Compressed air dew point: what −40°C and −70°C mean, and when you actually need each
Executive summary
Key takeaways
- Use pressure dew point (PDP), not “dew point” in general. PDP is dew point at system pressure and it predicts whether water will form inside live pipework.
- Do not measure after expansion. Bleeding air down to atmosphere before the sensor can make the number look much “drier” without changing the moisture in the system.
- −40°C PDP is a common practical target. It provides strong winter margin for exposed lines and avoids many moisture-related reliability failures.
- −70°C PDP is specialist duty. It is justified when trace moisture causes product defects or reactions, or when it is written into an OEM/customer requirement.
- Dryer selection is an operating-cost decision. Refrigeration dryers typically suit around +3°C PDP; sub-zero targets normally mean desiccant drying, where purge and control method dominate cost.
What “dew point” means in compressed air
Dew point is the temperature at which water vapour in air starts to turn into liquid water (or frost). If air is cooled below its dew point, water forms. In compressed air, the number that matters is pressure dew point (PDP). PDP is the dew point measured at the pressure inside your compressed air system. It tells you whether water will form inside the live pipework.✓ PDP — Pressure dew pointMeasured at system pressure. Tells you whether water will condense inside live pipework. This is the number to specify, measure, and act on.
✗ ADP — Atmospheric dew pointMeasured at atmospheric pressure. Useful for weather forecasts. Not useful for deciding whether your ring main will produce condensation.
⚠ The common trap: measuring after expansion. If you vent compressed air to atmosphere before it reaches the sensor, the reading will drop because the pressure has dropped. That gives a false sense of dryness even though the air inside the system has not changed.
Worked example: why measuring after expansion misleads
Air leaves a dryer at system pressure with a PDP of −40°C. If someone bleeds that air down close to atmospheric pressure before it reaches the sensor, the measured dew point can appear around −60°C to −65°C. The air has not become “drier” in the pipework. You have changed the measurement conditions.Rule of thumbIf you want to manage condensation risk, specify and measure PDP at system pressure, in a live flowing sample.
Why dew point matters
Moisture problems in compressed air show up as reliability problems.🔩 Corrosion and debris
Water inside pipework drives internal corrosion and creates rust and scale. That debris breaks loose and travels downstream, where it can block small orifices and damage pneumatic equipment.⚙️ Sticking valves
Water causes sticking valves and inconsistent actuator performance, particularly after shutdowns when the system cools and more water drops out.❄️ Freeze risk
If any part of the system can fall below 0°C, water can freeze in drains, valves, instruments, and outdoor lines. Ice blockages restrict flow and create pressure loss where you least want it.🏭 Process quality
In quality-critical applications, moisture can affect the process outcome. Water in the wrong place creates rejects, stoppages, and maintenance work.
Common standards in plain English (and where the confusion starts)
ISO 8573-1 is the most common framework used to specify compressed air quality. It separates contaminants into categories, including water, particles, and oil. The key point is simple. Dew point is only about water. It does not tell you anything about oil aerosols or hydrocarbon vapours. That matters because many specifications blend these topics together in conversation. A site can have a very low dew point and still have oil issues if oil is not controlled and verified separately.ISO water classes in practical PDP terms
| PDP target | Typical reason for specifying it | ISO 8573-1 water class |
|---|---|---|
| +3°C to +10°C | Indoor general manufacturing where you want to avoid liquid water carryover and visible condensate. | Class 4 to 6 |
| −20°C | Moderate cold exposure or cold-store type environments where sub-zero margin is required. | Class 3 |
| −40°C | Strong winter margin for exposed lines and tighter moisture control for sensitive processes. | Class 2 |
| −70°C | Ultra-dry duty where trace moisture can cause defects or reactions. | Class 1 |
Where the confusion starts: “lower is always better” becomes the default. In reality, lower dew point usually increases cost and complexity. The right target is the one that prevents your real failure modes.
Minus 40°C PDP: where it fits and what problems it prevents
A PDP of −40°C is a common choice because it gives a wide safety margin. In plain terms, it means liquid water will not form inside the system unless some part of the system drops below −40°C. On UK industrial sites, that usually provides robust protection where pipework runs outdoors, crosses unheated spaces, or has drains and valves exposed to winter conditions. It is also a practical midpoint between “basic drying” and “ultra-dry”. Many sites specify −40°C because it is a clear and recognised threshold that sits on an ISO water class boundary.What −40°C does not do is fix poor system design or poor maintenance. If bulk liquid water is carried into the distribution system due to failed separators, failed drains, or poor receiver management, you can still get wet points of use. Dew point is a vapour measure. Liquid management still matters.
Minus 70°C PDP: when it is justified, and when it is overkill
A PDP of −70°C is an extreme moisture target. It is used where tiny amounts of moisture can create high-value defects, drift, or chemical reactions. It is also one of the most commonly over-specified numbers, usually because it is treated as a badge of “better air”. If the process does not need it, −70°C is rarely a sensible default. The practical consequences are straightforward. You typically need more specialist desiccant, larger towers, and more demanding regeneration. That can raise capital cost, increase energy use, and increase maintenance burden.Decision ruleIf −70°C is written into an OEM requirement, customer specification, or controlled process standard — follow it.
If it is not written down, ask what failure it prevents and whether −40°C already prevents it.
Dryer types and the practical trade-offs
Dryer choice determines both the achievable dew point and the running cost profile. Refrigeration dryers cool the air so water condenses and is drained away. They are commonly used for indoor applications and are typically associated with PDP targets around +3°C. Desiccant dryers remove water vapour by adsorption. They are used for sub-zero dew points, including −20°C, −40°C and −70°C. Within desiccant dryers, the trade-offs that most affect operating cost are purge and control method.
How regeneration method changes cost
Heatless purge
Uses already dried compressed air to regenerate the offline tower. Purge can be a significant proportion of the dryer’s rated flow.Heated purge
Reduces purge by using heat to improve regeneration, but introduces heater load and extra components.Blower purge
Uses a blower and heater to regenerate using ambient air. Reduces compressed air purge losses to very low levels, but adds electrical load and complexity.| Dryer type | Typical PDP | Main cost trade-off | Maintenance focus |
|---|---|---|---|
| Refrigeration | +3°C to +10°C | Low complexity, no purge, but not suitable for sub-zero PDP. | Drains, condenser cleanliness, separator performance. |
| Desiccant, heatless | −40°C to −70°C | Higher running cost due to purge air, especially at part-load. | Pre-filtration, drain performance upstream, desiccant condition. |
| Desiccant, heated purge | Often −40°C duty | Lower purge than heatless, plus heater energy and added complexity. | Heater condition, sequencing valves, filtration integrity. |
| Desiccant, blower purge | Often −40°C duty | Minimal compressed air loss, higher electrical load and more components. | Blower condition, heater performance, control stability. |
How to choose: five questions to ask before you spend money
Before you buy
- What is the coldest temperature any part of the system can reach, including outdoor runs and drains? This sets the dew point margin you actually need.
- What is the point-of-use requirement, and where is it written? If only one area needs very dry air, point-of-use drying may be more sensible than treating the whole site.
- What are the load swings in flow, pressure, and inlet temperature? Dryers are rated at specific conditions. Your site will move around those conditions.
- Where will dew point be measured, and will it be measured at system pressure in a live flowing sample? A good sensor in the wrong place gives unhelpful data.
- What is your calibration plan for dew point measurement? Sensors drift over time, particularly if exposed to contaminants, so calibration is part of keeping the data trustworthy.
Short case example: what changes when load swings and you have peaks
ScenarioA site installs a heatless desiccant dryer rated at 1,000 cfm for −40°C PDP. The dryer uses purge air as part of regeneration. Purge is linked to dryer rating, not to the site’s actual flow at that moment.
This is why the right decision is rarely just “pick −40°C or −70°C”. You also need to match dryer selection and control strategy to how the site actually runs.
📉 Weekend low-load
Demand falls to 500 cfm but purge remains unchanged. The purge becomes a much larger proportion of the useful air delivered. Compressor running hours and energy use rise without the site noticing — because the dryer looks “fine” on dew point.📈 Summer peak
Hotter inlet air carries more water vapour, increasing the moisture load. If the dryer was sized with limited margin, dew point can drift upwards during hot, high-flow periods even if it meets target in mild conditions.Get the right dew point target for your site
If you are specifying a new-build system, or reviewing an existing one, ALS can help you choose the right pressure dew point and drying approach. Call 01789 867640 or Send an enquiry Send your current dew point specification and a photo of the dryer nameplate. For new-builds, send the application, expected flow and pressure, and any customer or OEM air quality specification you have been given.References
- ISO 8573-1 (Compressed air — Contaminants and purity classes).
- Atlas Copco: Pressure dew point (PDP) explainer.
- Van Air Systems: Pressure dew point vs atmospheric dew point (effect of compression/expansion).