of an example the nozzles 8 for high pressure can have a diameter u03c6 of 0 8 mm or of 0 12 mm 0 14 mm 0 16 mm and can be distanced with a pitch p which varies from 1 mm up to 5 mm preferably 2 3 mm said nozzles 8 can also be arranged in several horizontal rows fig6 7 a 7 b illustrate different arrangements of high pressure nozzles 8 in the same number of nozzle holder jet strips 7 fig6 shows a first embodiment of the jet strip 7 containing nozzles 8 with diameter of 0 12 mm which are arranged at a distance p of 2 or 3 mm one from the other the nozzles 8 are also arranged in two horizontal rows along the line l which have a distance d one from the other of approximately 1 mm fig7 a shows a second embodiment of the jet strip 7 containing nozzles 8 with diameter of 0 12 mm arranged in two rows which have a distance d one from the other of approximately 1 mm next to each nozzle 8 of each row a second nozzle 8 is positioned at a distance f for example of approximately 0 8 mm in this way groups of nozzles 8 are formed along the line l each one formed by four nozzles 8 practically adjacent one to the other each group is distant one from the other by a pitch p which can be equal to 2 3 5 mm fig7 b shows a third embodiment of the jet strip 7 containing nozzles 8 with diameter of 0.