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APPENDIX – ABSORPTION DISTANCE CALCULATION
Step 1: Enter the Design Conditions
Percentage of Outdoor Air
OA:
Indoor Air Temperature
TIndoor:
Setpoint Indoor RH (for the leaving air)
RHLeaving:
Step 2: Find the ‘Moisture Content Factor’ for the
leaving air (MLeaving)
In Table 7, find the intersection of TIndoor and RHLeaving. The intersection of those
two points is the ‘Moisture Content Factor’ for the leaving air.
MLeaving:
Step 3: Find the ‘Moisture Content Factor’ of the
entering air (MEntering)
Use the following equation to calculate the moisture content of the entering air:
MEntering = MLeaving x (1 - OA/100)
Note: this step assumes that the outdoor air being brought into the system will
contribute an insignificant amount of moisture relative to what’s being added by
the humidifier.
MEntering:
For example: if Outdoor Air percentage is 20% and the
MLeaving factor is 2.4, the calculation would be:
MEntering = 2.4 x (1 - 20/100) = 1.92
Step 4: Find the RH for the entering air
In Table 7, find the row that corresponds to the temperature TIndoor. Then, slide
across this row and find the value closest to MEntering, which was calculated in Step 3.
Finally, slide
up this column to determine the RH for the entering air.
RHEntering:
Step 5: Find the Absorption Distance
Using the Absorption Chart, Figure 33, draw a vertical line up from the intersection
of RHEntering along the bottom horizontal axis to the intersection of the diagonal line
corresponding to
RHLeaving. From this point draw a line horizontally to the intersection
of the duct height. This is the minimum required absorption distance for the given duct
height and the above Design Conditions.
Note: ‘duct height’ assumes a horizontal flow system. For vertical drop systems, duct
height should be considered as the duct dimension perpendicular to the distribution tube.
Absorption Distance
:
Example: The design conditions of 40% outdoor air, 72°F indoor air temperature, and an indoor RH setpoint of 50% would
require an absorption distance of 20” in a system with a duct height of 18”.
(MLeaving would be 3.68, MEntering would be 2.20,
and RHEntering would be 30%)
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