With only one compressor and dryer, uninterrupted long term compressed air supply is improbable. This risk is reduced through the installation of a second compressor. Fig. 10.4.2.1 shows a typical installation with two compressors (item 1.0 and item 1.1) which are used in parallel. Compressor 1.0 is available as the base load, compressor 1.1 for covering peak loads. Installing a duty/standby compressor, receiver or dryer will result in the dryer being subjected to a varied load of 0-50-100%. In addition to the specified compressor performance of 0 - 50 - 100%, the receiver volume multiplied by pressure difference can, under certain circumstances, throw an additional load onto the adsorption dryer. This must be clearly identified and analysed from the outset in order to exclude undesired operating circumstances.
Figure 10.4.2.1
As adsorption dryers are designed for maximum moisture loads, deviating loads are, part loads, and thus smaller than 100%. A Dewpoint Dependent control system compensates for these part loads. To adapt the regeneration output of the adsorption dryer to the actual compressed air requirement calls for a continuous assessment of the load situation by means of the Dewpoint Dependent control system. A dewpoint meter adjustable with dewpoint sensor at the outlet of the adsorption dryer permanently registers the true pressure dewpoint of the compressed air. The pre-set dewpoint is used as a signal for switching over the dryer from adsorption to regeneration. The loading time is inversely proportional to the part load. Working on the assumption that every part load operation is immediately followed by full load working, the regeneration period is set at a constant value corresponding to full load operation. Thus the regeneration air requirement will be inversely proportional to the load situation, in contrast to the regeneration air quantity which remains constant. The ratio of regeneration time to loading time determines the saving in regeneration energy.
When a Dewpoint Dependent control system is used, the adsorption dryer must be switched on throughout the entire operating time. Shutting off the compressed air from the system through the adsorption dryer is thus made impossible and continued operation guaranteed. With adsorption dryers in the smaller ranges, the economic viability of employing load depending control systems has to be carefully scrutinised. For such cases, there is an alternative to a Dewpoint Dependent control system in the form of a direct linkage dryer/compressor, the principle has been described in section 10.4.1. When setting up such an interdependence, the following must, be considered: The adsorption dryer must be linked to the base load compressor. With standby or operation of both compressors sufficient control is achieved. However, 50 % load also means a 50% waste of regeneration energy as far as the adsorption dryer is concerned. The compressors alternate as the base load. In this case, an OR-linkage for the dryer control must be installed. Also, when one compressor is running, 50 % of the regeneration energy of the adsorption dryer must be regarded as an unnecessary waste.
