Humid Air : Relative air humidity
Definition of relative air humidity and associated air psychometric calculations
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1. Definition of relative air humidity
2. Calculation of relative air humidity
3. Use of relative air humidity
4. Interactive Relative Air Humidity Calculator
5. Relative air humidity calculator Excel
1. Definition of relative air humidity
The relative humidity is defined as the ratio of the partial pressure of water in the air to the saturation pressure of water at the dry‑bulb temperature of the considered humid air. Detailed explanations and additional psychrometric calculations are available in the free online Process Engineering Handbook on air psychrometrics.
2. Calculation of relative air humidity
Relative humidity formula :
With :
\(\varepsilon\) = Relative humidity of air (-)
\(P_{\text{water}}\) = Partial pressure of water in the humid air considered (Pa)
\(P_{\text{water}}^{\text{sat}}\) = saturation pressure of water at the dry bulb temperature of the humid air (Pa)
The relative humidity is often noted as RH and expressed as a percentage :
The relative humidity is an easy way to express how much water is in the air and how far we are from saturation, indicating how much more water the air can accommodate. If the relative air humidity reaches 100 %, the air cannot hold more water, and condensation will occur. For a straightforward method to determine water activity from equilibrium relative humidity, calculate water activity based on equilibrium relative humidity water activity prediction using the BET isotherm model.
Example : what does a relative humidity of 80% mean ? This means that the partial pressure of water in the humid air amounts to 80% of the saturation pressure of water at this temperature, this also mean that the air can contain 20% of the saturation pressure of water before reaching the dew point.
If the absolute humidity is constant whatever the temperature and pressure (provided there is no condensation), the relative humidity is changing according to pressure and temperature. For a deeper understanding of absolute air humidity, see absolute air humidity definition and calculation.
- If the temperature increases, the relative humidity decreases as the saturation pressure, in the denominator of the equation above, is increasing (and reverse if the temperature decreases, the relative humidity increases, that is why relative humidity is high in winter)
- If the pressure increases, the relative humidity increases as the partial pressure of water, which is in the numerator of the equation above, is increasing (and reverse if the pressure decreases, the relative humidity decreases)
3. Use of relative air humidity
The relative air humidity is often used in dry bulk handling where a relative humidity of 45‑50 % at a maximum of 25 °C is normally a good practice to reduce the risk of caking of the materials. For optimal storage of produce, maintaining these humidity levels is essential; see optimal storage guidelines for produce. It is also used in all drying processes (spray drying, etc.) and in evaporative salt production rate calculations.
Relative humidity is also used in meteorology.
💡 Practical Plant Engineering & Design Rules of Thumb
- Powder Storage & Handling: Maintain relative humidity below 45–50% RH at temperatures \(\le 25^\circ\text{C}\) to prevent water sorption, cohesive bridging, and caking in silos, hoppers, and pneumatic conveying lines.
- Condensation & Dew Point Safety Margin: Process air streams should maintain at least a 5°C to 10°C margin above the dew point temperature to prevent localized pipe wall condensation.
- Spray Drying Systems: Exhaust air relative humidity is typically controlled between 20% and 50% RH to ensure proper particle drying without scorching or sticky wall deposition.
- Compressed Air Systems: Relative humidity must be driven near 0% (Dew Point < -40°C) using desiccant dryers to prevent line corrosion and instrument air valve freezing.
4. Interactive Relative Air Humidity Calculator
Calculation Summary Results
5. Relative air humidity calculator Excel
MyEngineeringTools.com has developed a free Excel calculator that allows you to calculate the relative humidity of air, knowing the partial pressure of water in air and the temperature, it allows also many other calculations related to humid air : Humid air enthalpy calculator Excel
Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.
