- Author
- Byron Raal, CAS Founder-Editor About the author
- Checked against
- ISO 8573-1ISO 1217
- Date last checked
- 18 August 2026
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Australian summer takes capacity off your compressed air system at exactly the moment you cannot spare it. A rotary screw swallows a fixed volume of air per revolution, so hotter air means less mass in, less air out, same power. That air brings more water with it, and the aftercooler turns the surplus into liquid your drains have to catch. Meanwhile the cooling side of the package loses headroom, so the margin to the high-temperature trip setpoint shrinks. Those effects can happen on healthy equipment. Summer can also expose a real fault, a fouled cooler, a failed drain, a blocked ventilation path, so do not read them as proof the machine is fine. It is design-condition physics meeting a 45 degree plant room, and the fixes cost far less in November than they do on the day the line stops.
Hot plant room, healthy machine
Same power in, less air out, more water, less margin
The three effects a 45 degree plant room has on a rotary screw, before any fault is involved.
1. Hot air is thinner
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Swept volume per revolution is fixed, so the same cubic metres go in with less mass in each. Shaft power barely moves, so specific power about 10 per cent worse.9%
less mass, 20 to 45 degree intake
2. More water comes in
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Compression removes none of it. The air saturates in the aftercooler, as it cools back towards room temperature, and everything past saturation becomes liquid.65%
more water per hour, 20 to 45 °C
3. Cooling headroom goes
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Air reaching your dryer tracks the room almost one for one. Heat takes away the margin that hides everything else, like the fouled cooler a machine shrugs off in July.6 to 8 °C
approach to room air, air-cooled
ISO 1217 reference inlet condition: 20 degrees Celsius, dry air. Maximum inlet air temperature in your installation manual: commonly 35 to 40 degrees. The second number is the one that governs your plant room.
Source: CAS summer heat guide; ISO 1217 reference values as published in the literature.
The data sheet is referenced to 20 degrees, not to your plant room
If someone tells you a compressor is “rated at 25 degrees”, two numbers have been blurred together, and the confusion costs sites real capacity.
ISO 1217 is the international acceptance-test standard for displacement compressors. Annex C is the simplified acceptance test for electrically driven packaged displacement compressors, which is what most packaged rotary screw flow figures are quoted against. Annex F is titled Reference conditions, and the standard’s introduction says it specifies standard inlet conditions for reference purposes. Both of those we read in the ISO-published document.
The values are another matter, and we would rather say so than let you assume. The literature publishes them as 1.0 bar absolute, 20 degrees Celsius and zero relative water vapour pressure. We did not read them off the annex table, and the technical article we took them from files them under Annex E rather than Annex F, which is what annex lettering does across editions. Take them as well-attested industry figures, not as a clause we checked.
Note the zero. The reference condition is deliberately dry air, because humidity varies too much between sites to be useful in a comparison rating. So the widely repeated “25 degrees at 60 per cent relative humidity” is not the ISO 1217 reference condition.
The number that governs your plant room is different again: the maximum inlet air temperature in your installation manual, commonly 35 to 40 degrees. Two numbers, two jobs.
The three things summer heat actually changes
Hot air is thinner, so you get less out for the same power in
A rotary screw is positive-displacement: the swept volume per revolution is fixed, so it inhales the same cubic metres per minute whether the room is 20 degrees or 45. Density varies inversely with absolute temperature, so 20 to 45 degrees strips about 8 per cent of the mass out of each cubic metre, before water vapour displaces dry air on top. Shaft power barely moves, because the compression work depends on inlet pressure and swept volume and neither has changed. The machine does not work harder. It does the same work and delivers less: roughly 9 per cent less mass, specific power about 10 per cent worse. On the floor that reads as pressure sagging at the far end of the ring main all afternoon.
More water comes in, and the aftercooler is where it lands
Water enters as vapour in the intake air and compression removes none of it. Compression is close to adiabatic, so the air leaves the airend hot and at low relative humidity: it does not saturate during compression. It saturates in the aftercooler, as that air cools back towards room temperature. Compress two to one, cool back to the intake temperature, and anything above about 50 per cent inlet relative humidity is already past saturation. At 7 bar the margin is far wider. Everything past saturation becomes liquid, and hotter, wetter intake air means more of it arriving per hour: about 65 per cent more in the worked example below. You see it at the point of use, often with no dryer alarm, because the dryer is still doing its rated job and that job is no longer big enough.
Cooling headroom goes on the days you can least afford it
An air-cooled package rejects heat into the room and cools the compressed air to within an approach temperature of that room air. Published figures sit at 10 or 15 degrees Fahrenheit, roughly 6 to 8 Celsius, or higher, quoted at the cooler’s design conditions, and the same source notes that intake air above 30 per cent relative humidity pushes the approach up further. So the air reaching your dryer tracks the room almost one for one, and the approach degrades as fins and filters load up. Above that sits the high-temperature trip, which is correct behaviour and is not the problem. The problem is that heat takes away the margin that hides everything else: the fouled cooler a machine shrugs off in July is the one that stops the line in January. Pull your controller’s event log and see when last summer’s warnings landed.
Size the dryer for a January inlet, not for the brochure number
This is the most fixable of the three, because the arithmetic is published.
Find the rating point. A refrigerated dryer’s nameplate flow is quoted at stated conditions, and there is more than one convention. The North American point is commonly described as 100/100/100: 100 psig, 100 degrees Fahrenheit inlet air and 100 degrees Fahrenheit ambient, about 7 bar gauge and 38 Celsius. Reference conditions vary by model, manufacturer and market, and you cannot infer them from the units. The same machine can be listed in cubic metres per minute, litres per second and cfm against one single set of conditions. We could not get the metric convention out of a standard we could read, which is exactly why the instruction is this: read your own data sheet and find out what conditions your number was quoted at.
Use the correction factors the right way round. Manufacturers publish capacity correction factors for inlet pressure, inlet air temperature and ambient temperature. Multiply the three together, then divide your required flow by the result. Getting that division backwards is an easy mistake and an expensive one. One published Australia and New Zealand sizing example runs 8 bar gauge, 35 degrees ambient and 40 degrees at the dryer inlet: 1.04 x 0.97 x 0.83 gives 0.837, so the dryer delivers about 84 per cent of nameplate and a 5.5 cubic metre per minute duty needs a 6.57 rating. Tables are model-specific, so use your supplier’s.
Do not stack the water load on the correction factor. They are the same physics from opposite ends: the factor already accounts for hotter air carrying more moisture. A 0.84 capacity factor already puts the nameplate flow you need at 1 divided by 0.84, about 1.2 times duty. Apply a 1.7 times water multiplier on top of that and you get 1.7 divided by 0.84, about 2.0 times duty, which is roughly double the original duty but about 70 per cent above the corrected 1.2 times duty requirement. Use the manufacturer’s correction factor, not both.
Desiccant is not an escape hatch. Adsorption capacity also falls as inlet temperature rises, and purge is a live cost and it depends on how the dryer regenerates. Sullair’s current published averages are 15 per cent of process air for heatless, 8 per cent for externally heated, and 2 per cent for heated blower purge. The heated blower design uses blower air and a heater for regeneration, but Sullair still publishes 2 per cent average purge air, so do not treat it as zero compressed-air consumption. Separate those three before you compare running costs, and use the figure on your own model’s data sheet. And ISO 8573-1 is a classification framework, not a mandate: the water class you need comes from your own risk assessment, not a blanket food or pharmaceutical rule. See the refrigerated versus desiccant comparison, the ISO 8573-1 explainer, the dew point calculator and the dryers guide.
Thermal trips, and what a stop costs on a peak day
We will not tell you what your downtime costs. Anyone quoting a per-hour figure without asking what you make is selling something. But the method takes ten minutes, and the number written down before summer turns a maintenance request into an approved one.
Cost of an unplanned stop = hours down x [(units per hour x contribution margin per unit) + (people idled x loaded hourly rate)] + restart, scrap and rework + any penalty exposure. Use contribution margin, not sale price, or finance will correctly pull the number apart.
One illustration, and each input is an assumption you should replace with your own: a line running 3,000 units an hour at 35 cents contribution margin is $1,050 an hour of lost margin, nine people idled at a loaded $58 an hour adds $522, and allow $400 for restart and scrap. A two hour stop costs about $3,500. That assumes the output is gone rather than made up later, so if your line catches up next shift, cross the margin out and cost the labour and the scrap. Set the answer against what it protects you from, which is mostly cheap: a cooler clean, a filter change, a louvre nobody closed after winter, a drain failing since March.
Ventilation and intake placement, the seasonal part only
Room design is a separate discipline and the compressor room design guide covers it properly: ventilation sizing formula, Bureau of Meteorology climate-zone design temperatures, inlet air quality, service clearances. Three seasonal points a correctly designed room can still get wrong:
- Design-day is not average-day. Ventilation sized against an annual average is comfortable for ten months and short for the two that matter.
- Fouling bites hardest in summer. Coolers and intake filters load up across the year. In cooler conditions you may have enough thermal margin to absorb moderate fouling. On hot design days that margin is smaller, although severe fouling can cause trouble in any month.
- Winter recirculation is a summer trap. Rooms that duct warm exhaust back inside for winter heating need those louvres physically closed and verified before summer. A damper stuck part-open is easy to miss.
Motor and VSD derating in a hot room
Motors carry their own ambient and altitude ratings, separate from the compressor package. IEC 60034-1 sets standard site conditions of 40 degrees Celsius ambient and 1,000 metres altitude, and those apply unless the machine is rated for something else. Plenty are. They are reference conditions, not a universal ceiling. Read the conditions marked on your own motor, and if your site is hotter or higher, ask for the manufacturer’s derating curve. The nearest published rule we could verify is written for air-cooled alternators in generator sets rather than compressor motors: roughly 3 to 5 per cent of available output for every 5 degrees above 40, which puts a 50 degree ambient at about 6 to 8 per cent on that source’s own example. Same standard, same physics, different machine, and we are not going to dress it up as a compressor figure. Get the OEM curve.
Drives need their own check, and this is the part that catches people. A variable speed drive can derate earlier than the motor it drives, because the power electronics have their own ambient limit, and drive ratings are often referenced to a cabinet or heatsink temperature rather than the room. Check both, and where each is measured. The case for a drive rests on your demand profile rather than ambient temperature: see VSD compressors and demand profiling.
Heat recovery, because you are already making the heat
More than 80 per cent of a compressor’s electrical input leaves the package as heat, and January is when you least want that heat inside the building. So the seasonal question is not only what the heat is worth. It is where you are putting it.
Two paths. A ducted exhaust that goes outside in the hot months and diverts back in for winter space heating, and a water-side loop serving hot water or process pre-heat, which does not care what month it is. The switchover is a damper and a control decision, and it is the part most often left un-commissioned.
Before you cost it, understand what recovery actually buys, because this is where it gets oversold. It does not give you the electricity back. It offsets the fuel you would otherwise burn for heating, so the saving is set by the fuel you displace, not by the size of your compressor bill. The heat recovery ROI calculator runs that arithmetic on a stated basis.
Worked example, a 45 kW compressor in a Sydney plant room
Assumptions, all stated. A 45 kW oil-injected rotary screw, air-cooled, sea level, Sydney metropolitan site. Package inlet volumetric flow taken as 7.5 cubic metres per minute, a mid-range figure for the class and an assumption rather than a measurement. About 6,000 hours a year at an illustrative $0.30 per kWh, with nominal motor kW standing in for electrical input at full load, the same basis as our running costs guide. Mild condition: intake at 20 degrees, 60 per cent relative humidity. Summer condition: 33 degrees outside at 50 per cent relative humidity, plant room 12 degrees above outside ambient, so intake at 45 degrees, the conservative end of the 10 to 30 degree room rise on our room design page. Psychrometrics from Magnus-Tetens, the same basis as our dew point calculator.
| Mild day (20 C, 60% RH) | Summer day (45 C intake) | Change | |
|---|---|---|---|
| Dry air density at intake | 1.187 kg/m3 | 1.082 kg/m3 | 8.9% lower |
| Dry air delivered | about 534 kg/h | about 487 kg/h | 8.9% lower |
| Specific power | baseline | about 10% worse | |
| Water arriving with the air | about 4.7 kg/h | about 7.7 kg/h | 65% more |
| Water arriving per 12 hour shift | about 56 litres | about 92 litres | 36 litres more |
| Air temperature reaching the dryer | about 28 C | about 53 C | 25 C hotter |
The line that should stop you is 53 degrees at the dryer inlet: 45 degrees of room plus an 8 degree approach, past the roughly 49 degree inlet ceiling many refrigerated dryers carry. The correction factor tables we could find stop well short of that, which is itself the answer: you are off the chart, so you ask the manufacturer rather than interpolate.
Cross-check. Our dryers page publishes roughly 0.1 litres of condensate per kWh in temperate conditions and up to about 0.3 in hot and humid ones. Net off the vapour a dryer leaves in the air at a plus 3 degree pressure dew point and this example condenses about 52 and about 89 litres across 540 kWh of full-load running: 0.10 and 0.16. Independent arithmetic, same published band.
What it costs. Two different bills, and people mix them up. The energy penalty lands whether or not you have headroom, because specific power is about 10 per cent worse and the same air simply costs about 10 per cent more to make. On roughly 1,500 compressor hours in the summer quarter at 45 kW and $0.30 per kWh the base is about $20,000, so call it $2,000. A machine running loaded most of the time sees close to all of that; a lightly loaded load/unload machine sees less, because the extra loaded time comes out of unloaded time and an unloaded compressor still draws power. The capacity penalty is separate, only bites if you are already flat out, and shows up as product you cannot make rather than as a line on the bill. Neither is the big number. The big number is the stop you did not have. Run your baseline through the running costs guide and the leak cost calculator.
The pre-summer checklist
Cheapest first. A plant manager with a logger, a torch and the equipment manuals can work through most of it in an afternoon, and nothing needs a purchase order until item 12.
- Log the intake temperature and the outside air together, not the wall. A logger where the compressor breathes, left a week in late spring, gives you the rise from outside air to intake. Put your summer design-day outside temperature through that rise to estimate January, then confirm it on a genuinely hot day.
- Clean the coolers and change the intake filter. The cheapest capacity you will buy this year.
- Walk the ventilation path with the machine running. Louvres clear, exhaust fan turning the right way, hot air leaving the building, nothing else drawing air out of that room.
- Confirm any winter recirculation damper is closed, and stays closed.
- Find your dryer’s rating point on its data sheet: the inlet air temperature, ambient temperature and pressure its nameplate flow was quoted at. If the data sheet is gone, ask the supplier in writing.
- Request the correction factor table for your model. Run worst-case summer conditions through it, divide your required flow by the result, and compare against the dryer you own.
- Log the dryer inlet temperature and pressure dew point on a hot afternoon, not a mild morning in September.
- Test each condensate drain. A failed drain shows up as wet air and gets blamed on the dryer. Zero-loss drains fail closed as well as open.
- Pull the controller’s high-temperature event history. Count last summer’s warnings and trips and note which days they fell on.
- Check the motor and drive ambient ratings, and ask for the OEM derating curve. Note where each is measured, room or enclosure.
- Cost one hour of an unplanned stop, in writing. Get finance to agree the contribution margin before you need it.
- Set your summer pressure band deliberately, then consider capital. Roughly 7 per cent of compressor energy rides on each bar, so being short at 6.5 costs less than being short at 7.5. With items 1 to 11 done you can narrow it to ventilation, dryer sizing, maintenance, pressure and control, or genuine capacity. Narrowing is not proving. Before you sign for a bigger compressor, log flow, pressure and compressor power or load state, because that is what separates a real capacity shortfall from storage, distribution restriction and control problems. A logged energy audit is the structured version of the same sequence.
Book your pre-summer compressed air check
If you would rather have someone independent work through that list with you, send your site postcode, compressor size and age, dryer type and rating point, operating hours, and what your plant room does on a hot afternoon.
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Frequently asked questions
What temperature are air compressors actually rated at?
The flow figure on a packaged rotary screw compressor is normally quoted against ISO 1217, whose Annex C sets out the simplified acceptance test for electrically driven packaged displacement compressors and whose Annex F is titled Reference conditions. The compressed air literature publishes those reference inlet conditions as 1.0 bar absolute, 20 degrees Celsius and zero relative water vapour pressure. We read the annex titles in the ISO-published document but not the values themselves, so treat the numbers as well-attested industry figures rather than as a clause we checked. The reference condition is deliberately dry air, because humidity varies too much between sites to be useful in a comparison rating. Either way it is a different number from the maximum inlet air temperature in your machine’s installation manual, which commonly sits somewhere in the 35 to 40 degree range and is the number that governs your plant room. Do not treat the two as interchangeable, and be careful with the widely repeated 25 degrees and 60 per cent relative humidity figure, which is not the ISO 1217 reference condition.
Why does my compressed air get wet in summer?
Water enters the system as vapour in the intake air and compression does not remove any of it. Hotter intake air simply carries more of it. The condensation happens in the aftercooler rather than during compression, because compression leaves the air hot and at low relative humidity, and it is the cooling back down towards room temperature that pushes the air across saturation. Everything past saturation becomes liquid water that your separator and drains have to catch. In a worked Sydney example on this page, moving from a 20 degree intake at 60 per cent relative humidity to a 45 degree summer intake raised the water arriving each hour by about 65 per cent, which took a 45 kW machine from roughly 56 litres to roughly 92 litres of water arriving across a 12 hour shift. If the drains or the dryer cannot keep up with that, the surplus reaches your tools and your product.
Will my refrigerated dryer keep up on a 40 degree day?
Check the rating point before you assume either way. A refrigerated dryer’s nameplate flow is quoted at stated conditions, and the North American convention is commonly described as 100 psig with 100 degrees Fahrenheit inlet air and 100 degrees Fahrenheit ambient. Reference conditions vary by model, manufacturer and market, and you cannot infer them from the units, so the only reliable move is to read the conditions off your own data sheet. Manufacturers publish capacity correction factors for pressure, inlet air temperature and ambient temperature. Multiply the three factors together, then divide your required flow by that multiplier to get the dryer size you need. One published sizing example at 8 bar gauge, 35 degrees ambient and 40 degrees at the dryer inlet gives a combined factor of about 0.84, meaning the dryer delivers about 84 per cent of its nameplate. Correction factor tables are model-specific, so use the one for your machine, and note that many refrigerated dryers carry a maximum inlet temperature around 49 degrees Celsius.
Why does my compressor keep tripping on high temperature in summer?
Because the cooling side of the package has lost its margin. An air-cooled compressor rejects heat into the room and cools the compressed air to within an approach temperature of that room air, so the discharge temperature tracks room temperature almost one for one. Add a plant room running 10 to 30 degrees above outside ambient, coolers and intake filters that have been quietly loading up across the year, and a hot afternoon, and the discharge temperature reaches the controller’s limit. The trip itself is correct behaviour and is protecting the machine. Before assuming you need a bigger compressor, clean the coolers, change the intake filter, verify the ventilation path is genuinely open, check that any winter recirculation damper is closed, and pull the controller’s event history to see how many times you got close last season.
How much compressor output do I lose in a hot plant room?
A rotary screw is a positive-displacement machine with a fixed swept volume, so it keeps drawing the same volume of air regardless of temperature. What falls is the mass inside that volume, which varies inversely with absolute temperature. Going from a 20 degree intake to a 45 degree intake takes about 9 per cent of the delivered mass away, while shaft power changes comparatively little, so your specific power gets about 10 per cent worse. Separately, the motor has its own rating: IEC 60034-1 sets standard site conditions of 40 degrees Celsius ambient, which apply unless the motor is rated for something else, and the nearest published derating rule we could verify, written for air-cooled alternators in generator sets rather than for compressor motors, puts the reduction at roughly 3 to 5 per cent of available output for every 5 degrees above that reference, or about 6 to 8 per cent at 50 degrees on that source’s own example. Get the OEM curve for your machine rather than relying on it. A variable speed drive can also derate earlier than its motor because the power electronics carry their own ambient limit, so check both ratings and note where each is measured.