Rotary Screw vs Rotary Vane vs Scroll Compressors

Most of the Time Pick Screw

Factory buying compressor, eight out of ten end up with screw machine. Not that screw is perfect. It's that power and flow coverage is too wide. 5 hp to 670 hp all have mature products. Other types can't do that. Vane tops out at 100 hp. Scroll caps at 30 hp. Large flow situations, no other choice.

Screw expensive? Not expensive in big power range. Small power range, yeah it's expensive. Under 10 hp same displacement, screw is a chunk more than piston. That's where vane and scroll have room to survive.

When Not to Pick Screw

Three situations.

First, tight budget, small air demand. Under 10 hp needs, screw value isn't there. Vane is cheap, simple structure. 2 hp to 100 hp range competing head to head with screw. OEM market, vane has good share. Printing machines, packaging machines, these complete equipment need air source. Equipment makers count costs precisely. Vane is small and cheap. Save where you can save. Auto shops, small paint booths are also vane customer base.

Gotta say something about vane structure: rotor mounted eccentric in cylinder, blades in slots, spinning the blades get flung out by centrifugal force pressing against cylinder wall forming compression chambers. Simple is simple. Blade wear is ongoing cost. Few thousand hours to 20,000 hours, change a batch of blades. Depends on conditions and oil. Factor this money in when selecting.

Second, hard noise requirements. Scroll is 50 to 65 decibels, screw with enclosure 62 to 82 decibels, vane 65 to 78 decibels. Clear gap. Hospitals, clinics, labs, scroll is almost standard. That small compressor next to dental chair, quiet like a computer tower, 80% chance it's scroll.

Third, must be oil-free. Food direct contact, medical breathing, precision electronics. Applications strict on oil content. Oil-free scroll tech is most mature. Scroll plate clearance control is precise, seals effectively without oil injection. Under 140 CFM flow needs, oil-free scroll is first choice. Larger flow scroll can't cover, gotta go oil-free screw, much pricier.

Some say oil-injected with aftertreatment can also meet oil-free standards. Theoretically yes. Multi-stage filtration plus adsorption drying can push oil content very low. Problem is filters need changing, adsorbent needs changing, plus leakage risk. For applications that really care about air quality, source oil-free beats aftertreatment oil removal. Less hassle.

Screw Machine Pitfalls

Screw is good, but selection and operation have a few places easy to trip.

Industrial Application

Screw Compressor Installation

Intake filtration gets underestimated. Screw maintenance interval 8,000 hours, rotor assembly design life ten years plus. Sounds easy. But premise is intake filtration is done right. Factory surroundings dirty, lots of dust, air filter not changed often enough, dust gets in compression chamber, rotor surface wears grooves, that's major overhaul or scrap. Seen cement plant screw machines, three years and rotor was shot. Air filter money saved, airend money thrown in.

One more thing, oil-injected screw compressed air carries oil. Need to add oil separator downstream. Higher requirements need precision filter too. Selection, just looking at main unit price isn't enough. Aftertreatment equipment cost needs adding.

Vane and Scroll Boundaries

Vane ceiling is 100 hp and 145 psi. Bigger power, no products. Higher pressure, internal leakage goes up, efficiency drops badly. Within this boundary, vane competes: smaller than same displacement screw by a size, low speed low vibration, price usually lower too. Mobile compressors, vane structure is common. Can handle bouncing around on work vehicles.

Scroll ceiling is 30 hp and 140 CFM. Scroll plate diameter gets big, machining difficulty skyrockets, thermal deformation can't be controlled. Manufacturers don't want to make them. So scroll is destined for small flow market. Medical, dental, labs, small oil-free supply. Can't get out of this circle. Need 350 CFM? Buy three scroll machines to make up? Equipment cost, footprint, maintenance workload, all don't work out.

Scroll plate is precision part. Breaks, replace whole thing, can't repair. Good thing is scroll machines are low power, run smooth. Keep intake filtration good, scroll plate life is pretty solid.

Middle Ground Selection

Tricky part is the middle ground: flow demand 175 to 530 CFM, power 20 to 100 hp, pressure 100 to 145 psi. This range both screw and vane can handle. Pick which?

Calculate total cost of ownership. Screw equipment cost high, maintenance interval long, parts expensive but change less often. Vane equipment cost low, blades need regular changing, other parts cheap. Electricity about the same, this power range efficiency is close.

Consider maintenance capability too. Vane blade change you can do yourself. Screw rotor has problems, need professional people. Profile is core technology. Parts supply, screw market is big, available everywhere. Vane some proprietary parts gotta go to OEM.

Specific Scenarios

Automotive

Paint Line

Medical

Dental Clinic

Packaging

Production Line

Auto plant paint line. Big air volume, continuous operation, high stability requirement. Screw. No other choice. Multiple units with coordinated control, start-stop based on header pressure.

Dental clinic. Small air volume, noise sensitive, needs oil-free. Scroll. Quiet and clean. Put it next room from treatment room, doesn't affect doctor-patient conversation.

Packaging machine bundled supply. Sold with equipment, cost sensitive, space limited. Vane. Small and cheap. Equipment integrator's standard choice.

Lab gas chromatograph. Small flow need, extremely high purity requirement. Oil-free scroll. With appropriate drying and filtration system.

Job site mobile work. Harsh conditions, frequent start-stop, space limited. Vane. Rugged, handles bumping and vibration.

Medium machine shop. Dozen-plus CNCs. Medium air volume, long runtime. Screw. Total cost of ownership works out lower.

Final Words on Selection

Pressure need over 145 psi, pick screw. Vane and scroll can't handle high pressure range.

Flow need over 530 CFM, pick screw. Other types can't cover it.

Must be oil-free, small flow pick scroll, large flow pick oil-free screw. Aftertreatment oil removal is backup plan, not first choice.

Hard noise spec, scroll is way ahead. Screw with sound enclosure can improve, price is bigger size and worse heat dissipation.

Tight budget, small air volume, short runtime, vane or piston might be better deal than screw.