It is with regard to regeneration that the vacuum method pursues an entirely new path. The pump, designed for vacuum operation, sucks ambient air into the adsorber. There, the desorption process takes place. The air, enriched by moisture, is ultimately discharged via the vacuum pump.
Diagram 5.6.3.1
The suction effect causes lower than atmospheric pressure in the adsorber. The degree of vacuum depends on the pressure gradient, right through the adsorber, of the quantity of air drawn through the adsorber. The level of vacuum is also determined by the adsorber geometry. The vacuum amounts to about 0.08 - 0.1 bar. From this difference, in contrast to the pressurising blower with about 1.1 bar absolute, arise the theoretically effective advantages of vacuum regeneration through
- less humidity entering the adsorber from the surrounding air
- a lower desorption final temperature
However, in the course of desorption, the vacuum pump is located in the hot air zone. The vacuum pump must be designed to cope with this extreme temperature situation. After desorption is completed, the heating is switched off via a thermostat. Immediately afterwards, ambient air flowing in the same direction is used to cool the adsorber. Cooling is automatically terminated by the low point contact of the thermostat.
There is no need to purge with dry compressed air, as the process conditioned pre-loading of the adsorber with ambient moisture affects the wet zone only. When desorbing in the same direction of flow as when adsorbing, the drying medium is exposed to the highest temperature levels on the inlet side of the adsorber. A temperature adequate for desorption must be achieved particularly in this zone, as it is this which determines the dryness of the compressed air atthe adsorber outlet. This causes the heating period to be theoretically longer than when desorption takes place on the countercurrent principle. The moisture evaporated by the heated regeneration air current is carried right through the entire bed of drying medium. As the drying medium at the adsorber outlet is not loaded up right to full saturation during the adsorption phase, double adsorption takes place here when the humidity loaded desorption air passes through. A gain in additional heat through the vacuum pump does not take place when desorbing in the vacuum range. Through the longer heating time and through double adsorption, up to 20 - 25% additional heating energy is required as compared with desorption by countercurrent. However, this additional expenditure in heat energy is just about compensated by the system conditioned advantages of the vacuum principle, such as: At a regeneration pressure of below 1 bar absolute and with a constant quantity of regeneration air, working within the vacuum range means that the process calls for a lower regeneration temperature. Cooling is more advantageous from an energy view point through a lower temperature rise with vacuum operation. The drawn in quantity of moisture from the surrounding air is lower with vacuum operation and diminishes the amount of humidity per cycle. The drawn in humid surrounding air loads up the moist entry side of the adsorber during regeneration and not the dry layer at the outlet. During the entire desorption process, no compressed air whatsoever is required to be taken from the system. The volume flow at the outlet equals the volume flow at the inlet to the dryer.
