For air compressors it is usually necessary to account for water vapor content. It is important to know the moisture content accurately when a process requires a definite quantity of dry air. Furthermore, the moisture in the inlet air affects the power requirement, and water drop-out in intercoolers and aftercoolers.

Note that water-vapor content must also be accounted for as a component in process streams, if present. In those cases, the content is usually available from process engineering, gas and chemical engineering, etc.

The following information discusses how to account for water content in air.

Relative humidity, in percent, may be determined from the following relationship:

Pv = Partial pressure of actual water vapor content
Psat = Partial pressure of water vapor when air is fully saturated at the temperature of interest (can be found in steam tables)

Specific Humidity is the ratio of the weight of the water vapor content to the weight of dry air at the existing conditions of pressure and temperature, and is determined as follows:

Wv = Weight of water vapor
Wda = Weight of dry air
P = Total pressure of the gas mixture (usually atmospheric), in absolute

Relative and specific humidity may be obtained from a psychrometric chart when the wet bulb and dry bulb temperatures are known. However, most psychrometric charts are based on the International Standard sea-level pressure of 14.7 psia, and are, therefore, accurate only for that barometric pressure.

For example, if a standard (14.7 psia) psychrometric chart were used for conditions of 5000 feet elevation (12.23 psia), 80°F dry bulb, and 60°F wet bulb, the indicated specific humidity would be low by about 25%, and the relative humidity low by 10%.

If the altitude is more than 200 or 300 feet above sea level, the following equation should be used instead of a psychrometric chart:

Pv(wb) = Vapor pressure in psia corresponding to wet bulb temperature
(from steam tables)
t = Dry bulb temperature, °F
twb = Wet bulb temperature, °F

Knowing Pv, the relative and specific humidities can be calculated with Equations 100-9 and 100-10. The volumetric or mole percent of the water vapor can be calculated from Equation 100-6 as follows:

The mole percent of dry air is then 100 minus the mole percent of the water vapor. The other properties of the mixture of air and water vapor (molecular weight, MCp, etc.) may then be calculated.

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