THE SIZE AND RELIABILITY OF SEPARATOR-DRUMS. 125 



(1) In the case of two exactly similar separator-drums making 

 an equal number of revolutions per minute, but one twice as broad 

 as the other, the acceleration in the former at the spot furthest 

 from the centre is double that of the latter. 



(2) In the case of two perfectly similar separator-drums of 

 exactly the same size, but one of which revolves at double the rate 

 of the other, that is, has twice the speed of the other, the centrifugal 

 acceleration on the spot furthest removed from the centre is four 

 times as great in the former as in the latter. 



From this it will be seen that the action of centrifugal force on 

 the milk may be increased in a double manner, namely, either by 

 increasing the size of the separator or by increasing its speed. 

 Since, however, in the case of doubling the diameter of the drum 

 the action is only increased twofold, but, in doubling the speed, 

 fourfold, the second method will be seen to be the more efficacious. 

 The second method is also to be recommended on other grounds. 

 Large drums are less handy, and are more difficult to work than 

 small ones. Since large masses of metal of a perfectly uniform firm 

 quality are more difficult to be obtained than small masses, there is 

 required for large drums a greater degree of security; in other 

 words, the revolving speed of large drums must be measured with 

 very special care, and this can only be done at the expense of 

 efficiency. Finally, it must be borne in mind that large drums 

 which effect the creaming of large quantities of milk in a compara- 

 tively short time require a comparatively large driving power, and 

 that it is more convenient, and generally also cheaper, to utilize a 

 comparatively low driving power for a longer time than a large 

 driving power for a short time. For this reason the use of large 

 drums, such as were formerly largely made, has decreased in the 

 course of time more and more, since they have not been found to be 

 suitable in practice. Separators under 50 kilograms (212 Ibs.) in 

 weight are now generally used with drums, and they can only hold 

 when in work a few kilograms (10 to 20 Ibs.) of milk, generally 

 between 4 and 8. The smaller of these drums is worked at a 

 speed of 6000 to 9000 revolutions per minute. In the case of 

 most of the separators at present in use, milk which has to be 

 separated only remains a short time in the drum, often not one 

 minute, and rarely more than three. 



When the separator is in use, the sides of the drum have to stand 

 very considerable internal strain, owing to the pressure of the milk 



