TM-5-855-4
2-8. Moisture loads.
conditions the latent heat of evaporation is approximately 1050 Btu per pound of evaporated water. The
latent heat of evaporation is transferred by the resulting vapors from the wet interface to the place where
the vapors recondense. Water vapor in the air will recondense on any exposed surface at a temperature
below the air dewpoint.
b. The vapor pressure PW of the water, in pounds per square inch absolute (psia), increases rapidly
with the temperature T in 1? according to the Tentens formula.
(eq 2-10)
The saturated vapor pressure Ps of air at dry-bulb temperature Ta is computed by setting T = Ta in
equation 2-10. The actual vapor pressure Pa of the air is then computed from its known relative humidity
which, by definition, is the ration of Pa to Ps . The air dewpoint temperature Tb is also computed from
equation 2-10 by setting Pw
(eq 2-11)
positive above the air dewpoint. Air movement is also a factor since it prevents vapor buildup and
saturation of the air above the evaporating surface. For a surface L feet long with air flowing parallel to it
at a velocity v in fpm, the average mass transfer coefficient in lb/h ft 2 per psi saturation deficit is
approximately:
(eq 2-12)
For more complicated interface geometries, Gp is computed by analogy with the heat transfer coefficient h'
in Btuh/ft 2 F, using the Lewis relation for moist air at 14.7 psi absolute pressure, 0.24 Btu/lb F specific
heat, and 0.622 water to air molecular weight ratio.
(eq 2-15)
d. Underground, the computation of moisture loads will be based on site specific data. A site survey
will determine the amount of water entering trough fissures, collected in pools, and the excess water to be
drained or pumped away. Determining rock seepage and other hidden or intermittent sources will
require extended observations. Moisture from equipment, materials, processes, personnel, fresh air,
infiltration from uncontrolled areas, and other sources will be included in the design. Experience
indicated that failure to account for these loads disrupts the entire humidity control process.
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