Engineering) problems: 8. 1, 8. 8, 8. 13, 8. 20 Posted at: http: //www. ce. utexas. edu/prof/Novoselac/classes /CE 397 b/Handouts/Chapter%208_Psychrometric %20 Processes%20 and%20 Applications. pdf (both textbook section and problems)
Swimming pool energy and mass balance
Evaporation • Heat loss by evaporation: pool ≈ hm pool Ws Tpool Ws W Tpool Tair Wair Depends on the air speed and even more on the condition of the water surface (people splashing in the water) Further analysis can be conducted in the psychrometric chart
Process in AHUs • Example AHUs for swimming pools MENERGA Thermo. Cond 19
Swimming pool heating
Dehumidification using outside air in Winter and Summer
More advance control MENERGA Thermo. Cond 29
Dehumidification using outside air in winter Condenser in the air stream Evaporator Compressor of the heat pump Condenser in the swimming pool
Recirculation of air with dehumidification catalog
- Slides: 22