Once an adsorber is fully charged with moisture, it must be regenerated. With the regeneration processes discussed previously, the quantity of regeneration air is branched off from the current of dried compressed air as a partial current and thus no longer available for production purposes. In contrast, regeneration by means of external blower air uses only small quantities of compressed air for purging. The schematic representation in Fig. 5.5.3.1 shows the most important elements of blower regeneration and the simple function of adsorption dryer systems using external heat regeneration.
Figure 5.5.3.1
Taking up the same period of time as adsorption, regeneration takes place in parallel. Via a blower (item 8) with inlet noise attenuator (item 7), ambient air is drawn in and heated (item 9) to the temperature of regeneration. Electrical energy, steam, hot water or also heated oil can be used as sources of heat. The ambient air which has been drawn in by the blower and subsequently heated, is ducted to the vessel to be regenerated via the upper switching valve. In countercurrent to adsorption, the heated regeneration air flows through the adsorber, thus heating the drying medium. Regeneration by countercurrent ensures that the moisture to be extracted is not conveyed through the entire bed of drying medium. The humidity stored in the drying medium evaporates and parts company from the drying medium.
Diagram 5.5.3.1
The heated blower air, humidified by the desorbed moisture, now leaves the adsorber via the lower valve (item 1) and the downstream exhaust valve (item 6). At the end of the heating phase, monitored by a thermostat (TS), the cooling phase begins. The heating is switched off and unheated cool ambient air is ducted through the system via the same path. Drying medium and adsorber are thus cooled down to a low operating temperature. The cooling phase is terminated after an accurately specified period. This limitation of cooling is necessary in order to avoid a dewpoint peak when switching over from regeneration to adsorption. As the ambient air, required for regeneration, has a certain water content, it is unavoidable, with this principle of regeneration, that a slight pre-loading with moisture takes place in the upper layer of drying medium when cooling with humid ambient air. This pre-loading causes a dewpoint peak because compressed air dried through adsorption impinges on this very moist zone, reentraining the moisture and conveying it into the compressed air network.
Diagram 5.5.3.2
In order to reduce this dewpoint peak, the vessel is purged with a fraction of already dried compressed air from the system via the purging air line (item 11) during a limited time after the cooling phase. Purge air quantity and purging time result from the heat requirement. As a guidance value, about 5-12 % purge air quantity for a period of 1 hour. Seen from the point of view of the overall cycle, this loss is on average smaller than or equal to 2%. The exhaust valve (item 6) closes and the pressure build-up phase follows. Blower and heater are protected from pressure bursts through a non-return valve (item 10). The drying installation remains ready for immediate use right up to the switch-over. After the switch-over, the exhaust valve (item 5) is opened and the regenerated vessel depressurised to atmospheric pressure via a silencer (item 4) fitted at the outlet. Following this, the exhaust valve (item 6) opens and the regeneration process starts again for the adsorber previously loaded. In the course of regeneration or activation, the adsorbent material is exposed to considerably higher mechanical and physical stress than during adsorption. Economically optimum operation is achieved with externally regenerated adsorption dryers, when correct setting leads to the following conditions of regeneration :
Diagram 5.5.3.3
Maximum exploitation of the loading capacity of the adsorbent material with humidity through dewpoint monitoring. The energy consumption for individual regenerations is lowered as the frequency of regeneration is reduced and pore blockage can be avoided. Regenerating countercurrent to the direction of adsorption. The water to be desorbed from the charged inlet layer is not carried right through the bed. The water fronts run out, in the opposite direction to the regeneration gas, downward, by gravity. This causes this water front to impinge onto charged silica gel only. Regeneration velocity of at least 0.08 m/s. With this linear speed, an even distribution of air can be achieved even inside large adsorbent beds, thus largely eliminating undesired recondensation within the adsorbent bed. Cooling the adsorption material after completed desorption, in order to avoid a temperature shock with correspondingly higher moisture content in the compressed air which is given off at the beginning of adsorption.
