Water-Lubricated Oil-Free Screw Air Compressor: Working Principle & the Near-Isothermal Advantage

What “water-lubricated oil-free” actually means

In a water-lubricated oil-free screw compressor, water (not oil) is the sealing, cooling and lubricating medium inside the compression chamber. Air never contacts oil during compression, so the delivered air starts clean by design — no downstream oil-removal filter is needed to reach oil-free status. CMN builds this on a single-screw air end: one main rotor with helical grooves engages two star wheels on either side, squeezing air in the groove volume as the rotor turns.

The near-isothermal advantage

Compression generates heat. In a dry-type oil-free screw, the rotors run oil-free and rely on anti-wear coatings (PTFE/PEEK) and timing gears, with discharge temperatures typically 180–220 °C. In a water-lubricated design, the injected water absorbs compression heat directly, holding discharge temperature close to 40–50 °C — a near-isothermal process.

Why this matters to a plant engineer:

• Lower thermal stress on rotors, bearings and seals → longer service life and fewer coating-related failures.

• Stable performance across load and ambient swings, because water carries heat away continuously.

• Lower noise and vibration than dry-running alternatives at comparable duty.

• No consumable coatings to reapply at overhaul.

Single-screw vs the compression path

The single-screw geometry balances radial load: the two star wheels act on opposite sides of the main rotor, so thrust cancels. This gives smooth, low-vibration operation and, with high machining accuracy, efficient sealing with low water consumption. CMN pairs the air end with a permanent-magnet variable-speed (PMVSD) drive option for part-load efficiency.

Where it fits

Water-lubricated oil-free units suit any process where oil contamination is unacceptable: food & beverage, pharmaceutical and medical air, PET bottle blowing, electronics, and instrument air. They are available across CMN’s 2–40 bar range and 15–2,100 cfm flow band.

**Spec note:** Pressure and flow ranges above are CMN’s published product-range figures. Confirm the exact model against the current CMN catalogue before quoting.

Need a sizing proposal? Send us your required pressure, flow and duty cycle, and our engineering team returns a recommendation and an energy estimate.