How to Size Air for a PET Blowing Line (Bottles/Hour to CFM)

How to size air for a PET blowing line: bottles per hour to CFM at 40 bar

Published 16 October 2026

The quickest honest answer: each litre of bottle volume needs roughly 50 litres of free air at 40 bar, once you allow for the air left in the blow nozzle, stretch rod and pipework. Multiply that by your bottles per hour and you have the compressor size, within about 20%. The blow moulder's own data sheet is always the final word, but this method gets you to the right model before you have it.

Below is how the number is worked out, a ready table for common bottle sizes and speeds, and how to match it to a compressor.

Why PET blowing needs two kinds of air

A stretch blow moulding line uses compressed air in two ways:

• Service air at 7 to 10 bar for preform handling, mould clamping, valves and pneumatic cylinders. A normal oil-injected screw air compressor with a dryer supplies this.
• Blowing air at 25 to 40 bar, which shapes the bottle. It touches the inside of the bottle, so it must be oil-free. This is the air that decides the size and cost of the system, and it is what this article sizes.

The calculation in three steps

1. Free air per bottle. When a bottle is blown at 40 bar gauge, the air inside is at about 41 bar absolute. Expanded back to atmospheric pressure, that is 41 times the bottle volume. A 1 litre bottle therefore holds about 41 litres of free air when it is full.

2. Add the dead volume. The bottle is not the only space that fills at 40 bar each cycle. The blow nozzle, the stretch rod passage, valves and the short pipe runs to them also fill and are vented. A practical allowance is 25%, which brings a 1 litre bottle to about 51 litres of free air per bottle. Machines with long blow lines need more; machines with short, compact valve blocks need less.

3. Multiply by output. Air demand (litres per minute) = free air per bottle × bottles per hour ÷ 60. CFM = litres per minute ÷ 28.3. Then add 15 to 20% for leaks, wear and the day the line runs faster than planned.

Worked example

A two-cavity machine making 1 litre water bottles at 4,000 bottles per hour:

• Free air per bottle: 1 L × 41 × 1.25 = 51 litres
• Air demand: 51 × 4,000 ÷ 60 = 3,400 litres per minute, about 121 CFM
• With 20% margin: about 145 CFM at 40 bar

That fits a Colt 3HFY27N+ oil-free reciprocating compressor (160 CFM at 42 kg/cm²).

Ready table: blowing air needed at 40 bar

Figures include the 25% dead-volume allowance. The last column adds 20% margin and names the smallest standard Colt model that covers it.

Bottle size	Bottles per hour	Air needed (CFM)	With 20% margin (CFM)	Colt model that covers it
250 ml	6,000	45	54	3HFY27N (120 CFM)
250 ml	12,000	90	109	3HFY27N (120 CFM)
500 ml	6,000	90	109	3HFY27N (120 CFM)
500 ml	12,000	181	217	3HF25N+ (265 CFM)
1 litre	4,000	121	145	3HFY27N+ (160 CFM)
1 litre	9,000	271	326	3HF23N (410 CFM)
2 litre	4,000	241	290	3HF24N (305 CFM)
2 litre	9,000	543	652	CSW2-20/40 booster (706 CFM)
5 litre	2,000	302	362	3HF23N (410 CFM)
5 litre	4,000	603	724	3HF43N (730 CFM)

Reciprocating figures are from the Colt oil-free PET range at 42 kg/cm²; booster figures are from the CSB/CSW screw-piston booster range at 40 bar. Larger lines, up to 2,000 CFM, are covered on request.

Things that change the answer

• Blow pressure. Thin, lightweight bottles for still water often blow at 25 to 30 bar; carbonated soft-drink bottles usually need the full 35 to 40 bar. Lower pressure means proportionally less air: at 30 bar, use 31 instead of 41 in step 1.
• Air recovery. Many modern blow moulders reuse part of the blowing air for the pre-blow stage or the service air. If yours does, take the maker's figure with recovery switched on; it can be noticeably lower than the formula.
• Pre-blow at 8 to 12 bar. Some machines take pre-blow air from the high-pressure line, some from the service line. Check which, so it is counted once.
• More than one machine. Add the demands together, then size for the total. A single larger compressor, or two units so one can be serviced while the other runs, are both common.
• Future bottles. If a 2 litre bottle is likely to be added later, size for it now. Changing a compressor after the line is installed costs more than buying the right size once.

Matching the demand to a compressor

Up to about 700 CFM, an oil-free reciprocating compressor (3HFY27N to 3HF43N, 120 to 730 CFM at 42 kg/cm²) is the simplest choice: one machine, intercoolers and aftercooler included, PLC panel with VFD.

For larger lines, or plants that already have 7 bar screw air, the CSB/CSW oil-free screw-piston booster (212 to 1,059 CFM at 40 bar) puts an oil-free screw stage and a two-stage oil-free piston booster on one skid. Compressing in two stages with intercooling keeps each stage cooler and uses about 15% less power than single-stage compression at the same flow and pressure. See the PET bottling air compressor page for both model tables, and the oil-free booster page for how the booster works.

Whichever you choose, the system needs a high-pressure air receiver, a dryer and high-pressure filters so the air reaching the mould is dry and clean. See air receivers, dryers and filters.

Frequently Asked Questions

Send us your blow moulder model, number of cavities, bottle sizes and bottles per hour, and our engineers will size the compressor, receiver, dryer and filters for your line.