The compressor room is dry. The pipe crossing an external loading bay is not. Once that pipe cools below the system’s pressure dew point, water condenses inside it and may freeze before the morning shift starts.
Choosing a pressure dew point for your process means matching the driest required air to the coldest pipework, the product-quality risk and any applicable standard. Design Air, Atlas Copco authorised distributor in Scotland, uses that specification to select the dryer, monitoring equipment and distribution arrangement. A lower figure isn’t automatically better. It may increase capital cost, purge-air consumption and maintenance without improving the process.
What Pressure Dew Point Measures
Pressure dew point is the temperature at which water vapour condenses into liquid at the compressed air system’s working pressure.
This is different from atmospheric dew point. Atmospheric dew point describes condensation in non-pressurised air. Pressure dew point, usually abbreviated to PDP, describes the moisture condition while the air remains compressed. That distinction directly affects air-quality specifications and dryer selection.
Compression concentrates the water vapour carried by the intake air. Cooling in the aftercooler removes part of that moisture as condensate, but the remaining air stream still contains water vapour. The dryer reduces its moisture content until the required PDP is reached.
If pipework cools below that PDP, the air reaches saturation and liquid water begins to drop out of the air stream.
The consequences are physical rather than theoretical. Water droplets can wash lubricant from pneumatic cylinders, interfere with instrumentation and cause internal corrosion in pipework or CNC machinery. In cold sections, the condensate may freeze and block the air line.
PDP should always be stated with the system pressure. A reading taken after depressurising a sample describes different pressure characteristics and cannot be compared directly with an in-line measurement.
How to Choose a Pressure Dew Point for Your Process
Start with the point in the system where moisture would cause the greatest operational consequence. That may be the point of direct food contact, an instrument-air connection, a breathing-air outlet or an external pipe run between buildings.
A reliable selection process has five steps:
- Define the process requirement. Identify whether moisture could damage machinery, affect product quality, support microbial growth or breach a regulated air-quality limit.
- Find the lowest pipework temperature. Include unheated plant rooms, external distribution lines, loading areas and shutdown periods. The normal compressor-room temperature is rarely the controlling value.
- Confirm the required air-quality class. For most industrial systems, the water class comes from ISO 8573-1. Medical and breathing-air systems have additional requirements.
- Specify PDP at the process pressure. Record the normal line pressure, peak flow, inlet temperature and expected operating range alongside the PDP.
- Select and verify the dryer. Apply the manufacturer’s correction factors, then provide an appropriate measurement point and alarm strategy.
Pressure and dryness solve different problems. Raising line pressure does not correct an inadequate PDP and can increase leakage and artificial demand. Our guide to choosing the right pressure for your air tools explains how to set pressure around the application rather than using it to mask distribution faults.
The result should be a written performance specification. “Dry compressed air” is not measurable. “Maximum -40°C PDP at 7 bar(g), at the dryer outlet and at the critical process connection” is.
ISO 8573-1 Turns the Process Risk Into a Measurable Water Class
The internationally recognised ISO 8573-1:2010 framework classifies compressed-air purity for particles, water and oil. A three-part classification such as `[1:2:1]` gives the class for each contaminant. The second digit is the water class.
The relevant thresholds include Class 1 at no more than -70°C PDP, Class 2 at no more than -40°C and Class 4 at no more than +3°C, according to the ISO air-quality class guide (atlascopco.com).
The table is a starting point, not a substitute for process assessment. Class 4 can be suitable for a heated production hall in Perth but unsuitable for the same facility’s external air line. Class 2 may be required by a food-contact risk assessment even when every pipe remains indoors.
Scottish Winter Conditions Can Set the Dryer Specification
Set PDP below the coldest pipework temperature with a margin that reflects the exposed route, local cold spots, seasonal conditions and the process consequence. The site’s winter design temperature, rather than a general weather forecast, should control the decision.
Consider a manufacturing site with a pipe running outdoors between two buildings. If the winter design temperature is -5°C, a +3°C refrigerant dryer leaves an 8°C shortfall before condensation prevention has even been considered. The air cools below its PDP, liquid water forms and can freeze inside the pipe.
Applying the 10°C buffer produces a target of -15°C PDP or lower. In practice, the next standard dryer specification may be -20°C or -40°C. Procurement should compare that operating requirement with the dryer’s guaranteed outlet PDP after correction for inlet temperature, pressure and flow.
For outdoor pipework in Scotland’s variable climate, this margin prevents condensation, freezing and physical blockages. It matters in Aberdeen supply yards, exposed food-processing sites in Fife and unheated plant areas where overnight temperatures differ sharply from daytime production conditions.
Refrigerant and Desiccant Dryers Serve Different PDP Targets
Refrigerant dryers use a heat exchanger to cool compressed air, condense the moisture and discharge it through a separator and drain. They typically provide a +3°C PDP, have a relatively low initial cost and are energy-efficient for general indoor industrial duty. This makes them the standard technology for ISO 8573-1 Class 4.
They do not make a system winter-proof. If any downstream pipe can fall below +3°C, condensation remains possible.
Desiccant air dryers remove water vapour by passing compressed air through twin towers containing a hygroscopic material such as activated alumina or silica gel. One tower dries the air while the other regenerates. These adsorption systems can achieve -40°C or -70°C PDP, which is why they are used for Class 2 and Class 1 duties.
The additional dryness has an energy cost. Standard heatless adsorption dryers use a proportion of the dried compressed air to regenerate the saturated tower. Heatless adsorption dryers use a share of dried compressed air for regeneration. The energy effect must be calculated from the selected dryer’s purge rate, operating pressure, flow and annual running hours.
Dryer capacity also has to match the compressor’s output at the actual operating conditions. A nominal flow rating may require correction when inlet air is hotter, line pressure is lower or demand exceeds the catalogue reference condition. An undersized dryer can show an acceptable PDP at light load and lose moisture control during the production peak.
Regulated Processes Need Their Own Moisture Limits
UK healthcare and hazardous working environments are tightly regulated. NHS Health Technical Memorandum 02-01 governs medical gas pipeline systems, while BS EN 12021 governs moisture in compressed gases used for breathing apparatus.
These requirements should be treated separately from a general ISO Class 4 specification.
Food, Beverage and Pharmaceutical Production
Food and beverage processes need a documented compressed-air specification where air can contact the product, packaging or product-contact surfaces. The required water class follows the site’s hazard analysis and the process risk, rather than a universal food-industry default. The low moisture level inhibits microbial growth inside the distribution system and protects product quality. The relationship between the -40°C water class and contamination control is covered in this ISO 8573-1 air-quality guidance (airbestpractices.com).
For direct food contact, the compressed-air specification should form part of the site’s Hazard Analysis and Critical Control Point risk assessment. A Class 2 target then becomes a process control with defined measurement points and records, rather than a nominal dryer setting.
These industries generally require desiccant drying systems. A refrigerant dryer providing +3°C cannot achieve a -40°C PDP, regardless of its filtration arrangement.
Breathing Air Under BS EN 12021
For breathing-air systems operating below 40 bar, the BS EN 12021 moisture guidance (hse.gov.uk) sets two important conditions:
- If usage and storage temperatures are unknown, the pressure dew point must not exceed -11°C.
- If those temperatures are known, the pressure dew point must be at least 5°C below the likely lowest temperature.
The 5°C requirement is specific to this breathing-air condition. It should not be confused with the more conservative 10°C industrial design margin used for exposed Scottish pipework.
For high-pressure systems operating between 40 bar and 200 bar, water content is limited to 50 mg/m³ or less, as detailed in this breathing-air testing guidance (alsindustrial.co.uk). The UK national foreword to EN 12021 advises testing and analysis at least every three months, with shorter intervals where the risk assessment requires them.
Medical Air Under HTM 02-01
Procurement documents sometimes describe HTM 02-01 Part A (england.nhs.uk) as mandating a -46°C PDP for medical air. The precise medical-air moisture criterion is an atmospheric dew point of -46°C, equivalent to a water-vapour concentration no greater than 67 parts per million by volume.
That distinction matters when an engineer selects the sensor and converts a reading taken at pipeline pressure. Atmospheric and pressure dew point figures cannot be interchanged without accounting for the measurement pressure.
Standard refrigerant dryers cannot achieve this moisture level. Medical-air plant therefore uses specialised multi-stage desiccant dryers. HTM 02-01 also requires continuous moisture monitoring, with the plant emergency alarm triggered if the dew point exceeds the -46°C threshold. UK healthcare moisture compliance therefore depends on both the drying equipment and the alarmed measurement system.
Dew Point Measurement Must Reflect the Real Process
A dryer display only confirms conditions at its own sensor. It does not prove that air remains within specification at the furthest process connection, after a wet receiver or across an older section of pipework.
For a representative reading:
- Install the primary measurement point downstream of the dryer and final moisture-separation equipment.
- Add a second point near the critical application when long pipe runs, multiple buildings or regulated production make downstream verification necessary.
- Measure at the normal process pressure or record the pressure used for any sampled measurement.
- Prevent ambient air from entering the sample line through leaks, open fittings or unsuitable tubing.
- Select a sensor with a measuring range that covers the specified PDP and expected fault condition.
- Maintain calibration records and set alarms above the normal operating value but below the process failure limit.
Rapid pressure changes can disturb a reading. If the compressor cannot maintain line pressure, confirm whether the fault is on the supply or demand side before condemning the dryer. The Isolate and Verify procedure for diagnosing pressure-building issues provides a practical test. Persistent air compressor pressure problems should also be corrected before using dew point trends to judge dryer capacity.
Where a regulated process requires evidence, online monitoring can provide a continuous record of water vapour and hydrocarbons alongside the commissioning and calibration records. The monitoring method and alarm limits should be set from the process specification.
The Final Specification Should Be Auditable
A complete dryer enquiry should state the required PDP, water class, normal and minimum process pressure, maximum flow, inlet temperature, lowest pipework temperature and measurement location. It should also identify any applicable food, breathing-air or medical standard.
That information lets the supplier apply correction factors and calculate total cost of ownership. It also gives the commissioning engineer a measurable acceptance test. Without it, the buyer receives a dryer model rather than a verified moisture-control system.
Design Air’s dipCAM-qualified engineers can assess the coldest point in your distribution network, confirm the appropriate pressure dew point and size the required dryer at your operating conditions. To arrange a compressed-air quality and dryer assessment for a site in Airdrie, Perth, Fife or elsewhere in Scotland, contact Design Air.