Why is it Necessary to Set Oil Traps?
When there is a large height difference in the refrigeration piping system between evaporator unit (indoor unit) and condensing unit ( outdoor unit), must install oil traps on the vertical pipe to prevent refrigerant oil from failing to return effectively to the compressor, which could otherwise affect the compressors’ service life.
The Role of Oil in Refrigeration Systems
Oil is essential in refrigeration compressor because it lubricates moving parts, reduces friction, and helps seal components to maintain pressure.
In most systems, oil mixes with refrigerant and travels through the piping. This is necessary for compressor’s health, but it also means the oil must cycle back properly after leaving the compressor.
In low-temperature refrigeration, for instance, the oil’s viscosity changes will make return more challenging.
In summary, proper oil management is key to preventing compressor failure, especially in systems handling extreme colds.
| Oil Type | Viscosity (cSt at 40°C) | Miscibility with Refrigerants | Common Use | Data on Performance |
|---|---|---|---|---|
| Mineral Oil | 32-68 | Low with HFCs | Legacy systems | 10% lower efficiency in modern setups |
| POE (Polyol Ester) | 22-100 | High with HFCs/HFOs | Modern AC | Improves return by 20% in low-temp |
| PAG (Polyalkylene Glycol) | 46-150 | High with R-134a | Automotive | Reduces migration by 15% |
Compare Oil Types in Refrigeration Systems
Common Issues Without Oil Traps
| Issue | Description | Impact Data |
|---|---|---|
| Oil Shortage | Low return to compressor | 25% higher failure rate |
| Efficiency Drop | Evaporator oil accumulation | 5-15% heat transfer loss |
| Downtime from Slug | Oil clumps damage valves | 10-20% elevated repair costs |
Benefits of Installing Oil Traps
Install oil traps offers multiple benefits that make them a must-have in many refrigeration setups.
They protect compressor by preventing oil starvation or slugging, which extends the system’s lifespan and reduces maintenance costs.
Energy efficiency improves as the evaporator stays clear of oil films that hinder heat exchange.
In commercial applications, this translates to reliable operation during peak loads.
| Metric | With Traps | Without Traps | Benefit Data |
|---|---|---|---|
| Efficiency | 90-95% | 75-85% | 10-20% uplift |
| Life Span | 10-15 years | 7-10 years | 20-30% extension |
| Energy Cost | $800/year | $1,000/year | 15-20% savings |
When Should Install Oil Traps?
1. When condensing unit is higher than evaporator unit
There is a vertical pipe between evaporator and the main suction pipe. Since refrigerant oil doesn’t evaporate inside the evaporator, it accumulates at the bottom. When too much oil accumulates, it will block the suction pipe.
If install oil traps at the bottom of evaporator, the amount of oil stored in the trap won’t be excessive. As soon as the trap is nearly full, the pressure difference on both sides will be sufficient to “pump” the oil out of the trap, push it up to the horizontal suction pipe at the top, and then allow it to flow back to the compressor.
If the vertical copper pipe is very long and there is concern that the oil may not be pumped all the way to the top, should install oil trap at intervals along the riser (for example, every 6~10 meters) to ensure the oil will return to compressor in stages.
2. When condensing unit is lower than evaporator unit with a large height difference
Even without oil trap, the oil would automatically flow back to compressor by gravity. However, the concern here is that too much oil might return at once, causing compressor “liquid slug (oil slug).”
To avoid this, should install oil traps at intervals (for example, every 6~10 meters) along the suction pipe so that the oil returns to compressor gradually in stages.
3. During low-load operation
Oil may accumulate in the trap, but due to the limited refrigerant velocity, the return effect driven only by the pressure difference when the trap is nearly blocked is very small. The key is to regularly increase compressor output to boost suction velocity and improve oil return.
If the suction velocity can increase to a sufficiently high level, then there is no need to add additional oil traps. However, in reality, under low-load conditions, the internal heat exchange effect causes compressor output to increase excessively, which can lead to system control issues such as excessively low suction pressure and lack of suction superheat.
This means the increase in suction velocity is limited. Therefore, when there is a large height difference, it is necessary to use oil traps to return the oil step by step.
Principles for Installing Oil Traps
1. When there is a height difference between indoor and outdoor units, install an oil trap on the vertical section of the gas (suction) pipe every 8~10 meters from bottom to top.
The oil trap can be made using two “U”-shaped bends or one “O”-shaped bend, with a height 3~5 times the pipe diameter.
What’s more, install an oil trap at the lowest point and a trap bend at the highest point.
2. The discharge pipe should design in the same way as the suction pipe, must consider: control pressure drop, ensure proper oil return, and prevent liquid slugging, noise, and vibration.
| A | A | R | C |
|---|---|---|---|
| mm | inch | mm | mm |
| Φ19.05 | 3/4 | ≥34 | ≤105 |
| Φ22.2 | 7/8 | ≥31 | ≤150 |
| Φ25.4 | 1/1 | ≥45 | ≤150 |
| Φ28.6 | 9/8 | ≥45 | ≤150 |
| Φ34.9 | 11/8 | ≥60 | ≤250 |
| Φ38.1 | 12/8 | ≥60 | ≤350 |
| Φ41.3 | 13/8 | ≥80 | ≤450 |
| Φ44.45 | 7/4 | ≥80 | ≤500 |
| Φ54.1 | 17/8 | ≥90 | ≤500 |
Dimensions for Oil Traps and Trap Bends
(Remarks: “A” means Copper Outer Diameter, “R” means Min. Bend Radius of Oil Traps, “C” means Max. Straight Pipe Length Between Oil Traps)
Technical Requirements for Installing Oil Traps
Outdoor Unit Below Indoor Unit
Outdoor Unit Above Indoor Unit
Conclusion
Oil traps are essential in refrigeration systems to ensure proper oil return to compressor, especially when there are vertical height differences or low-load conditions.
Properly installed oil traps prevent oil accumulation, reduce risk of compressor damage, and help maintain efficient and reliable system operation.
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