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Psychrometrics : the Science of Moisture In 4/16/2018 Psychrometrics: The Science of Moisture in Air Presented by Tom Peterson, CEO and Founder Climate by Design International www.cdihvac.com Owatonna, MN 507-451-2198 4 – 02/2014 1 Psychrometrics or psychrometry The field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. 2 1 4/16/2018 Psychrometric Chart 3 Dry Bulb Temperature The temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. 4 2 4/16/2018 Why Dry Bulb? • Dry Bulb (DB) is what most of us see on the thermometer and it is our best point of reference. 5 Dry Bulb Temperature Dry-bulb temperature (DBT) is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry. It is typically the abscissa (horizontal axis) of the graph. 6 3 4/16/2018 Moisture in Air – H2O 7 I am but a vapor in air 8 4 4/16/2018 Relative Humidity Percentage of saturation at a given temperature ALWAYS relative to a temperature In order to be useful for calculations the corresponding Dry Bulb Temperature is needed. 9 Why Relative Humidity? • Because it’s what we feel. We give up moisture through evaporation our bodies are more in tune with Relative Humidity, or the surrounding air’s ability to take on that moisture. • Because things happen as we approach 100% RH • Fog • Condensation 10 5 4/16/2018 Relative Humidity Relative Humidity The proportional amount of humidity contained in a sample of air compared to the maximum amount of humidity that the sample could hold. 11 Relative Humidity Increasing temperature EXPANDS the air Then that air can hold more moisture The ABSOLUTE amount of water doesn’t change (unless of course you add some) 12 6 4/16/2018 Wet Bulb Temperature Humidity expressed in terms of the instrument used to measure it. Relies on Evaporative Cooling Effect Temperature- dependant Humidity Measure 13 Why Wet Bulb? Wet Bulb (DB) became the first recognized, accepted and used (still is) for communicating moisture in air. 14 7 4/16/2018 Wet Bulb Temperature Air temperature as measured by a thermometer with a wet sensing bulb. As air passes over wet sock water evaporates and reduces its temperature. When the evaporation rate reaches equilibrium with moisture content of surrounding air the result is the wet bulb temperature 15 Enthalpy Specific Enthalpy (h) is the sum of the internal (heat) energy of the moist air in question, including the heat of the air and water vapor within. Enthalpy is given in BTU per pound of dry air. 16 8 4/16/2018 Why Enthalpy? Enthalpy (h) – Primarily used so we can determine energy required to change states. 17 Enthalpy In the approximation of ideal gases, lines of constant enthalpy are parallel to lines of constant WBT. Measured in BTU / lb 18 9 4/16/2018 Air Density Measured in quantity of cubic feet of dry air to make a pound. 19 Why Density? Air Density – Ft3 per Pound • We ultimately are concerned with MASS of air we need to know that mass. • Volume - CFM has become our normal means of discussing volumes of air due to the measurements used in fan air movers, distribution. 20 10 4/16/2018 Example – FT3 per Pound How many pounds of (dry) air are moved with a 13,500 SCFM fan? 1) Find FT3 per Pound 2) SCFM/FT3 per Pound = Pounds (per minute) 3) 13,500 SCFM / ? = ? Pounds per minute. CDI is the Premier Provider of Desiccant and Air Handling Systems 21 Air Density 22 11 4/16/2018 Air Density • Find FT3 per Pound • SCFM/FT3 per Pound = Pounds (per minute) • 13,500 SCFM / 13.5 = 1,000 Pounds per minute. 23 Absolute Humidity -Expression of Moisture Content Irrespective of Temperature -Dewpoint Temperature -Ratio of weight of water to weight of air Pounds of Water per Pound of Dry Air -Grains of Water per Pound of Dry Air TM -Vapor Pressure - inches of Hg Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 201824 – 02/2014 12 4/16/2018 Definitions: Dewpoint -Dewpoint is the Temperature to which Air must be Cooled for condensation to occur. -Many Dehumidification Applications are Dewpoint Applications. TM Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” 20 – 02/2014 www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 25 Dewpoint – Where the 65 Dew falls out F 55 F 45 F 35 F 25 Psychrometrics : The Science of Moisture in AirF “CDI is the Premier Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 26 13 4/16/2018 Dewpoint Dewpoint : is the Temperature to which Air must be cooled for condensation to occur. 27 Weight Ratios water : air -Most commonly used term for moisture calculations - It is a RATIO of weight - Water to Air - Pounds of Water per Pound of Dry Air = too small of a number 14 4/16/2018 Grains per Pound • Grains of Water per Pound of Dry Air • One Pound of Water = 7,000 Grains 29 Grain – a measurement of weight An “average” drop of water weighs about 4 grains….. 30 15 4/16/2018 Grains of Moisture Humidity Ratio: Grains of Moisture per Pound of Dry Air 31 Vapor Pressure -Describes Partial Pressure of the Moisture in the Air. -This is the Motive Force that Drives Water Vapor from High Humidity Areas to Low Humidity Areas. TM Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 550604/16 5 07-451-2198 20 – 02/2014 CDI is the Premier Provider of Desiccant and Air Handling Systems 32 16 4/16/2018 WelcomeVapor Pressure Example You want to control a building interior to 65ºF and 10 Grains/lb. The building is located in Atlanta where the summer design outdoor ambient is 82º FDB and 74º FDP (Dewpoint). Can you eliminate moisture infiltration by pressurizing the room? To what level will he have to raise the room pressure to stopTM Psychrometrics : The Science of Moisture in Air moisture“CDI is the Premier infiltration? Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 550604/16 5 07-451-2198 20 – 02/2014 CDI is the Premier Provider of Desiccant and Air Handling Systems 33 Vapor Pressure Vapor pressure: Describes Partial Pressure of the Moisture in the Air 34 17 4/16/2018 Vapor Pressure Outdoor = 0.847.847 hg hg or 11.513 W.C. Therefore, the space pressure would need to be increased by 10.6” W.C., which is the outside air minus the space (11.513” W.C. – 0.928” W.C.). Indoor = 0.0683in Hg = 0.928” W.C. 35 18 4/16/2018 19 4/16/2018 WelcomeMoisture Calculations • Most Moisture Calculations Rely Upon Absolute Humidity Units • Moisture Capacity = 4.5 x CFM x (Grains in – Grains out) • Permeation = A x Wp x (VP out – VP in) • Evaporation = A x Ht x (VP air – VP water) x (Latent Heat of Vaporization) TM Note: Constants are based on standard conditiPsychrometricsons and : The will Science change of Moisture for non-standard in Air conditions “CDI is the Premier Provider of Desiccant and Air Handling Systems.” 20 – 02/2014 www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 40 CDI is the Premier Provider of Desiccant and Air Handling Systems 4/16 20 4/16/2018 Dewpoint : Recap Humidity Ratio: Vapor pressure: 41 TM Two Parameters identify all the others TM Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 42 21 4/16/2018 Dry Bulb Temperature 63⁰⁰F Relative Humidity 38% RH Specific Humidity 100% Saturated 36 F dewpoint 32 grns / pd Enthalpy Psychrometrics : The Science of MoistureTotal in Air Energy “CDI is the Premier Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 543 07-451-2198 Psychrometric Calculator Electronic Tools are available Psychrometric Calculator - any two points identifies all TM others. Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” 20 – 02/2014 www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 44 CDI is the Premier Provider of Desiccant and Air Handling Systems 22 4/16/2018 Example: Product is stored in a 30ºF cooler. It leaves the cooler and enters an “ambient” loading dock with exterior dock doors. Will there be condensation on the product? 45 Step 1: Determine Ambient Climatic Data Tool Climatic Data 46 23 4/16/2018 Step 2: Plot Conditions •30 DBT •DP •Ambient •Bin Data Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 67° F Step 3: Are there points above? Yes it will condense Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 48 24 4/16/2018 Example -Ambient Design Charleston SC The ASHRAE 0.4% Cooling Design Ambient for is 94ºFDB & 78ºFWB.
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