The pressure dew point is the temperature at which the water left in compressed air condenses, measured at network pressure. Air with a +3 °C dew point stays dry as long as it does not cool below +3 °C. To choose one, start from the coldest point the air will meet, and pick a dew point well below it.
This guide explains the concept and gives the figures. For air quality classes, see our ISO 8573-1 guide. To compare dryer technologies, read which air dryer to choose.
What a dew point measures
Air always contains water vapour, but it can only carry a limited amount, which depends on its temperature. The colder it is, the less it holds. When you cool humid air, it reaches a point where it is saturated: its dew point. Below that, the surplus water forms droplets.
Relative humidity, in %, depends on temperature. The dew point, on the other hand, directly describes how much water is present. That is why it is used for compressed air.
How much water can air carry?
Maximum water vapour in 1 m³ of saturated air. These are standard physical values, rounded:
| Temperature | Water in 1 m³ of saturated air |
|---|---|
| 35 °C | about 39.6 g |
| 30 °C | about 30.4 g |
| 20 °C | about 17.3 g |
| 10 °C | about 9.4 g |
| 3 °C | about 6.0 g |
| 0 °C | about 4.8 g |
| −20 °C | about 0.9 g |
This table explains why drains discharge more in summer: air drawn in at 30 °C can carry almost twice as much water as at 20 °C.
Under pressure or atmospheric: do not mix them up
Compressing air reduces its volume but keeps the same amount of water. That water is concentrated, so it condenses at a higher temperature. You therefore need to state the pressure:
- The pressure dew point is measured at network pressure. It is the figure that matters for your pipes and tools, and the one given by dryers and by ISO 8573-1.
- The atmospheric dew point is measured after expansion to ambient pressure. It is always lower, and therefore more flattering.
As an order of magnitude, using the usual ideal gas approximation: air with a +3 °C dew point at 8 bar has an atmospheric dew point of about −23 °C. If a data sheet states a dew point without saying which, ask.
A step-by-step calculation: where the water goes
Take a compressor that draws in air at 20 °C and 60% relative humidity and compresses it to 8 barg, about 9 bar absolute. We work per 1 m³ of intake air.
- Water drawn in: 17.3 g × 60% ≈ 10.4 g.
- Volume after compression: 1 m³ ÷ 9 ≈ 0.11 m³.
- Water this volume can hold at 20 °C: 0.11 × 17.3 ≈ 1.9 g. As soon as the compressed air cools back to 20 °C, in the receiver or the pipes, about 10.4 − 1.9 = 8.5 g condenses.
- After a dryer with a +3 °C dew point: 0.11 × 6.0 ≈ 0.7 g remains. In total, about 9.7 g of water has been removed from each m³ drawn in.
Applied to the flow of a K 110, 1.7 m³/min at 8 bar according to its data sheet, or 102 m³ per hour: 102 × 9.7 g ≈ 1 litre of water per hour of loaded running, to be removed by the drains. It is an order of magnitude, which rises sharply in hot, humid weather. For removing this water, see water in compressed air.
How to choose your dew point
1. Find the coldest temperature along the route. A pipe running along an outside wall, under a canopy or into an unheated building sets the rule, not the workshop temperature. Take the lowest value of the year, nights included.
2. Keep a margin. The dew point must stay well below that temperature. A network that never drops below 15 °C can make do with a refrigerated dryer. A pipe that reaches 2 °C in winter needs a negative dew point.
3. Check the process requirement. Some manufacturers of machines, spray guns or automation equipment specify an ISO 8573-1 water class, and therefore a maximum dew point. In that case, it decides.
4. Deduce the technology. A refrigerated dryer cannot go below 0 °C, or water would freeze in its heat exchanger: it gives a few degrees above zero. A negative dew point calls for a desiccant dryer.
What spoils the real dew point
- An undersized dryer for the flow, or one installed in a hot room: it dries less well than its rated value.
- Air entering the dryer hot. The S 110, for example, delivers air at no more than 15 °C above ambient temperature, according to its data sheet: a hot room gives hot air, and a ventilated room helps the dryer.
- A blocked drain: separated water goes back into the network.
- A bypass left open after a service visit.
You check it with a dew point sensor at the dryer outlet. Failing that, water in a filter far from the dryer is a clear sign.
What AIRSTAHL compressors offer
- The K series 4-in-1 integrates a refrigerated dryer and filtration on the same frame as the compressor and the receiver.
- The L series, for laser cutting, integrates a refrigerated dryer rated for a dew point of 3 to 10 °C, with precision filtration.
- For the standalone S series compressors, we offer refrigerated dryers sized by power, and desiccant dryers for networks exposed to cold.
The configurator sizes the compressor from your tools. For air treatment, describe your network route and your requirements in a quote request: our technicians will define the dew point and air class with you.