The lubricating oil (refrigeration oil) of the low temperature chiller compressor is the core medium that ensures the lubrication, sealing, cooling and noise reduction of the compressor. Its condition directly affects the service life, operating efficiency and system reliability of the compressor. Regular oil sample testing can provide real-time monitoring of the deterioration trend of lubricating oil, while the scientific replacement cycle needs to be comprehensively determined based on the operating conditions and test results. The following provides practical maintenance guidelines from two aspects: the oil sample testing system (indicators, methods, frequency) and the replacement cycle (influencing factors, judgment criteria, operation procedures).

I. Regular Oil Sample Testing: Core Indicators and Implementation Logic

The essence of oil sample testing is to predict the risks of lubrication failure (such as wear, oxidation, and contamination) by analyzing the physical, chemical, and contaminant characteristics of lubricating oil, and to prevent sudden compressor failures (such as bearing burnout and valve plate jamming).

Key detection indicators and their significance

Reference values for specific indicators, detection purposes and abnormal impact standards (taking R134a/ mineral oil as an example)

The physical property viscosity (40℃, mm²/s) reflects the thickness of the lubricating film: excessively high viscosity → large flow resistance and high energy consumption; Too low → Oil film rupture and accelerated wear. New oil: 30-45; After deterioration: ±15% or more requires attention

Moisture (ppm) : Moisture reacts with refrigerants/lubricating oil to form acidic substances, which corrode metals. Freezing at low temperatures can clog capillary tubes/expansion valves < 100 (precision instruments < 50). More than 200 require urgent handling

Particle size (ISO 4406) Particle contamination (metal shavings, impurities) can scratch bearings, pistons and clog oil filters < 18/16/13 (Cleanliness grade, the lower the value, the cleaner it is)

Chemical properties: Total acid value (TAN, mg KOH/g). An increase in the acid value indicates oxidation/hydrolysis of the lubricating oil and corrosion of metal parts (such as copper tubes, coils). New oil: < 0.1; > 0.3 needs to be replaced

Oxidation stability (rotating bomb method) is used to assess the antioxidant capacity of lubricating oil. The lower the value, the more prone it is to oxidation and coking, which can clog the oil passage for more than 100 minutes (industrial standard).

Metal debris contaminants (ferrography/spectral analysis) : Excessive iron, chromium, copper and other elements (such as Fe > 200 ppm) indicate wear of bearings and gears, and mechanical faults need to be investigated. Fe < 100 ppm, Cu < 50 ppm, Cr < 30 ppm

Refrigerant dissolution amount (GC-MS) Excessive dissolution of the refrigerant (such as R134a dissolution amount > 30%) will reduce the viscosity of the lubricating oil, resulting in lubrication failure < 15% (strongly correlated with the type of refrigerant)

2. Detection methods and tools

• On-site rapid testing (weekly/monthly) :

• Moisture test pen (conductivity method) : Quickly screen whether the moisture content exceeds the standard;

Viscosity test paper: Determine viscosity deviation by comparing color gradation;

• Oil level observation window: Based on the operating conditions (for example, the oil level should be between 1/2 and 2/3 of the sight glass after 10 minutes of shutdown), determine whether the abnormal oil level is caused by leakage or emulsification.

• Laboratory precision testing (quarterly/semi-annual) :

Send the oil sample to a third-party laboratory (such as SGS or a cooperative institution of the equipment manufacturer), and conduct a comprehensive analysis through equipment such as spectrometers (for metal elements), particle counters (for contamination levels), and titrators (for acid values).

3. Detection frequency: Based on the operation stage and working conditions

The key indicators to focus on during the operation stage detection frequency

During the commissioning period of the new machine (< 500 hours), metal grinding swarf (check for installation/running-in wear) and the amount of refrigerant dissolved should be checked every 100 to 200 hours

During the stable operation period (500 to 2000 hours), moisture content, viscosity and total acid value should be measured once every quarter

For high-load/harsh environments (such as high-temperature and high-humidity areas, continuous operation > 8 hours /d), conduct particle size, oxidation stability, and refrigerant dissolution amount once a month

Moisture content (anti-corrosion during shutdown), acid value/viscosity (lubrication during restart) before and after long-term shutdown/once before shutdown + once before restart

Ii. Lubricating Oil Replacement Cycle: Dynamic Judgment and Practical Operation Standards

The replacement cycle should break away from the rigid model of "fixed time/hour", and be comprehensively determined based on the operating time, oil sample test results, and working conditions to avoid premature replacement (cost waste) or too late replacement (causing malfunctions).

The core factors influencing the replacement cycle

• Operating time and load: The longer the cumulative operating time and the higher the load rate (such as > 80% of the rated load), the faster the lubricating oil deteriorates.

• Environmental conditions: High temperature (> 35℃) accelerates oxidation, and high humidity (relative humidity > 70%) increases the risk of moisture intrusion.

• Refrigerant type:

• Refrigerants with strong miscible with oil (such as R134a and PAG/POE oil) : Lubricating oil is easily carried away by the refrigerant, and attention should be paid to the stability of the oil level.

Refrigerants with weak miscibility with oil (such as R22 and mineral oil) : Sludge deposition is prone to occur, and particle size monitoring needs to be strengthened.

• Compressor types: Screw type (high speed and heavy load, high cleanliness requirements) > Centrifugal type > scroll type > piston type (intermittent operation, slower deterioration).

2. Replacement cycle judgment criterion: "Dual-track system" decision-making

The trigger conditions for changing the judgment dimension

Time/hour basic cycle: Mineral oil ≤2000 hours/year, synthetic oil (POE/PAG) ≤4000 hours/year (subject to correction in accordance with working conditions)

The oil sample test results need to be replaced if any of the following conditions are met: ① Total acid value > 0.3 mg KOH/g; ② Moisture content > 200 ppm; ③ Particle size > ISO 18/16/13; ④ Metal shavings Fe > 200 ppm; ⑤ Viscosity deviation > ±20%

System abnormalities are associated with abnormal noise from the compressor (bearing wear), persistently high exhaust temperature (poor lubrication), and oil filter pressure difference > 0.1 MPa (contamination and blockage).

3. Change the operation process and precautions

Step 1: Shut down and isolate from the system

Turn off the power of the compressor and hang a "Under Maintenance" warning sign.

Close the high and low pressure stop valves to isolate the compressor from the condenser/evaporator and prevent the backflow of refrigerant/lubricating oil.

Step 2: Drain the old oil

Open the oil drain valve at the bottom of the compressor and collect the old oil into a dedicated container (the discharge volume needs to be recorded as a reference for refilling new oil).

For multi-stage compressors (such as two-stage compressors), the lubricating oil in the intercooler and the two-stage compression chamber needs to be drained separately.

Step 3: System Cleaning (Key!)

• If the old oil deteriorates severely (such as acid value > 0.5mg KOH/g, moisture > 500ppm), a dedicated cleaning oil circulation flushing system is required:

Inject cleaning oil to the center line of the sight glass, start the compressor and run it at low speed for 30 minutes (to avoid oil sludge detachment and blockage due to high speed).

Stop the machine and let it stand for 1 hour. Drain the cleaning oil and impurities. Repeat this process 2 to 3 times until the oil discharged is clear and free of particles.

Step 4: Add new oil and check for leaks in the system

• Add the specified type and quantity of synthetic oil as per the equipment manual (such as POE oil commonly used for screw compressors and PAG oil for scroll compressors);

Manually turn the wheel 3 to 5 times to ensure smooth oil circulation.

• Check the crankcase oil level (before operation: 1/3 through the sight glass; after operation: 1/2-2/3), and add oil if insufficient.

Restore the high and low pressure stop valves, and use an electronic leak detector to check the flanges and valve interfaces. After confirming no leakage, evacuate the air (-0.1 MPa, no rebound after 30 minutes).

Step 5: Verify after replacement

Start the compressor and monitor the oil pressure (oil pressure difference > 0.1 MPa) and exhaust temperature (screw compressor < 90℃, scroll compressor < 80℃).

After running for 24 hours, take another oil sample for testing to confirm that the viscosity and acid value have returned to normal.

Iii. Long-term Maintenance Suggestions

1. Establish an "oil sample file" : Record each test data (viscosity, acid value, moisture content, etc.), draw a deterioration trend chart, and predict replacement needs 300 to 500 hours in advance;

2. Distinguish between "regular replacement" and "condition-based replacement" : Under stable working conditions, the inspection results should be the main basis. Under harsh working conditions (such as frequent starts and stops, high temperatures), replacement should be prioritized according to the time cycle.

3. Avoid mixing lubricating oils: Oils of different base types (mineral oil vs. synthetic oil) and oils of different brands (due to differences in additive formulas) must not be mixed; otherwise, deterioration will be accelerated.

4. Synchronous maintenance of related components: When changing the lubricating oil, replace the oil filter (with a precision of ≤10μm) and the oil separator core (for screw machines) simultaneously to prevent secondary contamination.

Summary

The lubrication management of low temperature chiller compressors should be based on the principle of "oil sample testing as the core and dynamic replacement cycle" : By regularly testing indicators such as viscosity, moisture, and metal debris, the status of lubricating oil can be monitored in real time. Based on the operating time, working conditions and test results, scientifically determine the replacement timing. At the same time, when replacing, the old oil must be thoroughly drained and the system cleaned to avoid contamination and residue. Through meticulous maintenance, the failure rate of compressors can be reduced by more than 40%, their service life can be extended by 2 to 3 times, and the reliability of the chiller system can be significantly enhanced.

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