Rotary screw air compressors are a popular choice in plants thanks to their continuous, quiet operation and high efficiency. The most common question when investing is whether to choose an oil-lubricated (oil-flooded) or an oil-free machine. Each type suits different compressed air quality requirements.
Oil-flooded rotary screw air compressors
In an oil-flooded machine, oil is injected into the compression chamber to seal the clearance between the two screw rotors, and to cool and lubricate. After compression, the oil is separated from the air by the separator tank and separator element, but the discharged compressed air still contains a very small amount of oil.
- Lower capital cost, simple design, easy maintenance.
- Suitable for most applications: metalworking, construction, general textiles and garments, operating pneumatic equipment.
- When cleaner air is needed, add an air dryer plus fine filters and activated carbon filters.
Oil-free rotary screw air compressors
In an oil-free machine, there is no oil at all in the compression chamber; the rotor clearances are maintained by timing gears, and cooling is by water or air. The discharged compressed air contains no oil from the compression process and can achieve ISO 8573-1 Class 0 for oil content.
- Eliminates the risk of oil contaminating the product.
- Required or recommended for food, beverage, pharmaceutical, electronics and medical applications, and for some textile and painting processes.
- Higher capital cost, but lower filtration costs and less risk of product spoilage due to oil contamination.
Quick comparison
| Criterion | Oil-flooded | Oil-free |
|---|---|---|
| Compressed air quality | Traces of oil remain; additional filtration needed | Oil-free; can achieve ISO 8573-1 Class 0 |
| Capital cost | Lower | Higher |
| Typical applications | General industry | Food, pharmaceuticals, electronics |
| Risk of oil contamination | Depends on the filtration system | None from the compression process |
Don’t forget energy costs
Over the life of an air compressor, electricity usually accounts for the largest share of the total cost, far more than the purchase price. If compressed air demand varies from shift to shift, a machine with a variable speed drive (VSD) adjusts its speed to the actual flow demand and saves significant energy compared with a machine running in load/unload mode.
Selection tips
- Define the compressed air quality your product requires (oil, water, particles) according to ISO 8573-1.
- Measure or estimate the required flow and pressure, including variations between shifts.
- Compare the total life-cycle cost (capital + energy + maintenance), not just the purchase price.
Dong Tam supplies oil-flooded rotary screw air compressors and oil-free air compressors from Ingersoll Rand, together with installation and maintenance services.
Need specific advice for your plant? Contact Dong Tam for support from our engineers.