in a unit employed to service aluminum fused salt reduction cells comprising combined alumina feeder and crust breaking device the air expelled on the return stroke of the piston can be usefully employed if it is injected in the region of the portioned feeding device into the conical part of the day &# 39 ; s supply silo . this improves the flow characteristics of the alumina at the lowest part of the silo where the pressure is greatest , and does so without having to expend special energy for this purpose . in selecting the dimensions of the piston rod cross section or its out diameter , two factors have to be taken into account : the smaller the piston rod the smaller is the difference in force between the negative and positive side acting on the piston i . e . the smaller is the power supplied by the piston . the , in general , vertical or almost vertical working cylinder of crust breakers for fused salt reduction cells must also be capable , even with reduced pressure , of raising a breaker chisel which becomes stuck in the crust opening . consequently the outer diameter of the piston rod must not be too great , even though this would be desirable with respect to efficiency . it has been shown advantageous to design the piston rod with an outer diameter which amounts to 25 - 85 %, preferably 40 - 70 %, of the inner diameter of the working cylinder . as piston rods of the kind described above must in general have a relatively large outer diameter , they are usefully tubular in shape . when returned to its non - operative resting position , the crust breaker must have withdrawn the chisel out of the region of the carbon anodes in order that no damage occurs during anode changes or other cell operations . with this in mind it has been found that it is advantageous for the working cylinder to have a stroke of length 400 - 600 mm . for economic reasons ( consumption of compressed air , wear ) the compressed air circuit is operated , under normal operating conditions , at reduced pressure ; that circuit is therefore supplied with compressed air via a pressure reducing valve which reduces the pressure coming from the overall compressed air network by 35 - 75 %, preferably by 45 - 55 %. the pressure in the network generally lies in the range 6 - 8 bar . in practice these pressure reduction valves , and the other valves , are mounted on a common base plate , usefully on the cylinder head of the working cylinder . this lies out with the hot region of the cell and is easily accessible from outside . referring to the drawings , a pipe 12 , branching out from a conventional industrial network 10 for the supply of compressed air , leads to a slide valve 14 which , usefully , can be operated manually . the network 10 for compressed air supply is fed by a compressor and is stabilized by means of a compressed air reservoir . a compressed air pipe 18 leads away from slide valve 14 to the 5 / 2 channeling valve 20 . the compressed air pipe 22 running from the 5 / 2 channeling valve leads to the 3 / 2 channeling valve 24 . branching out from pipe 18 is a compressed air pipe 26 which leads to the reduction valve 28 . the air at reduced pressure is fed via pipe 30 into pipe 22 . compressed air pipe lines 26 and 30 and the pressure reduction valve 28 form therefore a by - pass round the 5 / 2 channeling valve . a compressed air pipe 32 runs from the 3 / 2 channeling valve 24 to the negative side of the working cylinder , in other words to the space 40 formed between the cylinder head 36 and the piston 38 . the working cylinder 34 has an inner diameter d 1 , the piston rod 42 which can move with a stroke h coaxially within the cylinder 34 has an outer diameter d 2 . the space 46 delimited by the inner face of the working cylinder 34 , the piston rod 42 , the piston 38 and the base 44 of the cylinder is defined as the positive side of the working cylinder . this space 46 is connected to the 5 / 2 channeling valve 20 via a compressed air pipe 48 . when the pneumatic drive system is in the non - operating position , space 40 is evacuated via pipe 32 , 3 / 2 channeling valve 24 and outlet pipe 50 , while space 46 is kept at reduced pressure . at the start of the working phase the microprocessor 16 reverses the 3 / 2 channeling valve 24 via connection a and actuating facility 68 ; the space 40 on the negative side of the working cylinder is subjected to reduced pressure . if the thrusting force of the working cylinder 34 has to be increased briefly , then the microprocessor 16 reverses the 5 / 2 channeling valve 20 via connection b and actuating facility 66 ; the pressure reducing valve 28 is closed and the space 46 is evacuated via compressed air line 48 , 5 / 2 valve and outlet pipe 52 . outlet pipe 52 leads to lines 70 , 74 and 76 . valve 82 may direct evacuated air from lines 52 , 70 and 74 to the atmosphere via line 76 or to the conical part 78 of silo 80 containing alumina 86 for feeding to the electrolytic cells via feeding means 84 . outlet line 50 can also be connected to line 70 via line 72 . this permits the air expelled on the return stroke of the piston to be usefully employed by injecting same in the conical region of the proportioned feeding device and improves the flow characteristics of the alumina at the lowest part of the silo where pressure is greatest while doing so without expending extra energy . fig2 shows the upper region of the working cylinder 34 in which the piston 38 can be moved in the axial direction . the cylinder head 36 is mounted , air - tight , on a pipe of inner diameter d 1 . an evacuation channel 32 is provided in the cylinder head 36 . the cylindrical shaped piece 56 projecting beyond the piston 38 and featuring sealing ring 54 fits into a corresponding recess 58 in the cylinder head 36 . leading out from this recess 58 to the outside is an evacuating channel 60 and such that this outlet can be blocked off with a regulating valve 62 . the piston 38 features three sealing rings 64 . in order to save material and weight the piston rod 42 of outer diameter d 2 is tubular in form . in the non - operative position the piston of the working cylinder is preferably acted on by the reduced network pressure on its positive side while the negative side of the working cylinder is evacuated . the piston is pressed e . g . on a stop plate on the cylinder head . if the piston is to be fixed in the non - operative position for an extended period , in particular when removing the crust breaking device , the piston can be held in place by means of a locking mechanism . when the cell is in operation , it must be possible to determine whether the chisel has fully penetrated the crust or not . for this purpose a mechanically or pneumatically operable end switch is provided e . g . in the interior of the working cylinder . using an electrical circuit the moment of immersion of the chisel in the electrically conductive molten electrolyte can also be employed to signal the end position . with respect to the process for operating the device the object is achieved by way of the invention in that the microprocessor , in a first selectable time interval , alternately feeds a control signal to the 3 / 2 channeling valve which is in the resting position and which has been evacuated , via a compressed air line , in the space inside the working cylinder between the cylinder head and the piston , as a result of which the closed circuit is formed and , if necessary , unlocks the piston , and , after the end piston has been reached , cancels the control signal for the 3 / 2 valve and , if necessary , locks the piston in place . should the breaker chisel not reach the end position during a second selectable time interval , the microprocessor switches over the 5 / 2 channeling valve by means of a control signal . the reduction valve is thus cut out and the interior of the working cylinder which is penetrated by the piston rod , the positive end , is evacuated via a compressed air line and the 5 / 2 channeling valve . the chisel failing to reach the end position means that the crust is not penetrated completely , and the alumina fed to that spot does not reach the electrolyte . by switching over the 5 / 2 channeling valve the force from the pneumatic drive system acting on the chisel is increased greatly : the pressure acting on the piston on the negative side is increased and with that also the force . by evacuating the positive side of the working cylinder the counter - pressure is eliminated , as a result of which the force acting on the piston is increased further . if , in spite of cutting out the pressure reduction valve , the chisel does not reach the end position , then the microprocessor causes the striking action to be repeated at brief intervals until the crust is penetrated . to achieve the maximum saving in energy , the microprocessor control can be arranged such that by switching over the 5 / 2 channeling valve the full force is applied only when the chisel is in the lowest part of its stroke e . g . in the last 100 mm of the downward movement . by repeating the movement with full force at short intervals the chisel is then moved only in the lowest part of its stroke so that correspondingly less compressed air is consumed . with all variations of switching arrangements the return movement to the negative side of the cylinder always takes place under reduced pressure . the compressed air drawn from the distribution network for the electropneumatic drive system is usually 6 - 8 bar , the reduced pressure 3 - 4 bar . in practice , in smelter operations , the first selectable time interval for normal operation of the device is usefully in the range of 0 . 5 to 5 minutes . the second selectable time interval , for initiating the higher pressure , is 0 - 3 times the first time interval . if the chisel does not reach the end position ( completely penetrated crust ), then the system switches over to application of full force preferably immediately or after a few seconds . according to another mode of operation the lowering of the chisel can be repeated first with reduced pressure at shorter time intervals than the first selected time interval , before switching over to the application of full force . also within the scope of the invention is to employ a closed circuit comprising working cylinder , 5 / 2 channeling valve , 3 / 2 channeling valve and the related compressed air lines , such that the said circuit is not fed with compressed air from the pressure reducing valve but directly from the pipe branching out from the compressed air network . the maximum possible force can be achieved , however , if not only the pressure reducing valve is cut out but also if the positive side of the working cylinder is evacuated . of course in these cases the consumption of compressed air is correspondingly greater . the following numerical examples show the different amounts of compressed air consumed by a conventional working cylinder and by a working cylinder according to the invention as employed for the pneumatic drive of crust breakers in electrolytic cells . when considering these examples , it must be taken into account that the saving is repeated at short intervals and that several hundred such working cylinders are in operation in an aluminum smelter pot room . the diameter d 1 of the working cylinder is 200 mm , the piston rod diameter d 2 is 50 mm , the stroke h is 500 mm and the pressure p in the compressed air network is 7 bar . this working cylinder is operated according to the conventional procedure i . e . without a closed circuit . all of the air emerging from the working cylinder is simply expelled to the surroundings ______________________________________air consumption for a downwards movementfilling the negative side : ## str1 ## = 109 . 9 dm . sup . 3evacuating the positive side : ## str2 ## ## str3 ## total 212 . 9 dm . sup . 3air consumption for an upwards movementfilling the positive side : ## str4 ## = 103 . 0 dm . sup . 3evacuating the negative side : ## str5 ## ## str6 ## total 212 . 9 dm . sup . 3______________________________________ in all therefore for a downwards and upwards movement the amount for air consumed is 425 . 8 dm 3 . a working cylinder with an internal diameter d 1 of 200 mm features a piston with tubular shaped piston rod of outer diameter d 2 equal to 100 mm . the length of stroke h is 500 mm , the reduced working pressure p red is 3 . 5 bar . this working cylinder is built into an electropneumatic drive system according to the invention . the thrust movement of the cylinder takes place in the normal case by means of a closed circuit . ______________________________________air consumption for a downwards movementfilling the negative side making use of the airexpelled from the space on the positive side : ## str7 ## = 13 . 7 dm . sup . 3air consumption for a downwards movementfilling the positive side ## str8 ## = 41 . 2 dm . sup . 3evacuating the negative side ## str9 ## ## str10 ## total for a downwards and an upwards movement 109 . 9 dm . sup . 3______________________________________ the consumption of compressed air or energy is therefore lowered by 26 % compared with the conventional practice used up to now . this saving is realized during normal servicing of the cell ; when the force is increased briefly the saving decreases . the above calculation of compressed air consumption refers to cylinder dimensions and pressure ranges in the compressed air network such as are to a large extent normal in today &# 39 ; s operating conditions . in an aluminum smelter with 200 reduction pots , each fitted with 6 crust breakers , the daily savings in compressed air with servicing at 3 minute intervals and a reduced pressure of 3 . 5 bar is :