While air can be liquefied under specific extreme conditions of high pressure and very low temperature, it's not possible to compress air in a typical air compressor to the extent that it turns into a liquid due to the significant temperature increase that occurs during the compression process.
The Challenge of Air Liquefaction in Air Compressors
The primary reason compressed air does not turn into a liquid in conventional air compressors lies in the physics of compression itself. When air is compressed, its molecules are forced closer together, increasing the pressure. However, this process also generates heat, causing the air's temperature to rise substantially.
As stated in the reference: "Unfortunately, when you compress air in an air compressor, you typically end up raising the temperature. And so, it's not possible to compress air to the extent that it turns into a liquid." This increase in temperature prevents the air from reaching its critical temperature, which is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied. For air (primarily nitrogen and oxygen), these critical temperatures are extremely low (e.g., around -147°C for nitrogen and -118°C for oxygen).
The reference also notes a contrast: "But you do (kind of) do this with water." This highlights that water vapor, unlike the air components, has a much higher critical temperature, making it relatively easy to condense into liquid water during compression if its partial pressure reaches saturation at the elevated temperatures.
Why Temperature is Key
To liquefy a gas like air, both high pressure and extremely low temperatures are required. The high temperature generated by typical compression methods keeps the air molecules moving too rapidly to settle into a liquid state, even under increased pressure. For air to become liquid, its temperature must be brought down to cryogenic levels (typically below -190°C or -310°F), far colder than what can be achieved by simple compression alone.
What Happens Instead? (Moisture Condensation)
Instead of the air itself turning into a liquid, what often happens in compressed air systems is the condensation of water vapor present in the air. As hot, humid air cools after compression, its ability to hold moisture decreases, causing the water vapor to condense into liquid water. This is why air compressors often have water separators or dryers.
Comparing Typical Compression vs. Industrial Liquefaction
The table below illustrates the fundamental differences between how air behaves in a standard compressor and the conditions required for true air liquefaction in industrial processes:
| Feature | Typical Air Compression (in an Air Compressor) | Industrial Air Liquefaction (Cryogenics) |
|---|---|---|
| Primary Goal | Increase pressure for power or storage | Produce liquid nitrogen, oxygen, argon |
| Temperature | Increases significantly | Cooled to extremely low (cryogenic) temperatures |
| Pressure | Increases | Increases substantially |
| Result | Air remains gaseous (but water vapor condenses) | Air turns into a liquid |
| Reference's Stance | "Not possible to compress air to the extent that it turns into a liquid" | Not directly discussed but implied through the "not possible" under those conditions |
In conclusion, while compressed air in the common sense (from an air compressor) does not become liquid, air can be liquefied under specific, highly controlled industrial processes that involve both extreme pressure and deep cooling.
