Short answer: A low-pressure air compressor for the textile / ceramic / glass / cement industry is specified the same way in all four: measure the pressure each process genuinely needs, set the discharge band to match it (15–300 HP at 3–5 bar on our low-pressure screw series), then size the receiver on the peak. Done properly that band typically returns around 33% less energy per cubic metre than an 8 bar machine on the same duty (based on 0.08 USD/kWh · 6,000 h/yr · 75% load).
By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.
Four industries that look nothing alike — air-jet looms, spray dryers, container-glass lines, cement silos — keep arriving at the same compressor room. Each buys an 8 bar package because that is what the site already had, then spends years paying compression ratio for headroom no process consumes. This is the specification method we use on site visits, written out so it can be applied without us in the room.
What does “low pressure” mean for these four industries?
It means a discharge band of roughly 3–5 bar instead of the 7–8 bar a general-purpose room is built around, matched to processes that do not need more. Our low-pressure screw series covers that band in one family, 15–300 HP at 3–5 bar — a different compression ratio, not a smaller version of the same machine.
What do textile, ceramic, glass and cement plants have in common on compressed air?
They all run the bulk of their air volume in the same low band, and all of them were probably sized by someone who inherited an 8 bar number.
| Industry | Main air consumers | Typical pressure band | What drives the sizing |
|---|---|---|---|
| Textile | air-jet insertion and relay nozzles, yarn texturing, blow-off | typically 3–5 bar | flat 16–24 h demand, so the delta repeats all year |
| Ceramic | spray-dryer atomisation, press support, conveying of body and glaze | typically 3–5 bar | continuous dryer load plus batch peaks |
| Glass | forming support, mould and ware cooling, blow-off, cold-end handling | typically 3–5 bar | pressure stability — drift shows in ware quality first |
| Cement | pneumatic conveying, silo air cannons, bag-filter pulse cleaning | typically 3–5 bar | duty cycle and pulse storage, not average flow |
The differences are real, but they sit in the duty cycle and in how tight the band must be held — not in the pressure itself. That is why one method covers all four, and why the four guides link back to each other.
How do you find the pressure each process actually needs?
Walk the plant with a logger and list every air consumer with its required pressure, flow and duty pattern. On the last five audits we ran across these four industries, consumers above 5 bar were under 25% of flow in every case (based on our own site audits of textile, ceramic, glass and cement plants). The 8 bar supply served the minority and set the cost for everyone else.
Two rules keep this honest. Take the pressure from the equipment manual, not from what the existing header reads — the header shows what you supply, not what the process needs. And log for a full production week: a duty that looks flat across one shift is often very different across seven.
What does the sizing calculation look like on a shared duty?
Take a plant logging 16 m³/min at 4.5 bar across conveying, cleaning and process air, running 6,000 hours a year at 0.08 USD/kWh and 75% average load:
- Air actually needed: 16 m³/min at 4.5 bar average, from a one-week site log, peaks recorded separately.
- 8 bar incumbent: a 100 kW (135 HP) general-purpose package, average input 100 × 0.75 = 75 kW.
- Its specific power: 75 kW ÷ 16 m³/min = 4.69 kW/m³.
- Low-pressure unit, same flow: a 63 kW (85 HP) low-pressure screw package delivering 17 m³/min at 4.5 bar, average input 63 × 0.75 = 47.25 kW.
- Its specific power: 47.25 kW ÷ 16 m³/min = 2.95 kW/m³.
- Difference: 4.69 → 2.95 kW/m³, about 37% less energy per cubic metre at identical delivered flow.
- Annual energy: 450,000 kWh against 283,500 kWh → 166,500 kWh saved, or 13,320 USD a year at 0.08 USD/kWh.
Our planning benchmark for this family is a ~35% saving against a fixed-speed machine sized for the same duty — a screening figure, not a guarantee. Run the same arithmetic on your own logged numbers before you accept anyone’s estimate, ours included.
What did the numbers look like in a real installation?
We publish measured results rather than only method. Our 75 kW PMV VSD installation case study documents an iron processing plant in Ho Chi Minh City: a 15,000 m² facility with more than 200 employees running 16 hours a day, where compressed air cost over 3,200 USD a month, with pressure swinging ±0.5 bar and maintenance at 850 USD a month on the old fixed-speed unit. That ±0.5 bar decides how the whole system behaves, and it is a storage symptom, not a compressor symptom.
A second case study replaced a 75 kW machine, held plant pressure to ±0.1 bar and paid back in 14 months.
How much storage does a low-pressure system need?
Size the receiver on the largest pulse, never on the average. Multiply the pulse flow by its duration and convert to volume at your working band: a 45 m³/min pulse lasting 5 seconds needs about 3.75 m³ of usable air, which at a 1 bar differential is roughly 3.75 m³ of receiver volume. Cement and glass are the two where this step pays most, because their pulses are the sharpest.
Why is an 8 bar machine the wrong default for all four?
Because compression ratio is charged on every cubic metre, and an unloaded machine is charged for nothing at all. A 75 kW (100 HP) airend delivering roughly 12 m³/min at 8 bar typically delivers 17–18 m³/min at 4.5 bar on the same motor (based on manufacturer flow curves across the 3–8 bar band). Meanwhile an unloaded screw compressor typically still draws 20–30% of full-load power while delivering no air (based on manufacturer part-load data for oil-injected packages) — and all four industries have plenty of unload time between batches and shifts.
When should you keep an 8 bar machine in the plant?
Whenever a genuine high-pressure minority exists. Packing machinery, palletising actuators, workshop tools, instrument air and most laser-cutting or blow-moulding branches need the higher band, and typically account for 20–30% of plant flow (based on our site audits). Keep those on the existing 8 bar unit and move the bulk volume across — usually the cheapest configuration on the table: one new package rather than a compressor-room rebuild.
How should you compare low-pressure compressor quotations?
Ask every supplier for the same four numbers, in writing, because quotes that look nothing alike on paper are often identical machines once normalised:
- Flow (FAD) at your discharge pressure, not 8 bar, tested per the ISO 1217 acceptance-test method.
- Specific power in kW/m³ at your duty point — the most comparable figure there is.
- Ambient rating and derate curve — our packages are specified for full output at 50°C ambient, which matters in all four of these industries and is the same at 4 bar as at 8 bar.
- Receiver and dryer sized on your pulse and your ISO 8573-1 class, not on a rule of thumb.
What should the acceptance test measure after installation?
Measure the same four things you specified, with the plant in normal production: discharge pressure band, delivered flow, specific power in kW/m³ and dew point at the point of use. Log them for a week, not an afternoon: a machine that holds ±0.1 bar at commissioning can drift to ±0.5 bar once drying and filtration loads settle.
Which industries should not go low pressure?
Any process that genuinely needs 8 bar or more, and anyone whose air is a small share of utility spend. Blow moulding at 30–40 bar, some laser cutting and most pneumatic tooling need the higher band, and if low-pressure consumers are under about 30% of your flow the savings rarely justify a second compressor class. Fix leakage and storage first — usually 15–20% of the bill for a fraction of the capital.
Key takeaways
- One method covers textile, ceramic, glass and cement: measure the real pressure per process, set the band to match, size storage on the peak.
- The band pays about 33–37% less energy per cubic metre against an 8 bar machine on the same duty (based on 0.08 USD/kWh · 6,000 h/yr · 75% load).
- Consumers above 5 bar were under 25% of flow in the last five audits across these four industries — keep those on the old machine.
- Compare quotes on kW/m³ at your duty point, plus ISO 1217 flow and ambient rating, never on headline kW.
- Add a low-pressure class when the low band is 30%+ of flow; below that, fix leakage and storage first.
Frequently asked questions
What is the low-pressure band for these four industries?
Typically 3–5 bar, and our low-pressure screw series covers it in one family at 15–300 HP — a different compression ratio, not a derated 8 bar package.
Does a low-pressure system need a different dryer or piping?
No. Air treatment is chosen from the ISO 8573-1 purity class the process needs, and pipe sizing from flow and allowable pressure drop. Both are independent of the discharge band, which is why the switch is a compressor-room change rather than a plant rebuild.
Can one low-pressure machine serve two of these industries on one site?
Yes, and it is common where a plant runs more than one product line. Size on the combined logged average at the shared band and give the sharpest pulse to storage; the four industries’ duty cycles complement each other more often than they collide.
How long is the payback across these industries?
Commonly 12–24 months where the plant runs 6,000 hours or more a year and the incumbent is a fixed-speed 8 bar package. Below 4,000 hours a year we usually recommend fixing leakage and storage first.
Sources and standards
- ISO 11011:2013 — Compressed air — Energy efficiency — Assessment. The method behind every baseline and load profile above.
- ISO 1217:2009 — Displacement compressors — Acceptance tests. Defines how delivered flow (FAD) is verified, making a 4.5 bar flow curve comparable with an 8 bar one.
- ISO 8573-1:2010 — Compressed air — Contaminants and purity classes. Relevant for instrument branches and wherever air touches the product.
- ISO 9001:2015 — quality management certification held by AirSpace Machinery Co., Ltd., with CE marking under Machinery Directive 2006/42/EC.
- Our published references — the low-pressure compressor series and the pressure-selection note on low vs medium pressure configurations.
About the author
Johnny Wayne (Wei Zhuang) is General Manager of AirSpace Machinery Co., Ltd., the Shanghai manufacturer behind ChinaCompressor.org. The company builds oil-injected and Class 0 oil-free screw compressors from 2 to 540 HP (1.5–400 kW), including the low-pressure series at 15–300 HP for 3–5 bar duty, on a 4,000 m² production facility with 20 years of engineering history. Products carry CE marking and ISO 9001:2015 certification, and we sell through distributors rather than around them.
The four industry guides referenced above are the textile mills, ceramic plants, cement plant and glass industry notes.
If you have a duty point, send the logged pressure profile, flows, running hours and your tariff to sales@chinacompressor.org, or start with the free compressed air kWh forecast and system review.
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