Difference between oil-injected screw and oil-free screw

Technical comparison between oil-injected screw air compressor and oil-free screw air compressor

oil-injected screw air compressor and oil-free screw air compressor are the mainstream compressed air preparation equipment in the industrial field, and their technical characteristics directly affect the equipment selection and system design. Now from the working principle, performance characteristics, application scenarios and operation and maintenance management four dimensions of professional comparative analysis:

1. working principle comparison

1. oil-injected screw air compressor
   the twin-screw rotor meshing compression technology is used to inject special lubricating oil into the compression chamber to form an oil-gas mixture. Lubricants assume three core functions:

• lubrication and wear reduction: an oil film is formed on the surface of the rotor to reduce direct metal contact friction
• seal cooling: Filling the rotor gap improves volumetric efficiency while absorbing the heat of compression
• noise reduction buffer: suppress mechanical vibration and extend bearing life
  after the oil-gas mixture is treated by the separator, the oil content of the compressed air is usually controlled within the range of 3-5ppm.

2. Oil-free screw air compressor
   oil-free compression through high-precision rotor machining technology, core technological innovations include:

• rotor coating technology: special alloy steel substrate with diamond-like coating, surface roughness of Ra0.2μm or less
• multi-stage compression design: usually equipped with two-stage compression host, single-stage pressure ratio control within 3.0, improve thermodynamic efficiency
• shaft seal system: double mechanical seal and nitrogen protection device to ensure that the lubricating oil does not enter the compression chamber
  the oil content of the final output compressed air is less than or equal to 0.01ppm, reaching ISO 8573-1 Class 0 clean standard.

Comparison of 2. performance characteristics

1. energy efficiency performance

• fuel injection machine type: the specific power can reach 5.2-5.8kW/(m & sup3;/min) under rated working conditions, and high efficiency operation can be maintained through frequency conversion adjustment under partial load.
• Oil-free model: the two-stage compression design improves the isothermal efficiency by 15-20%, and the specific power is optimized to 5.0-5.5kW/(m & sup3;/min)

2. operation stability

• oil injection machine type: lubrication system provides double protection, bearing life can reach more than 80,000 hours
• oil-free model: the use of active vibration monitoring system, rotor dynamic balance accuracy of G0.4 level, to ensure long-term stable operation

3. environmental Adaptability

• fuel injection machine type: allowable intake dust content ≤ 1 mg/m & sup3;, adapt to the ambient temperature of -10 ℃ to 45 ℃
• oil-free model: pre-filter (accuracy ≤ 0.1 μm) is required, and the ambient temperature range is extended to -20 ℃ to 50 ℃

comparison of 3. application scenarios

1. oil injection screw air compressor application field:

• general manufacturing: pneumatic tools, automated production lines, spraying equipment, etc.
• Construction works: foundation construction, tunneling, bridge construction, etc.
• Energy industry: oil and gas extraction, wind power equipment maintenance, etc.

2. Oil-free screw air compressor application field:

• food and medicine: aseptic packaging, drug synthesis, fermentation process, etc.
• Electronic semiconductors: wafer manufacturing, chip packaging, clean room gas supply, etc.
• Precision processing: laser cutting, 3D printing, precision measuring instruments, etc.

Comparison of 4. operation and maintenance management

1. oil injection screw air compressor maintenance points:

• consumables replacement: air filter replacement every 2000 hours, lubricating oil replacement every 4000 hours
• oil circuit maintenance: regular detection of oil viscosity and acid value, to ensure that within the standard range
• separation system: replace the oil and gas separation core when the pressure difference reaches 0.1MPa to avoid oil running failure

2. oil-free screw air compressor maintenance points:

• precision filtration: check the pre-filter pressure difference every 500 hours to ensure the cleanliness of the intake air
• drive system: detect rotor clearance every 8000 hours, calibrate coaxiality with laser alignment instrument
• cooling system: clean the cooler fins every quarter to ensure the heat exchange efficiency ≥ 90%

decision-making recommendations for 5. selection
enterprise selection needs to follow the three-dimensional evaluation model:

1. air quality requirements: according to the ISO 8573-1 standard to determine the cleanliness level, the food and pharmaceutical industry must choose oil-free models
2. full life cycle cost: 30-50% higher initial investment for oil-free models, but 20-30% lower five-year operating costs
3. system compatibility: when the oil content of the existing pipe network exceeds the standard, an independent oil-free gas supply system shall be configured.

Enterprises should establish an air compressor technology evaluation system, combined with GB/T 13279 "fixed reciprocating piston air compressor for general use" and GB/T 3853 "positive displacement compressor acceptance test" standards, to carry out equipment selection technology demonstration. It is recommended to conduct energy efficiency benchmarking every three years, focusing on core indicators such as specific power, leakage rate, and vibration intensity, and continuously optimize the operating quality of the compressed air system.