Psychrometrics : the Science of Moisture In

Home , Enthalpy, Humidity, Psychrometrics

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.

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Psychrometric Chart

Dry Bulb Temperature

The temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture.

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Why Dry Bulb? • Dry Bulb (DB) is what most of us see on the thermometer and it is our best point of reference.

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.

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Moisture in Air – H2O

I am but a vapor in air

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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.

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

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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.

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)

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Wet Bulb Temperature

 Humidity expressed in terms of the instrument used to measure it.  Relies on Evaporative Cooling Effect  Temperature- dependant Humidity Measure

Why Wet Bulb?

Wet Bulb (DB) became the first recognized, accepted and used (still is) for communicating moisture in air.

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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

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.

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Why Enthalpy?

Enthalpy (h) – Primarily used so we can determine energy required to change states.

Enthalpy

In the approximation of ideal gases, lines of constant enthalpy are parallel to lines of constant WBT. Measured in BTU / lb

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Air Density Measured in quantity of cubic feet of dry air to make a pound.

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.

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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.

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Air Density

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Air Density

• Find FT3 per Pound • SCFM/FT3 per Pound = Pounds (per minute) • 13,500 SCFM / 13.5 = 1,000 Pounds per minute.

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

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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

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Dewpoint Dewpoint : is the Temperature to which Air must be cooled for condensation to occur.

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

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Grains per Pound • Grains of Water per Pound of Dry Air • One Pound of Water = 7,000 Grains

Grain – a measurement of weight

An “average” drop of water weighs about 4 grains…..

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Grains of Moisture Humidity Ratio: Grains of Moisture per Pound of Dry Air

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

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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

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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.

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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

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Dewpoint : Recap Humidity Ratio: Vapor pressure:

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

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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

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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?

Step 1: Determine Ambient Climatic Data Tool Climatic Data

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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

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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

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Example -Ambient Design

PSYCHROMETRIC CHART Normal Temperature

O.A. Dewpoint

O.A. Cooling

Answer: Dewpoint design is higher Enthalpy

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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. -Process line must obey rules that describe the specific process.TM

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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 TM Psychrometrics : The Science of Moisture in Air “CDI is the Premier Provider of Desiccant and Air Handling Systems.” a “constant-enthalpy” process. 20 – 02/2014 www.cdihvac.com 2100 Park Drive, Owatonna, MN 55060 5 07-451-2198 52 CDI is the Premier Provider of Desiccant and Air Handling Systems

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Sensible Heating or Cooling

PSYCHROMETRIC CHARTLatent Cooling Normal Temperature

Latent Cooling

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PSYCHROMETRIC CHART Normal TemperatureEvaporative Cooling

Evaporative Cooling

PSYCHROMETRIC CHART NormalDesiccant Temperature Dehumidification

Desiccant Dehumidification

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PSYCHROMETRIC CHART Normal Temperature

Combined Process

Reactivation Out

PSYCHROMETRIC CHART Normal Temperature

Reactivation Inlet Reactivation Heated Process Inlet

Process Outlet Both Sides of a Process - Desiccant

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PSYCHROMETRICEnthalpyCHART RecoveryOutdoor Air Normal Temperature

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

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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

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Control Conditions

New 6i T PSYCHROMETRIC CHART Normal Temperature Lo H Hi T Hi H Lo T Control Level

Lo H Lo T

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PSYCHROMETRIC CHART Normal Temperature

60% A/A Design Range 68F db / 50% RH 51 gr/# 48.7F dewpt 30%

68F 75F

PSYCHROMETRIC CHART Normal Temperature

60% A/A Design Range 51 gr/# 68F db / 50% RH 48.7F dewpt 30% Practical Experience 68F 75F

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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.

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0.20 0.25 0.30 0.35 0.40 SENSIBLE HEAT RATIO = Qs / Qt I-P Units 55 60 SEA LEVEL

BAROMETRIC PRESSURE: 29.921 in. HG 90 200 5 1.3 0 1 5

. 5 0.45 190

1 5 85 . 0 1.2 180

85 170 W 0.50 E 1.1 T B U LB 160 TE M 80 P 4 E 0 R A 1 TU 150 0.55 R 6 E 0 - Outside Air °F 80Outside Air IR 140 A Y .9 R 0.60 D 3 75 F 5 130 O 1 4

D . N 5 U O 120 0.65 .8 P 75 R 5 E 5 P U 3 110 70 T 0 0.70 B - .7 Y P L 100 A 7 0.75 H 0 T N E 65 90 0.80 .6 25 50

1 POINTDEW TEMPERATURE°F - 6 4 0.85 5 . % 0 80 0 9 V 60 ENTHALPY - BTU PER POUND OF DRY AIR -ENTHALPY POUND OF BTU DRY PER O 0.90 .5 INCHES OF PRESSURE -VAPOR MERCURY L U 25% % 0 M 70 0.95 8 60 E 2 - 0 C 55 1.00 % U

0 . 7 F 60 .4 T

. 5 5 P % 50 45 0 E 6 R 1 50 1 3 5 L . 5 5 B 0 . .3 % % 45 50 D 15 R 40 Mixed Air Y 1 4 A 0 Mixed Air 5 % 40 0 I 4 R Y IDIT UM 40 E H 30 35 .2 Post Cooling 1 TIV 3 0% LA . 3 E 5 3 0 R 5 8% 30 4

HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR POUND OFHUMIDITY RATIO GRAINS OF DRY MOISTURE - PER 0 Post Cooling 20 30 25 Return Air 20% 6% HEATSENSIBLE RATIO = Qs / Qt 25 1 20 .1 0 2 IDITY . Return Air M 2 U % 5 E H 4 0 ELATIV 10 15 10% R 10 1 1 5 0 2 2% 0 . 0 0

-10-50 5 101520253035404550556065707580859095100105110115120125130

DRY BULB TEMPERATURE - °F Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com 66 35

0 5 10 15 20 25 30

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What are the Possible Discharge Conditions?

55 6 15 50 5

1 0 Mixed Air 45 4

40 Post Cooling 1 3

. 35 0 30 Return Air 25 1 2 2 . 0 5 15 1 1 0 2 . 0

10 15 20 25 30 35 40 45 50 55 60

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Copyrights

CDI presentations use materials from the following Sources:

-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

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