2803 



(NUMBER OF OPENINGS) 



6 7 8 9 10 II 12 13 14 15 



I I 



TOTAL PRESS TIME (MIN.) 



Figure 23-42. — Relationship between line speed, press time, press length, and number 

 of openings. See text for explanation of examples indicated. (Drawing from Suchs- 

 lond and Woodson 1985.) 



pressure, less heating medium leakage, and possibly pH control are cited as 

 advantages of the high pressure hot water system. On the other hand, hot water 

 requires forced circulation; and in most mills steam is already available because 

 it is required for the operation of digesters, pressurized refiners, and dryers. 



Press temperatures vary, but average about 400°F. One cycle of a 4- by 8-foot, 

 20-opening press may expend up to 1.5 million Btu, so both steam- and water- 

 heated presses use accumulators to provide such capacity. Since the water 

 accumulators operate at constant pressure, boiler pressure is less than for steam 

 systems, where accumulators operate between 25 and 20 atmospheres. Press 

 platens are about 21/2 inches thick with heating channels 1 '/s inches in diameter. 

 Platens must be plane and their surfaces parallel within tolerances of 0.005 inch. 



Pressure is applied to the press platens by a series of cylinders and rams, so 

 dimensioned that they will be able to provide the required specific pressure on 

 the fiberboard mat with a reasonable hydraulic working pressure. Larger and 

 fewer cylinders are the trend, because they reduce maintenance costs. Hydraulic 

 working pressures are between 3,000 and 5,000 psi. To provide a specific 

 pressure of 1 ,000 psi requires a ratio of platen area to total ram area of from 3 to 1 

 to 5 to 1 . 



