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
1. Definition of relative air humidity
The relative humidity is defined as ratio of the partial pressure of water in the air over the saturation pressure of water at the dry bulb temperature of the considered humid air.
2. Calculation of relative air humidity
Relative humidity formula :
With :
ε = Relative humidify of air (-)
Pwater = Partial pressure of water in the humid air considered (Pa)
Pwatersat = 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 allows easily to express how much water is in the air, and how far we are from saturation, which means how much more water the air can contain. If the relative air humidity is reaching 100%, the air cannot hold more water, and there will be condensation.
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.
- 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
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The relative air humidity is often used in dry bulk handling where a relative humidity of 45-50% at max 25c is normally a good practice to reduce the risk of caking of the materials. It is also used in all drying processes (spray drying...).
Relative humidity is also used in meteorology.
4. 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.
