210 



CHAPTER XI 



The conclusions drawn by the writer were intended to be only general, 

 and to apply solely to a layer of hagasse flowing uniformly over the trash plate 

 without interruption. In actual operation it is doubtful if such a condition 

 ever obtains. 



£^ap3o/f. 



'CapSaV- 



Fig. 112 



Pressure Regulators. — In a rigid mill in which the position of all the rollers 



is fixed by means of caps and 

 tie-rods, any variation in the 

 quantity of cane, or more 

 strictly of fibre, passing in a 

 unit of time, is accompanied by 

 a variation in the pressure to 

 which the material is subjected. 

 If the quantity of fibre is less 

 than corresponds to the mini- 

 mum opening or clearance, the 

 pressure tends to vanish ; and 

 if the quantity increase in- 

 definitely, the mill will either 

 choke, or a fracture of some 

 part will occur, provided that the engine develops sufficient power. In rigid 

 mills it is then necessary to keep the quantity of fibre passing as constant 

 as is possible, and to control the setting and speed in relation to the quantity 

 of cane desired to be miUed. WTien the volume occupied by a unit weight 

 of bagasse with a pre-arranged water content is known, the opening and 

 the speed of rotation can be arranged to suit. Only a first approximation 

 can be made, however, since, in proportion to the actual opening, the volume 

 occupied by the grooving and the inequalities of the shells forms a very con- 

 siderable percentage. 



In order that the disadvantages referred to above may be overcome, 

 rigid mills have become largely a thing of the past. At present one roller 

 of the mill is arranged to lift under a predetermined pressure. When, 

 as is general, the top roller which coacts with both the lower rollers is selected 

 as the moving element, the sum 

 total of the pressures exerted 

 vertically on the top roller is a 

 constant, whatever is the quan- 

 tity of cane passing, provided 

 sufficient is passed to cause the 

 top roUer to lift. As explained 

 elsewhere, however, the distribu- 

 tion of the load as between top 

 and front rollers, and the top and 

 back ones, will vary with every 

 variation in the feed of cane. 



The necessity of this pressure 

 regulation, which also acts as a safety device, was recognised at an early 

 date. The first edition, 1855, of Richardson's "Chemistry as applied to the 

 Arts and Sciences" figures a mill invented by a Demerara engineer, Moore, 

 and built by Pontifex & Woods, in which the front and back rollers were 

 free to slide outwards, being maintained in position by a system of weights 

 and levers. This device appears in a number of early American patents and 



Fig. 



113 



