Air is designated as:

Explanation:

Air Refrigerant Designation (R729)

• In refrigeration and air conditioning systems, refrigerants are substances used for heat transfer by undergoing phase changes (such as evaporation and condensation). Refrigerants are assigned unique identifiers, known as "R-numbers," standardized by ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers). These numbers help classify and identify refrigerants based on their chemical composition and properties. Air, as a refrigerant, is designated with the number R729.
• Air is a natural refrigerant that operates on the principle of reverse Brayton (or Joule) cycle, where compressed air is cooled and then expanded to produce refrigeration. In refrigeration systems using air, the cycle involves the following key processes:
  • Compression: Air is compressed, increasing its pressure and temperature.
  • Cooling: The compressed air is cooled using heat exchangers to remove heat.
  • Expansion: The cooled air is expanded through an expansion device, causing its temperature to drop further.
  • Heat Absorption: The low-temperature air absorbs heat from the space or substance to be cooled, completing the refrigeration cycle.

Applications: Air as a refrigerant is commonly used in applications where environmental impact is a critical factor, such as in aircraft refrigeration systems and cryogenic applications.