Compressed air is atmospheric air stored under pressure in a small volume, such as a tank or cylinder. Pure compressed air is definitively non-flammable and will not ignite or burn. Its fundamental chemical composition prevents it from acting as a fuel source in a combustion reaction.
The Chemistry of Non-Flammability
Flammability requires three components: a heat source, an oxidizer, and a fuel, often called the fire triangle. Compressed air inherently lacks the fuel component necessary to sustain a fire. Atmospheric air is primarily a mixture of nitrogen (about 78%) and oxygen (nearly 21%). The vast majority of air is nitrogen, which is chemically inert and acts as a diluent, slowing down the burning of other materials. Trace gases like argon and carbon dioxide are also non-flammable, confirming that the chemical makeup of air contains no combustible material.
Oxygen’s Role: Supporting Combustion, Not Fuel
The presence of oxygen often leads to the mistaken belief that compressed air is flammable. Oxygen is not a fuel; rather, it is an oxidizer, a chemical agent that supports combustion by chemically combining with a separate fuel source. Compressing air increases the concentration of oxygen molecules in a given volume. While this does not make the air flammable, it intensifies any existing fire. If a fire starts near a compressed air leak, the concentrated oxygen supply will cause the fire to burn hotter, faster, and more aggressively.
Hidden Dangers: When Compressed Air Systems Ignite
Although pure compressed air is non-flammable, two primary mechanisms can create a fire or explosion hazard, usually involving external materials or the physics of compression. The first hazard involves contaminants that act as a fuel source. Industrial compressors use hydrocarbon-based lubricating oils to keep internal components operating smoothly. Over time, oil mist or vapor can leak past seals and mix with the compressed air, forming a combustible mixture with the concentrated oxygen. Consumer “canned air” products also pose a risk, as they often contain flammable propellants like difluoroethane instead of actual compressed air.
Adiabatic Heating
The second danger comes from the physical process of compression itself, known as adiabatic heating. When gas is compressed quickly, mechanical energy converts into thermal energy. Since this happens rapidly, the heat cannot dissipate into the environment, causing the internal temperature of the air and system components to rise dramatically. If this heat reaches the autoignition temperature of oil mist, an internal fire or explosion can occur without any external spark. For instance, while typical lubricating oil autoignites around 365°C at atmospheric pressure, this temperature can drop significantly to 215°C to 255°C under the high pressures found within a compressor system.