SUCTION PRESSES

197

peculiarities of the machine, the speed, the quality of the stuff and the system
of cleaning, etc. One of the questions that is being continually asked by
paper-makers of each other is: 'How long do your felts last?' Conditions
vary so much that the answer may be either ten days or ten weeks. To illustrate
this, suppose we are making a well-fibrillated bank at from 100 to 150 feet
per minute. We will perhaps find that we can have little pressure on the
coucher at, say, 120 feet, and still less at 150 feet. Then the sheet will enter
the press rolls with a higher percentage of water. This is throwing more of

[Wibutyr
Re. 75.—SKETCH SHOWING USUAL AsxAitG&ssin: or PIBSSES (AIOVB) AND DUAL PMSS (Snow)
The saving in space is very considerable
the work of water extraction on to the felt, not only by increased speed, but
in quantity, greater in proportion to the ratio of speed. In addition, the in-
creased speed reduces the efficie&cy of the rolls and felt. The forward impetus
of the water carries it through the rolls in greater quantity. The time factor
of Ac nip is reduced. The water flowing through the felt and running
down the bottom roll has less time to get away and remains nearer the
nip. The porosity of the felt, though actually unaltered, is less able to
allow of the extra water getting through. The pressure of the nip must,
if possible, be increased to keep the paper relatively as dry as at the lower
speed.