Compressed air is ambient air forced into a smaller volume, resulting in a pressure higher than the surrounding atmosphere. This process involves drawing in standard air and storing it under immense force, making it a versatile energy source for industrial, medical, and commercial applications. The fundamental molecular composition remains identical to the air taken in. The difference between free air and compressed air lies not in the type of gases present, but in the physical state and purity of those gases.
The Gaseous Components of Ambient Air
The gaseous composition of compressed air is a direct reflection of the atmosphere from which it is drawn. Air is predominantly a mixture of two gases. Nitrogen (N2) is the most abundant, accounting for approximately 78.08% of the dry volume of air. Oxygen (O2) is the second largest component, making up about 20.95% of the total volume.
The remaining fraction, just under 1%, consists of various other gases. Argon (Ar) is the third most common gas at roughly 0.93%. A variety of trace elements and compounds, such as carbon dioxide (CO2), neon, helium, and methane, complete the remaining fraction. Water vapor (H2O) is also present, but its concentration is highly variable, ranging from near zero in dry climates to as much as 4% in humid tropical air.
Physical Changes During Compression
The act of compression introduces significant physical changes that fundamentally alter the quality of the air. As a compressor reduces the volume of air, the molecules are forced closer together, dramatically increasing both the pressure and the temperature, a relationship described by the Ideal Gas Law. This compression process generates intense heat, which must be managed by intercoolers and aftercoolers to prevent system damage and to prepare the air for storage.
The increase in pressure coupled with subsequent cooling causes a major reduction in the air’s moisture content. When the air cools, water vapor condenses into liquid water, known as condensate. This process decreases the pressure dew point, a measure of how dry the air is. Liquid water must be removed because it can cause rust and corrosion in pneumatic equipment.
The compressed air also becomes significantly cleaner than ambient air due to filtration steps. Particulate matter, such as dust, rust, and pipe scale, along with oil vapor carried over from lubricated compressors, must be removed before the air is used. Proper treatment ensures the compressed air is dry and free of contaminants that could damage downstream equipment or compromise the end product.
Purity Grades and Specific Uses
While the gas composition of standard compressed air is consistent, the required purity varies drastically depending on the intended application. Industrial compressed air used for powering pneumatic tools or general manufacturing typically requires moderate cleanliness. Applications involving direct contact with sensitive materials or human respiration, however, demand extremely high purity.
The International Organization for Standardization (ISO) 8573-1 standard defines air purity using a three-part code. This code specifies the maximum allowable levels of solid particles, water, and oil. Medical air used in hospitals for ventilators or the air supplied to deep-sea divers requires stringent filtration to remove specific gaseous contaminants like carbon monoxide. Industries like pharmaceutical manufacturing and food and beverage processing often mandate the highest purity levels, sometimes requiring oil-free compressors to ensure zero risk of oil contamination in the final product.