Short answer: In a 40°C+ factory, cooling decides how much air you actually get. A 75 kW package at 75% load puts roughly 51 kW of heat into the room, which needs about 15,800 m³/h of ventilation at a 10 K rise; without it the room passes 45°C and a standard air-cooled package loses output (based on a 0.9 heat-to-room ratio · 1.17 kJ/m³·K air at 30°C). Our packages are rated for full output at 50°C ambient, which moves that threshold.
By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.
Our tropical customers — Vietnam, the Philippines, Indonesia — run plants where the ambient is 35°C and the compressor room is worse. Cooling is the first thing that fails there, and it fails quietly: no alarm, no shutdown, just a machine delivering less air than the nameplate promised while the meters keep turning. We see it on every tropical site visit.
What is the difference between an air-cooled and a water-cooled screw compressor?
An air-cooled package carries its own fan and radiator and rejects all of its heat into the room it stands in. A water-cooled package uses shell-and-tube heat exchangers and dumps that heat into a cooling-water circuit — tower, chiller or process water. The airend and controls are the same; what changes is where the heat goes, and what your building must do with it.
Why does a 40°C factory change the answer?
Because air-cooled output depends on the temperature of the air the radiator can pull, not on the nameplate. Packages are commonly quoted with full output to 40°C ambient and a derate above it; in a room that reaches 50°C the shortfall is material (based on typical manufacturer derate tables for oil-injected packages). A water-cooled machine is far less sensitive to room temperature, because it is not using room air as its heat sink.
So in the tropics the question is rarely “air or water” — it is “have you designed the room so an air-cooled machine can actually see 40°C”. Our packages are specified for full output at 50°C ambient, and that rating holds at 4 bar exactly as at 8 bar.
How much heat does a screw compressor put into the compressor room?
Nearly all of the input power ends up in the room, minus the share that leaves with the compressed air. Take a 75 kW package at 75% average load:
- Average input power: 75 × 0.75 = 56.25 kW.
- Heat rejected to the room: 56.25 × 0.9 = about 51 kW (based on a 90% heat-to-room ratio for oil-injected, air-cooled packages).
- Ventilation needed at a 10 K rise: 51 kW ÷ (1.17 kJ/m³·K × 10 K) = about 4.4 m³/s, or 15,800 m³/h.
- The same figure at an 8 K rise: about 19,700 m³/h — the number to use when inlet air is already 35°C.
- Heat a water-cooled unit sends to the circuit instead: the same 51 kW, which at a 5 K water rise is about 2.4 L/s, or 8.8 m³/h of cooling water.
What did we measure in a tropical installation?
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 in electricity, with pressure swinging ±0.5 bar and maintenance at 850 USD a month on the old fixed-speed unit. The ±0.5 bar started the conversation, but thermal behaviour aged the old unit: in a tropical room a package that cannot reject its heat runs hot, and hot oil shortens the service interval long before the airend wears out.
In practice the same pattern recurs wherever the room cannot reject the heat — see our notes on overheating screw compressors and tropical versus cold-climate selection.
How do tropical textile, ceramic, glass and cement plants handle compressor heat?
The four industries that dominate our tropical enquiries — textile, ceramic, glass and cement — share one spec issue: demand is flat for 16–24 hours a day, so a thermal problem is not an occasional event but the steady state.
| Industry | Typical pressure band | Heat load pattern | Practical cooling note |
|---|---|---|---|
| Textile | typically 3–5 bar | flat 16–24 h, rarely off | room temperature is the whole derate story |
| Ceramic | typically 3–5 bar | continuous dryer load plus batch peaks | dryer heat and compressor heat are usually in the same building |
| Glass | typically 3–5 bar | steady within a shift | pressure stability is lost first when oil runs hot |
| Cement | typically 3–5 bar | pulse-heavy, long hours | heavy dust shortens radiator cleaning |
A low-pressure air compressor for the textile / ceramic / glass / cement industry runs at 15–300 HP and 3–5 bar in our series, and the lower compression ratio means less heat per cubic metre delivered — an underrated reason the low-pressure route helps in a hot plant.
When is water cooling worth the extra complexity?
When cooling water is genuinely available and cold, and the room cannot be ventilated. Water cooling wins where a plant already runs a cooling tower or process-water loop — many textile and ceramic sites do — because the machine stops caring about room air. It loses where the “cooling water” is a small, warm, shared circuit: a 40°C supply gives a poor approach temperature and the derate problem merely moves from air to water.
The honest rule: choose water cooling for the water circuit, not the brochure number. If nobody can tell you the supply temperature and flow at the compressor room, an air-cooled package in a properly designed room is the lower-risk answer.
How much ventilation does an air-cooled compressor room need?
Design for the heat, not a rule of thumb, and size for the worst day, not the average. Using the example above, a 75 kW package at 75% load needs roughly 15,800 m³/h at a 10 K rise — and that covers the compressor only. Add the dryer, other packages and solar gain on a metal roof, then check that intake and exhaust are not short-circuiting, the commonest defect we find on site.
Three checks: intake low and exhaust high, a few metres apart, and discharge air able to leave the building rather than pressurising the roof space. If the room still will not work, a ducted outside intake is cheaper than switching to water.
What temperature should the compressor room run at?
Design for a room temperature no higher than 40°C on the worst day, then choose a package whose ambient rating covers the number you measured rather than the number you hope for. Every one of our packages is rated for full output at 50°C ambient — about 10 K of margin above that design point, margin that matters when a dryer or a second machine is added later.
There is a commercial reason to design to 40°C rather than 45°C: a design that almost works means a derated machine that still costs full price.
How do you compare air-cooled and water-cooled quotations?
Normalise both quotes to four numbers before deciding:
- Ambient rating and derate curve — full output at what temperature, and output at your worst-case room temperature.
- Heat rejection in kW, with the ratio used, so you can size ventilation or a water circuit.
- Specific power in kW/m³ at your duty point, per the ISO 1217 acceptance-test method.
- Water quality and treatment requirements, if water-cooled — flow, supply temperature and any treatment the heat exchangers need.
A cheaper air-cooled package that derates at 42°C is not cheaper than one rated to 50°C: over 6,000 hours the difference typically outweighs the price gap (based on 0.08 USD/kWh and a 10% shortfall).
What changes in maintenance with each cooling method?
Air-cooled packages live or die on radiator cleaning, and in a cement or ceramic plant that interval is set by dust, not hours. Water-cooled packages trade that for descaling and water treatment — scale is the failure mode, and a neglected circuit can be worse than a dusty radiator. In both cases, oil and separator life is a temperature story.
Key takeaways
- In a 40°C+ factory, cooling decides delivered air, not the nameplate — a derated package still costs full price and full maintenance.
- A 75 kW package at 75% load puts about 51 kW into the room, which needs roughly 15,800 m³/h of ventilation at a 10 K rise (based on a 0.9 heat-to-room ratio).
- Water cooling is the right answer when a real cooling-water circuit exists — the same 51 kW leaves in about 8.8 m³/h of water at a 5 K rise.
- Design the room to 40°C and buy to 50°C. Full rating at 50°C ambient gives 10 K of margin for the dryer and the next machine.
- The low-pressure band helps here too: 3–5 bar means less compression work and less heat per cubic metre delivered.
Frequently asked questions
Can an air-cooled compressor work in a 40°C tropical factory?
Yes, if the room is designed for the heat load rather than assumed to be ventilated. Size ventilation from the rejected kW at a 10 K rise, keep intake and exhaust apart, and buy a package rated for full output at your worst-case ambient — 50°C on our packages.
How do I know if my compressor is already derating?
Compare logged delivered flow against the nameplate at the same discharge pressure, and check oil and discharge temperatures at the hottest part of the day. A 10% shortfall that recovers overnight points at the room, not the machine — typically the intake is pulling its own hot exhaust.
Is water cooling cheaper to run than air cooling?
Not inherently. Water cooling moves heat more efficiently but adds pump energy, treatment and descaling; air cooling spends fan power and needs ventilation.
Does the low-pressure band produce less heat?
Yes, less heat per cubic metre delivered, because the compression ratio is lower. The same rating — 15–300 HP in our low-pressure series at 3–5 bar — covers these four industries, and the lower band is one reason the route works in hot plants.
Sources and standards
- ISO 11011:2013 — Compressed air — Energy efficiency — Assessment. The method behind the load profiles and specific-power figures above.
- ISO 1217:2009 — Displacement compressors — Acceptance tests. Defines how delivered flow (FAD) is verified, which makes a derate claim checkable.
- ISO 8573-1:2010 — Compressed air — Contaminants and purity classes. Relevant to dryer selection in a hot, humid climate.
- 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 energy-efficient compressor manufacturing page.
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.
If you have a tropical duty point, send the room size, measured room temperature, machine rating and tariff to sales@chinacompressor.org, or start with the free compressed air kWh forecast and system review.
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