When is checking superheat primarily applicable?

The correct choice emphasizes that checking superheat is primarily applicable in systems using thermal expansion valves (TXVs). Superheat is a measure of the temperature of a vapor above its boiling point at a given pressure, and it is an essential parameter for ensuring that the refrigerant is fully vaporized before entering the compressor. In systems with TXVs, the superheat is critical because these valves modulate the flow of refrigerant based on the amount of superheat present.

In contrast, systems using fixed orifice metering devices do not require superheat checks in the same way because these devices operate under different principles. They do not control the refrigerant flow based on superheat and are less responsive to changes in load conditions. Consequently, the focus on superheat is less pertinent for those systems, making them less applicable for evaluating superheat settings.

Understanding the context of various system designs is essential. In the case of low ambient temperatures and commercial freezers, while superheat can still be relevant, it's the TXVs that necessitate careful monitoring to optimize system efficiency and prevent issues like liquid refrigerant returning to the compressor. Thus, the relationship between superheat and systems with thermal expansion valves is the critical factor in determining when checking superheat is most applicable.