A rare application is one where compressed air is further dried after already having been dried to a low pressure dewpoint. This occurs when, downstream of an adsorption dryer, a proportion of the total compressed air must have an even lower pressure dewpoint Compressed air with an inlet temperature of about ti = 35°C into the adsorption dryer (item 4) has a humidity level of hi = 39.9 g/m3. At the outlet from the dryer, a pressure dewpoint of Pdp = -25°C is achieved. At this pressure dewpoint, the remaining humidity level is to about ho = 0.55 g/m3. The moisture content at the inlet to the adsorption dryer (item 4.1) in the branched-off air flow is less than 1%. Such a low relative humidity at the inlet of the adsorber calls for a completely new way of reviewing at the design criteria of the dryer. Having selected a suitable drying medium and looking at dimensional criteria, this type of application always calls for a check whether flow takes place in an intermittent turbulent state and the dwell time in the adsorber is sufficient.
Figure 10.4.10.1
These considerations may lead to an adsorber which is larger than in a standard installation. Dwell time depends on the pressure drop of inlet to outlet of the air dryer. However, in extreme cases the intermittent flow in the turbulent flow, adequate dwell time, or high pressure drop, cannot be resolved. Under these circumstances, reliable after-drying is questionable. Regeneration has be to looked at from the same points of view. Logically, the quantity of regeneration air must be higher. For these part flow dryers, time and load depending control systems are used. These must be set with a safety margin and also fitted with a time cut-off.
