Application Note 11

Industrial Dehumidifier Sizing

I ntroduction

This application note will highlight the primary sources of INDUSTRIAL FACILITY
moisture in industrial facilities. Different methods of solving HEATING
COIL
moisture problems are outlined along with formulas for
CONDENSATION / HUMID ENVIRONMENT
estimating the moisture content. A questionnaire is incorporated
WARM, MOIST
to obtain the minimum information required for proper sizing of OUTSIDE AIR
EXHAUST AIR
SUPPLY
dehumidification equipment.

D ry Air
Quality and productivity are an ever increasing concern for
Figure 1 - Make-up Air Method
today’s businesses. The effect moisture laden air has on these
two concerns is becoming more important as industry strives for Using compression to dry air is effective when small quantities
tighter operational tolerances. Manufacturers must insure are needed. When air is compressed, the dew point is raised,
that products maintain specific quality specifications and that is, the temperature at which water vapor will condense is
efficiency is maintained throughout the four seasons. raised. This method has high installation and operational costs
and is most common when less than 100cfm of dry air is
There are many commercial and industrial applications which
required.
require dry air. To eliminate the moisture problem at a reasonable
cost, the specifier needs to know how much moisture is present, BYPASS DAMPER

how did it get in the facility and how to select the proper dehu- CONDENSER COIL
DRY
midification system. WARM
AIR
BLOWER

COMPRESSOR
M ethods of Drying Air
RECEIVER TANK
MOIST
WARM
There are several methods of drying air. Each method has AIR

advantages and disadvantages. The common types are:

• Make-up air method EVAPORATOR COIL

• Compression
• Refrigerated dehumidification Figure 2 - Refrigeration Dehumidifier Schematic
• Desiccant dehumidification
Refrigeration dehumidifiers reduce the moisture in the air by
The fist method uses the principle of dilution, removing a passing the air over a cold surface, removing the moisture by
portion of the moisture laden air from a space and replacing it condensation. A detailed discussion on this technique is
with drier air. The net result is a lower average moisture content. explained in Desert Aire Technical Bulletin #1. This method is
This method is relatively inexpensive to install, but relies on the effective for desired conditions down to 45 percent RH for stan-
fact that drier air is available. Since the most common source is dard applications. Specially designed systems can achieve
outside make up air, this method is difficult to apply in summer dew points as low as 35°F. This method has moderate capital
months and expensive to operate in winter due to heating costs. costs and can recover much of the latent energy thus offsetting
operating costs.
 REGENERATED
AIR OUT effect on the vapor migration. Desert Aire has used some basic
models to make assumptions to estimate moisture infiltration
and permeation.

HUMID The Combined infiltration and permeation load can be

HEATING AIR IN
COILS approximated from the following equation:
V x AC x GR x MF x CF
Lb/HR Moisture =
7000 x 13.5
Where
V = Volume of room to be conditioned (cu. ft.)
ROTATING
REGENERATION DESICCANT AC = Air change factor from Table 1
AIR INLET FILLED WHEEL
∆GR = The deviation from the outside to the desired
DRY AIR OUT
conditions (grains/LB)
Figure 3 - Desiccant Dehumidifier Schematic
MF = Migration factor is ∆GR ÷ 30 (min. value = 1.0)
Desiccant dehumidifiers use special materials that absorb or hold CF = Construction factor from table 4
moisture. The material is unique in that it does not change its 13.5 = Conversion factor for CU. Ft./LB.
size or shape when acquiring the moisture and can be regenerat- 7000 = Conversion factor for GR/LB
ed by applying heat. This technique is used effectively to dry air
According to ASHRAE, the median number of air changes per
in the range of 0 to 50%RH. It has a relatively expensive capital
hour is 0.5. The actual number of air changes is influenced by
expense as well as a high operational cost.
several factors, the most dominate being the size of the room.
S ources Of Moisture The larger the room the longer it takes to convert one volume.
The following table compensates for the reduction in infiltration
There are many sources of moisture in a facility. A list of the /permeation on larger or smaller volumes.
Common ones follows:
VOLUME (CU FT.) AC VOLUME AC
• Infiltration • Permeation • Ventilation and make-up air
• Door and window openings • People • Processes Less Than 10,000 0.65/HR 40,001-60,000 .45
10,001-20,000 0.60 60,001-100,000 .40
• Product
20,001-30,000 0.55 100,000-200,000 .35
30,001-40,000 0.50 Greater than-200,000 .30
I nfiltration and Permeation
Table 1 - Air Changes for Specific Volumes

Infiltration and permeation are often considered the same thing. The rate of infiltration is a function of the magnitude of
Infiltration is the movement of water vapor through cracks, joints imbalance between the outside absolute humidity and that
and seals. Permeation is the migration of water vapor through inside the conditioned space. The greater the difference, the
materials such as brick and wood. One of the physical laws of greater the driving force to make the vapor pressures equal.
nature states that all conditions must be balanced. In the case of The migration factor compensates for this influence.
water vapor the partial pressure of the water vapor must be the
The ∆GR (grain/lb) deviation must be obtained from the
same on either side of a barrier. For this reason water vapor will
Psychrometric chart. By locating the outside and inside
migrate through brick walls to get to the less humid side. The
conditions on the chart an absolute humidity in grains/lb can
rate of migration in an unbalanced situation exceeds the rate of
be obtained. The formula uses the difference in grain/lb
air through cracks and seals and will in effect find a path to
between these two conditions. Refer to Table 2 and 3 for
attempt to balance partial pressures.
humidity values for specific locations and inside design
Moisture load in a space due to infiltration and permeation is not conditions. For other values the Psychrometric chart must
easily measured. Factors such as the actual moisture deviation, be utilized. Please refer to Desert Aire Technical Bulletin
materials of construction, vapor barrier and room size all have an Number 3 if assistance is required to read the chart.
 APPLICATION NOTE 11
Industrial Dehumidifier Sizing

Another primary factor is the amount of moisture that is allowed into account since it will vary depending on the location of the
to permeate through the walls, floor and roof. The construction opening with respect to the wind source. Local weather stations
factor takes into account the effect good vapor barriers and can provide details on the normal prevailing direction and speed.
construction materials will have on the moisture migration. However, a guideline is 12 CFM of outside air per square feet of
Table 4 gives factors for common construction materials. This opening.The amount of air can be estimated by the following
factor will vary between 0.3 and 1.0. A composite wall must be formula.
modeled and a factor estimated.
AREA x OPEN x ∆GR x 12
LB/HR =
RELATIVE HUMIDITY 7000 x 13.5
40% 50% 60% 70%
Where:
55 25* 32* 40* 45
AREA = Surface area of opening ( Sq. Ft.)
60 31* 39* 46 54
65 37* 46 55 65 OPEN = Minutes area is open per hour
DRY BULB

70 42 55 66 78 ∆AG = The deviation from the outside to the

75 53 66 78 91
desired conditions (grains/LB)
80 62 77 93 108
85 72 91 109 128 12 = Estimated ingress of moisture (CFM/Sq.Ft.)
90 85 108 128 152 13.5 = Conversion factor for CU.FT./LB
Table 2 - Grains/LB at specific temperature and RH 7000 = Conversion factor for GR/LB
*Below refrigerant dehumidification capability
When this equation is used for a fixed opening such as a
window, the minutes open/hr will equal 60.
DESCRIPTION CF FACTOR

Frame construction, no vapor barrier 1.00

Masonry, no vapor barrier 1.00
Masonic, vapor proof paint .75
OUTSIDE AIR MOISTURE CONTENT TO BE REMOVED
Plastic modules .75
City Gr. City Gr. City Gr. City Gr.
Frame construction, vapor proof paint .75 Anchorage Fort Wayne
AK 59 121 Las Vegas 82 Lubbock 111
IN NV
Frame construction, mylar vapor wrap .50 AL
Birmingham 126 Indianapolis 130 Reno 59 TX Odessa 109
Mobile 137 KS Wichita 120 Albany 109 San Antonio 128
Sheet metal, good seals .50 AR Little Rock 132 KY Louisville 125 Buffalo 108 UT Salt Lake City 76

Glass .30 AZ Phoenix

Long Beach
102
91 LA
Baton Rouge
New Orleans
136 NY New York
Rochester
121
116
Norfolk
Richmond
124
143 VA 130
Los Angeles AP 96 Shreveport 134 Syracuse 110 Roanoke 113
Table 4 - Construction Factor CA
Sacramento 72 MA Boston 112 Cincinnati 120 VT Burlington 105
San Diego 103 MD Baltimore 120 OH Cleveland 116 Seattle 71
San Francisco AP 67 Caribou 102 Columbus 119 WA Spokane 61
ME
Santa Barbara 85 Portland 106 OK Oklahoma City 125 Yakima 63

D oor Openings CO
Stockton
Denver
72
78
Detroit
Flint
114
117
OR
Eugene
Portland
73
72 WI
Green Bay
Madison
117
115
MI
CT Hartford 111 Grand Rapids 116 Erie 114 Milwaukee 115
DC Washington Nat’l 129 Sault St. Marie 102 Philadelphia 124 WV Charleston 120
Another source of moisture is the opening of DE Wilmington 121 Duluth 98
PA
Pittsburgh 116
CANADA
Daytona Beach 137 MN Rochester 120 Scranton 114
doors and windows to the conditioned space or Fort Myers 143 St. Paul 114 RI Providence 114 AL Calgary 69
Jacksonville 134 Kansas City 126 Charleston 136 BC Vancouver 76
other openings such as conveyor passages. In Miami 137
MO
St. Louis 132
SC
Columbia 122 MN Winnipeg 97
FL
Orlando 136 Jackson 136 SD Sioux Falls 119 NB Saint John 87
these cases, the amount of moisture is directly Pensacola 143
MS
Meridian 127 Bristol 118 NF St. John’s 89
MT
proportional to the frequency of the opening, the Tallahassee
Tampa
136
136
Billings
Cape Hatteras
70
142 TN
Chattanooga
Knoxville
126
124
NS Halifax
Ottawa
100
101

difference in indoor and outdoor moisture con- GA

Atlanta
Augusta
123
128
NC Charlotte
Raleigh
122
126
Memphis
Nashville
132
126 ON
Sudbury 93
91
Thunder Bay
tent and the wind velocity at the opening. The HI Honolulu 117 ND Fargo 109 Brownsville 136 Toronto 108
Des Moines 122 NE Omaha 125 Corpus Christi 141 Windsor 115
IA
wind velocity will be the most difficult to take Davenport/Moline 127 NH Concord 109
TX
Dallas 121
QC
Montreal 106
ID Boise 59 Atlantic City 123 El Paso 99 Quebec 100
NJ
Chicago 118 Newark 121 Fort Worth 123 SK Regina 80
IL
Rockford 119 NM Albuquerque 80 Houston 135

Table 3 - Geographic Outdoor Design Criteria (ASHRAE FUNDAMENTALS 1%)

P roduct, Process and People V entilation and Make-up Air
The three “P’s”, product, process and people must also be If the facility is using fresh outside make-up air for ventilation as
included in the moisture evaluation. If the product has an affinity required by some building codes, then this air can contribute to
for water, then it may also release the water in the conditioned the moisture load. This is especially important in the summer
room. For example, wet wood brought into a conditioned months when high humidity is common. As with the calculation
warehouse will release the water at a specific rate. This can be for infiltration the difference in absolute humidity must be used,
determined by measuring the products weight loss over time. along with the volume of make-up air being brought in by the air
handling system. The formula for calculating moisture load is:
The process itself may generate moisture. If there are open water
tanks or cooking vessels, they will add moisture. A model must CFM x ∆GR x 60
LB/HR Moisture =
be developed for each process. 7000 x 13.5

In the case of, open water tanks, the evaporation rate can be CFM = Volume of outside air introduced
calculated with the following equation.
∆GR = The deviation from the outside to the
desired conditions (grains/LB)
LB/HR = 0.1 x AREA x (VPH2O - VPAIR)
60 = Conversion factor for min/hr

Where: 13.5 = Conversion factor for CU.FT./LB

Area = Surface area of water (square feet). 7000 = Conversion factor for GR/LB

VPH2O = Vapor pressure of water at water temperature

VPAIR = Vapor pressure of air at it’s corresponding C onclusion
dew point.
To properly select and size a dehumidification system to condi-
The above equation assumes 10 to 30 FPM air velocity in room. tion a facility requires careful planning. The engineer or facility
Vapor pressures can be obtained from technical publications. operator must specify the operating conditions that must be
Consult Desert Aire if you need assistance. Finally people give off maintained. Then he must evaluate all of the potential sources of
moisture. This is a function of the number of people and their water and the outside ambient conditions. This information can
activity: a worker lifting boxes will generate 4 to 8 times the then be used to size the system. The enclosed worksheet
moisture of a worker at a lab bench. ASHRAE’S data on the is provided to organize the collection of minimum information
amount of water added per person is reproduced in table 5. required for selection and sizing. The formulas will provide an
approximation of the moisture load. An engineer should be
consulted to confirm that the assumptions are appropriate for
WORK TYPE EVAP. RATE
the application.
Seated in theater 0.10 Lb/Persons/HR
Office work, light work 0.200
Medium factory work 0.475
Heavy factory work 0.965
Athletics 1.095

8300 West Sleske Court

Table 5 - Evaporation Rates of People
Milwaukee, WI 53223
(414) 357-7400
FAX: (414) 357-8501
www.desert-aire.com
111 3/99