At altitude, as in high heat, the air a compressor draws in is less dense: each litre contains less air. For the same power, the machine delivers a little less real air, and it cools less well. At around 1,500 m, standard atmospheric pressure falls to about 0.85 bar: each litre drawn in contains about 16% less air than at sea level.
Why the air "weighs" less
The amount of air in a litre depends on two quantities: pressure and temperature. It rises with pressure and falls as temperature goes up. This is the ideal gas law, a standard law of physics.
Altitude lowers the pressure. The values below are those of the standard atmosphere, an international physical reference. Actual pressure varies slightly with the weather.
| Altitude | Standard atmospheric pressure | Air per litre drawn in, compared with sea level |
|---|---|---|
| 0 m | 1.013 bar | 100% |
| 500 m | 0.955 bar | about 94% |
| 1,000 m | 0.899 bar | about 89% |
| 1,500 m | 0.846 bar | about 84% |
| 2,000 m | 0.795 bar | about 78% |
| 3,000 m | 0.701 bar | about 69% |
Heat expands the air. At equal pressure, air at 35 °C contains about 5% less air than air at 20 °C, and about 6% less at 40 °C. The calculation is done in kelvin: (273 + 20) / (273 + 35) = 0.95.
The two effects add up. A workshop at 1,000 m that reaches 35 °C in summer gets both.
What changes for flow
The flow on a data sheet, in l/min or m³/min, is a volume of free air measured at intake conditions. At the same speed, a screw compressor draws in roughly the same volume at altitude. But that volume contains less air, and tools need a certain amount of air to work.
The pressure gauge always shows gauge pressure: a compressor set to 8 bar delivers 8 bar above ambient pressure, in the mountains as on the plain. The compression ratio, however, goes up. At sea level, at 8 bar, it is about 9.0 / 1.0 = 9. At around 1,500 m, it rises to about 8.85 / 0.85 = 10.4. The machine works harder for each litre drawn in, and runs hotter.
Worked example at 1,500 m. An S 150 delivers 2,200 l/min at 8 bar according to its data sheet.
- Altitude factor, read from the table: 0.84.
- Air actually available, as sea-level equivalent: 2,200 × 0.84 = about 1,850 l/min.
- If your tools need 1,900 l/min, calculated with their usual consumption figures, the S 150 becomes too tight. The S 220 PM, 3,600 l/min at 8 bar on its data sheet, gives about 3,600 × 0.84 = 3,020 l/min with the same factor.
This is a cautious estimate, to be checked with our technicians. It leaves out other, smaller effects that work in both directions. To calculate what your tools need, follow our guide on what size compressor you need.
What changes for cooling
Our screw compressors are air-cooled. The fan moves a volume of air through the oil cooler. Two situations reduce that cooling:
- At altitude, the air is less dense: the same volume carries away less heat.
- In high heat, the temperature gap between the oil and the room air narrows: each cubic metre cools less.
The compressed air also leaves hotter. On our screw compressors, it leaves at no more than 15 °C above ambient temperature: in a room at 40 °C, it can enter the dryer at 55 °C. The electric motor, also cooled by air, is affected in the same way.
Each compressor's manual states its operating conditions, including the maximum ambient temperature. Compare it with the actual room temperature on a summer afternoon, and mention your site's altitude before ordering.
Water, heat brings more of it
Warm air can hold much more water vapour. Standard physical values, for saturated air:
| Air temperature | Maximum water content |
|---|---|
| 10 °C | about 9 g per m³ |
| 20 °C | about 17 g per m³ |
| 30 °C | about 30 g per m³ |
| 35 °C | about 40 g per m³ |
| 40 °C | about 51 g per m³ |
Between 20 and 35 °C, the amount of water can more than double. The refrigerated dryer and the drains have that much more to remove. Our article on a compressor in a hot workshop at 35 °C gives the factors our configurator applies to the dryer, and how to duct the hot air.
Cold does not always help either: below zero, condensate freezes in the receiver, drains and dryer. The room must stay frost-free.
What it means for choosing and installing
- State the altitude and the maximum summer temperature in your request: the sizing takes them into account.
- Allow a flow margin at altitude, using the factor from the table, before applying the usual margin.
- Take care of ventilation: fresh air inlet low down, hot air outlet ducted outside, as described in our article on compressor room installation.
- Size the dryer for summer, or choose a standalone S series compressor with a dryer placed in the coolest area, if the room stays very hot.
- Clean the cooler and the intake filter more often: a layer of dust reduces cooling that is already limited.
If the machine still trips, our article on compressor overheating lists the checks. For a new project, calculate your needs in the configurator, then send us a quote request with the site altitude and the room temperature: our technicians check the calculation with you.