A dryer outlet can show a steady +3°C pressure dew point in August and still leave an outdoor airline full of condensate in January. The dryer hasn’t necessarily failed. The original specification may simply have ignored the coldest part of the distribution system.
Choosing a pressure dew point for your process means working back from product risk, air quality requirements and the lowest temperature the compressed air will encounter. Design Air, Atlas Copco authorised distributor in Scotland, uses that information to specify the required ISO 8573-1 water class and the dryer technology capable of maintaining it under actual site conditions.
The right target is dry enough to prevent condensation, corrosion and contamination without paying to remove more moisture than the process requires.
Pressure Dew Point Is a Process Specification
Pressure dew point is the temperature at which water vapour condenses into liquid water at the compressed air system’s operating pressure.
A PDP of +3°C means the compressed air can cool to +3°C at line pressure before liquid water begins to form. A PDP of -40°C means substantially more water vapour has been removed. The lower the PDP, the drier the compressed air.
The reference to line pressure matters. Pressure dew point isn’t the same as the atmospheric dew point reported in a weather forecast. When compressed air expands to atmospheric pressure, its water vapour becomes distributed through a larger volume and its atmospheric dew point falls. A PDP result is only useful when the measurement pressure is known.
PDP also isn’t a pressure setting. Raising or lowering a distribution system from 7 bar to 8 bar changes air density, compressor work and tool performance, but it doesn’t replace moisture treatment. Operating pressure and dew point must be specified separately. Our guide to choosing the right pressure for your air tools explains the pressure side of that decision.
Start with the air at its point of use. Ask what happens if that air contains liquid water, excessive water vapour or ice. A pneumatic cylinder in a heated assembly hall presents a different risk from air contacting pharmaceutical packaging or operating an outdoor actuator in a Scottish winter.
Compression Turns Atmospheric Moisture Into a Plant Problem
Atmospheric air always contains water vapour. The compressor draws that moisture in with the air required by the plant.
At 7 bar gauge pressure, the compressor has packed roughly eight volumes of atmospheric air into one system volume. The water carried by those intake volumes is concentrated as well. The compression process initially heats the air, which allows much of the water to remain as vapour. When the aftercooler and distribution pipework remove that heat, the air’s capacity to retain water falls and condensate forms.
A separator and drain remove bulk liquid produced after compression. They don’t remove enough remaining water vapour to control the pressure dew point. That is the dryer’s job.
If the air subsequently cools below its PDP, more water condenses inside the distribution system. The result can include internal pipe corrosion, blocked or frozen control lines, displaced pneumatic lubrication and damage to sensitive instruments. In direct or indirect product-contact applications, the same moisture can support microbial growth or carry contamination to the process.
This is why dryer selection can’t begin with nominal compressor flow alone. Flow determines dryer capacity. The process consequence and lowest temperature determine how dry the air must be.
Use This Method to Choose a Pressure Dew Point for Your Process
A defensible PDP specification can be built in six steps. The order matters because selecting a dryer before defining the process requirement usually leads to either inadequate drying or unnecessary energy use.
1. Define the Consequence of Moisture
Identify every way compressed air interacts with the process. Air may power tools without contacting the product, operate control valves, clean components, transport material, contact packaging or come into direct contact with food or pharmaceutical products.
Record the consequence of moisture at each use point:
- Internal corrosion in tools, valves, actuators or pipework
- Ice formation in outdoor lines, loading bays or unheated buildings
- Water marks, coating defects or failed paint adhesion
- Product spoilage or contamination
- Instrument drift or unreliable control response
- Failed customer, quality or regulatory audits
- Unplanned downtime while wet equipment is drained and repaired
The most moisture-sensitive legitimate use normally controls the central dryer specification. An alternative is to dry the main system to a practical baseline and install point-of-use treatment for a small critical demand. That can be more efficient than supplying -40°C PDP to an entire factory when only one instrument line requires it.
2. Establish the Required Air Purity Class
ISO 8573-1 classifies compressed air purity separately for particles, water and oil. The three classifications shouldn’t be collapsed into one description such as “clean air”.
For water, Classes 1 to 6 use pressure dew point limits. Class 4, for example, specifies a PDP no higher than +3°C. Class 2 specifies a PDP no higher than -40°C. “No higher” is the key wording because a lower temperature represents drier air.
Oil Class 0 doesn’t automatically mean Water Class 0 or -40°C PDP. A pharmaceutical process might specify Class 0 for total oil and Class 2 for water. The complete ISO 8573-1 purity designation needs to state the requirements for all relevant contaminants.
For food and beverage applications, the BCAS Food and Beverage Grade Compressed Air Best Practice Guideline (bcas.org.uk) helps processors define, maintain and audit compressed air purity. The current BPG102-1 guidance covers equipment, installation, maintenance, testing and the relationship between compressed air and food-safety controls. It doesn’t make one water class correct for every food process. The site’s risk assessment still has to distinguish direct product contact from non-contact utility air.
3. Find the Lowest Temperature in the Entire Air Path
Inspect the route from the dryer outlet to the furthest point of use. The relevant temperature isn’t always the compressor room minimum.
Check pipework passing through:
- External walls and roof spaces
- Unheated stores and loading bays
- Outdoor process areas
- Underground or exposed distribution sections
- Cold rooms and chilled production areas
- Coastal buildings exposed to wind and low winter temperatures
- Idle branches where air remains static long enough to cool
The coldest section may be a short outdoor drop leg rather than the main ring. That small section can control the PDP for the whole system because it is where vapour first condenses.
4. Apply a Temperature Safety Margin
Where no stricter process requirement exists, the target PDP should be at least 10°C below the lowest temperature experienced by any part of the pressurised system. This follows the instrument-air selection guidance described by Atlas Copco’s ambient-condition guidance (atlascopco.com).
The selection relationship is:
Target PDP ≤ the lower of the process limit or the minimum system temperature minus 10°C.
Consider an airline that reaches a minimum winter temperature of -5°C. Applying the 10°C margin produces a target PDP of -15°C or lower. A +3°C refrigerant dryer would leave an 18°C gap in the wrong direction. A dryer capable of at least -20°C PDP would be the logical ISO-class starting point.
The safety margin accounts for measurement uncertainty, local cold spots and short-term temperature changes. It shouldn’t be used to weaken a stricter process specification. If the process requires -40°C PDP, an ambient calculation of -15°C doesn’t permit the target to be raised.
5. Check the Dryer Rating at Actual Inlet Conditions
Dryer catalogue capacity is based on stated reference conditions. A unit rated for the compressor’s nominal flow may be undersized once actual inlet temperature, operating pressure, ambient temperature and seasonal load are applied.
Higher inlet temperature increases the water load reaching the dryer. Lower operating pressure increases volumetric flow through the dryer for a given mass flow. A poorly ventilated compressor room can raise both the dryer’s thermal load and the temperature of the compressed air entering it.
Use the manufacturer’s correction factors rather than matching nameplate flow to nameplate flow. Include maximum summer inlet temperature, minimum and maximum operating pressure, peak flow, expected pressure drop and future demand. The dryer has to meet the PDP at the worst credible combination, not only at an average Tuesday load.
6. Decide Where the PDP Will Be Verified
Specify the measurement point before procurement. A reading at the dryer outlet confirms dryer performance. A second reading at the critical point of use confirms the distribution system hasn’t introduced a new moisture problem.
The test record should include operating pressure, air temperature, system load, sensor range and stabilisation time. Without those conditions, a dew point figure can’t be compared reliably with the design target.
ISO 8573-1 Links PDP to a Water Purity Class
The table below gives the pressure dew point limits for ISO 8573-1 water Classes 1 to 6 and the usual technology route. These are purity limits, not automatic recommendations for every process.
Atlas Copco’s ISO 8573-1 air-purity guide (atlascopco.com) sets out the -70°C, -40°C and -20°C limits for Classes 1, 2 and 3. Its UK guidance also identifies +3°C, +7°C and +10°C as the limits for Classes 4, 5 and 6 (atlascopco.com).
An application description alone isn’t enough to assign a class. Two packaging plants may use compressed air differently. One may use it only for enclosed pneumatic cylinders. Another may blow directly onto open containers before filling. Their water, particle and oil requirements won’t be identical.
For Scotland’s life-sciences sector, -40°C PDP is a common engineering specification where the validated process requires ISO 8573-1 Water Class 2. It may be paired with Class 0 oil-free compression when oil contamination presents an unacceptable product risk. Atlas Copco confirms that its Class 2 dryer range is designed around the required -40°C pressure dew point (atlascopco.com).
That equipment pairing supports a compliant system design. It doesn’t prove Good Manufacturing Practice compliance by itself. Qualification, maintenance, sampling, documented test methods and change control still form part of the audit trail.
Refrigerant or Desiccant Dryer: Which Fits the Target?
Refrigerant dryers cool compressed air so that water vapour condenses and can be separated. A correctly selected unit commonly delivers ISO 8573-1 Water Class 4 at +3°C PDP. This is often appropriate for pneumatic equipment inside a continuously heated production building.
Variable-speed refrigerant dryers can reduce electrical consumption when flow and thermal load vary. The refrigeration circuit adjusts its output rather than operating continuously for the maximum design condition. That makes sense for a Falkirk packaging site with clear shift changes, provided every pressurised line remains safely above the specified PDP.
A refrigerant dryer becomes the wrong choice when pipework can fall below +3°C or the process needs Water Class 3, 2 or 1. Cooling the air further would risk freezing condensate within the dryer. Deeper drying requires technology that removes water vapour by adsorption or another suitable drying mechanism.
Desiccant dryers pass compressed air through a material that attracts and retains water molecules on its internal surface. One vessel dries the air while another regenerates. The regeneration method has a direct effect on energy use.
A heatless dryer uses a proportion of dried compressed air for regeneration. That purge air has already been compressed, cooled and treated, so losing it creates an additional compressor load. Heated-purge and blower-purge designs reduce the amount of dried compressed air used for regeneration by supplying heat, ambient blower air or a combination of both. Heat-of-compression dryers can use thermal energy already produced by an oil-free compression process.
The right comparison is based on total cost at the required PDP and flow. Purchase price alone misses purge loss, electrical heating, pressure drop, desiccant replacement, filtration and maintenance. It also misses the cost of a dryer that cannot protect the process during winter.
Scottish Winter Conditions Set the Real Limit
A heated compressor room in Motherwell can remain at 15°C while an airline crossing an external loading canopy falls below freezing. Specifying the dryer from compressor-room temperature would protect the machine room and leave the exposed process line at risk.
The same issue appears in fish-processing facilities around Fraserburgh, outdoor plant in Ayrshire and distribution branches serving unheated stores across the Central Belt. Wind exposure, overnight shutdowns and low-flow periods allow compressed air to cool further than it does during full production.
A +3°C PDP is suitable only where every relevant pressurised section remains above that temperature with a sensible margin. If any section may reach 0°C, -5°C or lower, the selection should be recalculated from that minimum.
Pipe insulation can slow heat transfer, but it doesn’t create heat. During a long shutdown, an insulated line will still move towards ambient temperature. Trace heating may protect a defined section when it is correctly designed and monitored, but the failure mode must be included in the risk assessment. Where frozen valves or wet product would stop production, specifying a lower PDP is usually the more dependable control.
Seasonal assessment also has to cover the dryer’s inlet side. Summer brings higher intake temperatures and potentially greater water loading. Winter creates the lowest downstream pipe temperature. The selected dryer must handle both conditions.
A Lower PDP Has an Energy and Maintenance Cost
Drying to -40°C when the process only needs +3°C adds equipment, regeneration and maintenance costs without improving production. The driest technically achievable air isn’t automatically the correct specification.
Heatless desiccant dryers are particularly sensitive to this decision because regeneration consumes dried compressed air. If the dryer is oversized, poorly controlled or left on during low demand, purge losses continue to load the compressor. Heated and blower-purge systems may reduce that loss, though their heaters and blowers introduce their own electrical and maintenance requirements.
Pressure drop matters as well. Every restriction between the compressor and the point of use forces the compressor to operate at a higher discharge pressure to maintain the required process pressure. Dryer sizing, pre-filtration, after-filtration and the condition of those filters all affect resistance.
This is another reason to keep pressure and PDP specifications together during system design. If downstream equipment is losing performance, use a pressure profile and flow measurement before raising the compressor setpoint. Our guide to how do you diagnose pressure building issues in your compressor separates compressor capacity faults from distribution-side losses.
The commercial target is the highest PDP that still satisfies the process standard and remains below the coldest system temperature with the required margin. That gives the dryer less work without accepting condensation risk.
Verification Must Happen at Operating Pressure
A dryer specification is complete only when it includes an acceptance test.
Measure PDP after the dryer has reached stable operating conditions and while the system is carrying a representative load. For variable-demand sites, repeat the measurement close to peak flow. An overloaded dryer may appear satisfactory during a quiet shift and lose its target when production demand rises.
Take a second measurement at the most critical or coldest use point. A difference between dryer-outlet and point-of-use readings can indicate wet receiver carryover, failed drains, contaminated pipework, a bypass valve passing untreated air or a local ingress problem.
Dew point sensors also need the correct measurement range and protection from liquid water, oil and particulate contamination. A sensor designed around +3°C duty may not provide dependable readings at -40°C. Sampling arrangements should follow the instrument manufacturer’s flow, pressure and installation requirements.
Trend data is more useful than a single isolated reading. A slowly rising PDP can expose desiccant ageing, valve leakage, reduced purge flow, excessive inlet temperature or growing demand before liquid water appears in the plant. Design Air can combine periodic calibrated testing with remote monitoring of compressor and system conditions where the installation supports it.
Monitoring doesn’t replace verification. The plant’s maintenance plan should define the target PDP, alarm threshold, test location, calibration interval and action required when the limit is exceeded.
The Final Specification Should Be Unambiguous
A procurement schedule that says “supply dry air” leaves the critical engineering decisions unresolved. A usable specification records:
- The ISO 8573-1 water class and maximum permitted PDP
- Separate particle and total-oil classes where applicable
- The required flow at minimum, normal and peak demand
- Minimum and maximum operating pressure
- Maximum dryer inlet and ambient temperatures
- Lowest downstream pipework or point-of-use temperature
- Permitted system pressure drop
- Dryer redundancy requirements
- Measurement locations and acceptance conditions
- Monitoring, alarm and maintenance requirements
For a heated indoor production line, the result may be Water Class 4 at +3°C PDP from a variable-speed refrigerant dryer. For exposed Scottish pipework with a design minimum of -5°C, the 10°C margin points towards -20°C capability. For a validated pharmaceutical process, the documented requirement may be Water Class 2 at -40°C with a separate Class 0 oil specification.
Those are three different engineering decisions, even if the compressor flow is identical.
Design Air’s dipCAM-qualified engineers can measure your site conditions, identify the coldest part of the air path and define the required pressure dew point before equipment is selected. To arrange a compressed air quality and dryer assessment for a site in Scotland, contact our Airdrie engineering team.