The layout of adsorption dryers, using the principle of heatless regeneration, is clearly structured. For continuous operation, the adsorption dryer based on heatless regeneration (Fig. 5.3.1.1) consists of two vessels filled with drying medium.
Figure 5.3.1.1
Flat sieve bases (item 2) are used at the inlet side of the adsorber, whereas cylindrical wire mesh (item 3) is fitted at the outlet side in order to retain the drying medium in the adsorbers. Both adsorbers are interconnected at the inlet and outlet by piping. In order to switch over from adsorption to desorption, i.e. from adsorber A to adsorber B, interconnecting piping is fitted with valves.
Figure 5.3.1.2
Dryers for smaller outputs (Fig. 5.3.1.1) have direct controlled 2/2-way directional control solenoid valves (item 1 and item 6) on the inlet side. Main valves switch the dryer to adsorption or desorption respectively, via exhaust valves and silencers (item 7), leading to desorption and the pressure build-up. Adsorption dryers in the higher performance ranges (Fig. 5.3.1.2), on the other hand, are fitted on the inlet side with a 4/2-way pneumatically piloted directional control valve as main control element (item 1). A 2/2-way pneumatically piloted directional control valve is fitted after the main valve as exhaust valve. At the outlet side of the adsorption dryer, the flow of dried air is fed into the compressed air piping via non-return valves (item 4). In parallel to the piping at the outlet of the dryer, a purge flow of dried compressed air is directed via a by-pass line with perforated screen (item 5) for desorption. The diameter of the perforated screen is determined by the quantity of air for desorption and the pressure difference at the perforated screen. Adsorption takes place at operating or line pressure, desorption at atmospheric pressure. Heatless regenerated adsorption dryers achieve operating pressure dewpoints of -25°C given a dwell time of about 4 seconds. Different dewpoints call for a corresponding dwell times. For example, for a pressure dewpoint of -70°C, a dwell time of about 7.5 seconds is necessary. Differing pressure dewpoints, moreover, also require different quantities of regenerating air. The heatless regenerated adsorption dryer is filled with activated alumina drying medium for normal operating conditions. Molecular sieves are utilised for even lower pressure dewpoints. Heatless regenerated adsorption dryers need few valves. These valves are controlled directly or indirectly with reference to time. For this reason, the control of cold regenerated adsorption dryers calls for only a few control functions per cycle. These control systems are thus arranged to contain few complications. Using an appropriate time relay or switch system, the changeover from adsorption to desorption is brought about. Fully automatic operation makes demands on the control system, calling, for instance, for a programmable logic controller in combination with a dewpoint measuring gauge.
