The adsorption dryer using heated regeneration (Fig. 5.4.1.1) with built-in heating consists of two (or more) vessels filled with drying medium. Depending on the size of the adsorption dryer and the design, two or more heaters are used per adsorber.
Figure 5.4.1.1
The most practical solution for arranging the heaters inside the adsorber consists of distributing star shaped heating elements in one or several central part circles. This achieves a relatively even distribution of heat inside the bed of drying medium. Because of thermal expansion, the heating elements must have upward play so that heating element and drying medium do not suffer damage. Fitting the integrated heating system into the bed of adsorbent diminishes the usable cross-section of the adsorber vessel and the volume of drying medium which can be installed. The correct way of compensating for this is by means of a larger diameter but not through a longer adsorber.
For construction and design reasons, the flow velocity through internally heat regenerated adsorption dryers with identical adsorber diameters would be slightly higher compared to other systems. As with heatless adsorption dryers, adsorption dryers with internal heat regeneration (Fig. 5.4.2.1) likewise use flat sieves (item 2) and cylindrical wire meshes (item 3) at the inlet and outlet fitting. This prevents drying medium from being carried over into the downstream compressed air piping during operation, even at high flow velocities. The adsorber vessels are interconnected by piping at the inlet and outlet. The valves for switching over from adsorption to regeneration are integrated into this piping. A by-pass with purge oriifice (item 5) is fitted at the outlet of the adsorption dryer with heat regeneration in parallel to the interconnecting piping. A fraction of the already dried compressed air is branched-off via this by-pass for regeneration.
Adsorption takes place at operating pressure, regeneration at atmospheric pressure with simultaneous input of heat through the heating elements. The timing of regeneration and the subsequent building up of pressure is controlled through the exhaust valves (item 6) to the outlet to which silencers (item 7) are fitted. Adsorption dryers with heat regeneration and internal heating are designed for a pressure dewpoint of -25°C at a dwell time of about 4.5 seconds. Pressure dewpoints deviating from this call for a correction of the dwell time, heating capacity as well as changed regeneration and purge air quantities, depending on the overall heat requirement. Low pressure dewpoints down to -70°C, with powerful heat capacities and correspondingly large quantities of regeneration and purge air, form the economic limit for this system. The drying media most frequently used in heat regeneration adsorption dryers with heat regeneration are based on a silica gel mixture from water resisting material on the inlet side, and high performance drying material on the outlet side or, alternatively on a complete molecular sieve filling. The silica gel mixture is used for inlet temperatures up to 45°C and pressure dewpoints down to -45°C. Molecular sieve is always utilised for higher inlet temperatures up to 55°C and at low pressure dewpoints down to -70°C. The electrical control system of heat regenerated adsorption dryers incorporates a time sequence of the individual functions. Thermostats (item 9) limit the heating phase during regeneration, monitoring instruments signal malfunctions. The contactor controls which used to be commonly applied to thisdryer system have, in the course of time, been replaced by programmable logic controllers in combination with dewpoint measuring instruments.
