An Introduction to Air and Moist Air

Compressed Air Compressed Air Wiki Basic Theory

If you are breathing, do you actually know what you are breathing in? Apart from being vital to life on earth, it is of course the most important ingredient in compressed air. Understanding air is the first step in understanding compressed air. Let us take a closer look.

What is air?

Air is a colorless, odorless and tasteless gas mixture. It is a mixture of many gases, but is primarily composed of oxygen (21%) and nitrogen (78%). This composition is relatively constant, from sea level up to an altitude of 25 kilometers. Air is not a pure chemical substance, but a mechanically-mixed substance. This is why it can be separated into its constituent elements, for example, by cooling. Atmospheric air is always more or less contaminated with solid particles, for example, dust, sand, soot and salt crystals. The degree of contamination is higher in populated areas, and lower in the countryside and at higher altitudes.

What is moist air?

Air can be considered a mixture of dry air and water vapor. If it contains water vapor it’s called moist air, but the air’s humidity can vary within broad limits. Extremes are completely dry air and air saturated with moisture. The maximum water vapor pressure that air can hold increases with rising temperatures. A maximum water vapor pressure corresponds to each temperature. Air usually does not contain so much water vapor that maximum pressure is reached. Relative vapor pressure (also known as relative humidity) is a state between the actual partial vapor pressure and the saturated pressure at the same temperature. The dew point is the temperature when air is saturated with water vapor. Thereafter, if the temperature falls, the water condenses. The atmospheric dew point is the temperature at which water vapor starts to condense at atmospheric pressure. The pressure dew point is the equivalent temperature with increased pressure. The following relation applies:


P = total absolute pressure (Pa)

Ps = saturation pressure at actual temp. (Pa)

φ = relative vapor pressure

V = total volume of the moist air (m3)

Ra = gas constant for dry air = 287 J/kg x K

Rv = gas constant for water vapor = 462 J/kg x K

ma = mass of the dry air (kg)

mv = mass of the water vapor (kg)

T = absolute temperature of the moist air (K)

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an illustration about compressed air in the atlas copco air wiki.

What is Compressed Air?

Compressed air is all around us, but what is it exactly? Let us introduce you to the world of compressed air and the basic workings of a compressor.