Rotary Screw vs Reciprocating Compressors – Air Compressor Manufacturers

Fifteen or so years ago buying air compressors, screw and piston machines could still compete. Back then screw machines were expensive. One 50 hp screw machine could buy two or three piston machines same power. Many small factory owners couldn't make the numbers work, figured fixing piston machines when they break still cheaper than buying a screw machine. Looking back now, those factories that insisted on piston machines either switched to screw machines long ago, or still dealing with repair workers and parts suppliers.

Screw machines eating up medium and large flow market took about ten years. Foreign brands like Atlas Copco, Ingersoll Rand, Sullair led the way. Domestic brands like Kaishan, Xinlei, Jaguar followed. Screw machine prices kept dropping, piston machine cost advantage gradually disappeared.

Two types of machines compress air in completely different ways.

Piston Machine

Piston machine principle similar to car engine. Crankshaft drives pistons up and down in cylinders. Downstroke, intake valve opens drawing in air. Upstroke, exhaust valve opens pushing compressed air out. This setup been around over a hundred years. Structurally simple. Old repair shop masters can understand at a glance, take apart and fix. Many compressor repair workers old days crossed over from diesel engine work, principles are similar, parts look alike too.

Screw Machine

Screw machines more complex. Core components are a pair of screw rotors, one called male rotor with four lobes, one called female rotor with six flutes. Two rotors mesh and rotate together. Air enters from one end, gets trapped in tooth spaces pushed forward, space gets smaller and smaller, pressure gets higher and higher, comes out the other end as compressed air. No valves, no pistons, no reciprocating parts. Rotor profile design is core technology. Swedish SRM company's profile patents sustained most of the industry for decades. After patents expired, domestic airends gradually developed.

Fundamental reason screw machines pushed out piston machines is continuous air supply capability.

Screw rotors keep turning, air keeps coming out, flow is steady, pressure fluctuation small. Piston machines can't do that. Pistons reciprocate once for each discharge cycle, airflow comes in bursts, pressure jumps around. Atlas Copco website states clearly, piston machine duty cycle around 50-60%, meaning can only run 30-35 minutes per hour, have to stop in between to cool down, otherwise overheat and get damaged. This means piston machine systems must have larger air receivers to buffer pulsation and stabilize pressure. Same air supply, receiver capacity often two to three times what screw machine systems need.

Airflow pulsation causes another problem. Downstream equipment doesn't like it. Refrigerated dryer evaporators can freeze up when flow fluctuates violently. Desiccant dryer adsorbent gets repeatedly pressure-shocked and accelerates powdering. Screw machines don't have these issues.

Many people underestimate noise as a factor.

Atlas Copco China website technical documents say screw machines typically 65-75 dB, piston machines 85 dB and up. Specific models, GA11 VSDS rated 67 dB(A), GA15-GA30 series 65-70 dB(A). Data from Grandlink Group shows piston machines typically over 75 decibels, twin screw machines 64-78 decibels, permanent magnet VFD screw machines can do 60-63 decibels. 10 decibel difference might not sound like much, but sound pressure level is logarithmic. 10 decibels means perceived loudness more than doubles.

Environmental noise pollution prevention laws specify industrial equipment daytime noise must not exceed 70 decibels, nighttime must not exceed 55 decibels. Bare piston machines exceed limits. Screw machines with sound enclosures easily comply. Many factories these years replanning compressor station locations. Moving from standalone machine rooms to inside workshops or near point of use. Eliminating long pipe runs, reducing pressure drop. Only screw machines can support this layout. Put piston machine in workshop, workers can't stand it, environmental compliance can't pass either.

Industrial compressor — Modern screw compressor installation

Maintenance, screw machines much less hassle.

Screw machine routine maintenance just oil and filter changes. Oil filter every 1,500-2,000 hours. Oil separator around 3,000 hours. Air filter depends on environment, clean places 2,500-3,000 hours, dusty places need to shorten. Airend bearing design life 25,000-30,000 hours, proper maintenance can last long time. Data cited in Baidu Baike "screw compressor" entry says airend design life reaches 30 years. Bolaite technical materials mention imported brand screw machines lasting 15 years or so are common, domestic ones 5-10 years normal. Changing filters something ordinary electrician can do with half day training, doesn't need professional repair workers.

Wear parts list for piston machines scary when you pull it out: piston rings wear and leak, need checking every 1,000-2,000 hours, basically need changing at 2,000-3,000 hours. Intake and exhaust valve plates repeatedly open and close under impact, breakage and sealing surface damage routine. Connecting rod big end bearings, small end bushings bear alternating loads, worn ones will knock and jump. Crankshaft oil seals age and leak. All these things need people to inspect and repair, need parts inventory, need repair skills. Use a piston machine five years, accumulated parts costs not small.

China's GB 19153-2019 "Positive displacement air compressor energy efficiency limits and efficiency grades" implemented July 1, 2020 incorporated VFD screw machines into energy efficiency evaluation system. Standard specifies VFD machine specific power is weighted value: 100%, 70%, 40% of full load volumetric flow multiplied by weighting factors 25%, 50%, 25% respectively then summed. Someone on Zhihu calculated specific power of Atlas GA45VSD+ at 5.9 kW/(m³/min), first tier efficiency. Atlas website promotes VSDS saving 60% compared to fixed speed machines. That number optimistic, but 30-40% energy savings is solid.

Compressor equipment — Compressor technology comparison

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Said so much good about screw machines, should piston machines be phased out?

No.

Small flow air supply still piston machine territory. Displacement under about 35 CFM, screw rotors can't be made that small, or made small but machining precision can't hold up, tooth clearance leaks, efficiency actually worse than piston machines. Price gap also large. 20 CFM small piston machine costs $300-400. Same spec screw machine over $1,500.

Intermittent air use suits piston machines better. Auto repair shops, dental clinics, small metalworking shops. Cumulative daily air use might be just two or three hours, constantly starting and stopping. Piston machines start and stop freely. Screw machines can't. Screw machines have load/unload processes during startup. Frequent start-stop causes oil separator tank pressure to fluctuate repeatedly, lubricating oil tends to emulsify, bearing conditions deteriorate. Seen people buy small screw machines and use them like piston machines, starting and stopping dozens of times a day. Airend had problems within two years. That usage pattern wrong to begin with.

High pressure compression is absolute piston machine territory. Baidu Baike "screw compressor" entry clearly states screw machine discharge pressure generally doesn't exceed 1,450 psi. Atlas GR series high pressure screw machines doing 290 psi already considered high. Going higher requires piston machine multi-stage compression. Two-stage, three-stage, four-stage in series, compress air to hundreds or thousands of psi. Cylinder filling, PET bottle blowing, high pressure air storage industries, can't do without multi-stage piston compressors. Screw machines can't enter this market.

Small oil-free machines also favor piston. Labs, medical, small food plants needing oil-free compressed air, small oil-free piston machines are mainstream choice. Oil-free screw machine technology barriers high, rotors need special coatings, timing gears require precision. Same spec prices several times oil-free piston machines. Value for money completely doesn't compete at small flow ranges.

Industrial equipment — Application-specific selection

So selection judgment actually simple: displacement over 100 CFM, continuous operation 8 hours or more per day, care about noise and maintenance cost, screw machine, no hesitation. Small flow, intermittent air use, tight budget, need high pressure or need cheap oil-free air source, piston machine still reasonable choice. Two technologies will coexist long-term, just each holding different territory.