About the authorCondensation in Refrigerant Vapor Lines
Here is a example of a fundamental principle that may explain some of your past issues with liquid slugging. Liquid slugging occurs when a volume of liquid refrigerant suddenly flows into a compressor. This may be difficult to imagine how this occurs if the accumulator upstream of the compressor(s) is performing properly.
Here is one way to consider this phenomena. Vapor exists in two states: saturated or superheated. Superheated vapor is simply vapor heated above its saturation temperature. When the superheat is removed the vapor state approaches the saturation temperature. Once the vapor reaches the saturation temperature, any further removal of heat can result in the vapor condensing into a liquid.
It is the pressure/temperature relationship of the refrigerant we must be aware of when determining the cause and effect of potential problem issues. Vapor in a dry suction line may exist with a slight amount of superheat or saturated. This is dependent on the system operation and equipment types installed.
Consider this: If the suction pressure has been relatively constant, the suction line wall temperature will approach the saturation temperature of the refrigerant vapor. If the pipe wall temperature is warmer than the saturation temperature, nothing happens. However, if the pipe wall temperature is below the saturation temperature condensation can occur.
One way the pipe wall temperature can be below the saturation temperature is when the compressor unloads (decreases in capacity). When the compressor capacity is decreased the suction pressure rises slightly. This is a change in the pressure/temperature relationship of the vapor. If the pressure increases, the saturation temperature of the refrigerant vapor is increased. When this occurs the suction line wall temperature is now colder than the saturation temperature of the the refrigerant and the vapor begins to condense.
If this takes place over a period of time a sufficient volume of liquid refrigerant can accumulate in the dry suction line leading to the compressors. Then, when the compressor capacity is again increased the higher volume of vapor flow will pull the liquid back into the compressor. And that is how you can liquid slug a compressor when the accumulator is working and designed properly. For this reason, it is important to recognize how the refrigerant behaves in various situations and design the system accordingly.
Tags: accumulator, dry suction line, liquid slugging, vapor condensation
