How a Compressed Air Desiccant Dryer Works
Innovative Compressed Air Desiccant Dryers
The term desiccant dryer can refer to a broad range of compressed air dryers which are also known as heat regenerative dryer, twin tower dryers or adsorption dryers.
The compressed air is passed through a pressure vessel with two towers or chambers in a modular dryer. These are filled with a media such as activated alumina, silica gel, molecular sieve or another desiccant material. This desiccant material removes water and moisture from the compressed air via a process called adsorption (The accumulation of molecules of a gas to form a thin film on the surface of a solid). As the water build up in the desiccant, the desiccant bed becomes saturated. The dryer is timed to switch towers/chamber based on a timed cycle, once this cycle completes some compressed air from the dry side of the dryer is used as purge air. This is flowed in the reverse direction through the saturated desiccant bed which dries to desiccant and is exhausted from the system to atmosphere via a valved outlet which is fitted with a silencer to reduce noise.
While one tower/chamber is in operation the alternative one is in regenerative mode, this is carried out by a times sequence of valves which are automated by the main dryer controller.
The job of the desiccant bed is to reduce the pressure dew point (PDP) of the compressed air flowing through it to a level in which the water will no longer condense, alternatively to remove as much water from the compressed air as possible. A standard dew point that is expected by a regenerative desiccant dryer is −40 °C (−40 °F).
This means that when the very dry air that leaves the dryer outlet has as much water in the compressed air as if it had been cooled down to −40 °C (−40 °F). The required dew point is dependent on the application and in some applications −70 °C is needed.
The regeneration of the desiccant tower/chamber can be done using one of three different methods:
1. Heatless Pressure Swing Drying
This uses part of the dry compressed air coming from the online tower/chamber to dry the desiccant in the offline tower/chamber being regenerated at lower pressure. Typically a percentage of 17-20% purge rate is required.
2. Heated Regenerative Dryer
This uses a hot air blower or internal electrical heating system to unsure that there is no loss of compressed air. This type uses less than 7% Purge Rate.
3. Heat of Compression
This is less common and can only be used with an oilfree compressor.
Inlet filtration to remove bulk water, dirt particles, oil mists and aerosols originating in the atmosphere and the lubricated compressor as well as after filtration to remove and desiccant dust carry over are recommended. If an oil free compressor is used, then only a pre and after filter are required. Further downstream filtration may also be required depending on the application.