When we say that there is “flash gas” at a particular point in the system it can either be a bad thing or a good thing depending on where it is occurring.
Flash gas is just another term for boiling.
It is perfectly normal (and required) that refrigerant “flashes” or begins boiling directly after the metering device and as it moves through the evaporator coil. In order for us to transfer heat from the air into the refrigerant in large quantities we leverage the “latent heat transfer of vaporization”. In other words we transfer heat into the boiling refrigerant, or “flash gas”.
In a boiling pot of water we create flash gas by increasing the temperature of the water until it hits the boiling temperature at atmospheric pressure.
Inside of a refrigeration circuit we get flash gas when the pressure on the liquid refrigerant drops below the temperature / pressure saturation point or if the temperature of the refrigerant increases above the same point.
This “flashing” can occur in the liquid line when the liquid line is long or too small and also in cases with line kinks and clogged filter/driers. All of these instances result in a pressure drop and a drop in the saturation temperature.
It can also occur in the liquid line if it is run uninsulated through a space that is hotter than the liquid saturation temperature like on a hot roof or in an unconditioned attic. This is more rare and will generally only cause flashing in conjunction with another issue.
This flashing can be prevented by keeping line lengths and tight bends to a minimum, insulating the liquid line where it runs through very hot spaces and keeping the refrigerant dry and clean with one properly sized filter/drier.
It can also be prevented in most cases by maintaining the proper levels of subcooling. A typical system that has 10°+ of subcooling will not experience flashing in the liquid line under normal conditions.
When you walk up to a liquid line near the evaporator and you hear that hissing/surging noise or when you look in a sight glass and see bubbles you are seeing refrigerant that is at saturation, meaning it is a mix of vapor and liquid. This doesn’t necessarily mean it is “flash gas”, it could very well be that the refrigerant was never fully condensed to liquid in the condenser in the first place. This can be due to low refrigerant charge and in these cases the subcool will be at 0° Even when taken at the condenser.
True liquid line “flash gas” issues are cases where you have measurable subcooling at the condenser coil outlet but still see, hear or measure boiling/flashing refrigerant in the liquid line before the metering device.