Oil: More Than Just Lubrication
- MySafety LLC
- 10 hours ago
- 3 min read
When asked what the oil in an ammonia screw compressor is responsible for, most people immediately answer, "lubrication." While that's certainly true, lubrication is only one of several critical functions oil performs within the compressor.
This oil lubricates bearings and moving components, absorbs heat generated during compression, and creates a dynamic seal between the rotors and compressor housing. This sealing effect reduces internal leakage, improves volumetric efficiency, and allows the compressor to operate more effectively.
Industrial ammonia refrigeration systems commonly utilize screw compressors because of their reliability, efficiency, and ability to handle fluctuating refrigeration loads. Most modern ammonia screw compressors are classified as oil-flooded screw compressors, meaning oil is intentionally injected directly into the compression chamber.
Oil-flooded screw compressors use two interlocking helical rotors, a convex male rotor with lobes and a concave female rotor with flutes, that rotate in opposite directions (outwards). As ammonia vapor enters the suction port, it becomes trapped in the cavities formed between the rotors. As the rotors continue to turn, the trapped vapor is progressively compressed along the length of the rotors and discharged at a higher pressure.
One of the lesser-known benefits of oil-flooded screw compression is its impact on the compression process itself. Let's explore why oil is truly the lifeblood of the ammonia screw compressor:
Oil Is Actually the Primary Cooling System
Many think the oil cooler exists to cool the compressor.
In reality:
The oil is cooling the compressor.
The oil cooler is cooling the oil after it has absorbed compressor heat.
Oil is injected directly into the compression process and absorbs heat generated during compression.
Without oil injection:
Discharge temperatures would be too high.
Rotor expansion could cause contact.
Bearing life would be reduced.
The oil functions as a secondary cooling medium, absorbing heat generated during compression and transporting it away from the compression chamber. By limiting discharge temperatures, the oil reduces thermal stress on rotors, bearings, seals, and lubricant itself while improving compressor efficiency and reliability.
Oil Is a Dynamic Seal, Not Just a Lubricant
One of oil's most important functions is sealing the clearances between the male and female rotors and between the rotors and housing. This is similar to the role oil plays in an internal combustion engine. The oil forms a thin protective film between moving surfaces, preventing direct metal-to-metal contact.
Without oil:
Internal leakage (slip) increases dramatically.
Volumetric efficiency decreases.
Compressor capacity drops.
Oil essentially fills microscopic leakage paths and creates a hydraulic seal that improves compression efficiency. In an oil-flooded screw compressor, oil serves as a dynamic sealing medium. Much of the machine's volumetric efficiency is achieved through oil reducing internal leakage. Oil Viscosity considerations:
Higher viscosity:
Better sealing.
Better film strength.
But also:
Higher pumping losses.
More difficult oil return.
Reduced heat transfer.
Lower viscosity:
Easier circulation.
Better cooling.
But:
Reduced sealing.
Reduced bearing protection.
Operators/technicians often focus on preventing high oil temperatures but oil that is too cold can cause problems:
Increased viscosity.
Poor oil separator performance.
Poor oil flow.
Capacity control sluggishness.
Oil Creates Hydraulic Forces Inside Capacity Controls
Many screw compressors use oil pressure to actuate:
Slide valves
Vi controls
Unloaders
These devices frequently rely on hydraulic power supplied by the compressor oil to operate. In addition to transmitting hydraulic force, the oil must also provide adequate lubrication and wear protection for the internal components of the capacity control system. If the oil becomes contaminated, degraded, or loses its protective properties, increased wear can occur, often leading to erratic operation, reduced reliability, and premature component failure.
FINAL THOUGHTS
Carefully selecting the proper lubricant to balance these competing factors is critical to maximizing the performance, efficiency, and service life of an ammonia screw compressor. To meet these demands, lubricant manufacturers such as Camco Lubricants have developed specialized formulas designed specifically for industrial ammonia refrigeration applications, providing enhanced performance and protection compared to conventional lubricant options (Camco Lubricants product catalog below). Oil selection is not simply a maintenance decision; it's a reliability decision. Choosing a lubricant specifically engineered for ammonia refrigeration can help improve compressor efficiency, reduce wear, extend oil life, and contribute to the long-term reliability of the entire refrigeration system.
Stay tuned to the MySafety blogs. Reach out to us if you have additional questions or concerns. Our team will be pleased to assist you.
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