CONDENSED MILK CONDENSING 75 



unless it really is kept clean at all times, it may become a serious 

 source of contamination. 



The Vacuum Pump. The vacuum pump is, strictly speaking, 

 not a part of the vacuum pan, but its intimate connection with the 

 pan makes it necessary to briefly 

 consider it at this point. The 

 suction end of the vacuum 

 pump is connected with the end 

 of the condenser farthest from 

 the pan. The vacuum pump 

 exhausts the pan, forming a 

 partial vacuum. There are prin- 

 cipally two types of vacuum 

 pumps used in the milk con- 

 densery, the dry-vacuum pump and the wet-vacuum pump. The 

 dry-vacuum pump is used in the factories with the dry-vacuum 

 system, i. e., where the cooling water and the condensation water 

 escape to the sewer direct and without passing through the vacuum 

 pump, as is the case with the surface condenser and the barometric 

 condenser. The wet-vacuum pumps are used with the wet-vacuum 

 system, where the cooling water and the condensation water pass 

 through the cylinder of the pump. The dry-vacuum pumps have 

 the advantage of permitting the operation of the machine at a 

 higher piston speed than the wet-vacuum pumps in which the water 

 must be displaced at the end of each stroke. The cylinders of the 

 dry- vacuum pump are cooled by water jackets. The initial cost of 

 the dry-vacuum pumps, however, is greater than that of the wet- 

 vacuum pumps. 



The efficiency of the vacuum apparatus depends very largely 

 on the vacuum pump. Rapid evaporation at a relatively low tem- 

 perature necessitates the maintenance of a high vacuum. The type, 

 material, construction, workmanship, installation and operation of 

 the vacuum pump should be such as to insure the maximum effic- 

 iency. 



The pump should be placed on a good foundation and as near 

 the vacuum pan as practicable in order that the full benefit of the 

 vacuum may be realized. The suction pipe and all connections must 

 be tight. The suction pipe must be of the size directed by the man- 



