The adsorption dryer of closed loop design forms a complex system in conjunction with the compressor. In this, the adsorber vessels are subjected to flow from inlet to outlet of the adsorption dryer, one after the other and also in parallel, while the functions called for by the process are fulfiled in sequence :
Desorption/adsorption
Cooling/adsorption
Adsorption
In order to give a clear explanation of the individual functions of the adsorption dryer of closed loop design, it is necessary to describe dryer and compressor as forming one unit. The principle of full flow regeneration forms the basis of this description. The principle of full flow regeneration ensures a short desorption period and problems of part-flow regeneration3 under conditions of part load are thus avoided in principle. Working on the basis of full flow regeneration causes :
Adsorption with full flow
Desorption with part-flow
Cooling with part-flow
Adsorption with full flow
Desorption with full flow
Cooling with part-flow
Figs. 5.7.2.1 to 5.7.2.3 illustrate the explanation and also help the understanding for adsorption and desorption as well as cooling in closed loop adsorption dryer systems. Pipe lines link the oil free compressor and the adsorption dryer to form one unit. After the last compression stage and before the principal cooler of the compressor, it is necessary to fit a 3/2-way valve (item C). This provides two alternative paths from compressor to dryer. Firstly the hot air connection for desorption and secondly the cold air connection via the principal cooler of the compressor, with the cyclone separator (item A) for adsorption and cooling of the compressor fitted downstream. 3artificial pressure gradient for regeneration gas mixture
Figure 5.7.2.1
The hot and cold air connections are each ducted to a 3/2-way valve and linked to each other via a hinged shaft complete with pneumatic rotary drive, so that only one connection is open at any one time. This provides simple and reliable switching of the dryer. Fig. 5.7.2.1 shows an opened hot air inlet and a cold air inlet, closed, in the opposite direction. Compressed air heated by compression flows via the valves (item 5 and item 4) from top to bottom under operating pressure and uncooled in the direction of gravity through the adsorber already saturated with moisture, thus warming up the latter. If the heat of compression of the air fails to reach the required regeneration temperature, the required level can be achieved by means of an additional heater. When dimensioning the adsorber, the change of volume through regeneration temperature has to be taken into account. The humidity stored in the adsorber separates from the drying medium and is re-entrained by the quantity of hot unsaturated air.
Diagram 5.7.2.1
At the beginning of desorption, when hot regeneration air with a dewpoint of about 60°C impinges upon the still cold drying medium, condensation of the water vapour in the drying medium bed takes place. It must be possible to regenerate the drying medium adequately by means of regeneration air at temperatures of around 140 - 160°C and with dewpoints around 60 - 70°C. Via valves (item 1 and item 6) of the lower interconnecting piping, the moisture loaded hot compressed air is ducted to the water cooled regeneration cooler (item 7) with the separator (item 8) fitted to its outlet. The hot compressed air is cooled down to 30 - 35°C within the regeneration cooler and the moisture contained in the compressed air is condensed. The condensed humidity is separated from the compressed air inside the cyclone separator (item 8) and drained off. At the outlet of the water cooled after cooler, the compressed air is always 100% saturated with respect to a temperature which lies about 10°C above that of the cooling water. Given a cooling water temperature of around 20 - 25°C, the outlet temperature will then amount to around 30 - 35°C. Cooled but moisture saturated compressed air is now ducted from bottom to top through the valve (item 1) to the second adsorber. Here the compressed air is dried to the dewpoint governed by the regeneration temperature. The dried air reaches the compressed air network via another valve (item 4).
Figure 5.7.2.2
When the regeneration temperature is reached, the hot air connection (item 5) is closed and the cold air connection (item 6) opened simultaneously. The signal for the switch over is emitted by the temperature switch (TS) and is also directed at the same time to the 3/2-way valve (item C) between compressor and main cooler. Valve setting is made clear by Fig. 5.7.2.2. The full quantity of compressed air now flows via the main cooler of the compressor and 100% saturated, while at a temperature of about 30 - 35°C, through the lower 3/2-way valve (item 6) and the 4/2-way valve (item 1) into the previously heated adsorber. The cooled compressed air takes up the heat contained in the heated adsorber and ducts this heat, in the same direction of flow, towards adsorption via the upper 4/2-way valve (item 4) and subsequent 3/2-way valve (item 5) to the installation’s regeneration cooler (item 7). In the course of the decrease in temperature within this cooler, the re-entrained humidity is separated from the compressed air and discharged via the drain. The dehydrated compressed air reaches the adsorber for adsorption via the lower valve combination (item 1 and item 6), in order to pass out of the installation fully dried via the upper 4/2-way valve (item 4), as already described.
Diagram 5.7.2.2
Contrary to the desorption time period, cooling time is a fixed quantity. The actuation of the lower 4/2-way valve (item 1) is signalled and switched in line with the calculated time period. Valve setting is shown in Fig. 5.7.2.3. The cold air arriving from the compressor is ducted direct into the adsorber for adsorption via the lower valve combination. This valve position is maintained right up to the end of the drying period.
Figure 5.7.2.3
After adsorption, the other 4/2-way valve (item 4) is rotated by 90° at the same time as the coupled 3/2-way valve (item 5 and item 6) and the entire sequence now starts with the sides reversed taking the process steps previously described.
