What Is a Screw Air Compressor?

Strip the shell off a screw air compressor, not much inside. An airend, a motor, an oil separator tank, a cooler, a control panel, the rest is pipes and valves. The airend matters most, costs the most too, 30 to 40 percent of the whole machine. Motor is 15 to 20 percent, cooler about 10, controls under 10, the sheet metal shell 5 percent, and the rest is odds and ends of valves and piping.

Industrial Application

Modern Screw Compressor Installation

Two rotors inside the airend.

Male rotor has four lobes, female has six, 4+6 setup. The male rotor connects to the motor and spins, the female meshes along with it. Air goes in one end, rotors turn, the space between the lobes gets smaller and smaller, air gets squeezed, pressure comes up, out the other end. Oil-injected models spray lubricating oil in there. The oil does three things: plugs up the gaps between rotors so air doesn't leak back, carries away the heat from compression, lubricates the tooth surfaces to cut down wear. The air coming out has oil mist mixed in, needs to go through separation before you can use it.

Rotor material is ductile iron, QT500-7 or QT600-3. QT500-7 is softer, 7% elongation, not prone to brittle fracture. QT600-3 is harder, stronger, only 3% elongation. Surfaces get coated, PTFE or molybdenum disulfide, makes them slippery, so if the two rotors happen to touch they won't seize up. Coating is very thin, tens of microns, wears off after running long enough.

The shape of the rotor teeth is called the profile. SRM profile was developed by the Royal Institute of Technology in Sweden, patent expired years ago, anybody can use it. GHH profile is from the German company GHH. There's also profiles copying the Atlas Copco A-type, some domestic manufacturers use those. Different profiles, efficiency differs by a percent or two, not a big deal. The gap between rotors matters more. Gap too big, air leaks back, efficiency drops. Gap too small, friction goes up, dangerous too. Domestic airends get clearances down to 0.05 to 0.08 mm. Imported ones can do 0.03 to 0.05 mm. Just those few hundredths of a millimeter, and the price gap is huge.

The airend, OEMs generally don't make it themselves.

There are dedicated airend manufacturers. Atlas Copco is Swedish, GHH is German, VMC is Italian. Domestic ones include Baosi, Hanbell, UnairS, Fusheng. OEMs buy airends from these guys, pair them with a motor, cooler, controller, valves and pipes, stuff it in a shell, slap their own name on it and sell.

This creates a thing. Same airend, inside different brands, price can differ by 50%. Imports cost more. You're paying for the brand premium, imported components, nationwide service coverage. Tier-one domestics are cheaper, performance is about the same, service in the main industrial areas. Tier-two and three domestics are even cheaper, problem is thin service coverage. Something breaks, hard to find someone to fix it, parts take a while. Waiting a month, it happens.

Manufacturing

Assembly Line

Automotive

Production Facility

Food & Beverage

Processing Plant

Airend breaks, basically means replacing the machine. Repair cost is close to buying a new airend, add in teardown and reinstall plus downtime, might as well get a new one. This is why the airend is 30 to 40 percent of the cost. The core is right here. Can't afford it breaking.

Air enters the airend through the intake valve.

Several types of intake valve. Butterfly type is the most common. A butterfly-shaped plate, rotate it, intake opens bigger or smaller, controls how much air comes in. There's also piston type, piston moves back and forth, good seal, used for high-pressure. Rotary valve type, valve core rotates to adjust, needs high machining precision, not used much.

Air comes out after compression, mixed with oil, into the oil separator tank.

Inside the tank there's a filter element called the oil separator element, fiberglass. Oil-laden air goes through the element, oil mist gets caught, coalesces into droplets that run down the element wall. Clean air goes out the other side. The element clogs over time. How clogged, look at the pressure differential. New element, about 0.2 bar. Keeps climbing as you use it. Hits 0.8 to 1.0 bar, time to change.

Compressed air is hot. Needs cooling before you can use it.

Cooling Technology

Heat Exchange Systems

Air-Cooled

Ventilation Setup

Water-Cooled

Circulation System

Two types, air-cooled and water-cooled. Air-cooled is fans blowing on finned heat exchangers, pushing heat into the air. Fin spacing about 2.5 mm. Too tight and air can't push through, dust builds up easy. Too loose and not enough heat exchange area. Air cooling has a limit. Ambient goes high, cooling doesn't cut it. Over 40°C, temperature differential shrinks, heat can't get out, machine throws a high-temp alarm and shuts down. Air-cooled machines need good ventilation. Stuffy corner of the factory in summer, forget it.

Water-cooled uses circulating water to carry heat away. More efficient than air-cooled, doesn't care about ambient temperature. The headache with water cooling is water quality. Too much calcium and magnesium, high hardness, scale builds up on heat exchanger inner walls. Scale conducts heat terribly. One layer of scale, heat exchange efficiency drops a chunk. Severely scaled heat exchangers lose 10 to 15 percent efficiency per year. Needs periodic acid washing to strip it off. Either use softened water or put in water treatment equipment.

On the control panel there's a controller that runs the whole machine.

Common controller brands, MAM series, Pulite, these are generic, functions are adequate, most domestic machines use them. Atlas Copco has their own Elektronikon, pairs well with their machines, more features, but service has to go through Atlas Copco.

Control Systems

Digital Interface

Monitoring

Real-time Data

Two or more machines supplying air together, you need a sequencing system. Sequencing lets the machines take turns, keeps the runtime balanced. Without sequencing, each machine does its own thing. Pressure setpoints differ by just a tiny bit, and one machine becomes the workhorse. Full load every single day, the others sit idle. Workhorse wears out fast, breaks first. With sequencing, the system schedules which starts and which stops based on demand. Everyone takes turns. Overall lifespan longer.

Discharge temperature on the control panel. Pay attention to this number.

Normal range 75 to 95°C. Below 75 is bad. Water vapor in the compressed air hits the cold and turns to water, mixes with the lube oil, emulsifies. Emulsified oil looks milky white. Lubrication gone, wear speeds up. Winter or light loads, watch for it dropping too low. Above 95 is also bad. High temp, oil oxidizes fast, degrades, forms sludge, clogs oil passages. Run at high temp long-term, oil change intervals get shorter, maintenance costs go up.

Temperature sensor is at the oil separator tank outlet. Temperature here reads 10 to 15°C lower than the airend outlet. When it shows 95°C, the airend outlet is already past 105°C.

When buying a machine people look at specific power. How much power consumed per unit of air output. Lower number, better efficiency. Problem is this specific power number, different manufacturers don't measure the same way.

Specific power at 7 bar and 8 bar is different. Higher pressure, higher specific power. Full load and partial load specific power are different. Partial load has idle losses baked in. Whether you use FAD (measured free air delivery) or theoretical displacement, numbers come out different. Theoretical displacement doesn't account for losses, looks prettier on paper. Whether power is shaft power or input power, also different. Shaft power is what comes out of the motor shaft. Input power is what the motor pulls from the grid. Difference between them is motor efficiency.

Sizing screw compressors for a factory, look at discharge volume and discharge pressure. Volume based on total demand from the air-using equipment, leave some margin, 10 to 20 percent, for load swings. Standard industrial air pressure is 7 to 8 bar.

Logistics

Distribution Center

Pharmaceutical

Clean Room

Textile

Production Line

Imports like Atlas Copco, Ingersoll Rand, Sullair. Expensive, lots of service locations, fast parts. Tier-one domestics Fusheng, Kaishan, Xinlei. Performance is close, cheaper, service in major industrial areas. Tier-two and three domestics cheaper still, service coverage limited, long wait for parts.