Flash-gas

In refrigeration, flash-gas is refrigerant in gas form produced spontaneously when the condensed liquid is subjected to boiling. The presence of flash-gas in the liquid lines reduces the efficiency of the refrigeration cycle. It can also lead several expansion systems to work improperly, and increase superheating at the evaporator. This is normally perceived as an unwanted condition caused by dissociation between the volume of the system, and the pressures and temperatures that allow the refrigerant to be liquid. Flash-gas must not be confused with lack of condensation, but special gear such as receivers, internal heat exchangers, insulation, and refrigeration cycle optimizers may improve condensation and avoid gas in the liquid lines.

The most common causes driving the fluid to change its phase when in the liquid line are the excessive length of the pipeline, the small diameter of the pipelines, and the lack of subcooling. These create low pressure loci that may induce the fluid to begin changing its phase, especially if the pipeline is somehow exposed to heat absorption. If subcooling is not generous enough, the refrigerant remains in thermodynamic conditions close to saturation easing flash-gas appearance.

The heat may be absorbed not only from high temperature sources, but from diverse energy sources such as friction in the pipeline. On the other hand, pressure in the liquid line may also be upset by artifacts and conditions including vertically rising pipes that reach too high; gear too small for the size of the piping; devices pumping refrigerant through the line, and several other strangling obstructions. Eventually, heat absorption and pressure losses in the liquid line modify the saturation conditions of the refrigerant in such manner that the refrigerant may boil and produce flash-gas.

Refrigerant may also explain flash-gas occurrence. When a system lacks refrigerant, or has a leakage, it may exhibit flash-gas as the volume in the piping exceeds the capacity of the refrigerant to fill it as liquid. This may force the refrigerant into a phase change. On the other hand, lack of refrigerant can sometimes also produce the opposite effect: an overall increase on the subcooling (and superheating) which will depend on the size and design of the system and its piping. If the system’s refrigerant is degenerate, flash-gas may also be produced, as physical properties of the fluid change. This happens because the piping is designed for a specific refrigerant mixture that allows liquid in the liquid line, given certain thermodynamic conditions. If the refrigerant mixture changes its composition considerably, the original designs will not be adequate. Degenerated refrigerant produced by leakage, chemical decomposition or loading with gas when using a glided refrigerant; will most likely make the system perform very poorly, alter oil’s circulation or composition, and may eventually render gear inoperative.

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