Introduction
This
application note will
highlight the primary
sources of moisture in industrial facilities.
Different methods of solving moisture problems are outlined along with formulas for estimating the moisture content.
A questionnaire is incorporated
to obtain the minimum
information required for proper sizing
of dehumidification equipment.
Dry Air
Quality and productivity are an ever increasing concern
for today’s businesses. The effect moisture
laden air has on these two concerns is becoming more important as industry strives
for tighter operational tolerances. Manufacturers must insure that products maintain specific
quality specifications and efficiency is maintained throughout the four seasons.
There
are many commercial and industrial applications which require dry air. To eliminate the moisture problem
at a reasonable cost, the specifier needs to know
how much moisture is present, how
did it get in the facility and how to select the proper dehu- midification system.
Methods of Drying Air
There
are several methods
of drying air. Each method has advantages and disadvantages. The common types are:
• Make-up air method
•
Compression
•
Refrigerated dehumidification
•
Desiccant dehumidification
The fist method uses
the principle of dilution, removing a portion
of the moisture laden air from
a space and replacing it with
drier air. The net
result is a lower average
moisture content. This method
is relatively inexpensive to install, but
relies on the fact that drier air
is available. Since
the most common
source is outside make
up air, this method
is difficult to apply in summer
months and expensive to operate in winter due to heating
costs
Using compression to dry air is effective
when small quantities are needed. When
air is compressed, the dew point
is raised, that is,
the temperature at which water
vapor will condense
is raised. This method has high installation and operational costs and is most common when less than 100cfm of dry air is
required
Refrigeration dehumidifiers reduce the moisture in the air by
passing the air over a cold surface, removing the moisture
by condensation. A
detailed discussion on this technique is explained in Desert
Aire Technical Bulletin #1.
This method is effective for desired
conditions down to 45 percent
RH for stan- dard applications. Specially designed systems
can achieve
dew
points as low
as 35°F. This method
has moderate capital costs and can recover much of the latent energy
thus offsetting operating costs.
Desiccant dehumidifiers use special materials that absorb or hold moisture. The material is unique in that it does not change its size or shape when acquiring the moisture and can be regenerat- ed by applying heat. This technique is used effectively to dry air in the range of 0 to 50%RH. It has a relatively expensive capital expense as well as a high operational cost
Sources Of Moisture
There
are many sources
of moisture in a facility. A list of the
Common ones follows:
• Infiltration • Permeation • Ventilation and make-up air
• Door and window openings • People • Processes
• Product
Infiltration and Permeation
Infiltration and permeation are often
considered the same thing. Infiltration is the movement of water vapor
through cracks, joints and seals. Permeation is the migration of water vapor
through materials such as brick and
wood. One of the physical
laws of nature states
that all conditions must be balanced. In the case
of water vapor the
partial pressure of the water
vapor must be the
same on either side of a barrier. For this reason
water vapor will migrate through brick walls
to get to the less humid side.
The rate of migration in an unbalanced situation exceeds the rate of air through cracks and seals and will
in effect find a path to attempt to balance partial pressures.
Moisture load in a space due to infiltration and permeation is not
easily measured. Factors
such as the actual moisture deviation, materials of construction, vapor
barrier and room size all have an
effect on the vapor migration. Desert Aire has used some basic
models to make assumptions to estimate moisture infiltration and permeation.
The Combined infiltration and
permeation load can be approximated
from the following equation:
Lb/HR Moisture = V x AC x GR x MF x CF
7000 x 13.5
Where
V = Volume of room to be conditioned (cu. ft.) AC = Air change factor from Table 1
DGR = The deviation from the outside to
the desired
conditions (grains/LB)
MF = Migration factor
is DGR
÷ 30 (min. value = 1.0)
CF = Construction factor from table 4
13.5
= Conversion factor
for CU. Ft./LB. 7000 = Conversion factor for GR/LB
According to ASHRAE, the
median number of air changes per hour is 0.5. The actual
number of air changes is influenced by several factors, the most dominate being
the size of the room. The larger the room the longer
it takes to convert one volume.
The following table compensates for the reduction in infiltration
/permeation on larger or smaller volumes
VOLUME (CU FT.) AC VOLUME
Less Than 10,000 0.65/HR 40,001-60,000 0.45
10,001-20,000 0.60 60,001-100,000 0.4
20,001-30,000 0.55 100,000-200,000 0.35
30,001-40,000 0.50 Greater than-200,000 0.3
The rate of infiltration is a function
of the magnitude of imbalance
between the outside absolute humidity and that inside the conditioned space. The greater
the difference, the greater the driving force to make the vapor pressures equal. The migration factor
compensates for this influence.
The DGR (grain/lb) deviation must be obtained from the
Psychrometric chart. By locating the outside and inside conditions on the chart an absolute
humidity in grains/lb
can be obtained.
The formula uses the difference in grain/lb between these two conditions. Refer to Table 2 and 3 for
humidity values for specific locations and inside design conditions. For other values
the Psychrometric chart
must be utilized. Please refer to Desert Aire Technical Bulletin Number 3 if assistance is required to read the chart.
Another primary factor is the amount
of moisture that
is allowed to permeate through the walls,
floor and roof.
The construction factor takes
into account the effect good vapor barriers and construction materials will have on the moisture
migration.
Table 4 gives factors for common
construction materials. This factor will vary
between 0.3 and 1.0. A composite wall
must be modeled and a factor estimated.
RELATIVE HUMIDITY
40%
|
50%
|
60%
|
70%
|
|
55
|
25*
|
32*
|
40*
|
45
|
60
|
31*
|
39*
|
46
|
54
|
65
|
37*
|
46
|
55
|
65
|
70
|
42
|
55
|
66
|
78
|
75
|
53
|
66
|
78
|
91
|
80
|
62
|
77
|
93
|
108
|
85
|
72
|
91
|
109
|
128
|
90
|
85
|
108
|
128
|
152
|
DESCRIPTION CF FACTOR
Frame
construction, no vapor barrier 1.00
Masonry, no vapor barrier 1.00
Masonic, vapor proof paint 0.75
Plastic modules 0.75
Frame construction, vapor proof paint 0.75
Frame construction, mylar vapor wrap 0.50
Sheet
metal, good seals 0.50
Glass 0.30
Door Openings
Another source of moisture
is the opening of doors
and windows to the conditioned space or other
openings such as conveyor passages. In these cases, the
amount of moisture is directly proportional to the frequency of the opening,
the difference in indoor and outdoor moisture
con- tent and the wind velocity
at the opening. The wind velocity will be
the most difficult
to take
into account
since it will vary depending
on the location of the opening with respect to the wind
source. Local weather
stations can provide
details on the normal prevailing direction and speed. However, a guideline is 12 CFM of outside
air per square
feet of opening.The
amount of air can be estimated by the following
formula.
LB/HR = AREA x OPEN x DGR x 12
7000 x 13.5
Where:
AREA
= Surface area
of opening ( Sq. Ft.) OPEN = Minutes area
is open per hour
DAG = The deviation from the outside to
the
desired conditions (grains/LB)
12 = Estimated
ingress of moisture (CFM/Sq.Ft.)
13.5 = Conversion factor
for CU.FT./LB 7000 = Conversion
factor for GR/LB
When
this equation is used for a fixed
opening such as a
window, the minutes open/hr will equal 60.
Product, Process
and People
The
three “P’s”,
product, process and people must
also be included in the moisture
evaluation. If the product has an affinity for water,
then it may also release
the water in the conditioned room. For example, wet wood brought
into a conditioned warehouse will release the water at a specific
rate. This can be
determined by measuring the products weight
loss over time.
The process
itself may generate
moisture. If there are open watertanks or cooking vessels,
they will add moisture. A model must be developed for each
process
In the case of, open water tanks,
the evaporation rate can be calculated with the following equation.
LB/HR = 0.1 x AREA x (VPH2O - VPAIR)
Where:
Area = Surface
area of water (square feet).
VPH2O = Vapor
pressure of water
at water temperature VPAIR = Vapor
pressure of air at it’s corresponding
dew point.
The above equation assumes 10 to 30 FPM air velocity in room. Vapor pressures can be obtained from technical publications. Consult Desert Aire if you need assistance. Finally people give off moisture. This is a function of the number of people and their activity: a worker lifting boxes will generate 4 to 8 times the moisture of a worker at a lab bench. ASHRAE’S data on the amount of water added per person is reproduced in table 5.
Ventilation and Make-up Air
If
the facility is using fresh
outside make-up air for ventilation as required by some
building codes, then
this air can
contribute to the moisture
load. This is especially important in the summer months when high humidity is common. As with the
calculation for infiltration the difference in absolute humidity
must be used, along with the volume
of make-up air
being brought in by the
air handling system. The formula for calculating moisture load is:
LB/HR Moisture = CFM x DGR
x 60
7000 x 13.5
CFM
= Volume of outside air introduced
DGR = The
deviation from the outside to the
desired conditions (grains/LB)
60 = Conversion factor for min/hr
13.5 = Conversion factor for CU.FT./LB 7000 = Conversion factor for GR/LB
Conclusion
To properly select and size
a dehumidification system
to condi- tion a facility requires careful planning. The engineer or facility
operator must specify the operating conditions that must be maintained. Then he must evaluate all of the potential sources
of water and the outside ambient conditions. This information can then
be used to size the system. The enclosed worksheet
is provided to organize the collection of minimum information 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 the application
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