Drying Air

Overview

Engineering reference data for Drying Air in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Saturation Vapor Pressure Data

The table below shows saturation vapor pressure of water at various temperatures, essential for calculating drying force.

Example: Drying Force Calculation

Air is heated from 21°C and 50% relative humidity (A) to 38°C (B). The vapor pressure remains constant during heating.

At state A (21°C, 50% RH):

Using the saturation pressure from the table ($p_{ws} = 25$ mbar at 21°C):

$p_w = \frac{25 \text{ mbar} \times 50\%}{100\%} = 12.5 \text{ mbar}$

$DF_A = 25 \text{ mbar} - 12.5 \text{ mbar} = 12.5 \text{ mbar}$

At state B (38°C, same moisture content):

Heating doesn't change moisture content, so vapor pressure remains 12.5 mbar. With $p_{ws} = 65.6$ mbar at 38°C, the relative humidity drops to 19%:

$DF_B = 65.6 \text{ mbar} - 12.5 \text{ mbar} = 53.1 \text{ mbar}$

Result: Heating the air from 21°C to 38°C increases the drying force from 12.5 mbar to 53.1 mbar — a 4.2× increase in drying capacity. This provides:

• Increased moisture transport capacity of the air
• Increased evaporation capacity from water surfaces

Key Insight: Air temperature has a major influence on drying capacity due to the exponential relationship between temperature and saturation vapor pressure.

Important Properties

• Drying force is not a mechanical force (Newton) — it expresses the vapor carrying capacity of humid air
• When air is heated without adding moisture, the vapor pressure remains constant while saturation pressure increases, causing relative humidity to decrease
• The relationship between temperature and saturation vapor pressure is exponential, not linear

Interactive Charts

References

• Original Source