16 JACK PINE AND HEMLOCK FOR MECHANICAL PULP. 



tain the speed constant at all times. When the pressure on a pocket 

 is removed the speed is bound to rise considerably, especially when 

 the water wheels or turbines are operated without a governor. 



When hemlock wood was ground at low speed and low pressure 

 it was impossible to obtain anything more than a powder. Also when 

 this wood was ground at low pressure and high speed the product was 

 extremely short, but the pressures at which these results were obtained 

 are considerably lower than those ordinarily employed commercially, 

 and the results have little significance. If the stone is what is ordi- 

 narily called sharp, it is necessary to use a lower pressure, and when 

 dull, a higher one, but it is impossible to obtain the same quality of 

 pulp under both conditions. Speed and pressure affect quantity 

 rather than quality, and by the proper adjustment of both the maxi- 

 mum efficiency of grinding is attained. If a certain speed is selected 

 there must be a corresponding pressure which will yield the greatest 

 amount of pulp in 24 hours with the least consumption of power. 



By the term " constant pressure," wherever used in this report, is 

 meant constant pressure on the grinder cylinders. The pressure per 

 square inch of wood in contact with the grinding surface varies con- 

 siderably, chiefly with the size of wood ground and the area of the 

 pocket. Again, the length of the wood is not at all a constant quantity, 

 and this, too, can only result in a variable pressure per square inch of 

 wood. The pressure of the wood on the stone varies throughout 

 certain limits with any pressure on the grinder cylinder, and the 

 ranges of pressure of the wood on the stone are raised or lowered by 

 raising or lowering the cylinder pressure. This pressure variation, 

 however, can hardly be controlled commercially, and therefore has 

 not been considered in the tests discussed in this report. There is 

 also more or less pressure variation due to binding of wood in the 

 pockets, and this, too, is difficult, if not impossible, to control. 

 Figure 1 shows a measure of the power applied to the grinder. The 

 effect of pocket binding and the withdrawal of pressure from one of 

 the pockets will be noted. At one end of this chart the power con- 

 sumed is approximately 360 kilowatts, falling off graduaUy to 280 

 kilowatts, due to pocket binding. After raising the pistons and read- 

 justing the wood in the different pockets, the power to the grinder 

 motor had to be increased to 350 kilowatts on account of the added 

 load produced by eliminating the pocket binding. 



SURFACE OF STONE. 



The most efficient grinding condition is one where there is a maxi- 

 mum amount of grinding surface, and still a sufficient amount of 

 depression in the stone to allow for the carrying away of the ground 

 wood, or, as this is commonly called, for the clearing of the stone. 



