Drying Force of Air
The Drying Force of air may be expressed as the moisture holding capacity of the air and the evaporation capacity from a water surface to the air
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Air is commonly used in drying processes for heat and vapor transport.
The heat required to evaporate water from the product may be transferred from heating coils in the dryer to the drying product by continuously circulation air. Evaporated water vapor from the product is removed by replacing some of the circulating air with fresh make up air with a lower specific moisture content.
The drying force is the difference between the vapor pressure in the air and the saturation pressure at the same temperature. The drying force can be expressed as:
DF = pws - pw (1)
where
DF = Drying Force (mbar)
pw = vapor pressure (mbar)
pws = saturation vapor pressure at the actual dry bulb temperature (mbar)
Note! The drying force is not a force (Newton) as known from the mechanics. It express the most import variable for the vapor carrying capacity in humid air.
The table below can be used to determine the vapor saturation pressure in mbar at different temperatures:
| Temperature | Saturation Vapor Pressure (mbar, millibar, mb) |
|
| (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 |
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 a double effect:
- the moisture transport capacity of the air is increased
- the evaporation capacity from water surfaces in the air is increased
Note! The temperature has the major influence on the drying effect of air.
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