Sušičky vzduchu tlakový rosný bod +15 °C

What a +15 °C dew point means and why it matters

A pressure dew point of +15 °C is a relatively high value, meaning the compressed air is only lightly dried – the water vapour in it starts to condense as soon as the temperature drops to +15 °C. For comparison, such air still contains a considerable amount of moisture; it is far wetter than air from a standard refrigeration dryer (+3 °C). This dew point class matters mainly for specific, undemanding applications or as a limit value when no more efficient drying is available. In practice, a dew point of +15 °C means the air will stay free of condensation only in an environment warmer than 15 °C. As soon as it cools to that temperature or below, the water in it starts to condense. So why choose such a “high” dew point at all? The reason may be cost savings – reaching +15 °C does not require expensive equipment; minimal air treatment is often enough. Sometimes air with a dew point around +15 °C is a by-product of another process (e.g. partial cooling) and is perfectly adequate for the given purpose. As a general rule, +15 °C is usable only where a warm environment is guaranteed and where a small amount of moisture in the compressed air does no harm.

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Suitable applications and practical use

Compressed air this “wet” (dew point +15 °C) is used only in genuinely undemanding applications. One case is a situation where the compressor itself partially dries the air – a piston compressor with a well-sized aftercooler and a large air tank can reach a pressure dew point of around +20 °C once the air has cooled to ambient temperature. If the compressor room is at about +25 °C and the air cools a little more on its way through the pipework, further moisture separates out and the resulting dew point drops to roughly +15 °C even without a dedicated dryer. In simple systems – a home workshop, for example, where compressed air is used only for blowing off dust and the compressor stands in a warm garage – a dew point of +15 °C is acceptable. The air is barely treated, but if it is used immediately in the warm, no condensate has time to form inside the tool. Another use is in seasonal summer operations where work takes place only in warm weather (above 15 °C). Inflating bouncy castles, running pneumatic irrigation systems and similar tasks – there it may be enough to filter liquid water and coarse contaminants out of the air; the dew point itself stays high, but it does not matter, because no night-time cooling below that limit occurs. Membrane dryers at minimum flow can also supply air with a dew point of around +10 to +15 °C – useful when only very slight drying is needed to remove mist, but neither power nor space is available for a larger dryer. For field instrumentation (analysers, sensors), for instance, a small membrane dryer is sometimes enough to lower the humidity so that water does not condense during moderate temperature swings. Overall, a dew point of +15 °C is rarely specified as a target value in industrial practice, but it can occur as the result of very simple or emergency drying. You will use it when you know the compressed air will always stay warm and a small moisture content does no harm to the equipment.

Which dryer types achieve a +15 °C dew point

To obtain a pressure dew point of around +15 °C we often do not even need a dedicated dryer. As mentioned, the very process of compressing and cooling the air can lower its dew point from ambient level (e.g. +30 °C in summer) by several degrees, because part of the water condenses in the air tank and is discharged by an automatic drain. If that is not enough, cyclone separators and pre-coolers are available – they remove free water and thus lower the dew point from, say, +30 to +20 °C. That is still high, but better than nothing. To reach +15 °C, membrane dryers with a small purge flow are usually installed. A membrane dryer can work with no additional air bleed (i.e. without drying) – then the inlet dew point is retained – or with a minimal purge (e.g. 10 % of the flow), lowering the dew point by roughly 10–15 °C. That way an inlet pressure dew point of +30 °C (which corresponds to very humid summer air) can be turned into about +15 °C at the outlet. A membrane dryer is therefore one of the practical ways to achieve +15 °C in the field or on a small scale. A refrigeration dryer can of course also be run so that it reaches only +10 to +15 °C (if it is heavily overloaded beyond its rated flow, for example, the dew point rises). Older or simplified refrigeration dryers are sometimes rated at +10 °C dew points, which is really quite wet air – more a sign of weak unit performance. Deliquescent (chemical) dryers likewise hold the dew point only a few degrees below the air temperature, so in a warm environment of around 25–30 °C they could bring the outlet to roughly +15 °C. Such cases are, however, special. In summary: for +15 °C a combination of good condensate separation and possibly a membrane dryer is usually enough – nothing that would call for a major investment. But if you find you need more reliable drying, consider going straight for a refrigeration dryer for +3 °C or +5 °C, which will cover harsher conditions as well.

Advantages and disadvantages of a +15 °C dew point

Advantages: The biggest advantage is that reaching a dew point of +15 °C can be very cheap or completely free. Often no active drying equipment is needed at all – natural cooling and good draining of the air tank will do. If a dryer is fitted, it can be the simplest type, for example a small membrane cartridge that is inexpensive and maintenance-free. Air with a dew point of +15 °C still contains less free water than fully saturated air – water will not run through the pipework, it will merely be damp. For undemanding tools and tasks that may be enough, so you do not invest needlessly in more expensive technology you do not need. Running costs are minimal: no electricity consumption (if you rely on passive drying alone) and minimal air loss (a membrane with a 10 % purge is still more economical than a full adsorption dryer). The system is also simple – fewer components mean fewer potential failures. In a permanently warm environment no negative effect shows up, so why complicate things if this is sufficient.

Disadvantages: There are, however, plenty of disadvantages, because a dew point of +15 °C is at the very edge of usability. Above all, the air is still fairly wet – if it cools even slightly (below +15 °C), it condenses immediately. That means the system has practically no reserve. Any change in conditions (a cooler morning, a draught of colder air, air expansion causing cooling) can lead to water forming in the pipework. That can cause corrosion, fouled instruments or other problems, which is exactly what drying is meant to prevent. This is why +15 °C is an option only where you are certain of a consistently warm environment. Another disadvantage is that air this wet meets no industrial quality standard – even the lowest class 6 to ISO 8573-1 requires a pressure dew point of +10 °C, so +15 °C falls outside the classification. In practice this means it would not be acceptable for most serious applications because of the risks. Pneumatic systems with electronics, paint spraying, food production, healthcare – in all of these +15 °C would be an unacceptably high figure and a better-performing dryer would have to be used. It can be said that +15 °C protects only partially – it does eliminate running condensate at normal temperatures, but it does not prevent the increased humidity in the air itself. That can affect combustion quality (in air fed to burners), it can promote corrosion when mixed with oil aerosols (moist air + oil = a more aggressive emulsion) and so on. Moreover, if compressed air with such a high dew point expands abruptly (in a blow gun, for instance), adiabatic cooling and condensation occur right in the air stream – so the operator may see “mist” or droplets while blowing, which is undesirable. This does not happen at +3 °C, but at +15 °C it easily can. For these reasons a dew point of +15 °C is used only exceptionally. If you find that doing without a dryer altogether does not work (condensate does appear now and then), it is time to move up to better drying – a simple refrigeration dryer for +5 °C or +3 °C, which will solve these troubles. Summary of the drawbacks: +15 °C is usable only within narrowly defined conditions and carries the risk of condensation at the slightest drop in temperature. It is therefore chosen more as an emergency or interim solution.

Links to further information

If you are considering whether air this wet is enough for you, we recommend looking at drying to lower dew points as well, just to be safe. Even a dew point of +5 °C or +3 °C markedly improves the situation, and these dryers are not particularly expensive. If you are addressing air quality systematically, it may also be worth looking at the higher drying classes (-25 °C, -40 °C) described in further articles. Our range covers technology for all the dew points mentioned, from simple separators to high-performance adsorption dryers. In the air dryers section you will also learn more about the differences between refrigeration and membrane dryers, which may be relevant for achieving such a high or, conversely, a very low dew point. Remember that the goal is air that is as dry as necessary, but no drier than needed – a dew point of +15 °C is an extreme compromise and suits only those cases where you really know it is enough.

All compressed air dryers and more information about air drying and the dew point can be found in this article from our advice centre: Measuring the dew point temperature and Why and how to dry air