Externally heat regenerated adsorption dryers operating according to the vacuum principle are laid out in the opposite manner to those with blower regeneration, from the process technological point of view. Fig. 5.6.1.1 shows the layout principle, with two adsorbers including interconnecting piping at the inlet and outlet, as well as valves for switching over between adsorption and regeneration. Furthermore, there is the regeneration system, consisting of vacuum pump and heater. When comparing this with the external layout of adsorption dryers with pressure generating blowers, two essential differences can be seen: The external heater (item 9) on the regeneration air suction side in the atmospheric pressure zone, The vacuum pump (item 8) linked to the air outlet side by means of a flexible heat resisting tube. With these adsorption dryers, drying takes place at operating pressure in the advantageous direction of flow from bottom to top.
Regeneration, however, flows in this case in the same direction using drawn-in ambient air heated in the vacuum zone. The vacuum regenerated adsorption dryer is designed for pressure dewpoints down to -25°C. For this type of dryer, too, the rule concerning dwell times in order to reach the pressure dewpoints reliably, applies. Vacuum regeneration is capable of reaching pressure dewpoints down to about -70°C in continuous operation. Quantities of purge air from the system are not needed for this process. This is the materially important and ultimately also decisive advantage of vacuum regeneration. With vacuum regenerated adsorption dryers, the heat resisting drying medium silica gel forms the uniform filling material right through. This water resisting drying material is suitable for inlet temperatures up to maximum 45°C and pressure dew points down to approximately -55°C. Molecular sieves, on the other hand, are utilised when the very lowest pressure dewpoints have to be achieved.
Figure 5.6.1.1
Programmable logic controllers are used in line with the present-day state of the art. Such a controller, in combination with a dewpoint measuring instrument, brings about optimum adaptation to changing operating conditions. For reasons of energy saving, it is expedient to insulate the adsorber with mineral or slag wool, also as contact protection. Adsorption dryers of this type are constructed up to capacities of 50 000 m3/h and more. For smaller capacities, the complete system is manufactured as a compact unit.
