psychrometrics: the science of moisture in airwet bulb temperature air temperature as measured by a...

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4 – 02/20141

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

Psychrometrics or psychrometryThe field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures.

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Psychrometric Chart

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Dry Bulb Temperature

The temperature of airmeasured by a thermometerfreely 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.

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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

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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 correspondingDry Bulb Temperature is needed.

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Why Relative Humidity?

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• 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

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.

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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

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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

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EnthalpySpecific 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.

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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

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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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CDI is the Premier Provider of Desiccant and Air Handling Systems

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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• Find FT3 per Pound

• SCFM/FT3 per Pound = Pounds (per minute)

• 13,500 SCFM / 13.5 = 1,000 Pounds per minute.

Air Density

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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 – 02/201418 – 02/2014

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 24

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20 – 02/2014

Definitions: Dewpoint-Dewpoint is the Temperature to which Air must be Cooled for condensation to occur.

-Many Dehumidification Applications are Dewpoint Applications.

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65 F

55 F

45 F35 F25 F

Dewpoint –Where the Dew falls out

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Dewpoint

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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 = 7,000Grains

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Grain – a measurement of weight

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

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Humidity Ratio: Grains of Moisture per Pound of Dry Air

Grains of Moisture




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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.

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Welcome



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Vapor 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 youeliminate moisture infiltration by pressurizing the room? Towhat level will he have to raise the room pressure to stop

moisture infiltration? 4/16 33

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Vapor pressure: Describes Partial Pressure of the Moisture in the Air

Vapor Pressure

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Outdoor = .847 hgOutdoor = 0.847 hg or 11.513 W.C.

Indoor = 0.0683in Hg = 0.928” 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.).

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Vapor Pressure

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Welcome



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Moisture 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)

Note: Constants are based on standard conditions and will change for non-standard conditions

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Vapor pressure:Dewpoint :

Humidity Ratio:

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Recap



TM

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Two Parameters identify all the others

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EnthalpyTotal Energy

Relative Humidity38% RH

Dry BulbTemperature

63⁰⁰F

Specific Humidity100% Saturated36 F dewpoint32 grns / pd




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20 – 02/2014

Psychrometric Calculator

Electronic Tools are available

Psychrometric Calculator - any two points identifies all others.

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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?

Example:

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Clim

atic

Dat

a T

ool

Step 1: Determine Ambient

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•30 DBT

•DP

•Ambient

•Bin Data

Step 2: Plot Conditions

67° F



Step 3: Are there points above?

Yes it will condense

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20 – 02/2014

Example -Ambient DesignThe ASHRAE 0.4% Cooling Design Ambient

for is 94ºFDB & 78ºFWB.

Charleston SC

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

Answer: Dewpoint design is higher Enthalpy

O.A. Cooling

O.A. Dewpoint

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Example -Ambient Design

PSYCHROMETRIC CHARTNormal Temperature

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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.

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Psychrometric Processes-Energy goes from Hot to Cold

Left or Right Changes Temperature

-Moisture goes from Wet to DryUp and Down Changes Moisture

-A “Perfect” temperature/moisture exchange is a “constant-enthalpy” process.

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Sensible Heating or Cooling

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Latent Cooling

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Latent Cooling


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Evaporative Cooling

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Evaporative Cooling


Desiccant Dehumidification

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Desiccant Dehumidification


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Combined Process


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Reactivation Out

Reactivation HeatedReactivation Inlet

Process Outlet

Process Inlet

Both Sides of a Process - Desiccant


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Enthalpy Recovery

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Outdoor Air

Exhaust Air

Return Air

Supply Air

Both Sides of a Process –Enthalpy Heat Recovery


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Outside Air

Mixed Air

Return Air

Air Mixing


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TM

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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.

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Control Level

Lo H Hi T

New 6i T

Lo H Lo T

Hi H Lo T

Control Conditions

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A/A Design Range68F db / 50% RH

60%

30%

68F

51 gr/#

48.7F dewpt

75F


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A/A Design Range68F db / 50% RH

60%

30%

68F

51 gr/#

48.7F dewpt

75F

Practical Experience


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20 – 02/2014

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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BAROMETRIC PRESSURE: 29.921 in. HG

I-P UnitsSEA LEVEL

-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

DRY BULB TEMPERATURE - °F

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Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com

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0.95

0.90

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0.400.350.300.250.20SENSIBLE HEAT RATIO = Qs / Qt

SE

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Outside Air

Return Air

Mixed Air

Post Cooling

BAROMETRIC PRESSURE: 29.921 in. HG

I-P UnitsSEA LEVEL

Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com

Outside Air

Return Air

Mixed Air

Post Cooling

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What are the Possible Discharge Conditions?

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CDI is the Premier Provider of Desiccant and Air Handling Systems20 – 02/201468

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20 – 02/2014

CopyrightsCDI 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

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Keep it Dry, with CDI

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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psychrometrics: the science of moisture in airwet bulb temperature air temperature as measured by a...

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