It’s the temperature at which water vapor condenses at system pressure—critical for predicting condensation in compressed air lines.
Moisture is one of the biggest threats to compressed air systems. When water vapor condenses inside pipes and tools, it causes corrosion, damages equipment, and compromises product quality. The key metric for moisture control is pressure dew point (PDP)—the temperature at which water vapor turns to liquid at system pressure. Managing PDP ensures compliance with ISO 8573-1 water classes and protects your operations.
Understanding Pressure Dew Point
What Happens During Compression
Air always contains water vapor. When compressed, its temperature rises and its ability to hold water increases. As the air cools after compression, excess vapor condenses. PDP predicts when this condensation will occur under pressure, making it essential for system design and quality control.
Why PDP Is Different from Atmospheric Dew Point
Atmospheric dew point is measured at ambient pressure, but compressed air operates under higher pressure. PDP accounts for this, giving a realistic measure of moisture risk inside your system.
ISO 8573 Water Classes
ISO 8573-1 defines air purity classes for particles, oil, and water. For water, classes are based on PDP:
- Class 4: PDP ≤ +3 °C (≈ 37 °F) – typical for general industry
- Class 3: PDP ≤ −20 °C
- Class 2: PDP ≤ −40 °C
Most industrial applications aim for Class 4, which prevents liquid water under normal indoor conditions.
How to Achieve and Maintain PDP Targets
Cooling and Separation
Aftercoolers reduce air temperature, condensing water that can be removed by separators and drains. This first step lowers the load on downstream equipment.
Refrigerated Drying for Standard PDP
For most workshops and assembly lines, refrigerated dryers maintain PDP around +3 °C, balancing moisture control and energy efficiency. This is the ABAC-approved solution for general-purpose air treatment.
Filtration and Pressure Drop Management
Filters remove liquid and aerosols, but they also cause pressure drops. Plan for 0.3–0.5 bar loss across treatment stages and verify final pressure and PDP at the point of use.
Condensate Handling
Every time water is removed, condensate must be treated. ABAC recommends oil-water separators and compliant drains to meet environmental regulations.
Measurement and Verification
Install dew point sensors downstream of treatment and upstream of critical points-of-use. Ensure proper sampling flow and clean lines for accurate readings. Log data and maintain calibration records to prove compliance during audits.
Frequently Asked Questions
Class 4 water (PDP ≤ +3 °C) is common for indoor environments and prevents liquid water formation.
Use calibrated dew point sensors, log readings continuously, and align results with ISO 8573 water classes.
