We are often asked whether an adsorber should be used with or without valves. This decision depends on the plant and the application. It is important whether a long maintenance interval or a favorable purchase price have priority. Long maintenance intervals require the desiccant to be protected by valves. We explain why.

Valves are of particular interest for an aeration dryer if the air flow through it is infrequent during its service life. Then moisture diffuses through the holes in the adsorber bottom into the bed. This results in loading and a color change.

In a test setup we have tested 3 identical adsorbers with different connection holes.

  • Holes (standard for cheap solutions)
  • Holes with a hydrophobic membrane
  • Valves with an opening pressure of 6mbar




The mass of the aeration dryers is measured after defined time intervals. The ambient air of the aeration dryers is kept at a constant high relative humidity during the test with an air humidifier. The following figure shows the loading curve of the three different aeration dryers. The test period is 50 days.

The effects of the hydrophobic membrane and the valves are clearly visible. The adsorbed water volume of the aeration dryer with holes is 10% of the dry mass at the end of the test. The hydrophobic membrane is responsible for a loading of 3.6 % after 50 test days. At 0.6 %, the aeration dryer with valves has the lowest loading after 50 test days.

As can be seen, the aeration dryer with the simple holes, has the largest slope. Its loading increases on average 0.2% per day. The slope of the aeration dryer with valves is lower by a factor of 22.

The color of the aeration dryer with holes has become significantly darker. This discoloration is evenly spread over the whole height of the adsorbent bed. There is no formation of a typical color profile. The adsorbed water molecules are evenly distributed over the entire height of the bed. In the aeration dryers with membrane and valves, no change in color can be seen after 50 days of testing. The use of a hydrophobic membrane and valves has a great impact on aeration dryers during plant shutdowns. They reduce the amount of water diffusing through openings into the adsorbent. Valves are most effective in preventing loading by ambient air. The use of such valves significantly extends the service life of aeration dryers, which are rarely in use.


Adsorber with holes

Adsorber with membrane

Adsorber with valves