Appendix – absorption distance calculation – Aprilaire 1150 User Manual
Page 38
APPENDIX – ABSORPTION DISTANCE CALCULATION
Step 1: Enter the Design Conditions
Percentage of Outdoor Air
OA:
Indoor Air Temperature
T
Indoor
:
Setpoint Indoor RH (for the leaving air)
RH
Leaving
:
Step 2: Find the ‘Moisture Content Factor’ for the leaving air (M
Leaving
)
In
Table 7, find the intersection of T
Indoor
and
RH
Leaving
. The intersection of those
two points is the ‘Moisture Content Factor’ for the leaving air.
M
Leaving
:
Step 3: Find the ‘Moisture Content Factor’ of the entering air (M
Entering
)
Use the following equation to calculate the moisture content of the entering air:
M
Entering
= M
Leaving
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.
M
Entering
:
For example: if Outdoor Air percentage is 20% and the M
Leaving
factor is 2.4, the calculation would be:
M
Entering
= 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 T
Indoor
. Then, slide
across this row and find the value closest to
M
Entering
, which was calculated in Step 3.
Finally, slide up this column to determine the RH for the
entering air.
RH
Entering
:
Step 5: Find the Absorption Distance
Using the Absorption Chart,
Figure 33, draw a vertical line up from the intersection
of
RH
Entering
along the bottom horizontal axis to the intersection of the diagonal line
corresponding to
RH
Leaving
. 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”. (
M
Leaving
would be 3.68,
M
Entering
would be 2.20,
and
RH
Entering
would be 30%)
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