Prevent Condensation & Damage by drying compressed air

Moisture in (Compressed) Air

Let’s begin with water vapour. You can’t see it, you can’t smell it, but it’s always present. The warmer the air, the more water vapour it can hold (see table). Usually, the air contains less than the maximum possible amount of vapour — this is expressed as Relative Humidity (RH).

Example calculation: If the RH of air is 50% at 15°C, then the air contains 50% × 13 g = 6.5 g of water vapour per cubic metre (g/m³).

When warm air cools, it can hold less vapour. Once RH rises to 100%, the excess vapour turns into tiny droplets (condensate). The temperature at which the cooling air starts forming droplets is called the dew point — think of dew on the grass. The dew point is expressed in °C.

Even when air has not yet condensed, it still has a dew point. This dew point is always lower than the current temperature of the air. In the above example, 15°C air containing 6.5 g moisture will only start condensing once it cools below 3°C (see Table 1). Thus, the dew point is 3°C.

As long as the ambient temperature of the compressed air system remains above the dew point, no condensate will form inside. The lower the dew point, the drier the air.

Compressed air is at a much higher pressure than ambient air, which changes its behaviour. In this case, we speak of a pressure dew point. The principle remains the same: condensate will form once the temperature falls below the dew point.

Producing compressed air requires around 7–9 m³ of ambient air for every 1 m³ of compressed air (at 7–9 bar). This means that the same multiple of vapour and dirt particles is concentrated into that single cubic metre. However, the air at 15°C can only hold 13 g/m³. The surplus water vapour will condense already inside the compressor.

Example calculation: If RH = 50% at 15°C, then 6.5 g/m³ vapour is present. Compressed to 8 bar: 6.5 g × 8 = 52 g/m³ → 52 – 13 g/m³ = 39 g/m³ will condense. The compressed air is now fully saturated: RH = 100% at 15°C.

In practice, compression generates heat. Up to 85% of the electricity consumed by a compressor turns into heat, raising air temperature to 70–90°C. Hotter air can hold more vapour (see table). After compression, RH will almost always be 100%, and each m³ may contain up to 290 g vapour.

If this “wet” compressed air is not dried, it will inevitably cause condensation in your system as it cools back down to ambient temperature.

Condensate always forms at the coldest spot in the system, which is often not the compressor room.

Why Condensate Is Harmful

• Moisture has a corrosive effect on many metals, leading to irreparable damage in the long term.
• In agricultural environments, the high ammonia concentration in ambient air dissolves in condensate, creating a highly aggressive lye. Standard aluminium alloys are particularly vulnerable to this.
• Condensate washes away factory lubrication in pneumatic components, causing excessive wear of seals and seizure. A common failure is a pulled-out nose seal in a cylinder.
• Washed-out grease accumulates further down the line. In reaction with condensate, it forms a thick sludge that clogs filters and blocks air supply. In valves, this sludge can freeze in winter, preventing switching.
  

To prevent these issues, the dew point of compressed air must always be lower than the coldest temperature the milking parlour (or environment) will reach.

Drying Methods

There are three main types of compressed air dryers:

Refrigeration dryer (refrigerated air dryer)

• Works like a refrigerator, cooling the air so it can no longer hold as much vapour.
• Typically cools to 3–7°C. Below this temperature, condensate will still form.
• Energy use is lower than for adsorption or membrane dryers.
• Not suitable for agricultural environments: ammonia in ambient air attacks silver solder joints, leading to failure.
• Unsuitable for systems that regularly operate below 3–7°C.
  

Adsorption dryer

• Uses desiccant granules to capture water vapour. Together these beads have a massive surface area.
• One column dries air while the other is regenerated with purge air (~12% consumption).
• Robust and reliable construction, resistant to pressure surges and corrosion.
• Provides dew points down to –40°C.
• Desiccant is ammonia-resistant, making it highly suitable for agricultural use.
• Maintenance is simple: desiccant replacement costs only a few tens of euros.
  

Membrane dryer

• Uses hollow fibre membranes to separate vapour from compressed air.
• Consumes extra purge air (~12%).
• Achieves pressure dew points of –20 to –25°C, but only if preceded by a refrigeration dryer.
• Very sensitive to oil and pressure surges.
• Membranes are not ammonia-resistant and will eventually rupture.
• Entire unit must be replaced (membranes are not spare parts) → costly in the long run.
• Therefore, generally not recommended in agricultural environments.

Dew Point Measurement

We recommend fitting every dryer with a test connection for quick and easy dew point measurement.