Oil within a refrigeration system has but one function, and that is to lubricate the compressor.
Oil is pumped completely through the system and back to the compressor. Always check oil level after the compressor has been running for a while. This will allow you to get the true oil level in the compressor. On larger systems, a discharge oil separator is necessary. On these systems a surplus oil charge is not necessary.
On prolonged shutdowns, the refrigerant has a tendency to work its way to the compressor crankcase. On start-up of the compressor, damage will result.
On all systems, crankcase heaters are placed within the crankcase.
When using them, check the surface temperature to make sure it is between 120 and 200 degrees Fahrenheit.
When using compressors in multiples always make oil equalizer lines the same size as the equalizer plugs in the compressor. This is to maintain oil levels in compressor crankcases.
If the velocity within a system is to low, you will have poor oil return to the compressor. This will also effect upon the amount of oil that will settle in the evaporator. The length, size, and pitch of the suction line can affect proper oil return to the compressor crankcase.