Percentages change even without adding water
Heating without adding moisture lowers relative humidity, even though the water vapour has not disappeared. Cooling increases it towards saturation. Relative humidity percentage alone is therefore insufficient to compare water in winter and summer intake air; temperature and pressure are also needed.
Balance before and after the compressor
Compression changes air volume and temperature. Simply subtracting intake g/m³ from g/m³ in pressurised pipework would compare different air masses. To calculate condensate use a consistent dry-air basis and account for water remaining in the outlet gas.
What to check in practice
- Always state temperature and pressure with relative humidity.
- For g/m³, state whether the volume is at operating or reference conditions.
- To assess condensation risk, monitor the dew point at the correct pressure.
Example scenario
Two operations have 60% relative humidity at the intake, one at 10 °C and the other at 30 °C. They do not contain the same amount of water vapour. Under these conditions, warmer air brings more water per unit of intake volume and can place a greater load on aftercooling and drying.
What to watch out for
- Treating air heating as water removal.
- Comparing only percentages without temperature.
- Confusing g/m³ with g/kg of dry air.
Why is there more condensate in summer?
Warm, humid air often brings more water into the system than cold intake air. How much actually condenses also depends on flow, pressure and temperature after cooling. Drains should therefore not be sized from winter observations alone.