Note! The drying force is not a force as known from the mechanics (Newton). It express the most import variable for the vapor carrying capacity of humid air.
The table below indicates vapor saturation pressure related to temperature.
|Temperature||Saturation Vapor Pressure|
Air is heated from 21oC and 50% relative humidity (A) to 38oC (B).
With the saturation pressure from the table above and the expression for relative humidity the vapor pressure in (A) can be expressed as:
pw = (25 mbar) (50%) / (100%)
= 12.5 (mbar)
The drying force in A can be calculated as:
DFA = (25 mbar) - (12.5 mbar)
= 12.5 (mbar)
Heating the air from A to B don't change the moisture content. The vapor pressure remains constant but the saturation pressure increases. The relative humidity decreases to 19% - the Mollier diagram.
The vapor pressure in B can be calculated as:
pw = (65.6 mbar) (19%) / (100%)
= 12.5 (mbar)
The drying force in B can be calculated as:
DFB = (65.6 mbar) - (12.5 mbar)
= 53.1 mbar
Comparing A and B the "Drying Force" has increased from 12.5 mbar to 53.1 mbar. This has the double effect
Note! - air temperature has major influence on drying capacity
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