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Psychrometrics Psychometrics 5/5/2017 PSYCHROMETRICS PSYCHOMETRICS Chapter 1 2013 ASHRAE Handbook Fundamentals 1.Definitions 2.Processes 3.Evaporative Cooling Michael K. Fulton, PE Western Mechanical Solutions SUSTAINABLE HEATING COOLING Definitions Definitions Dry Air Psychrometrics: Water Vapor Moist Air Thermodynamics properties Saturation Analyze conditions and properties Perfect Gas Equation pV = nRT Moist Air Terms Terms Dry Bulb db – deg - Temperature of air Dew Point dp – deg - Surface Temperature with no evaporation occurring 72 deg where Condensation Occurs 46.4 Wet Bulb wb – deg - Temperature of air at Enthalpy h – BTU/lb – Pressure x Volume equilibrium with full evaporation occurring Energy and Internal Energy 26.2 Absolute Humidity – lb/lb – general number is 56.2 deg 0.008 lb/lb da , 1% lbs water / lb dry air Relative Humidity RH – % - Percentage of grains/lb – there are 7,000 grains in a lb moisture in the air compared to the Specific Volume V – ft3/lb – 16.5 ft3/lb maximum allowable AT THAT Density p – lb/ft3 - 0.061 lb/ft3 TEMPERTURE 40% Density and Specific volume are reciprocals of each other 1 5/5/2017 Terms Process Terms Sensible Heating Sensible cooling Dehumidification/Cooling coil line Sensible heat load Evaporative Cooling Humidification Comfort window Definitions Definitions Dry Bulb Dew Point Wet Bulb Dry Bulb Fog Occurs When Air Is Saturated Wet Bulb Dew Point Humidity Ratio Relative Humidity lb of dry air © American Standard Inc. 1999 Air Conditioning Clinic TRG-TRC001-EN 2 5/5/2017 Enthalpy Basic Processes HumidifyHumidification Cooling Heating De-Humidify Basic Processes Mixed Air Fan Heat Cooling Coil 3 5/5/2017 People Latent Latent Definitions Load 0.9 0.8 0.6 0.5 0.7 Enthalpy Enthalpy Definitions Difference Sensible & Latent Load Humid Day (81/65) = Total Load Summer Comfort Winter Comfort Window Window 4 5/5/2017 Psychrometric – Formula’s Hour Distribution • Moa = CFM / Voa • Moa = lb/min • CFM = ft3/min • Voa – Specific volume – ft3/lbm Psychrometric – Formula’s Psychrometric – Formula’s • Air heat of absorption / rejection Sensible • Heat Transfer • qs = Moa * Cp * (t1-t2) = Btu/hr • q = M * Cp * (t1-t2) = Btu/hr • qs = CFM * 60 * Cp * (t1-t2) / Voa = Btu/hr • M = mass of fluid lbm/hr • qs – Sensible heat transfer • Cp – Specific heat of the fluid – • Moa = mass of air lbm/hr BTU/lb*degF • Cp – Specific heat of moist air – BTU/lb*degF • t1 = entering temp degF • t2 = leaving temp degF • Moa = CFM / Voa • CFM = ft3/min • Voa – Specific volume – ft3/lbm Psychrometric – Formula’s Psychrometric – Formula’s • Air heat of absorption / rejection Sensible • Air heat of absorption / rejection Latent • Generally at sea level • ql = Moa * (W1-W2) * Hfg = Btu/hr • qs = CFM * 1.08* (t1-t2) = Btu/hr • ql = CFM * 60 * (W1-W2) * Hfg / Voa = • qs – Sensible heat transfer Btu/hr • CFM = ft3/min • ql – total heat transfer • Voa – Specific volume – ft3/lbm • Moa = mass of air lbm/hr • Generally at altitude • W1 – W2 = Difference in design humidity ratio • qs = CFM * 0.89* (t1-t2) = Btu/hr • Hfg – latent heat of vaporization at indoor • Density at sl = 0.075, Alt = 0.062 conditions – BTU/lbv • Density Ratio is 0.062/0.075= 0.827 • Voa – Specific volume – ft3/lbm • Multiply 0.827 * 1.08 = 0.89 6.
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