Purified compressed air is not a low cost item. It cannot be tolerated that unused compressed air is blown into the atmosphere upon regeneration. Adsorption dryers are designed for maximum full load operation. Deviating operating conditions thus, in principle, mean underutilisation of the adsorption dryer. If this is not corrected, it will always consume desorption energy as though it was running on full load. Adapting the adsorption dryer with heatless regeneration to differing operating situations therefore makes sense from the point of view of good housekeeping and energy efficiency.
Load dependent control systems lead to a variable cycle whenever the demand for compressed air fluctuates, inlet temperature with corresponding humidity load varies strongly between summer and winter operation but also if the takeoff depending pressure range is scattered over a wide band of levels. The differing moisture load at the inlet to the adsorption dryer, caused by volume, pressure or temperature fluctuations, leads to a change in the pressure dewpoint at the outlet of the dryer in the course of time. Depending on what level of residual humidity is acceptable, the pressure dewpoint of the compressed air is specified as a limiting value which forms the basis for regulation in face of a variable load situation.
Electronic systems in conjunction with a humidity measuring instrument (see part 9) are capable of detecting changes in operating conditions, evaluating these and passing on the result as reliable signals. Every part load of the adsorption dryer is thus consistently converted into prolonging the adsorption period while keeping the desorption time constant. When part load running, the adsorption period is extended proportionately in line with the moisture load and converted into a full load situation. The desorption time is not variably adapted with a correspondingly reduced desorption air quantity, as a part load can, at any time, be followed by full load and the required desorption must have been completed at this point in time. The saving in desorption energy results from the difference between variable adsorption time and constant desorption time.
Compensating for overloading the adsorption dryer by means of load depending control systems is impossible in principle. When using a load dependent control system, one special feature has to be observed. Assuming a 70% loading of the dryer with time dependent control and at full compressor output, the quantity of air for desorption is set for just this 70% of dryer capacity. On the other hand, when using the load dependent control system, this will be 100% because the dryer is utilised to 100% capacity per cycle via the load dependent control system.
