200 PASTEURIZATION 



cream from seeping in between the liner and the wood of the 

 cover, causing the latter to become a serious source of con- 

 tamination of the cream. Some of the most modern vat covers 

 are enveloped entirely in tinned copper and are proving highly 

 satisfactory from the sanitary standpoint. 



Connections for and the Circulation of the Heating and Cool- 

 ing Medium. 'There are two principle systems which serve to 

 circulate the heating and cooling medium, the hot and cold water 

 and the brine, through the revolving coils in the pasteurizing 

 vats, namely the self-circulating system and the positive circulat- 

 ing system. 



The self-circulating system operates on the principle 'that 

 air is lighter than water. It consists of admitting to the coil, 

 a small amount of air through an automatic air vent, installed 

 at the head of the coil shaft. As the air and water, or brine, 

 enter the coil, the air rises to the surface in the coil. If enough 

 air enters so that the surface of the water in the coil is below 

 the under side of the coil wall at its highest point, the air in 

 the top portion of the coil forms a solid plug, or partition, be- 

 tween the water columns in the left and right side of the coil. 

 When the coil revolves, this partition of air prevents the water 

 in the coil from staying or flowing back. The coil being a 

 spiral, causes the air plug to push the water column forward 

 with each revolution of the coil. This inevitably produces a 

 vacuum and suction behind the air plug and this suction draws 

 in more water and air, the operation thus repeating itself with 

 every turn of the coil. 



In the positive system of circulation the heating and cool- 

 ing medium are forced through the revolving coil by a pump. 

 In this system there is no air in the coil, the water fills the 

 entire coil. 



For heating, the coils should be fed with hot water only. 

 They should not be charged with steam, as the agitation of the 

 cream in the vat, is not sufficient to prevent excessive burn- 

 ing of the. cream on the surface of the steam heating coil, and 

 direct steam is also exceedingly destructive to the packing in 



