Two-Stage Air Compressor – Air Compressor Manufacturers

Single-stage screw compressors top out at 10 to 13 bar. Stuck on temperature.

300°C

Discharge Temp 10:1

200°C

Mineral Oil Flash Point

0.1mm

Design Clearance

Compress air from 1 bar to 10 bar, 20°C going in, 300°C coming out. Compression ratio is right there, temperature has to go up. No negotiating.

300°C. Mineral oil flash point is only 200°C. Oil will carbonize. Seals, don't even talk about it. Nitrile rubber handles 120°C, fluoroelastomer handles 200-something°C. No common material can survive 300°C long-term.

Precision Engineering

Male and Female Rotor Assembly

The rotor issue needs its own discussion, because this is the real reason screw machines fear high temperature.

The heart of a screw compressor is that pair of male and female rotors. Two screws meshing together, one with four lobes, one with six, turning to push air from this end to that end. Meshing has to be tight. Leak and the work is wasted. So clearances are designed very small. Between rotors, between rotors and housing, typically just a few hundredths of a millimeter to just over a tenth.

These clearances are precisely calculated. Machining has tolerances, assembly has deviations, rotors thermally expand during operation, all factored in. Normal discharge temperature eighty-something, ninety-something degrees, expansion is within the designed allowance. Just right.

Temperature shoots to 300°C, things go wrong.

Metal expands when heated, expansion can be calculated. Ductile iron linear expansion coefficient is about 11×10⁻⁶/°C. Meaning every 1°C rise, a 1-meter iron bar grows 0.011 mm. Doesn't sound like much, but the temperature difference is big. From 20°C to 300°C, 280°C gap. A 500 mm rotor grows over 1.5 mm.

1.5 mm. Design clearance is only around 0.1 mm.

Rotor expands outward, housing expands too, but the housing has different wall thickness, different cooling, expands at a different rate from the rotor. Result: rotor pushes up against the housing inner wall. Starts as light contact, friction makes heat, temperature goes higher, expansion gets worse, contact gets tighter. Vicious cycle. Then scoring, metal surfaces get grooved. Then seizure. Two rotors locked inside the housing, dead still. Motor overloads. Protection trips.

Damage

Thermal Scoring

Failure

Rotor Seizure

What does a seized airend look like opened up? I've heard from maintenance guys. Score marks all along the rotor surfaces. Bad spots, metal actually deformed from the squeezing. Matching scars on the housing inner wall. Mild cases can be ground smooth, re-set the clearances. Bad cases, rotors are scrap. Replace them. The airend is the most expensive part of a screw compressor. Replacing one airend can cost close to half a new machine.

So screw compressors are extremely touchy about discharge temperature. Every manufacturer's manual writes the normal discharge temp range, what temperature triggers alarm, what triggers shutdown. They're not being conservative. It has happened.

Two-stage compression exists to get around this temperature wall.

Idea is simple. Can't get there in one shot, do it in two. First stage compresses 1 bar to 4 bar, compression ratio of 4, discharge temp around 150°C. Safe. Hot air goes into an intercooler, water or air brings temperature back down to 40 to 50°C. Second stage picks up from 4 bar, compresses to 16 bar, compression ratio still 4, discharge temp still around 150°C. Two fours multiplied, total compression ratio 16, final pressure 16 bar. Neither stage exceeds the limit.

Intercooler is just a heat exchanger. Compressed air one side, cooling water or air on the other, carrying heat away. Also knocks out some moisture. Air temperature drops, water vapor condenses into droplets, drains from the bottom.

10-15%

Energy Savings

7-8bar

Factory Standard

16bar

Two-Stage Output

Two-stage uses less energy than single-stage. Reasoning isn't complicated. Closer the compression is to isothermal, less work it takes. Adding a cooling step in the middle moves it one step toward isothermal. Saves 10 to 15 percent. Compressors already use a ton of electricity. Saved energy adds up year after year.

Most factories need 7 to 8 bar. Pneumatic wrenches, spray guns, cylinders, all that pressure range. Single-stage screw covers it. No reason for two-stage.

High-pressure demand is in a few specific industries.

25-40 Bar

PET Bottle Blowing

16+ Bar

Laser Cutting

25-30 Bar

Marine Diesel Start

PET bottle blowing, making beverage bottles, water bottles. Plastic preform gets heated soft, into a mold, high-pressure air blows in and inflates it into bottle shape. Needs 25 to 40 bar, depends on bottle size and wall thickness. Two stages aren't enough. Need three or four. High-pressure machines in a blow molding shop and the general factory air supply are two separate systems. Nothing to do with each other.

Laser cutting sometimes needs high pressure too, mainly cutting stainless steel and aluminum. Nitrogen as shielding gas, prevents the cut edge from oxidizing and going yellow, also blows away the melt. Nitrogen pressure too low, slag doesn't blow clean, cut edge has a string of beads hanging off it. Looks bad, works bad. Some processes need nitrogen above 16 bar. Cutting carbon steel uses oxygen, leverages the oxidation reaction to help cut, doesn't need as much pressure.

Marine diesel engines start on compressed air, 25 to 30 bar. Those big engines can't be cranked with a starter motor like a car. Need high-pressure air pushing the pistons to get going. Air stored in cylinders, separate compressor on board charges them. Before going to sea, cylinders must be full. Regulations say so.

Most extreme pressure is scuba tank filling, 200 bar minimum, some go to 300 bar. Not screw compressor territory anymore. Need reciprocating machines, four or five stages going up, intercooler between each. Filling station equipment costs serious money. And because it's breathing air, contamination requirements are extremely strict. No oil, minimal water, CO and CO₂ both have limits. Filtration system is a whole setup, air samples go for lab testing on a schedule.

Higher the pressure, more expensive the equipment. That's a rule. Two-stage compression is just the first step past the single-stage ceiling.