The installation of a duty/standby plant (Fig. 10.4.3.1) consists of two compressors and two adsorption dryers and offers optimum assurance of the compressed air supply. Such an installation can be utilised in different ways: Depending on demand, compressor and adsorption dryers are switched on as required. The duty machines (items 1 - 5) take over the entire compressed air provision, The standby machines (items 1.1 - 5.1) are available as reserve. Receiver siting should be considered as in the example in section 10.4.1 and is, therefore, not detailed here. However, when considering this installation scheme, an automatic logic linkage between compressors and dryers provides a more effective solution. Both compressors (item 1.0 and item 1.1) and dryers (items 4 and 4.1) are connected in series and cross-over via one air receiver (item 2), but with interchangeably for automatic operation so that, should one unit fail, whether this be a compressor or a dryer, the stand-by unit takes over the supply of the compressed air.
Figure 10.4.3.1
So that the system automatically fulfils this task, both adsorption dryers must be equipped with a Dewpoint Dependent control system. Compressor 1 takes on the base load, compressor 1.1 is stand-by operation via a change-over switch,. The adsorption dryers are linked electrically via a programmable logic controller, which then signals the automatic operation. During operation, the pressure dewpoint is measured at the outlet of dryer 4 and the limiting value used for the load governed fully automatic control of the dryer. As long as the dewpoint is lower than the set limiting value, the valve behind the downstream filter (item 5) remains open so that there is free flow towards the points of use. However, as soon as the dewpoint falls off and reaches the set switching point, the valve of the first line is closed and dryer 4.1 activated at the same time. The latter then takes over the drying of the compressed air. It is necessary for automatic continuous operation that dryer 4 be regenerated. To achieve this, this adsorption dryer is regenerated at a predetermined interval until the pressure dewpoint is reached. Once the pre-set pressure dewpoint is reached at dryer 4, this line switches over to standby and becomes available again for operation if required. Regeneration takes place in parallel to the operation of dryer 4.1. Of course, other solutions may be selected as well. However, alternatives either call for more electro-technical resources or are limited in their degree of automatic response. Altering the control system to other operating conditions presents no problem. The increasing demand for compressed air for production, is covered by the parallel operation of both lines with automatic adaptation to a load in the range of 0 - 100%.
