Relative Humidity Production Process

Overview

Engineering reference data for Relative Humidity Production Process 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

Symbol	Description	Unit
$\omega$	Humidity ratio	kg/kg
$\phi$	Relative humidity	%
$P_v$	Vapor pressure	Pa
$P_{vs}$	Saturation vapor pressure	Pa
$T_{db}$	Dry bulb temperature	°C
$T_{wb}$	Wet bulb temperature	°C

Recommended Relative Humidity in Production Environments

The following table outlines general recommended relative humidity ranges for various industrial and storage applications to maintain product integrity, prevent corrosion, and inhibit microbial growth.

Microbial Growth and Process Considerations

The critical factor for microbial growth is water activity (a_w), which is directly related to relative humidity. General thresholds are:

• Bacteria: Generally require relative humidity above 90% to grow.
• Mold: Generally require relative humidity above 80% to grow.

The precise limit for a specific organism is influenced by other environmental factors, including:

• pH
• Oxygen availability
• The nature of the solutes present
• Nutrient availability
• Temperature

Important Safety Note: Low relative humidity (typically below 40%) can help prevent rust and corrosion. However, very low humidity (e.g., in environments for explosives) can generate static electricity, posing a risk of spark ignition. This must be considered in process design and material handling.

References

• Original Source