Should the receiver be installed upstream or downstream from the dryer? While this question appears ambiguous both forms of installation are potentially correct, however, there must be a full evaluation of the installation requirements if there is any likelihood of high short term demands, installation of both upstream and downstream receivers should be considered. Without considering marginal cases or abstract theories, the installation of receiver/dryer is operationally most practical. The precondition is that the dryer must be sized to the maximum compressed air consumption. Installing (Fig. 10.4.1.1) an adsorption dryer (item 4) downstream from the receiver (item 2) is expedient if an even, continuous loading through the compressed air is assured. The dryer is then system sized to this even compressed air consumption. A positive aspect of this arrangement is the additional cooling of the compressed air by the large receiver surface. Lowering the temperature accompanied by simultaneously reduced inlet humidity means theoretically that the dryer system fitted downstream can be reduced in size, at the same time achieving a saving in regeneration energy. With such an installation, sudden demand for compressed air has to be avoided in order to prevent an overload of the adsorption dryer. One also comes across plants with an intermittent compressed air consumption pattern. Sometimes the compressed air demand equals zero, i.e. there is no compressed air requirement. The adsorption dryer without dewpoint dependent switching with a rigid time cycle control will, however, permanently consume regeneration air during this low demand period.
Figure 10.4.1.1
There is a simple method of counteracting energy losses of this kind. The poor compatibility between zero load operation of the compressor and full load regeneration of the adsorption dryer is resolved by interlinking of the control system to the operational pattern. One simple solution for an installation with adsorption dryers with heatless regeneration consists of linking compressor (item 1) and adsorption dryer electrically so that simultaneous running of both components is maintained. The electrical supply of the dryer is linked to the pressure monitor of the compressor as signal emitter. Both compressor and adsorption dryer are simultaneously switched on or off, depending on the demand. For this, the pressure monitor must be arranged in such a way that the pressure in the piping system is detected. For an adsorption dryer with normally open inlet valves (see Fig. 5.3.3.1), the pressure monitor can be fitted to the compressor or to the receiver. For an adsorption dryer with normally closed inlet valves (see Fig. 5.3.1.1), the pressure monitor must be fitted downstream from the adsorption dryer, as otherwise, compressed air generation would be separated from the compressed air network through the closed dryer inlet valves in the power off condition. However, all this linkage achieves is an adaptation of the regeneration air consumption in relation to compressor running periods. Adaptation to fluctuations in pressure or temperature cannot be achieved through this control method. For small and medium installations, the above solution is practicable and expedient, as every operator of a compressor installation is capable of establishing this linkage without difficulty. For adsorption dryers in the high performance range, a Dewpoint Dependent Switching system (see section 5.3.6) should, ideally, be incorporated in the original project plan, with a view to achieving an economic balance between regeneration energy and a wide range of load situations.
