Illustrating HumidityAIHA Conference & Exposition

Philadelphia, PAJune 7, 2007

PO 128 – IEQ General Interest

Presented By : Paul Haas CSP, CIH

Teaching Psychrometrics

• We teach classes describing dehumidification

• Students range from PhDs to remediation workers

• Many students had difficulty understanding the concepts

• We developed animations to illustrate the behavior of water vapor

Water

• As everybody knows water is simply two hydrogen atoms bonded to one oxygen atom (H2O)

• Water Molecule• H2O as a single molecule is a gas (Water

Vapor).

Hydrogen Bond

• The molecules of water are charged. Positive on one side, negative on the other. This makes them want to stick together positive to negative in what is called a Hydrogen Bond.

• Hydrogen Bond

Kinetic Energy

• Heat makes the molecules of water vibrate and bounce off one another

• Kinetic Energy - Latent Heat• This vibration is kinetic energy as it is

energy in the form of movement of the molecules. The more heat the more vibration.

• If it were not for this kinetic energy we would have ice in a crystal structure (Ic)

Liquid Water

• If there is kinetic energy between 32° and 212°F the molecules bounce off one another sufficiently to allow movement as a liquid

• Liquid water behaves as large agglomerates of water molecules held together by hydrogen bonds

• It behaves more like H252O126

Liquid Water (Continued)

• This is why membranes like Tyvek™ work they have holes big enough to allow H2O to pass, but too small to allow H252O126 to pass.

• The first part of this animation shows how water molecules are held together to form liquid water – Coalescence - Evaporation

Evaporation

• At all temperatures above absolute zero there will be enough kinetic energy that the occasional water molecule will escape as water vapor– Below 32°F this is called sublimation– Between 32° and 212° F it is called evaporation– Above 212°F it is boiling releasing steam

• The second part of this animation illustrates evaporation

• Coalescence - Evaporation

Condensation

• Water vapor stays as a gas because there is enough kinetic energy to keep the individual water molecules bounding off each other

• If the temperature is cooled the molecules slow enough that the hydrogen bond sticks them together and they condense out as water

• Condensation

Condensation

• Typically this condensation occurs when the water vapor encounters a chilled surface

• Where there are mold problems with drywall the chilled surface is typically drywall panels that have been cooled by air conditioning

Psychrometrics

• Not surprisingly the behavior of water vapor is of great importance in air conditioning and other building issues

• The relationships were studied at length and equations developed to describe the phenomenon

• These relationships are expressed graphically in the psychrometric chart

Psychrometric Chart

30 40 50 60 70 80 90 100 110 120 130

25

50

75

100

175

Dry bulb temperature (°F)

Spe

cific

hum

idity

(Gra

ins

of w

ater

vap

or p

er p

ound

of d

ry a

ir)

Air

dew

poih

t (°F

) Air

dew

poih

t (°F

)(In

ches

of m

ercu

ry)

0.1

0.1

0.7

0.9

1.1

1.3

2030

40

50

60

70

80

86

90 %

70 %

50 %

40 %

30 %

20 %

10 %

2 %

Enthalp

y - B

tu pe

r pou

nd of

dry a

ir

Relativ

e Hu

midity

10

15

20

25

30

35

40

45

50

55 60

20 2530

3540

4550

55

60

65

75

80

85° F Wet bulb temperature

90

95

Dry Bulb Temperature

• Temperature of the air as measured by a dry thermometer

• When you refer to temperature you are generally referring to the dry bulb temperature70°

Dry bulb temperature

Relative Humidity

• Relative Humidity –The amount of moisture in the air as a percent of the amount of moisture the air can hold

• Warm air can hold more moisture than cool air

50%

Relative Humidity

Specific Humidity

• The amount of moisture in the air in grains of water vapor per pound of air

55 grS

peci

fic h

umid

ity

Vapor Pressure

• Pressure due to the water molecules in gaseous water

• Expressed in inches of mercury

• Convert to inches of water by multiplying by 13.6

0.37"V

apor

pre

ssur

e

Dew Point

• The temperature at which the air can no longer hold the moisture that is in it.

• At the dew point moisture condenses out as dew on a surface or as a fog.

50° 50°Air

dew

poin

t tem

pera

ture

Satu

ratio

n cu

rve

Dew Point

• Temperature of surface above dew point – Surface stays dry

• Temperature of a surface below dew point– Moisture condenses on surface

• Air below dew point– fog

Enthalpy

• The amount of heat in the air.

• This is the sum of the heat that is in the air temperature plus the heat of conversion to keep water in a vaporous form.

Enthalp

y

25.4 Btu

Wet Bulb Temperature

• Air temperature as measured by a wet bulb thermometer

• Wet bulb temperature when compared to dry bulb temperature can be used to determine relative humidity

58.5°

Wet bulb temperature

Psychrometrics

• Relative humidity – Changes with temperature– Not a measure of the total amount of moisture in the

air.• Specific Humidity and Dew Point

– both are a measure of the total moisture in the air.• Dew Point

– Depends on total moisture in air– Can be directly related to temperature measurements

of surfaces to determine condensation risk.

Example

• Consider a hot humid day 90°F and 90% RH

• We want to condition the air to 70°F at about 50% RH

• We do this by chilling the air far enough to condense out enough moisture to dehumidify it

• Our goal is air with 50 to 55 grains of moisture per pound of air

Using the Psychrometric Chart

• Plot 90° and 90% RH on the chart• There is 195 grains of moisture per pound

of air• Cool the air to 50 – Now there is 53 grains

of moisture and 100% RH. • 142 grains of moisture have condensed

out – the air is now dehumidified. • Cool from 90 degrees, 90% RH to 50

degrees

Using the Psychrometric Chart (2)

• The air is dehumidified, but cold (50°F) and is at 100% RH

• However it only has 53 grains of moisture• When warmed to 70°F the RH drops to

49%• Warm Back Up to 70 degrees• Summary

Tying It All Together

• We had hot humid air– chilled it to condense

the moisture out– warmed it back up to

reach our target humidity

• Chart at left gives the temp, RH and specific humidity at each step

53 gr49%70º

53 gr100%50º

195 gr.90%90º

Specific Humidity

Relative HumidityTemp

Tying It Together (2)

• Water molecules as a gas – more kinetic energy

• Water molecules as liquid – less kinetic energy

• Temperature and RH at each step

• Psychrometric Chart for each step

This animation ties the whole thing together• On the left we see what the water

molecules are doing• On the right we see the temp, RH and

Psychrometric chart• Dehumidification

Dehumidification

.6

.5

.4

.3

.1

.2

VAP

OO

R P

RES

SUR

E -

INC

HE

S O

F M

ERC

UR

Y

DEW

POIN

T TE

MP

ERAT

UR

E - °

F

85

80

75

70

65

60

55

50

45

40

35

30

2520

100

2%

4%

6%

8%

10% RELATIVE HUMIDITY

15%

20%

25%

30%

40%

50%

REL

ATIV

E HU

MID

ITY

60%

70%

80%

DRY BULB TEMPERATURE - °F

HU

MID

ITY

RA

TIO

- G

RAI

NS

OF

MO

ISTU

RE

PER

PO

UN

D O

F D

RY

AIR

DHUMIDIFICATION- 90° & 90% RH TO 50°

DEHUMIDIFICATION

90%

REL

ATIV

E HU

MID

ITY

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

25

10

10

10

35

40

45

ENTH

ALPY

- BT

U PE

R PO

UND

OF

DRY

AIR

SATU

RATI

ON

TEM

PERA

TURE

- °F

3035

40

45

50

55

60

65

70

75

80

85

50 55 60 651.3

1.2

1.1

1.0

.9

.8

.7

Dehumidification

• Chill air below dew point• Water condenses out on the fins of the

cooling coil• The leaving air has a reduced dew point

and specific humidity (50 to 55 grains/pound)

• The leaving air will be saturated and have a relative humidity of about 100%

• When warmed it will be at about 50% RH

Psychrometric Resources

• Munters Corporation– The Dehumidification Handbook, Second Edition– PsyCalc – Psychrometric calculator– Munters Corporation, 79 Monroe Street, Amesbury, MA 01913, (800) 422-6379,

web page: www.muntersamerica.com. • Linric Company

– PsyCalc – Psychrometric calculator– PsyFunc – Excel functions– Linric Company, P.O. Box 10303, Bedford, NH 03110, Fax: (603) 472-4823 web

page: www.linric.com• ASHRAE

– Psychrometric charts– ASHRAE Handbook of Fundamentals– Humidity Control Handbook– ASHRAE, 1791 Tullie Circle, NE, Atlanta, GA 30329, email: orders@ashrae.org,

phone (800) 527-4723, web page www.ashrae.org

Morse Zehnter AssociatesRoger G. Morse AIA504 Snake Hill Road

Poestenkill, NY 12140(518) 283-7671

rgmorse@mzaconsulting.com

Coauthor’s - Paul Haas and Dean Zehnter2240 Palm Beach Lakes Blvd., Suite 300

West Palm Beach, FL 33409(561) 712-4777

phaas@mzaconsulting.com