Air Dryers Explained

Why Dry Compressed Air?

Atmospheric air contains water vapour. When compressed, the same mass of water vapour occupies a smaller volume, so the relative humidity rises. As compressed air cools in piping, the water condenses, causing corrosion, freezing, and damage to pneumatic equipment.

The amount of water in compressed air is expressed as pressure dew point: the temperature at which water condenses at the system pressure. Lower dew point means drier air.

Dryer Types

• Refrigerated dryer: Cools the compressed air to about 3 degrees C, condensing moisture which is then drained. Achieves pressure dew point of +3 degrees C. Suitable for most general industrial applications. Energy-efficient and low maintenance.
• Desiccant dryer: Passes air through a bed of desiccant (activated alumina or silica gel) that adsorbs moisture. Achieves dew point of -40 degrees C or lower. Two towers alternate between drying and regenerating. For applications requiring very dry air (outdoor, instrument, medical).
• Membrane dryer: Uses a semi-permeable membrane that allows water vapour to pass through while retaining air. Compact, no moving parts, no electricity. Low capacity. For point-of-use drying of small air flows.
• Deliquescent dryer: Uses a hygroscopic chemical that absorbs moisture and dissolves. Simple but requires chemical refills. Achieves dew point about 10 degrees C below inlet temperature. Low cost, low efficiency.

Selecting a Dryer

Refrigerated Dryer Maintenance

• Clean condenser: The refrigerant condenser coil must be clean for heat rejection. Dirty coil causes high refrigerant pressure and reduced drying capacity.
• Check drain: The automatic condensate drain must function. A blocked drain allows water to re-enter the air stream.
• Check refrigerant level: Low refrigerant reduces cooling capacity. If the dryer cannot reach 3 degrees C, check for refrigerant leaks.
• Replace air filters: Pre-filter and after-filter elements should be replaced per manufacturer schedule (typically every 6 to 12 months).

Desiccant Dryer Maintenance

• Replace desiccant: Desiccant degrades over time (typically 2 to 5 years). Contaminated or oil-fouled desiccant loses drying capacity. Test dew point regularly.
• Check purge flow: Purge air (used to regenerate the desiccant) consumes 10 to 20% of compressor output. Verify purge flow is at the correct rate. Excess purge wastes energy.
• Check switching valves: The valves that switch between drying and regenerating towers can fail. A stuck valve allows wet air through. Check operation.
• Check exhaust silencers: Clogged silencers on the purge exhaust restrict flow and reduce regeneration effectiveness.