What phrase best describes the term "free air"?

The phrase that best describes "free air" is air at atmospheric conditions. Free air refers to the ambient air surrounding us, which exists at a pressure that is equal to atmospheric pressure and at normal temperature conditions. This concept is important in refrigeration and HVAC applications, as understanding the properties of free air is crucial for calculating processes involving heat transfer, pressure changes, and other thermodynamic principles.

In many refrigeration systems, the behavior of refrigerants when interacting with free air is a fundamental factor in determining efficiency and performance. Recognizing that free air is simply the air we breathe, under normal atmospheric conditions, is essential for engineers when designing and troubleshooting systems that rely on air as a heat transfer medium.