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Desiccant dryers (also knows as adsorption dryers) work on the principle of moisture always migrating to the driest medium possible. Therefore, water vapour is removed from compressed air by passing it over an adsorbent desiccant material. As the air contacts the adsorbent material, water vapour transfers from the wet air to the dry desiccant, however, adsorbent materials have a fixed adsorption capacity and once this capacity is reached, they must be regenerated or replaced. Therefore, to provide a continuous supply of clean, dry compressed air, adsorbent dryers utilise two chambers of desiccant material and at any one time, whilst one chamber is on-line, drying the incoming compressed air, the other is either off-line, being regenerated or is re-pressurised, ready to come on-line. All heatless desiccant dryers remove water in this manner.

Pressure Dew Point

The water content in compressed air is measured in PDP, or Pressure Dew Point. This is the temperature at which water vapour (a gas) condenses into water (liquid state) at the current working pressure.

Air (or gas) compression increases dew point, and gas expansion (or de-compression) lowers the dew point. Compressor installations with no air dryers after the compressor would usually produce compressed air that is saturated at ambient temperature. So if the compressed air is serving a process where having water present doesn't matter, for example an air compressor powering a pneumatic tool, there is no need for an air dryer.

In a compressed air network, the compressed air coming directly out of the compressor will be hot & so it can hold more moisture. Once it starts to cool down as it moves through the pipes or it goes into the air tank, the water will start to condense out. If this air is serving a process in a factory the ambient temperature causes water to form inside the pipes or air tank as it cools and this condensate could cause production line issues or contaminate the air if it comes into contact with the process it serves. For example, when paint spraying dry air is needed so the water doesn't contaminate the paint or in food packaging & processing, completely pure air ISO class 0 may be required, so we need to remove the water with the air dryer.

water content removal by dryer type

Why select a desiccant dryer over a refrigerant one ?

This graphic at the top of the page shows the different dryer technology types and how much water extraction is possible from the compressed air, for each type. It shows the PDP or Pressure Dew Point - the temperature at which water vapour will condense out of the air, as you can see for refrigerants it's higher than for desiccants. In summary, desiccant dryers can extract more water per m3 than refrigerants, but will use more energy in the process, fridge (refrigerant) dryers also require less maintenance in comparison to desiccant ones and the PDP can be used to help define the water content in the compressed air. GD's patented sub-freezing technology can remove more water than a standard refrigerant dryer and use less energy than a desiccant dryer. You can also see the different air quality classes achievable, up to class 0.

The size of the dryer you need is also dependant on the air flow rate and the air quality class you need depends on the process the compressed air is serving, some processes require purer air than others. Please contact us directly to understand which option is best for your process.

The installation image below, from left to right shows the air compressor, air tank (or receiver) to store the compressed air, then 2 air filters which remove solid particles and impurities from the compressed air, then an adsorption (or desiccant) dryer, with the twin towers to remove the water content from the compressed air, finally on the right hand side is the condensate drain which stores the oil/water mix which has been extracted from the compressed air. These drains can separate the oil water mix to be disposed of in the correct way.

air compressor installation