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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Related Topics
- Air Psychrometrics - The study of moist and humid air - air condition - psychrometric charts, Mollier diagrams, air temperature, absolute and relative humidity, moisture content and more .
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Related Documents
- Drying Temperature and Time - common Materials - Drying temperature and drying time for some common materials as coffee, fruits, lumber and more
- Evaporation from Water Surfaces - The amount of evaporated water from a water surfaces - like swimming pools or an open tanks - depends on the temperature in the water and in the air, and the humidity and velocity of the air above the surface - online calculator
- Heat Removed by Air - Heat removed with air - dry and wet bulb temperature
- Heating Humid Air - Calculating enthalpy change and temperature rise when heating moist and humid air without adding moisture
- Humidity Ratio of Air - Humidity ratio (GPP) is the ratio between actual mass of water vapor present in moist air - to the mass of dry air
- Mass and Weight - Weight and mass - the difference
- Mollier Diagram - The Mollier diagram is a graphic representation of the relationship between air temperature, moisture content and enthalpy - and is a basic design tool for building engineers and designers
- Relative Humidity of Air - The relative humidity of air can be expressed by partial vapor and air pressure - density of the vapor and air - or by the actual mass of the vapor and air
- Temperature and Moisture Holding Capacity of Air - The moisture holding capacity of air varies with temperature
- Water Vapor and Saturation Pressure in Humid Air - The saturation pressure of water vapor in moist air varies with temperature
