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
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
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
Moisture in Air H2O 7 I am but a vapor in air 8 4
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
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
Wet Bulb Temperature Humidity expressed in terms of the instrument used to measure it. Relies on Evaporative Cooling Effect Temperaturedependant 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
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
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
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
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
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 -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 TM 20 18 24 02/2014 12
Definitions: Dewpoint -Dewpoint is the Temperature to which Air must be Cooled for condensation to occur. -Many Dehumidification Applications are Dewpoint Applications. 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 TM 20 02/2014 25 Dewpoint Where the Dew falls out 65 F 55 F 45 F 35 F 25 F 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 26 13
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
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
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. 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 4/16 5 07-451-2198 20 02/2014 32 CDI is the Premier Provider of Desiccant and Air Handling Systems TM 16
Vapor Pressure Example Welcome 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 TM what level will he have to raise the room pressure to stop Psychrometrics : The Science of Moisture in Air CDI the Premier Provider of Desiccant and Air Handling Systems. moisture infiltration? www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 4/16 5 07-451-2198 CDI is the Premier Provider of Desiccant and Air Handling Systems 20 02/2014 33 Vapor Pressure Vapor pressure: Describes Partial Pressure of the Moisture in the Air 34 17
Vapor Pressure Outdoor = 0.847 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
19
Moisture Calculations Welcome 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) Note: Constants are based on standard conditions Psychrometrics and : The will Science change of Moisture for non-standard in Air conditions CDI is the Premier Provider of Desiccant and Air Handling Systems. 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 TM 20 02/2014 4/16 20
Recap Dewpoint : Humidity Ratio: Vapor pressure: 41 TM Two Parameters identify all the others 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 TM 42 21
Dry Bulb Temperature 63⁰⁰F Relative Humidity 38% RH Specific Humidity 100% Saturated 36 F dewpoint 32 grns / pd Enthalpy Total Energy 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 543 07-451-2198 Psychrometric Calculator Electronic Tools are available Psychrometric Calculator - any two points identifies all others. 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 CDI is the Premier Provider of Desiccant and Air Handling Systems TM 20 02/2014 44 22
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 46 23
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
Example -Ambient Design Charleston SC The ASHRAE 0.4% Cooling Design Ambient for is 94ºFDB & 78ºFWB. The ASHRAE 0.4% Dehumidification (Dewpoint) Design Ambient Is 84ºFDB and 78ºFDP. Plot these points on the chart. Which design ambient condition has the higher enthalpy? 49 20 02/2014 Example -Ambient Design PSYCHROMETRIC CHART Normal Temperature O.A. Dewpoint O.A. Cooling Answer: Dewpoint design is higher Enthalpy 50 25
Psychrometric Processes -Changes in temperature and/or moisture content can be described on a Psychrometric Chart. -Multiple blended air streams can be shown as process lines. -Relative volumes of blended air streams can be described. TM -Process line must obey rules that describe the specific process. CDI is the Premier Provider of Desiccant and Air Handling Systems 51 20 02/2014 4/16 Psychrometric Processes -Energy goes from Hot to Cold Left or Right Changes Temperature -Moisture goes from Wet to Dry Up and Down Changes Moisture -A Perfect temperature/moisture exchange is a constant-enthalpy process. 52 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 CDI is the Premier Provider of Desiccant and Air Handling Systems TM 20 02/2014 26
Sensible Heating or Cooling 53 PSYCHROMETRIC CHART Normal Temperature Latent Cooling Latent Cooling 54 27
Evaporative Cooling PSYCHROMETRIC CHART Normal Temperature Evaporative Cooling 55 Desiccant Dehumidification PSYCHROMETRIC CHART Normal Temperature Desiccant Dehumidification 56 28
PSYCHROMETRIC CHART Normal Temperature Combined Process 57 Reactivation Out PSYCHROMETRIC CHART Normal Temperature Reactivation Inlet Process Inlet Reactivation Heated Process Outlet Both Sides of a Process - Desiccant 58 29
PSYCHROMETRIC CHART Normal Temperature Enthalpy Recovery Outdoor Air Exhaust Air Supply Air Return Air Both Sides of a Process Enthalpy Heat Recovery 59 Outside Air PSYCHROMETRIC CHART Normal Temperature Mixed Air Return Air Air Mixing 60 30
Control Conditions Use the Chart to visualize control conditions Example You want to control a space to 70º F + 5º F and 40% Relative Humidity + 2%. Show this on the Psychrometric chart. TM CDI is the Premier Provider of Desiccant and Air Handling Systems 61 20 02/2014 4/16 Control Conditions New 6i T PSYCHROMETRIC CHART Normal Temperature Hi H Lo T Lo H Hi T Control Level Lo H Lo T 62 31
PSYCHROMETRIC CHART Normal Temperature 68F db / 50% RH 60% 30% A/A Design Range 51 gr/# 48.7F dewpt 68F 75F 63 PSYCHROMETRIC CHART Normal Temperature 68F db / 50% RH Practical Experience 60% 68F 30% 75F A/A Design Range 51 gr/# 48.7F dewpt 64 32
45 4/16/2018 Sensible to Total Heat Ratio Sensible Load / Total Load = Heat Ratio. Heat Ratio of 0.95 means mostly Sensible. Heat Ratio of 0.50 means LARGE Latent (moisture) Component. 20 02/2014 65 0.20 0.25 0.30 0.35 SENSIBLE HEAT RATIO = Qs / Qt 0.40 I-P Units SEA LEVEL BAROMETRIC PRESSURE: 29.921 in. HG 50 55 90 60 200 1.3 15.5 190 0.45 15.0 180 85 1.2 170 0.50 1.1 160 80 0 0 5 5 10 10 15 Post Cooling Post Cooling 12.0 20 15 30 25 20 Mixed Air 25 Mixed Air 30 12.5 35 ENTHALPY - BTU PER POUND OF DRY AIR 40 45 13.0 50 40 35 55 90% 80% 70% 60% 50% 40% 30% 20% 60 65 Return Air Return Air 13.5 70 10% RELATIVE HUMIDITY 75 80 14.0 VOLUME- CU.FT. PER LB. DRY AIR 85 WET BULB TEMPERATURE - F Outside Air Outside Air 14.5 25% 15% 8% RELATIVE HUMIDITY 6% 4% 2% HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 75 70 65 60 55 50 45 40 35 30 25 20 10 0 DEW POINT TEMPERATURE - F 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 SENSIBLE HEAT RATIO = Qs / Qt 1.9.8.7.6.5.4.3.2.1 VAPOR PRESSURE - INCHES OF MERCURY 60 55 50 45 40 ENTHALPY - BTU PER POUND OF DRY AIR -10-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com DRY BULB TEMPERATURE - F 66 35 0 5 10 15 20 25 30 33
What are the Possible Discharge Conditions? 7 55 6 15 15 10 10 Mixed Air 45 45 50 50 5 4 Post Cooling 40 25 30 12.5 35 35 13.0 Return Air 10 10 12.0 15 15 20 20 10 15 20 25 30 35 40 45 50 55 60 67 67 CDI is the Premier Provider of Desiccant and Air Handling Systems 68 20 02/2014 34
Copyrights CDI presentations use materials from the following Sources: -Internal CDI equipment pictures, installations, performance data, etc 1985-2017 Climate by Design International, Inc. -ASHRAE, AORN, ASHE and other public Standards and Engineering documents in the Public Domain -Pictures and graphics from the Public Domain CDI is the Premier Provider of Desiccant and Air Handling Systems. www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 507-451-2198 20 02/2014 69 A special thanks to the following organizations to help make this presentation possible: Scott Limberg of Limberg Productions for his video work. www.limbergproductions.com Jonny Peterson of Jonny P Design for the animation videos. www.jonnypdesign.com Shawn Turek for the Dr. Dry illustrations. www.shawnturek.com www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 507-451-2198 Keep it Dry, with CDI 70 35