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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