for certain applications fig1 also includes the control valves 150 160 for separately controlling the lower cylinders and upper cylinders respectively of the double felt drying group 17 as described above with reference to the embodiment shown in fig1 fig1 is a further alternative embodiment of fig5 instead of providing one control valve for each individual cylinder in a drying group 17 as in fig5 the embodiment in fig1 provides two control valves 27 and 27 39 the first control valve 27 is provided for controlling the steam pressure in the drying group 24 and the first few cylinders of the drying group 25 the second control valve 27 39 is provided for controlling the steam pressure in the last few cylinders of the drying group 25 and the cylinders of the drying group 26 each set of adjacent upper and lower cylinders in the drying groups 24 and 25 and the adjacent cylinders of the drying group 26 is provided with an isolation valve 50 each isolation valve 50 is preferably formed to be open or closed without being able to be partially open by an adjusting device the isolation valves 50 are connected in steam lines between the control valves 27 27 39 and the cylinders of the drying groups 24 26 if the last cylinder or last few cylinders of the drying group 23 are connected to the control valve 27 as shown in the dotted or broken line an additional isolation valve 50 can also be provided between the control valve 27 and the cylinders of the drying group 23 a plurality of isolation valves 50 39 are also provided.