Installation of adsorber/receiver is the second variant. With the equipment installed in this order, adsorption dryer size solely matches the compressor output so that an accurately specified load situation can be allocated to the dryer. With this installation overloading, the adsorption dryer through sudden high compressed air demand from the compressed air receiver is not possible, provided the design is correct. Piston compressors compress the air in a pulsating manner and may have a more significant effect on the adsorption dryer. These pulsations do not exert a damaging influence on the adsorption dryer if the volume between compressor and dryer corresponds to 50 times the final stage of the compressor, or if pulsations are removed by pulsation dampers between compressor and dryer. To install the adsorption dryer upstream of the receiver is possible but irregular, as varying demand for compressed air is to be expected. While irregular compressed air demand is not always totally evened out by a large compressed air receiver, it is balanced between suitable limits. Arguments presented for this type of installation is the reduced corrosion of the receiver, as the compressed air enters the system in a dry state once it leaves the dryer, no further moisture is precipitated, so elaborate condensate traps are not necessary. Varying load situations are possible for the operation of the adsorption dryer, i.e. continuous operation as well as partial loading, caused either by pressure and/or temperature fluctuations at the inlet of the dryer. In the unfavourable situation where, no compressed air is drawn off but there is a steady consumption of regeneration air.
Figure 10.4.4.1
As shown in section 10.4.1, the aim should be to achieve optimum performance from no load operation to full load regeneration between the adsorption dryer and the compressor. To this end, heatless adsorption dryers are electrically linked to the pressure monitor of the compressor system, thus forming a matched running unit. However, with this adaptation, the dryer running time is dependent on the compressor running time. Other criteria, such as temperature variations and/or pressure fluctuations, cannot be catered for by this linkage. This solution is expedient for small to medium installations and can often be arranged on site. With this linkage, the following is worth considering as a possible variant: The compressor pressure monitor emits the unload signal for compressor/dryer when the upper switching point is reached. The purge valves of the adsorption dryer (see Fig. 10.4.4.2) close when this status is reached, to block the escape of regeneration air into the atmosphere. The main valves remain open allowing the pressure build up at the receiver to feedback to the compressor pressure monitor. Pressure equalisation inside the adsorber will take place by the regeneration air repressurising the empty vessel. After pressure equalisation inside the adsorption dryer, the pressure may be lower than the minimum operating set at the compressor sensor non-return valves. The non-return valves in the outlet of the adsorption dryer prevents full pressure equalisation. In the worst case, the system is restarted.
Figure 10.4.4.2
The pressure level after pressure equalisation, depends on the volumes of both adsorbers, this forms a criterion for checking and, if necessary, correcting the pressure difference set at the pressure monitor. The pressure drop inside the adsorption dryer is compensated for by an additional compressed air return feed, Fig. 10.4.4.2. This return feed by-passes the non-return valves of the adsorption dryer. The direction of flow is controlled by an non-return valve (item 8). A leak proof valve (item 9) prevents dewpoint degradation in the course of normal operation. Alternatively, the pressure monitor is not fitted to the compressor but to the receiver. In this case, the pressure drop inside the adsorber can be ignored (excepting that it is not too high to cause the safety valve to blow off). With this arrangement, direct coupling of compressor and adsorption dryer cause no difficulty, and the return feed becomes unnecessary. However, for large heatless adsorption dryers (see Fig. 5.2.6) the adaptation to varying operating conditions can be achieved only by a Dewpoint Dependent control system. Using a governed control system with pre-set automatic regeneration, for long standby periods, one has to supply the regeneration air requirement of the adsorption dryer via a return feed line. Regeneration by means of dry air from the receiver, provided this has a sufficiently large volume, can be assured without compressor restart.
