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Drying Force of Air

The drying force of air can be expressed with moisture holding capacity of air and the evaporation capacity from water surface to air

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Air is used in product drying processes for heat and vapor transport.

It is common to transfer heat required to evaporate water from a product from heating coils in a dryer to the drying products by the continuously circulation of air. Evaporated water vapor from the product is removed by replacing some of the circulating air with fresh make-up air with lower specific moisture content.

The drying force of the air is the difference between vapor pressure in the air and saturation pressure at the same temperature. The drying force can be expressed as:

DF = pws - pw                                 (1)

where

DF = Drying Force (mbar, Pa, psi)

pw = vapor pressure (mbar, Pa. psi)

pws = saturation vapor pressure at the actual dry bulb temperature (mbar, Pa, psi)

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.

TemperatureSaturation Vapor Pressure
(10-3 bar)

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(oC)(oF)
-18 0 1.5
-15 5 1.9
-12 10 2.4
-9 15 3.0
-7 20 3.7
-4 25 4.6
-1 30 5.6
2 35 6.9
4 40 8.4
7 45 10.3
10 50 12.3
13 55 14.8
16 60 17.7
18 65 21.0
21 70 25.0
24 75 29.6
27 80 35.0
29 85 41.0
32 90 48.1
35 95 56.2
38 100 65.6
41 105 76.2
43 110 87.8
46 115 101.4
49 120 116.8
52 125 134.2
  • 10-3 bar = 1 millibar

Example - The Drying Force of Air

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

Saturated Air and Mass of Water

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