Air & Water vapor

Some theory

The air is a mixture of about 78% by volume of nitrogen and 21% of oxygen; the remaining 1% is composed of 14 other gases. The composition remains constant, from the sea level up to 20 km height, but the density decreases and varies together with temperature and pressure. At sea level, at a pressure of 1 bar and a temperature of 0°C, the density of air is 1,275 kg/m3 (at the standard pressure of 1,01325 bar, the density is of 1,292 kg/m3). Therefore, 1 kg of air, at sea level and at 1 bar, has a volume of 0.784 m3.

Normally air contains some water vapour. The total pressure of wet air is the sum of the partial pressures of dry air and of water vapor. The air is saturated when the partial pressure of the water vapor is equal to its saturation pressure at that actual temperature.

Absolute humidity is the actual grams of water vapour contained in one cubic metre of humid air. The dew point is the point at which the partial vapour pressure is equal to the saturation pressure. When cooling the air below the dew point, the contained water will be separated from the air, creating the so-called condensate. We know it very well.

Some practice

In all our plants, the air after been compressed, must be cooled, to reduce the water and oil vapor content, through condensation, prior to be sent to customer distribution network.

A typical example of pollution, downstream of the compressor, is as follows:

• Solid particles up to 20 mg /Nm3
• Water vapor up to 80 g/Nm3
• Oil aerosol up to 100 mg/Nm3
• Oil vapor up to 20 mg/Nm3
• Other gases (e.g. CO2, CO, NOx, SOx, H2S, hydrocarbons, NaCl) in concentrations (ppm or ppb) related to the local microclimate
• Microorganisms (to be considered only when we target a bacteriologically pure air).

To treat compressed air effectively and to remove such pollutants, we need to know, at the different points of the plant, the following parameters:

• Pression
• Temperature
• Specific humidity
• Relative humidity
• Enthalpy
• Dew point