16
november
2010

How to reduce oil carryover in refrigeration compressors

The majority of compressors used in refrigeration systems are oil-injected. Sometimes a compressor unit shows higher oil consumption than calculated.

Over the last five years we have investigated the most common causes of oil carryover. During our field investigations, we noted interesting findings related to oil carry over. We found that older compressor installations started to show more oil consumption than they had originally demonstrated.

Top three reasons for increased oil consumption:

  • The oil and refrigerant mixture is entering the oil recovery system outside the ideal separation area. The adhesion is too strong between lubrication oil and refrigerant. The time in the oil separator is too short to reach 100% separation.
  • The volume of oil/refrigerant flow is greater than the oil separator can handle. In VSD compressor units, the flow is variable in relation to variation in applications.
  • During production, oil can build up and become a different viscosity. In the compressor aggregates, this kind of oil can be separated to the original viscosity over time. The oil separator can be partly blocked and forced to work outside design limits.

The relationship between the main systems of a compressor unit:

diagram

How to reduce the oil carryover of your compressor system:

  • Find the ideal separation temperature area between your refrigerant and the used oil. Enter the oil recovery system with this temperature. The entry temperature of the oil recovery system is based on the heat generated by the particular compressor.  Some refrigerants generate temperature under the ideal separation temperature, which must be compensated for.
  • Calculate your ideal oil flow and use a compensation regulation if necessary. In most cases, corrections in entry temperatures can be used to correct the flow. Doing this after separation is always an additional option, if the existing oil recovery system is limited in size.
  • Using oil with a different viscosity in combination with a two-stage oil separator (in which the oil separator elements are located in the last stage) can cause blockage in the last stage. The elements will go outside the design limits by reducing the filtration area. If you use a three-phase oil separator with a mixing phase before entering the last stage, the filters will experience a homogenous mixture of oil and refrigerant and will not become blocked. This type of separator can also be used as the central after-oil separator in the refrigeration system. The captured oil can be sent to the compressor with the lowest oil level.