The flood-back of oil can be equally as damaging as the flood-back of liquid refrigerant in terms of liquid slugging only. A well designed piping system will promote a uniform movement of oil, preventing the accumulation of damaging oil slugs.
System piping is particularly critical, for instance with Systems that must Operate for extended periods of time at minimum load when the gas velocities required for oil movement may be marginal. If an improper piping design permits large amounts of oil to be trapped at minimum load, the oil may return as a slug when a compressor returns to higher capacity operation.
To avoid gas velocity problems associated with minimum capacity operation, it is absolutely necessary that accepted piping design and sizing practices be strictly adhered to.
If it is found that a piping design is allowing excessive amounts of oil to be trapped in the lines at minimum load, the problem may be solved temporarily by preventing the compressor from unloading to less than the part load rating where the velocities become marginal. However, when this is done, the compressor can be expected to cycle frequently. This may create a new problem of motor overheating and may require the addition of anti-cycle protection to prevent nuisance motor shutdown during this temporary period.
It must be understood that this is only a temporary solution to a piping problem. The piping design must be corrected at the earliest opportunity and the compressor return to full range of capacity control to properly corrected problem.