How compressed air is used in PET bottle production
PET bottles start as injection-moulded preforms. The preform is reheated in an infrared oven, stretched by a rod and blown into a mould in two stages: a lower-pressure pre-blow that starts the shape, and a high-pressure final blow that pushes the PET firmly against the mould wall to form detail, base and threads. After forming, the air in the bottle is vented. That vent is the main energy loss of the process, and also the opportunity for air recovery.
Around the blow moulder, the rest of the plant uses ordinary machine air: preform injection moulding machines, conveyors, grippers, valves, packaging and palletising. A plant may only make preforms, only blow bottles from bought-in preforms, or do both in line with a filler.
| User | Typical pressure | Flow pattern | Air quality (ISO 8573-1, typical approach) |
|---|---|---|---|
| Final blow (stretch-blow moulder) | Typically 25–40 bar, depending on bottle and machine | Large, in short repeated pulses | Oil-free, dry, finely filtered; air forms the inside of the container |
| Pre-blow | Lower stage, often fed from recovered or regulated air | Short pulses | Same quality as final blow |
| Blow moulder valves, stretch cylinders, mould locking | About 6–7 bar | Cyclic | Dry, filtered machine air |
| Preform injection moulding machines | About 6 bar | Intermittent: ejection, part removal, gate valves | Dry, filtered; preform-contact blow-off at oil-free quality |
| Preform dryer (PET resin) | Low pressure if compressed-air type | Steady | Very dry air; many dryers use their own desiccant blower system |
| Air conveyors and bottle transfer | Low pressure from blowers | Steady | Filtered; usually not from the compressor system |
| Packing, palletising, workshop | About 6 bar | Peaky | Standard dry machine air |
The blow moulder maker states the pressure and air consumption per cavity or per bottle for each bottle design. Those figures, not a general rule, set the size of the high-pressure system.
Why the LP/HP split matters
A PET plant that tries to run everything from one pressure pays heavily. Producing air at 40 bar and regulating it down to 6 bar for a cylinder wastes most of the compression work. The opposite mistake, raising a low-pressure station to make up for a weak high-pressure supply, does not work either, and every extra bar on the LP side costs roughly 7% more power.
The usual arrangement is:
- LP system at about 7 bar from screw compressors, with a refrigerated dryer (pressure dew point about +3 °C) and filters, feeding all machine pneumatics.
- HP system at the blow moulder’s required pressure, from either a dedicated high-pressure compressor or a booster taking air from the LP system, with its own dryer, filters and HP receivers.
- Recovered air from the blow moulder returned to the LP network or to the pre-blow circuit where the machine supports it.
Keeping the two networks separate also makes faults easier to find: an LP pressure problem is an LP problem, and a bottle-forming fault can be traced to the HP side without confusion.
High-pressure compressor or booster?
Both approaches are widely used. The choice depends on how many blow moulders run, how much LP air the plant already makes, and redundancy needs.
| Option | How it works | Strengths | Watch points |
|---|---|---|---|
| Dedicated HP compressor | Multi-stage oil-free piston compressor, or a screw first stage with piston high-pressure stages, compressing from atmosphere to final pressure | Independent of LP system; simple to understand | Many stages, coolers and valves to maintain; full compression from atmosphere |
| Booster from LP air | Oil-free piston booster takes dry 7–10 bar air from the LP system and raises it to blow pressure | Uses existing efficient LP screw capacity; fewer stages; good for expansions | LP supply and dryer become critical to bottle production; LP must be sized for booster demand |
| Combined packages | Integrated LP plus booster packages, sometimes with VSD | Compact; one controller | Single package can be a single point of failure |
Whichever route is chosen, the HP machines must be oil-free in the compression chamber or followed by treatment that reliably delivers oil-free quality. See our pages on booster compressors and high-pressure reciprocating compressors for the machine details.
Sizing the high-pressure side
Average flow on a PET line can be misleading because HP demand comes in rapid pulses. A sound sizing method is:
- Take the blow moulder maker’s air consumption for each bottle design at the planned output, including pre-blow and final blow, and any recovery fraction.
- Add the number of machines that run at the same time, and the largest bottle they will run in future.
- Confirm the minimum pressure the machine needs at its inlet during the final blow, then add losses across HP dryer, filters, pipe and valves.
- Size HP receivers so pressure stays above that minimum through the pulse, between compressor or booster responses. A simple storage calculation gives a starting volume; final volume depends on temperature, control dead band and vessel rating.
- Check the LP system has spare capacity for the booster’s intake, if a booster is used.
- Decide redundancy: N+1 boosters or HP compressors where a stop costs significant production.
Our receiver storage calculator helps with step 4. All HP receivers, pipes, valves and hoses must be rated for the working pressure with a safety margin and inspected under the applicable pressure-vessel rules.
Air recovery: reusing blow air
After final blow, the bottle contains air at high pressure that would otherwise be vented to atmosphere. Many modern stretch-blow moulders can recover part of this air in two ways:
- Recycling into pre-blow. Part of the exhaust feeds the next bottle’s pre-blow stage, reducing HP air needed per bottle.
- Return to the LP network. Remaining air, after pressure reduction, is fed into the plant’s 7 bar network, so the LP compressors make less air.
Recovered air reduces energy use, but it must be engineered carefully. Recovered air flow varies with machine output and stops when the moulder stops, so LP compressors must cover the full LP load when recovery is unavailable. The return point should have a non-return valve and pressure control so recovered air does not over-pressurise the LP network or upset compressor controls. Recovered air should also pass through filtration before reaching sensitive users.
Air quality for blow moulding
Blow air forms the inside surface of a bottle that will hold water, soft drink, juice, edible oil or other products, so it is treated as direct-contact air. Typical specifications start around ISO 8573-1 class 1 for oil and particles with a low pressure dew point, but the bottle customer’s and blow moulder maker’s specifications decide.
At high pressure, water condenses more readily: air that is dry enough at 7 bar can drop water when compressed to 40 bar and cooled. The HP side therefore needs its own drying, either a high-pressure refrigerated or desiccant dryer after the HP compressor or booster, or LP air dried to a low enough dew point before boosting. Water in blow air causes valve corrosion, mould marks and bottle defects. For dryer selection see our desiccant dryer page.
Reliability and maintenance of HP systems
High-pressure piston compressors and boosters work hard. The high-pressure stages run hot, valves and piston rings wear, and coolers must be clean for each stage to work within its design temperature. Typical points to watch:
- Interstage pressures and temperatures, logged daily: a change usually points to a valve or ring problem in one stage.
- Valve condition and replacement on the maker’s schedule.
- Cooler cleaning, especially in summer when Indian plant rooms run hot.
- Condensate drains at each stage, which must work reliably at high pressure.
- Safety valves, pressure switches and high-temperature shutdowns tested and in date.
- Booster inlet pressure from the LP system: low inlet pressure lowers booster output.
Planned stops for valve and ring work are best scheduled outside peak season. For long service or overhaul jobs, a rental booster or HP compressor may be possible depending on pressure, flow and availability; talk to us early.
Energy: where PET plants spend most
In many PET bottle plants, compressed air is one of the largest electricity users, with the HP system taking the largest share. Practical savings usually come from:
- Using the lowest final blow pressure the bottle design allows, confirmed with the moulder maker.
- Enabling and maintaining air recovery.
- Fixing leaks on HP pipes and fittings, which waste air that is very expensive to produce.
- Running boosters from an efficiently controlled LP station, with VSD trim where demand varies.
- Recovering compressor heat for resin drying or process water.
A measured compressed air audit with power and flow logging on both LP and HP systems shows where the energy goes.
Indian context: PET plants in Telangana, Andhra Pradesh and beyond
PET preform and bottle plants supply packaged water, soft drinks, edible oil, pharmaceuticals and household products, and are found in industrial estates around Hyderabad and across Telangana and Andhra Pradesh. Common local conditions:
- Hot plant rooms. Summer temperatures stress multi-stage HP compressors, whose stage temperatures rise with ambient; good ventilation and clean coolers matter more than on LP machines.
- Humidity in monsoon and coastal areas increases condensate load on every stage.
- Seasonal demand. Bottle demand rises sharply in summer, so HP capacity is often tightest exactly when equipment is hottest.
- Power quality. Voltage dips and DG changeovers can trip boosters; restart logic and receiver capacity should protect the moulder.
Common problems and fixes
| Symptom | Likely cause | What to check |
|---|---|---|
| Poorly formed bottles, weak base or threads | HP pressure dips during final blow | Pressure at moulder inlet during blow, receiver size, filter pressure drop |
| Booster cannot reach pressure | Low LP inlet pressure, worn valves or rings | Inlet pressure, interstage readings |
| Water at the blow moulder | No HP drying, failed drain | HP dew point, drains on each stage |
| LP pressure swings when moulders run | Uncontrolled recovered air or booster draw | Return point control, LP capacity |
| High stage temperature trips | Fouled coolers, hot room, valve leak | Cooler condition, ventilation, valve inspection |
How Reddy Compressors helps
Reddy Compressors is an independent multi-brand compressed-air specialist at Tarnaka, Secunderabad, working with industry since 2002. For PET preform and bottle plants we:
- Supply new and used LP screw compressors, oil-free high-pressure compressors and boosters, HP and LP dryers, filters and receivers, from makers such as ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic.
- Rent compressors from our own fleet, including high-pressure units, for breakdowns, overhauls and seasonal peaks, subject to the duty and air-quality need.
- Service and overhaul LP and HP machines of all major brands, on call or under AMC, with over 20 service engineers and technicians covering Telangana and Andhra Pradesh.
- Supply spare parts, including valves, rings and filters, identified from the nameplate and the old part with fit confirmed before quoting.
- Audit LP and HP systems: pressure at the moulder, flow, power and leak surveys.
Equipment, rentals and spares are supplied across India. We are not an authorised distributor of any brand. We have tie-ups with all major and reputed brands.
Our track record. We have supplied compressors to leading companies in PET bottle manufacturing across Telangana, Andhra Pradesh and beyond – new and used machines, long-term rental, and breakdown cover when a plant’s own compressor fails – backed by our own service team.
What to send us
- Blow moulder make, model and number of cavities, and the bottle sizes you run.
- Blow pressure and air consumption from the moulder maker, if available.
- Existing LP and HP compressors, boosters and dryers: make, model, nameplate photos, running hours.
- Whether air recovery is fitted and in use.
- The problem: bottle defects, capacity, energy, breakdown or expansion.
Frequently asked questions
What pressure is needed for PET bottle blowing?
Final blow pressure for stretch-blow moulding is typically 25–40 bar, depending on the bottle design, size, wall thickness and the machine. Carbonated drink bottles generally need higher pressure than plain water bottles. Pre-blow uses a lower pressure. Always use the blow moulder maker’s figure for your bottle and confirm pressure at the machine inlet during the blow, not only at the compressor.
Should I use a booster or a high-pressure compressor for PET blowing?
A booster raises dry low-pressure air to blow pressure, uses your efficient LP screw capacity and suits plants expanding with existing LP air. A dedicated high-pressure compressor works independently of the LP system. The best choice depends on number of moulders, LP capacity, redundancy needs and maintenance capability. In both cases the HP air must be oil-free and dry.
What is air recovery on a blow moulding machine?
After the final blow, high-pressure air inside each bottle is normally vented. Air recovery captures part of it, either to feed the next bottle’s pre-blow or to return reduced-pressure air to the plant’s 7 bar network. This cuts the air the compressors must make. The return must be controlled so it does not over-pressurise the LP network, and LP compressors must still cover the full load when recovery stops.
Why are my PET bottles not forming properly?
Air problems that cause poor forming include pressure dropping below the machine’s minimum during final blow, restricted HP filters or hoses, an undersized HP receiver and a worn booster or HP compressor. Process causes include preform heating, mould temperature and preform quality. Log pressure at the moulder inlet during blowing first; this quickly shows whether the air system is involved.
Does PET blowing air need to be oil-free?
Yes. Blow air forms the inside surface of the bottle, which will hold water, drink or other products, so it is treated as direct-contact air. Plants usually use oil-free high-pressure compressors or boosters, with fine filtration and drying after compression. Your bottle customer or brand owner may also specify an ISO 8573-1 air-quality class that must be met and checked.
How can a PET plant reduce compressed air energy?
The biggest savings normally come from the high-pressure side: use the lowest blow pressure the bottle allows, keep air recovery working, repair HP leaks and avoid feeding LP users from HP air. On the LP side, lower station pressure, control compressors as a group and repair leaks. Heat recovery for resin drying or process water is worth checking. Measure LP and HP separately.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Receiver storage sizing
Estimate the receiver volume needed to ride through a short burst of demand without the pressure falling below the limit.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Burst demand above compressor output (m³/min free air): 5
- Burst duration (s): 20
- Allowed pressure fall (bar): 0.7
- Receiver volume you already have (litres; 0 to skip the hold-up check): 1520
Result
- Receiver volume needed: 2,412 litres
- 2.41 m³
- Practical size (with 25% margin): 3,016 litres
- Hold-up time of the receiver you have: 12.6 s before the pressure falls by 0.7 bar
Method. V = Q × t × P_atm / ΔP, where Q is the shortfall in free air (m³/min), t the time (min), ΔP the allowed pressure fall (bar) and P_atm = 1.013 bar. The same relation gives the hold-up time of an existing receiver: t = V × ΔP / (Q × P_atm). Isothermal; for a real system add a margin.
What to take from it. A receiver buys time. It does not create capacity: once it is spent the pressure keeps falling until the compressors catch up.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.