Humidity and Dew Point
Water in the air is enemy #1 of compressed air.
Why Does Air Have Water?
Water constantly evaporates from oceans, rivers, lakes, and soil. This water vapor mixes with air and travels with it.
Air Capacity
The amount of water vapor air can hold depends on temperature:
| Temperature | Maximum content |
|---|---|
| 32°F (0°C) | 4.8 g/m³ |
| 68°F (20°C) | 17.3 g/m³ |
| 86°F (30°C) | 30.4 g/m³ |
| 104°F (40°C) | 51.1 g/m³ |
Hotter air = can hold more water
Relative Humidity
Relative humidity (RH) indicates what percentage of maximum capacity is occupied:
Example: At 77°F (25°C), air can hold ~23 g/m³. If it contains 11.5 g/m³:
The Problem: Compression
What happens when you compress humid air?
- You take 247 ft³ (7 m³) of air at 68°F and 70% RH
- You compress it to 100 psig (reduce to 35 ft³ or 1 m³)
- Same water in 1/7 of the space
- Air becomes supersaturated at 700%
- Excess condenses as liquid
For every 35,000 scf (1,000 m³) of compressed air at 100 psig, approximately 2.5-4 gallons (10-15 liters) of condensate is generated.
Dew Point
The dew point is the temperature at which air begins to condense water.
| Type | Description |
|---|---|
| Atmospheric dew point | At atmospheric pressure |
| Pressure dew point (PDP) | At system pressure |
A PDP of +37°F (+3°C) means if your air drops below 37°F, it will condense water.
Outdoor piping in winter? You need a lower PDP.
Why do we need dryers?
| Without dryer | With refrigerated dryer | With desiccant dryer |
|---|---|---|
| PDP = ambient temp | PDP = 37-50°F | PDP = -40°F or lower |
| Always condenses | Condenses below 37°F | Almost never condenses |