THE EXTR.\CTIOX OF THE JUICE BY MILLS 



225 



by the consumption of power much in excess of that computed proper tionallv 

 to the quantity of fibre milled. 



A different set of conditions obtains when the mill operates under a con- 

 stant load, as af- , 



forded by 

 draiihc gear. 



Fig. 131 



as 

 the hy- 

 Evi- 

 dently in this case 

 the power consimied 

 varies always in 

 proportion to the 

 quantity of fibre 

 passing, since the 

 opening between 

 the top and back 

 roll adjusts itself 

 automatically. In order that the mill should accommodate itself to the 

 passage of an increased quantity of fibre, more power must be developed 

 by an increased speed, or by an increase in the mean effective steam 

 pressure. In the first case the thickness of the blanket of bagasse mav be 

 maintained constant, and in the latter it may increase in depth propor- 

 tionate to the quantity of fibre passing. Eventually, when the engine has 

 developed its maximum power, more cane capacit}^ can be obtained by 

 decreasing the hydraulic load. 



The above discussion has only treated of the compression of fibre as be- 

 tween the top roll and the back roll, and has neglected the action of the top 

 roll and front roll, the friction on the trash bar, the crushing effect 

 between top roU and trash bar, and other minor influences. The results 

 obtained in milling depend not only on the top and back roller setting, 



but on this acting in 

 combination ^\ith the top 

 and front roller setting, 

 and, again, different con- 

 ditions arise dependent 

 on whether the mill is 

 rigid or is operating under 

 a constant hydraulic load. 

 In the former case, what 

 has been said \uth refer- 

 ence to top and back roll 

 appHes equally to top and 

 front roU, and the power 

 required to operate the 

 mill will be the sum of 

 that required to operate 

 the top and front and top 

 and back roller, considered 

 independently of each 

 other ; and for every alter- 

 in the position of either front or back roller relative to the top 

 there will be a different consumption of power. 



ation 

 roller 



In an hydrauhc miU, however, the sum total pressure exerted on the 

 bagasse is constant, and the problem the engineer has to consider is how 



