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# Air - Drying Force

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.

Air - Drying Force
TemperatureSaturation Vapor Pressure
(10-3 bar)

(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

.

## Related Topics

### • Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

## Related Documents

### Air - Humidity Ratio

The mass of water vapor present in moist air - to the mass of dry air.

### Air - Moisture Holding Capacity vs. Temperature

The moisture holding capacity of air increases with temperature.

### Drying Temperature and Time for Products

Drying temperature and time for products like food, coffee, fruits, lumber and more.

### Evaporation from a Water Surface

Evaporation of water from a water surface - like a swimming pool or an open tank - depends on water temperature, air temperature, air humidity and air velocity above the water surface - online calculator.

### Evaporative Cooling

Evaporative cooling tutorial.

### Food Products - Osmotic Pressures

Osmotic pressure in food products.

### Humid Air - Heating

Enthalpy change and temperature rise when heating humid air without adding moisture.

### Industrial Product and Production Processes - Climate Conditions

Recommended indoor temperature and humidity for common industrial products and production processes.

### Mass vs. Weight

Mass vs. weight - the Gravity Force.

### Moist Air - Enthalpy

Sensible and latent heat of moist air.

### Moist Air - Relative Humidity

Relative humidity in moist air is the ratio of partial vapor pressure to air pressure.

### Moist Air - the 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.

### Moist Air - Water Vapor and Saturation Pressure

Saturation pressure of water vapor in moist air vs. temperature.

### Moisture Content Calculation

Calculate the moisture content in products like wood on wet and dry basis.

### Relative Humidity in Production and Process Environments

Recommended relative humidity in production and process environments - like libraries, breweries, storages and more.

### Removing Heat with Air

Calculating heat removed with air by measuring the wet bulb temperature.

### Water Content in Food and other Products

Water content before and after drying - in food and other products cork, grain, soap, peat, wood and more.

### Wood Species - Moisture Content and Weight

Weight of green and air-dried fire wood.

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