First a quick summary of the role of oil in the refrigerant circuit –
The compressor requires oil for lubrication of the moving parts in the compressor. If we could, we would keep 100% of the oil in the compressor but since that is generally unrealistic we need to utilize oils and oil strategies that will circulate the oil through the system and return it back to the compressor where it belongs on a regular and continuous basis.
There are components called oil separators that can strip most of the oil from the discharge gas and return the oil to the compressor, these are often used on larger systems and they are still less than 100% effective by themselves.
In VERY large systems such as chillers, we are beginning to see oilless technologies with magnetic bearings like TurboCor from Danfoss (shown above), but these are still pretty rare in the field.
So we are left with circulating oil through the system and returning it to the compressor as a regular part of A/C and refrigeration system operation under normal running conditions.
First, let’s cover the oil considerations a service tech can easily diagnose and impact.
We are tasked with preventing liquid refrigerant from entering the compressor which can cause more rapid and potentially catastrophic oil loss. This is called “flooding” and it can occur while the system is running when the refrigerant superheat is allowed to stay at zero as it enters the compressor which indicates the presence of liquid refrigerant mixed with the suction vapor.
A flooded start is flooding that occurs during startup when liquid refrigerant was allowed to collect in the compressor, in the suction line or even in the evaporator. Both of these conditions can cause oil loss from the compressor as well as oil dilution which can result in rapid compressor wear.
Preventing flooding is a significant part of oil management and involves setting superheat properly and using other strategies such as crankcase heaters, non-bleed expansion valves and pump down on the off cycle to help keep liquid refrigerant out of the compressor.
Another factor in lubrication is oil breakdown that can occur at high temperatures. We should consistently monitor discharge temperatures exiting the compressor to ensure it doesn’t exceed 225° which equates to around 300° at the compressor discharge valves (on a reciprocating compressor). This helps to ensure that the oil doesn’t break down and “carbonize”. Now, this does vary based on the compressor type, system and oil type, but is a generally accepted rule in the absence of a more detailed guideline. On a properly functioning compressor, the mass flow rate (amount of refrigerant moving through the compressor) and the suction gas temperature are the primary factors that impact compressor discharge temperature. Often high discharge temperatures occur when the suction pressure is low, superheat is low or compression ratio is high (high head pressure, low suction pressure) or some combination of these issues.
Once the oil has left the compressor it must circulate through the system and return to the compressor crankcase and there are a few key factors that impact oil return-
- Oil/refrigerant miscibility (how well the oil mixes and moves with the refrigerant)
- Oil Viscosity (oil thickness)
- Refrigerant velocity throughout the circuit
The oil being utilized should be suited to the refrigerant type and of the proper viscosity for the compressor and the temperature application. Refrigerant velocity should be maintained according to manufacturer recommendations and low velocity will primarily be an issue in evaporator coils and suction lines when the suction pressure is lower than design due to improper tubing sizing, low evaporator load, metering device underfeeding or undercharge.
Keep in mind that the longer the refrigerant lines are, the larger the evaporator(s) the more oil will be out in the “circuit” and the more total oil the system will need to contain. Technicians who work in “built up” systems like market refrigeration are very aware of this and take an active approach to manage oil. Residential and light commercial HVAC techs may take the approach that it’s the “manufacturers job” to ship the system with the correct amount of oil but may fail to read long line guidelines that call for more oil to be added.
On the other hand, too much oil can also lead to compressor issues and poor system performance. This often occurs when a new compressor is installed with a new oil charge on a system that previously had oil return issues. The new compressor will only add more oil to the circuit making the situation worse and again leading to a reoccurring failure. This is why diagnosing fully and finding WHY the old compressor failed is a huge part of the process so that you can make some oil adjustments if an oil return issue was found and rectified.
Important Oil Terms
Miscibility – the ability of the oil to mix with and move with the refrigerant.
Viscosity – a measure of the oil’s flow resistance (how thick it is). Two units of measure are used with refrigeration oil. The older measure is Saybolt Universal Seconds (SUS), the newer is ISO viscosity grade number (ISO VG). In both cases a higher number is a thicker oil, just don’t mix up the two standards.
Hygroscopic – Many modern oils are hygroscopic which means they attract and hold moisture. It is very important to keep moisture away from hygroscopic oils to keep them from becoming contaminated.
Hydrolysis – decomposition due to a reaction with water. For example, POE oil decomposes into acids and alcohol in the presence of water which means that the once it decomposes it cannot be reconstituted with line driers or evacuation.
Mineral – is a product of gasoline production. Naphthene based mineral oils are suitable for refrigeration systems using CFC or HCFC refrigerants and has been the standard oil used for generations. Mineral oil worked well with refrigerants that contained Chlorine but is not miscible with modern HFC and HFO refrigerants.
Alkylbenzene (AB) – a synthetic oil suitable for refrigeration systems using CFC or HCFC refrigerants. It is compatible with mineral oil and compared to mineral oil, it has improved refrigerant miscibility at low-temperature conditions which is why it was and is often used with HCFC refrigerants in commercial refrigeration.
Polyolester (POE) – The most common oil utilized in refrigeration and air conditioning systems using HFC / HFO refrigerants. It is also suitable for systems using CFC, HCFC refrigerants.
Polyvinyl Ether (PVE) – a synthetic oil that is being used as an alternative to POE oil and is very common in ductless and VRF. It is more hygroscopic than POE oil, but PVE oil does not undergo hydrolysis in the presence of water. This means that while PVE will grab water more easily it is capable or being dehydrated again unlike POE.
Polyalkylene Glycol (PAG) – a synthetic oil primarily used in automotive air conditioning systems. It is more hygroscopic that either POE or PVE oils, but like PVE it does not undergo hydrolysis in the presence of water.
Refrigerant Piping for Oil Return
When oil does not return properly to the compressor it can cause compressor wear but it can also decrease system performance by coating the inside of the evaporator tubing walls and inhibiting heat transfer and can even cause restrictions.
Especially with mineral and AB oils it was very important to employ proper trapping strategies according to the manufacturers and industry guidelines such as THIS ONE FROM RSES
with newer oils like POE and PVE these trapping and oil return strategies have become less critical in high and medium temperature applications due to the strong miscibility of the oil in the refrigerant. As always, read and follow manufacturers piping guidelines as the lower the velocity the more likely the oil is to have issues returning especially in retrofit applications where some Mineral or AB may still be present with the POE or PVE oils.
One good practice to use when running long runs of horizontal piping is to pitch it back towards the compressors. This is a common-sense practice no matter the oil, refrigerant or application whenever possible.
When POE oils first gained common use it was widely rumored that mixing POE and mineral oil would result in “sludge”. This has been proven to be a myth, and to some extent manufacturers of retrofit, refrigerants have been suggesting adding small amounts of POE to mineral oil to help carry it through the system. It is always better to move to POE or PVE oils from mineral when retrofitting to an HFC refrigerant but small amounts of mineral oil have proven to be rather inconsequential in most cases.
New Oil Considerations
One thing that has become clear with the advent of POE oil is the importance of proper brazing practices (flowing nitrogen), proper deep evacuation and keeping the oil away from air and moisture during storage. Many poor practices that techs could get away with when CFC/HCFC and mineral oil were in common use can result in DISASTER with modern refrigerants and oils.
Keep the system clean and dry and use the correct oil in the correct amounts. Keep the oil from overheating and keep the compressor from “throwing” oil by preventing flooding. Maintain proper oil return through proper pipe sizing, pitching and trapping (as required) and by maintaining appropriate deisgn velocity of the refrigerant.