Why blowing needs a different class of compressor
A PET bottle is formed by inflating a heated preform against a mould. The pressure that does the work sits far above the 7 to 10 bar used for general workshop air, which puts the duty in the 20 to 30 bar class. You cannot reach that range efficiently by asking a standard single-stage screw compressor to run harder, because the compression ratio per stage becomes too high.
What two stages change
Splitting compression across two rotors keeps the pressure ratio per stage within a sensible band. The air is compressed, cooled between stages, then compressed again. The intercooling is the point: it removes the heat of the first compression before the second stage, which reduces the work needed and keeps discharge temperatures in a range the oil and seals can live with.
| Consideration | Practical implication |
|---|---|
| Duty pressure | Matched to the blow moulding machine, not to the plant's general air header |
| Air quality | Oil and moisture have to be removed before the air reaches the preform - a high pressure dryer and filter chain are part of the package, not an accessory |
| Storage | A receiver sized for peak blow demand, rated above the working pressure |
| Continuity | Blowing is usually continuous, so uptime and service access matter more than a small efficiency gain |
Separating blowing air from plant air
Most plants run two headers: a low pressure line for general actuators and a high pressure line for blowing. Keeping them separate avoids pressurising the whole factory to blow pressure, which would cost energy everywhere, and it lets the high pressure machine run at its design point rather than part loaded.
What to specify
State the blowing machine's required pressure and consumption, the number of cavities and cycle rate, whether a high pressure dryer is included, and the site voltage and frequency. Those answers decide the model far more than the bottle size does.
Send us your working pressure, required flow and site conditions and we will propose a configuration. Contact OPPAIR.