How To Change The Refrigeration Oil in A Refrigeration Compressor?

Many owners and maintenance personnel of refrigeration systems often overlook the critical task of changing compressor lubricating oil. Don’t underestimate its importance—failing to change oil regularly accelerates component wear, drives up energy consumption, and in severe cases, can directly lead to compressor failure.

This guide clearly explains “how to properly change oil,” “when to change it,” and “what pitfalls to avoid.” After reading, you’ll confidently handle compressor lubrication maintenance, ensuring your equipment runs smoothly and reliably.

What is Refrigeration Oil?

Refrigeration oil, also known as compressor lubricant, is a specialized lubricant made from highly refined base oils blended with additives like antioxidants, anti-wear agents, and rust inhibitors. It operates reliably under harsh conditions ranging from -40°C to 150°C and pressures of 2-25MPa, performing four core functions:

lubrication, sealing, cooling, and noise reduction. It forms a nano-level oil film on friction surfaces, reducing the friction coefficient below 0.01; It seals between pistons and cylinder walls to prevent leakage; controls discharge temperatures; and reduces compressor operating noise by 15-20 decibels, making it critical for ensuring refrigeration system operation.

Refrigeration Oil Functions

Lubrication and Friction Reduction

Between high-speed components like pistons and cylinders, or bearings and journals, refrigeration oil rapidly forms a uniform, resilient oil film through splash lubrication and pressure delivery. This film reduces the friction coefficient by 60%-80% and minimizes mechanical wear by over 90%. It significantly slows down wear, reduces metal fatigue damage, extends the lifespan of core compressor components by 2-3 times, and enhances equipment reliability and stability.

Sealing and Leak Prevention

Leveraging its excellent wettability and viscosity, refrigeration oil fills minute gaps between components like pistons and cylinder walls. During operation, the oil film dynamically deforms to form an adaptive seal layer. This prevents refrigerant leakage while balancing pressure differentials, controlling system pressure fluctuations within ±5% and maintaining stable refrigeration efficiency.

Heat Dissipation

During lubrication, refrigeration oil serves as an efficient thermal transfer medium. It rapidly absorbs heat generated by component operation, achieving thermal conductivity 1/10 to 1/5 that of metals. After absorbing heat, the oil circulates through the oil cooler to dissipate heat, maintaining compressor operating temperatures between 60-90°C. This effectively prevents component performance degradation and lubrication failure caused by overheating.

Capacity Control

In compressors equipped with capacity control mechanisms, refrigeration oil serves as the hydraulic transmission medium. When adjusting refrigeration capacity, the oil is pressurized to 8-12 MPa to drive the capacity control spool valve and cylinder.

By altering the compressor’s effective working volume, this achieves dynamic load regulation from 10% to 100%. This hydraulic drive method offers rapid response and high precision, adapting to varying refrigeration demands across different operating conditions.

Selecting the right refrigeration oil and performing regular oil changes not only boosts compressor efficiency but also prevents rust and corrosion, helping equipment “live longer.”

What Are The Types of Refrigeration Oils

• Polyalphaolefins (PAO): Synthetic oils polymerized from ethylene, structurally similar to mineral oils. Offers excellent low-temperature fluidity, suitable for a wide temperature range from -60°C to 200°C. Compatible with high-performance refrigerants and provides stable lubrication. While miscible with mineral oils, mixing reduces anti-wear and oxidation resistance; long-term blending is not recommended.
• Polyol Esters (POE): The most widely used synthetic refrigeration oil, highly compatible with HFC refrigerants. Compared to PAG oils, it exhibits lower moisture absorption and superior chemical stability, reducing risks of acid corrosion and lubrication failure. However, strict control of system moisture content remains essential.
• Polyalkylene Glycol (PAG): Maintains stable oil films at high temperatures due to its high viscosity index and thermal resistance. Its strong moisture absorption prevents refrigerant acidification, offering outstanding chemical stability and lubrication performance in CO₂ and ammonia refrigeration systems.
• Alkylbenzene (AB): Synthetic aromatic lubricant providing reliable performance from -40°C to 150°C. Naturally compatible with HCFC refrigerants and suitable for low-temperature applications. However, it separates from HFC refrigerants and degrades when mixed with mineral oils, requiring thorough system flushing during replacement.
• Mineral Oil: Derived from petroleum refining, historically used in CFC, HCFC, and ammonia systems. Widely adopted for low-temperature fluidity and cost-effectiveness. However, its poor chemical stability, susceptibility to high-temperature oxidation, and limited compatibility with modern eco-friendly refrigerants lead to gradual replacement.
• Hydrotreated Mineral Oil (HTMO): Specifically engineered for ammonia systems through deep hydrogenation to resist ammonia corrosion. Available in standard and high-viscosity grades for conventional and large-scale high-pressure equipment, respectively.

Requirements of Refrigeration Oils

Regardless of the oil selected, it must satisfy these core standards to prevent system performance issues:

1. Thermal Stability: Exhibits exceptional high-temperature tolerance, maintaining molecular structure integrity under severe conditions. Effectively suppresses sludge and acid formation to ensure clean, efficient system operation.
2. Chemical stability: Chemically inert with refrigerants and system components under normal operating conditions, preventing material corrosion and performance degradation from chemical reactions.
3. Oil-refrigerant miscibility: Exhibits excellent mutual solubility with refrigerants to form stable mixtures. Ensures uniform dispersion of lubricant in evaporators and smooth return flow to compressors via system pressure for continuous lubrication.
4. Hydrolysis and Moisture Resistance: Exceptional moisture-proof properties significantly reduce water absorption, effectively blocking hydrolysis reactions to comprehensively protect system components from corrosion.

How To Properly Change Refrigeration Oil?

Each step during oil replacement is critical. Strict adherence to the following process ensures safety and optimal results:

Step 1: Safely Evacuate Refrigerant

Pressurized operations pose significant risks; ensure complete refrigerant evacuation first:

• Sequentially close the compressor’s high-pressure discharge valve and low-pressure suction valve to fully block refrigerant circulation pathways, maintaining system containment.
• Locate the compressor service valve and open it gradually at a slow, steady pace to release refrigerant gas from the crankcase. Continue until crankcase pressure fully equilibrates with atmospheric pressure. This step effectively mitigates refrigerant leakage risks during oil change, establishing a solid safety foundation for subsequent operations.

Step 2: Drain Old Oil and Clean the Filter Screen

Thoroughly removing old oil and contaminants is critical:

• Using an Allen wrench or compatible tool, slowly unscrew the drain plug at the bottom of the crankcase. Tilt the unit to completely drain the old oil. If necessary, shake the unit to ensure no residual oil remains.
• Remove the oil filter screen and soak it in a dedicated refrigeration system cleaner. Gently scrub the screen pores with a soft-bristle brush. Focus on removing adhered gummy sludge and metal debris. After drying with compressed air, inspect the screen for integrity until it exhibits good light transmission with no blockages.

Step 3: Remove Residual Oil and Reinstall the Filter Screen

Even minimal residual oil can contaminate the new oil and must be thoroughly removed:

• Securely connect the nitrogen cylinder to the low-pressure charging valve. Slowly introduce nitrogen into the refrigeration system to create conditions for oil purging.
• Manually seal the drain port tightly. Allow internal pressure to gradually rise to an appropriate level to ensure complete oil drainage.
• Thoroughly air-dry the cleaned filter. Precisely reinstall it into the crankcase. Tighten the drain plug to prevent leaks.

Step 4: Injecting New Refrigerant Oil

Select the correct oil injection method based on compressor type to prevent operational errors:

• Method 1: Screw Compressors (Injection via Process Port)

1. Identify the oil injection port: Unscrew the process port plunger on the crankcase side and convert it into a dedicated oil injection port.
2. Slowly inject oil: Use a funnel or specialized oil injection tool to introduce new oil into the crankcase at a steady flow rate. Control the injection speed to prevent air bubbles caused by excessive flow, which can impair compressor performance.

• Method 2: Piston Compressors (Vacuum Suction Method)

1. Establish negative pressure: Connect the low-pressure hose of the refrigerant charging gauge to the compressor’s low-pressure service valve. Use a vacuum pump to evacuate the crankcase until it reaches negative pressure.
2. Connect the oil injection line: Disassemble another refrigerant charging hose. Insert one end into the new oil container and securely connect the other end to the valve needle at the low-pressure suction port of the oil pump.
3. Automatically draw in new oil: Utilize the negative pressure differential within the crankcase to uniformly and steadily draw new oil into the crankcase, completing the oil injection operation.

Step 5: Precise Oil Quantity Control

Oil quantity must be strictly controlled according to the following standards:

• Most compressors are equipped with oil sight glasses. It is recommended to control the oil quantity slightly above the lower limit mark on the sight glass. This level ensures adequate lubrication while avoiding wear risks caused by insufficient oil.
• Considering residual old oil may remain in the refrigeration system, filling to the upper limit of the sight glass risks excessive oil pressure and abnormal oil temperature spikes. Severe cases may cause compressor overload, impairing operational efficiency and shortening service life.

Step 6: System Vacuum Treatment

Air and moisture within the system severely degrade new oil performance, necessitating vacuum treatment:

1. Connect the vacuum pump to the system and activate it to evacuate air.
2. Operate according to manufacturer specifications (typically below 500 microns). After vacuuming, maintain the vacuum for 10–15 minutes to ensure no residual air or moisture remains.

Step 7: Refrigerant Leak Detection

Refrigerant leaks not only reduce system efficiency and waste resources but may also cause equipment malfunctions. Therefore, comprehensive detection using scientific methods is essential:

• System Pressure Recovery: Open the compressor’s high and low-pressure shut-off valves to restore the refrigeration system to normal pressure, creating optimal conditions for precise leak detection.
• Leak Detection and Handling:

1. Instrument Detection: Use a professional refrigerant leak detector, slowly moving it along critical areas such as pipe joints and valves to capture subtle leak signals.
2. Soap Solution Inspection: When instruments are unavailable, evenly apply soapy water to all connection points. Persistent bubbles indicate a leak. Immediately repair any detected leaks. Proceed only after repairs are complete and leak-free status is confirmed.

Step 8: Start the Unit and Check Lubrication

Proceed with caution during this final step. Confirm no abnormalities before normal operation:

• After opening valves, wait 20 minutes for system pressure and temperature to stabilize.
• Start the compressor, focusing on these two checks:

1. Oil Level: The oil level in the sight glass should remain at the midpoint, never falling below 1/4 of the sight glass height. Add oil promptly if insufficient.
2. Oil pressure: Screw compressors use differential pressure oil return without an oil pump. The suction side pressure of the oil pump should be close to the low-pressure suction pressure, with the oil pressure differential maintained between 1.4–3.5 bar. For models equipped with an oil pump, connect a pressure gauge to the needle valve to verify normal oil pressure.

What Risks If Not Changing Refrigeration Oil?

Over time, refrigeration oil degrades: its viscosity decreases, impurities contaminate it, and oxidation may produce acidic substances. Persistent failure to change the oil leads to problems:

• Degraded lubrication accelerates wear on critical components like crankshafts and pistons, causing scratches and pitting that shorten equipment lifespan and may lead to component failure.
• Reduced thermal conductivity impairs heat dissipation, forcing the compressor to operate under high loads and increasing energy consumption and operating costs.
• Sludge and acidic residues from oxidation block lubrication, clog lines, and corrode components, potentially causing pipe ruptures and refrigerant leaks.
• Reduced sealing performance leads to refrigerant leakage, disrupting the cooling cycle and failing to meet cooling demands.

When Should You Change the Refrigeration Oil?

Don’t wait for compressor failure to change oil—follow these intervals for optimal performance:

Operating Hours

New units require early oil changes: Replace oil after the first 2000 operating hours. During break-in, machining residues, assembly contaminants, and metal particles from friction can contaminate oil. Early replacement prevents irreversible compressor wear.

Subsequent Periodic Replacement: Replace every 3 years or after 10,000-12,000 cumulative operating hours, whichever comes first. High-temperature/high-pressure conditions, refrigerant chemical reactions, and moisture/contaminant ingress degrade the oil’s lubrication and sealing properties. Overdue replacement weakens compressor protection, increasing failure risk and accelerating equipment aging.

After Replacing Compressor

When a compressor fails (e.g., motor burnout) and is replaced, strictly adhere to maintenance protocols: Refrigeration oil must be replaced immediately after the new compressor has operated for 100 hours. This procedure is based on two considerations: First, the new compressor undergoes a “break-in period” during initial operation, requiring time for components to achieve optimal coordination.

Second, lubricating oil from the original system may contain contaminants like metal debris or carbonized impurities introduced during the failure. Continued use with such oil accelerates wear on the new compressor. Timely replacement with fresh, clean refrigeration oil is crucial for ensuring the new compressor operates at peak efficiency and extends its service life.

Early Replacement in Special Conditions

Some units require shorter oil change intervals:

• Field-assembled units: Impurities can easily contaminate during assembly. Replace refrigeration oil promptly after 100 cumulative operating hours to prevent component wear from contaminants.
• Units operating near maximum capacity: Prolonged full-load operation or high-temperature environments significantly accelerate oil degradation. Similarly, replace oil after 100 hours to maintain efficient, stable system performance.

Adjust Oil Changes Based on Refrigerant System Cleanliness

If the refrigerant system contains substantial impurities, dust accumulation, or suffers from poor ventilation/airflow, pay close attention to the refrigeration oil condition:

Immediately replace the oil if it appears cloudy, exhibits abnormal coloration (e.g., blackening, yellowing), or contains visible suspended particles. Continuing to use contaminated oil accelerates internal compressor wear and shortens equipment lifespan.

Safety Tips and Common Mistakes to Avoid

Strictly Prevent Moisture Entry into the System

POE oil in refrigerants exhibits extremely strong hygroscopic properties. Once it reacts with water, the resulting compounds cannot be completely removed even through vacuum extraction. Therefore, during operation, strictly observe the following:

• Minimize the system’s exposure to air during oil changes; use the opened oil container promptly.
• Avoid oil changes during rainy weather, as humid air significantly increases the risk of moisture ingress.

Thoroughly Remove Residual Oil After Compressor Replacement

When installing a new compressor, residual lubricant may remain in the old system. To prevent future failures, implement these measures:

Flush the system with nitrogen to ensure complete removal of residual lubricant. If residual oil remains, it may cause “liquid hammer” during startup—lubricant surging into the cylinder—resulting in severe damage to compressor valves and pistons.

Ensure oil compatibility with refrigerant

When replacing refrigeration oil, strictly select based on refrigerant type; do not arbitrarily change oil types. For example:

1. Use POE oil with HFC refrigerants;
2. For ammonia refrigerants, PAO or PAG oils are suitable.

Incompatible oils react chemically with refrigerants, compromising lubrication and potentially generating contaminants that cause system blockages.

Conclusion

Refrigerant oil replacement for compressors is not optional maintenance—it’s a critical operation that prevents major failures and saves repair costs. By following the steps outlined here—selecting the right oil, identifying the optimal replacement timing, executing each step correctly, and avoiding common pitfalls—you can maintain your compressor in excellent condition and minimize issues. Don’t wait until the compressor makes abnormal noises or stops working. Review your maintenance plan against this guide now, replace the refrigeration oil promptly, and extend your equipment’s lifespan!