5.4.4. Diffusion by membrane drying


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Fig. 5.9: The principle of a membrane dryer
The principle of the membrane dryer is based on the fact that water penetrates a specially coated hollow fibre 20 000 times faster than air.

The membrane dryer consists of a bundle of thousands of coated hollow fibre membranes. These hollow fibres are made of a solid, temperature and pressure-resistant plastic. Their inside surface is coated with an ultra-thin (less than the length of a light wave) coating of a second plastic. The hollow fibres ( membranes ) are installed in a pipe where the inner channel of the fibres is open at the end.
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<BR>Operating principle

The moist compressed air flows through the inside of the hollow fibres (internal flow). The moisture contained in the air penetrates through the layer of coating on the hollow fibres towards the outside. To do this a concentration gradient of moisture is required between the inside and outside of the hollow fibres.

A quantity of air for flushing is taken from the main volume flow of the compressors and relaxed (decompressed). Since the maximum air humidity depends on volume, the relative air humidity drops. The flushing air becomes very dry. The flushing air flows around the hollow fibres and provides the necessary concentration gradient of moisture. The flushing air can escape unfiltered into the open.

Features
    • Low level of particles in the air.
      A filter must always be connected upstream of the membrane dryer, in order to filter out particles up to a size of 0.01 µm. If installed directly downstream of the compressor, the filter should be connected to dust separator.
    • Low pressure loss in the dryer.
      The pressure loss Dp from the dryer is max. 0.2 bar.
    • Compact construction.
      The dryer can be installed as a component of the pipeline.
    • No servicing.
      There are no moving parts in the dryer.
    • No precipitation of condensate during drying
    • No additional energy costs.
    • Silent.
    • No fluorocarbons.
    • No moving parts.
    • No motor.