Functionality of the adsorber

The air sucked in by the adsorber is produced by a generated negative pressure in the hydraulic tank, the storage tank, a barrel or IBC container, and a gearbox. The air must open the inlet valve, flow through the entire silica gel bed and pass through the internal 3μm or 1μm filter.

Components for pressure buildup

Three main components are responsible for the build-up of differential pressures in the adsorber, which cause the pressure to rise in relation to the air volume.


Valves

The valves protect the desiccant from unnecessary loading and extend the maintenance interval. With the disadvantage that the adsorber increases the pressure. The graph below shows that the pressure increases from 20mbar to 80mbar for a DV-5 series adsorber at 400l/min.


Silica gel

When air passes through the silica gel, the water molecules are attracted to the surface of the grains and adhere to them. This occurs due to Van der Waals forces and is reversible depending on temperature and other influencing factors.

Since silica gel is a very porous material and water absorption takes place exclusively in the pores of the grains, the outer surface remains unaffected. Thus, the grains retain their diameter, and the entire bulk retains its volume and consistency. The pressure build-up through the silica gel fill is favored by the following factors:

  • Long adsorber, long paths through the fill.
  • Small diameter, small gap volume in the bulk
  • Small particle size, denser bed and smaller gap volume in the bed

Filter element

Star filter elements with a large surface area are installed in each adsorber. This ensures that the overall pressure build-up of the adsorber is not caused by the filter. The tested pressure build-up through the standard 3μm paper filter was a low 1-3mbar up to an air flow rate of 500l/min.

Pressure curve by state

In the course of using an adsorber, the silica gel is loaded with water molecules, i.e. the free surface in the pores is filled with water.
filled. In this process, the binding energies for further absorption decrease. Due to this, the newly inflowing humidity is no longer adsorbed and flows unhindered through the adsorber. Various tests have shown that the pressure does not change under different loading conditions.
Several adsorber variants have been tested in the graph. With increasing loading of the silica gel (X-axis) the pressure remains constant (Y-axis)

Of course, attention must be paid to heavy loading by dirt particles. If used for a very long time, far beyond the color change to green, the suction of dirt can lead to an increase of the pressure due to a clogging of the filter.

 

Conclusion and recommendations for action

The pressure build-up in the adsorber is primarily generated by the valves. These are responsible for the largest proportion of the differential pressure. Furthermore, an incorrectly designed adsorber (too narrow and high) will cause additional pressure as the air flow rate increases. If the pressure is critical in a system or if there is a very high air flow rate, an adsorber without valves is recommended. In this case the pressure build-up will remain low. The loading condition is not important. The pressure remains the same over the entire time (under normal ambient conditions). However, in the case of a high dirt load, e.g. paper mills, cement factories, concrete plants, etc., care should be taken to change the filter regularly. This can otherwise clog and create a rapid, high differential pressure.