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40 bar air for PET: reciprocating compressor or screw-plus-booster?
The short answer: up to about 700 CFM of blowing air, an oil-free reciprocating compressor is usually the simpler and lower-cost choice. Above that, or where power cost matters more than purchase price, a screw-plus-booster package is the better fit. In the middle of the range both can do the job, and the right one depends on your plant.
A PET blow moulder needs oil-free air at up to 40 bar, because the blowing air touches the inside of the bottle. The two systems get there in different ways, and that difference decides running cost, upkeep and how you prove the air is clean. If you do not yet know how much air your line needs, start with our guide on how to size air for a PET blowing line.
How the two systems make 40 bar
Oil-free reciprocating compressor. One machine takes in air at atmospheric pressure and compresses it to 40 bar (42 kg/cm²) in stages, with intercoolers between the stages and an aftercooler at the end. The pistons seal with dry-running PTFE rings, and a distance piece with oil scraper rings keeps crankcase oil away from the cylinders. No oil enters the air at any point, so it is oil-free at the source. Colt supplies these from about 120 to 730 CFM, with a PLC panel and VFD.
Screw-plus-booster package. The job is split in two on one skid. A lubricated screw compressor first raises the air to an intermediate pressure. That air then passes through a series of filters and an activated carbon tower, which take out the oil carried over from the screw. The cleaned air enters a two-stage oil-free piston booster, which takes it the rest of the way to 40 bar. Colt supplies these from about 212 to 1,059 CFM, and up to 2,000 CFM on request. They are water cooled, with an optional VFD and permanent magnet motor.
Side by side
| Oil-free reciprocating | Screw-plus-booster | |
|---|---|---|
| How oil is kept out | No oil in the compression chamber at all | Lubricated screw stage; filters and an activated carbon tower remove the oil before the oil-free booster |
| Flow at 40 bar | About 120 to 730 CFM | About 212 to 1,059 CFM, up to 2,000 on request |
| Power for the same air | Higher at large flows | Generally a little lower where the two ranges overlap |
| Cooling | Intercoolers and aftercooler in the package | Water cooled; needs a steady cooling water supply |
| Regular upkeep | Valves, PTFE rings, packings | Screw oil and separator, filter elements, carbon tower, booster valves and rings |
| Best fit | One or two blow moulders, up to about 700 CFM | Larger lines and long running hours |
For model-wise flows, power, cooling water and sizes, please refer to the Colt catalogue.
What each costs to run
Over the life of a PET compressor, electricity costs far more than the machine. Where both types can supply your flow, compare the power each needs for the same air at your working pressure. Across the range where the two overlap, our catalogue shows the screw-plus-booster package generally needing a little less power for each CFM. Its two booster stages with an intercooler run cooler and use about 15% less power than single-stage boosting at the same flow and pressure. When you compare two quotes, ask for the input power of the complete package at your pressure, not just the motor rating.
Three things can tip the balance the other way:
- Cooling water. The booster package needs a steady cooling water supply, which usually means a cooling tower and pump. If your plant has spare capacity, this costs little. If not, it adds to the project.
- Filter and carbon tower costs. On the screw-plus-booster package, the filter elements and the carbon in the tower are consumables. Count them in the running cost, along with the small pressure drop they add.
- Part load. If bottle sizes or line speeds change through the day, a VFD saves energy when demand drops. Both types can be supplied with one.
Upkeep: what changes
Oil-free reciprocating: the valves, PTFE piston and rider rings, and packings are the wear parts, serviced on a fixed schedule. It is one machine with one service plan.
Screw-plus-booster: there are two machines on one skid. The screw stage needs its oil, oil filter and separator changed like any lubricated screw compressor. The filters and the activated carbon tower must be changed on time, because they are what keep oil out of the booster and the bottle. The booster has its own valves, rings and packings. Done on schedule this is routine work, but it is more items to track than on a reciprocating machine.
Five questions that decide it
- How much 40 bar air does your line need, with margin? Below about 200 CFM, choose reciprocating. Above about 730 CFM, choose a screw-plus-booster package or two reciprocating machines. In between, read on.
- Do your customers or auditors test your compressed air? Oil-free at the source is the simplest to qualify. A screw-plus-booster package can deliver oil-free air too, but you will need records showing its filters and carbon tower are changed on time.
- Do you have cooling water to spare? If yes, the booster package's lower power is easy to use. If not, include a cooling tower in the comparison.
- How many hours a year will it run? On two or three shifts, a small saving in power per CFM adds up to a large saving in rupees. On a single shift, a lower purchase price often matters more.
- One machine or two, and will the line grow? Two units let you service one while the other runs. If a larger bottle or a second blow moulder is likely, size for it now.
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
The booster stage is oil-free. The screw stage is lubricated, so its air passes through a series of filters and an activated carbon tower before it enters the booster. Kept on schedule, that filtration delivers oil-free air at the outlet; the filter and carbon changes must not be skipped.
An oil-free reciprocating compressor is the simplest to qualify, because no oil is present in its compression chamber at all. A screw-plus-booster package can also deliver oil-free air, but you will need to show records that its filters and carbon tower are changed on time.
Yes, if it is sized for their combined demand. Many plants still choose two units so one can be serviced without stopping production.
Model-wise flows, power, cooling water and sizes are in the Colt catalogue, which you can download from our website. Our engineers can also compare both options for your line.
Send us your blow moulder model, number of cavities, bottle sizes, bottles per hour and running hours through our contact page, and our engineers will compare both options for your line and size the receiver, dryer and filters. You can also see both ranges on our PET bottling air compressor page and download the Colt catalogue.