Size the compressor from the blow-moulder, not the motor
A PET compressor is sized from what the blow-moulder actually consumes at its final blow pressure. Motor kW, “bottles per hour” on a brochure or the size of the machine you are replacing are poor guides. The four numbers that matter are the number of cavities, the blown volume per cavity, the final blow pressure and the cycles per hour. If the moulder maker gives an air consumption figure for your worst format, cross-check it with the arithmetic below.
The blown volume is the bottle’s internal volume plus the dead volume in the blow nozzle, stretch-rod passage and valve block, all of which are filled to blow pressure and then exhausted every cycle. Each litre filled from atmosphere to p bar(g) uses p ÷ 1.01325 litres of free air.
Worked example: 4-cavity, 1-litre machine (illustrative)
Assumptions: 4 cavities; 1-litre bottle with 1.2 litres blown volume per cavity including dead volume; final blow 30 bar(g); pre-blow 8 bar(g); 1,500 cycles per cavity per hour, so 6,000 bottles per hour.
- Free air per cavity per cycle, all from the high-pressure side: 1.2 × 30 ÷ 1.01325 = 35.5 litres.
- Blows per hour: 4 × 1,500 = 6,000. Free air: 6,000 × 35.5 = 213,200 litres per hour = 3.55 m³/min (125 cfm).
- Add 10% for valve timing, control air on the high-pressure side and small leaks: 3.55 × 1.10 = 3.91 m³/min (138 cfm) at 1.01325 bar(a).
- Correct for Hyderabad’s lower atmospheric pressure (about 0.950 bar(a)): a compressor here has to draw 3.91 × 1.01325 ÷ 0.950 = 4.17 m³/min (147 cfm) of intake air to deliver the same mass.
- If pre-blow comes from the 8 bar(g) plant network instead, the high-pressure share per cavity falls to 1.2 × (30 − 8) ÷ 1.01325 = 26.1 litres. High-pressure demand becomes 2.61 m³/min, or 2.87 m³/min with the 10% allowance, which is 3.06 m³/min (108 cfm) of intake at Hyderabad. The low-pressure side then supplies about 1.04 m³/min (37 cfm) more.
Two lessons follow. Compare quoted FAD at the same final pressure and intake conditions; a 140 cfm compressor tested at sea level is not a 140 cfm compressor in Hyderabad. And blow pressure drives air consumption almost linearly. Going from 30 to 40 bar(g) on the same bottle needs 33% more free air per blow, and each cubic metre also costs about 8% more ideal compression energy, so the total rises by roughly 44%. Trial the lowest blow pressure that gives good base and shoulder definition before buying a bigger compressor. The method behind this is set out in our guide to PET high-pressure forming air.
Dedicated 40 bar piston, screw plus booster, or air recovery?
There are three ways to make high-pressure blow air, and most plants end up combining two of them. The choice depends on how many moulders you run, how much spare capacity your 7–8 bar plant compressor has, and how much standby you need.
| Option | How it works | Energy reasoning | Suits | Watch out for |
|---|---|---|---|---|
| Dedicated three-stage piston compressor to 30–40 bar(g) | Draws atmospheric air and compresses it in three intercooled stages | Does all the work in one machine; at 40 bar(g) from Hyderabad intake, the stage ratio is about 3.5 and each stage discharges at about 175 °C (ideal adiabatic estimate) with 40 °C intake | Standalone moulders, plants without spare low-pressure air, sites wanting independence from the plant network | Three sets of valves to maintain; must be sized for the full blow demand; vibration foundations |
| 7–8 bar(g) plant screw feeding a booster | A screw compressor and dryer deliver dry 7–10 bar(g) air; a two-stage piston booster raises it to blow pressure | Compressing to 8 bar(g) is about 64% of the ideal work of reaching 30 bar(g), and the screw does that part and does it efficiently, especially with a VSD; the booster’s stage ratio from 7 to 40 bar(g) is only about 2.3 per stage in two stages | Plants with several moulders, existing screw capacity, line expansions | Booster output falls if inlet pressure sags; the screw must cover booster demand plus all plant air; two machines in series |
| Air recovery on the moulder | Exhaust air from final blow is captured and reused for pre-blow or fed to the low-pressure header | In the example above, exhausting from 30 to 8 bar(g) releases up to 26 litres of free air per blow, more than the 9.5 litres pre-blow needs; the surplus can offset low-pressure compressor load | Any moulder that offers it; strongest payback on large formats and high blow pressures | Real recovery is less than the theoretical ceiling; set-points must be coordinated or recovered air will not flow; keep low-pressure capacity for when recovery stops |
Neither of the first two is always cheaper to run. Compare total kW of every machine involved per m³/min delivered at blow pressure, using makers’ ISO 1217 data. Our booster compressor guide and high-pressure piston compressor guide cover the machines themselves in more depth.
High-pressure air quality: dew point, filters and oil on preforms
Blow air forms the inside surface of a food or beverage container, so it must be dry, clean and free of oil. Plants usually work to the bottle customer’s or moulder maker’s ISO 8573-1 classes; class 1 for particles and oil is a common starting point. Measure it rather than assume it.
Why 7 bar dryness is not 40 bar dryness
Air dried to a +3 °C pressure dew point at 7 bar(g) and then boosted to 40 bar(g) has its water vapour squeezed into a smaller volume. Its pressure dew point at 40 bar(g) becomes about +29 °C. That air condenses in an air-conditioned blow room, in pipework at night and in the moulder’s valve block. To hold +3 °C at 40 bar(g), which corresponds to about −37 °C at atmospheric pressure, you need a high-pressure refrigerated or desiccant air dryer rated for 40 bar on the booster or high-pressure compressor discharge.
Water load on the high-pressure side (illustrative)
For the 4.17 m³/min machine above in a Hyderabad monsoon (30 °C, 90% relative humidity), about 6.8 litres of water per hour enter the intake. After the final aftercooler at 45 °C and 40 bar(g), only about 0.4 litres per hour can remain as vapour, so roughly 6.4 litres per hour must leave through intercooler and aftercooler drains. A stuck stage drain floods the next cylinder and the dryer. Test every drain daily.
Oil carry-over
Lubricated piston compressors pass some cylinder oil downstream, and worn rings, overfilled lubricators or hot stages increase it. Oil reaches preforms as haze, spots or odour. Fit high-pressure coalescing filters and an activated-carbon stage rated for 40 bar, change elements on differential pressure, and consider oil-free cylinders where the customer requires it. More on treatment trains is in compressed air filtration and desiccant dryers.
Receiver sizing at 40 bar
Final blow draws air in sharp pulses. The high-pressure receiver near the moulder supplies these pulses and lets the compressor load and unload without the blow pressure sagging. Size it with V = Q × t × 1.01325 ÷ ΔP, where Q is demand in m³/min of free air, t is the ride-through time in minutes and ΔP is the pressure band you can tolerate in bar.
Worked example (illustrative): Q = 3.91 m³/min; ride-through 30 seconds (0.5 minutes) for a short compressor unload; receiver allowed to fall from 36 to 32 bar(g), so ΔP = 4 bar. V = 3.91 × 0.5 × 1.01325 ÷ 4 = 0.495 m³, so about 500 litres, rated above the highest safety valve setting. The same storage duty on a 7 bar network with a 1 bar band would need 1.98 m³. High pressure stores air compactly, but only if the receiver runs well above the moulder’s minimum blow pressure.
| Receiver item | What to check at 30–40 bar |
|---|---|
| Design pressure | Above the compressor’s final safety valve set pressure, with documented hydraulic test |
| Position | Close to the moulder inlet, with short, full-bore pipe; a second receiver at the compressor if lines are long |
| Safety valve | Sized for full compressor output, set at or below design pressure, tested and tagged |
| Drain | Rated automatic drain plus manual valve; high-pressure blow-down is violent, so pipe it to a safe point |
| Inspection | Statutory pressure-vessel inspection and thickness checks at the intervals that apply to your site |
Indian site conditions: heat, power and dust
- Heat. Compressor rooms in Telangana and Andhra Pradesh reach 45–50 °C in May. Intake temperature raises every stage’s discharge temperature and lowers mass flow. At 50 °C intake, our three-stage example reaches about 189 °C per stage instead of 175 °C. Duct cool outside air to the intake, exhaust cooler air out of the room and keep cooler fins clean.
- Power quality. Voltage dips and DG changeovers trip compressors mid-blow. A piston compressor must restart unloaded, with stage heads and intercoolers vented; a sticking unloader causes a loaded start that trips the motor or damages the crank. Check low-voltage protection and the restart sequence after a changeover.
- Starting. Star-delta starting cuts starting current to about one third of direct-on-line and is standard on larger units. A soft starter gives smoother torque and suits sites on DG sets. Size the DG for the compressor’s starting kVA, not just its running kW.
- Dust and humidity. Check intake filters weekly in the dusty summer and watch drains closely from June to September.
Buying a PET compressor: what drives the price
Price is driven by:
- FAD at your final pressure under ISO 1217 test conditions, not swept volume.
- Number of stages, rotational speed and construction: a slower, heavier machine costs more and usually lasts longer between overhauls.
- Lubricated or oil-free cylinders, and the filtration needed to reach your air-quality class.
- Air-cooled or water-cooled; water-cooled units need a cooling tower and water treatment.
- Motor, starter and controls: star-delta or soft start, pressure and temperature protection per stage.
- What is included: high-pressure dryer, filters, receivers, piping, drains, installation and commissioning.
- For used machines: hours since overhaul, condition of valves, cylinders and crank, and whether it is sold as used or fully refurbished.
Ask every supplier for FAD at your pressure, kW at full load, the stage pressures and temperatures and the scope of supply, then compare like for like with our supplier bid normalisation worksheet.
Checking a used PET compressor before you buy
A used high-pressure piston compressor is only as good as its valves, rings and running gear. Run it under load and check these points.
| Check | What good looks like | What a bad reading tells you |
|---|---|---|
| Stage pressures at full load | Close to the nameplate or manual values for each interstage | Low first interstage pressure points to first-stage valves or rings; high first interstage pressure points to second-stage valves leaking |
| Stage discharge temperatures | Within the maker’s limits and similar stage to stage | One hot stage usually means a leaking valve in that stage or a fouled cooler upstream |
| Pump-up time | Fills a receiver of known volume from a set pressure within the time its FAD predicts | Slow fill means lost capacity; see our pump-up test |
| Valves | Clean seats, unbroken plates and springs, no carbon build-up | Carbon and broken plates mean overheating or excess oil, and a valve overhaul is due |
| Running hours and records | Hours since last overhaul known, oil and valve changes logged | No records means you should price a full overhaul into the purchase |
| Crank and running gear | No knock, steady oil pressure, low crankcase blow-by | Knock or blow-by indicates bearing, ring or liner wear |
| Safety valves and receiver | Each stage valve lifts at its set point; receiver test record available | Untested or wrong valves must be replaced before use |
Our used and refurbished compressors carry a six-month warranty on the whole compressor from the delivery or invoice date, excluding consumables, misuse or wrong oil, service by others and accident or site damage. See used compressors for sale.
PET compressors on rent at 30–40 bar
We hire out PET high-pressure units at 30–40 bar from our own fleet. Typical reasons to rent:
- Breakdown cover while your own compressor or booster is repaired or overhauled.
- Summer peak. Bottled water and soft-drink demand rises sharply from March to June; a hired unit lets you run an extra moulder or a second shift without buying for a three-month peak.
- New line or trial before you decide on the permanent compressor.
- Planned overhaul of the plant’s main high-pressure compressor.
Tell us the blow pressure, the free air demand, your available power supply and where the unit will sit. See high-pressure air hire.
Service, overhaul and spares
On continuous PET duty, valves are the first wear item, followed by piston rings and coolers. Inspect earlier than the maker’s schedule if stage pressures or temperatures drift.
| Part | Why it matters | Service action |
|---|---|---|
| Suction and delivery valves (each stage) | Leaking valves cut output and overheat the stage | Inspect, lap or replace plates, springs and seats; replace as sets per stage |
| Piston rings and rider rings | Worn rings lose capacity and raise oil carry-over | Measure ring gap and wear; replace with liner check |
| Pistons and liners | Scoring causes blow-by and oil in air | Measure bore and ovality; hone or replace |
| Intercoolers and aftercooler | Fouled coolers raise next-stage temperature and water carry-over | Clean fins or tubes, pressure-test for leaks |
| Safety valves (each stage) | Protect each stage and the receiver | Test set pressure, replace if they leak or pass |
| Unloaders and drains | Unloaded starts and water removal | Clean, reseal, test operation |
| Bearings, crank and lubrication | Running-gear life | Oil analysis, bearing clearances, oil pressure check |
We supply spares for all major makes, identified from the nameplate and part numbers. For parts identification see compressor spare parts in Hyderabad; for maintenance contracts see compressor AMC.
How Reddy Compressors helps
- Independent multi-brand suppliers since 2002, based in Tarnaka, Secunderabad, with tie-ups with all major makes.
- New, used and refurbished PET high-pressure piston compressors and boosters, with matching dryers, filters and receivers.
- PET high-pressure units at 30–40 bar for hire from our own fleet.
- Service, overhaul, AMC and spares from a team of more than 20 service engineers and technicians.
- Coverage across Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad; supply across India.
What to send us
- Moulder make and model, number of cavities and every bottle format you run.
- Final blow pressure, pre-blow pressure and cycles per hour, or the maker’s air consumption figure.
- Nameplate photos of existing compressors, boosters, dryers and receivers.
- Whether air recovery is fitted and used.
- Power supply details: transformer or DG, available kW and starter type.
- The problem or plan: new line, replacement, breakdown, summer peak or overhaul.
Frequently asked questions
Where can I buy a PET compressor in Hyderabad?
Reddy Compressors in Tarnaka, Secunderabad, supplies new, used and refurbished PET high-pressure compressors and boosters of all major makes across Hyderabad, Rangareddy, the rest of Telangana and Andhra Pradesh. We size the compressor from your moulder’s cavities, bottle volume, blow pressure and cycles per hour, then match the dryer, filters and receiver. Call or WhatsApp 99590 60718 with your moulder details.
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Short explainers on this topic
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What compressor do I need for a 30 bar or 40 bar PET blowing machine?
You need a compressor whose FAD at your final blow pressure covers the moulder’s demand plus about 10%, corrected for Hyderabad’s lower atmospheric pressure. For 30–40 bar(g) this is usually a three-stage piston compressor, or a two-stage booster fed with dry 7–10 bar(g) air from a screw compressor. A 4-cavity 1-litre machine at 6,000 bottles per hour and 30 bar(g) needs roughly 147 cfm of intake air if all blow air comes from the high-pressure side.
Is a 2-stage or 3-stage high pressure piston compressor better for PET?
For compressing from atmosphere to 40 bar(g), three stages are better for continuous duty. With two stages the ratio per stage is about 6.6 and stage discharge reaches about 260 °C even in theory at 40 °C intake, which cokes oil and destroys valves. With three stages the ratio falls to about 3.5 and discharge stays near 175 °C. Two stages are normal for a booster, because its inlet is already at 7–10 bar(g).
What is a PET booster compressor?
A PET booster is a piston compressor that takes already compressed, dried air at about 7–10 bar(g) from a plant screw compressor and raises it to the 25–40 bar(g) final blow pressure. Its output depends on its inlet pressure, so the screw compressor must supply the booster’s demand plus all other plant air without the pressure sagging. Boosters suit plants with several moulders or spare screw capacity.
What is the price of a PET compressor?
Price depends on FAD at your blow pressure, the number of stages, speed and build, lubricated or oil-free cylinders, air or water cooling, the starter, and whether the dryer, filters, receivers and installation are included. Used machines also depend on hours since overhaul and on whether they are sold as used or refurbished. Send your moulder details and we will quote a matched system rather than a bare compressor.
Should I buy a used PET compressor?
A used PET compressor is good value if you check it under load. Compare stage pressures and temperatures with the maker’s values, run a pump-up test, inspect valves, ask for running hours and service records, and test every safety valve. Our used and refurbished machines carry a six-month warranty on the whole compressor, with the usual exclusions for consumables, misuse, service by others and site damage.
Can I rent a 40 bar compressor for PET blowing?
Yes. We hire out PET high-pressure units at 30–40 bar for breakdown cover, planned overhauls, new-line trials and the summer peak in bottled water and soft drinks. Tell us the blow pressure, free air demand, power supply and installation point so we can match the unit and its connections to your receiver.
Why is there water in my PET blow air?
Air dried at 7 bar(g) and then boosted to 40 bar(g) has a pressure dew point of about +29 °C at 40 bar, so it condenses in cool pipework and air-conditioned blow rooms. Fit a high-pressure dryer after the booster, check every intercooler and aftercooler drain daily, and confirm that coolers are clean. In the monsoon a 147 cfm system can drop more than 6 litres of water per hour.
Related pages
- PET blowing compressors: pressures, layouts and air quality
- PET high-pressure forming air interfaces
- PET bottle production utility integration
- Booster compressors for high-pressure air
- High-pressure reciprocating compressors
- Multi-stage reciprocating compressors
- High-pressure air hire
- Used compressors for sale
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.