342 



SORGHUM. 



Also, the rapidity of the operation is greatly increased, since the 

 evaporation in a vacuum proceeds very rapidly. 



The amount of vacuum in the pan is stated in inches, in the sugar 

 house, and is determined by a barometer connected with the interior 

 of the pan. Thus, 28 inches of vacuum means, that the pressure 

 within the pan is equal to only 2 inches of mercury at the sea level ; 

 20 inches vacuum means, that two-thirds of the pressure has been re- 

 moved from within the pan. 



It is obvious, that the pressure on the pan by the surrounding at- 

 mosphere, when any thing approaching a good vacuum is attained, 

 becomes enormous ; 28 inches vacuum is common wdth good pans, and 

 a pan 8 feet in diameter would then sustain an outside pressure of at 

 least 200 tons. The pan, therefore, must be constructed of material 

 able to sustain this pressure. 



The following table gives the boiling points for syrup in the vacuum 

 pan, for the several degrees of vacuum which are obtained in practice. 

 It is taken from Ures' Dictionary of Arts and Manufactures: 



BOILING POINTS IN' VACUUM PAN. 



In construction, the vacuum pan consists of a spherical or oval ves- 

 sel of copper or iron, from four to ten or more feet in diameter, pro- 

 vided with one or more coils of copper tubes, to either or all of which 

 steam may be admitted by means of cocks. These coils occupy about 

 one-fourth or one-third the height of the pan. From the top of the 

 pan a large pipe proceeds, which enters first a receptacle, called the 

 overflow, which is to catch and retain any portion of the contents of 

 the pan which may by accident boil over. From the bottom of this 

 receptacle a tube, provided with a stop-cock, passes back to the vacuum 

 pan, in order that such overflow may be returned at intervals. 



The large tube proceeding from the vacuum pan is connected with 

 the condenser, where the vapor escaping from the pan is condensed, 



