THE EXTRACTION OF THE JUICE BY MILLS 



219 



such as these obtain in Cuba, and are naturally combined with a lower 

 efficiency at the mills. The reverse condition,, namely, a cultivation which 

 has reached the limit of its extension, obtains in Hawaii, and here are found 

 lower capacities combined with the maximum of efficiency. 



In one and the same milling plant the means available to increase capacity 

 is principally an increase in the speed of the mill, whereby the grinding surface 

 developed in a unit of time is increased. This, of course, implies higher 

 engine speed, and the development of more power, which is the ultimate 

 factor controlling mill capacity. Other factors of influence are the roughness 

 of the roUers as affecting slippage, the setting of the trash turner, the care 

 taken in feeding the cane to the mill, and the means adopted in preparing 

 it for milling. Minor factors are details such as the angle of the crusher, 

 of the carriers, and of the feeding shoots. 



As regards the actual recorded 

 capacities of milling trains, as dis- 

 tinct from a mill, a study of a very 

 large number of results on record 

 of nine, twelve, fifteen and 

 eighteen-roller plants, preceded 

 by efficient crushers, has led the 

 wTiter to the formula : — Capacity 

 = 0-005 '^ ^^ ^" tons fibre per 

 hour, where n is the number of 

 mills in the train, I is the length, 

 and d the diameter of a roller ex- 

 pressed in feet. Values of this 

 expression, which has no rational 

 basis, and is entirely empirical, 

 are given belo\v. 



If besides capacity the eco- 

 nomics of high efficiency are 

 considered, it will be reasonable 

 to obtain this end with longer 



trains, rather than with larger units, because of the opportunity afforded 

 for compound maceration. 



I- d- OR Capacity in Toxs Fibre per hour. 



The Stresses on the Three-Roller Mill. — In Fig. 126 let the three circles 

 represent the three rollers of a standard three-roller mill, of which the ver- 

 tical angle is a. Let the cane pass from left to right. Let the 

 pressure exerted on the layer of bagasse in its passage between the top and 

 front roUer be p, and let that between the top and back roUer be n p. Let 

 these pressures act through the lines joining the centres of the rollers. Along 

 B A produced lay ofi A D = p. Resolve A D into vertical and horizontal 



