34 MARINE PRODUCTS OF COMMERCE 



the vacuum pans are both compUcated and expensive, they are coming into use 

 for the refining of sea salt because they result in the finest product. This process 

 makes use of the lowering of the boiling point of a liquid by decreasing the 

 atmospheric pressure above it. It is impossible to attain nearly complete exhaustion 

 of the chamber above the pan, but the partial vacuum maintained effects an 

 appreciable lowering of the boiling point and a distinct saving in fuel. In the 

 multiple-effect vacuum-pan system each pan acts, not only as an evaporator, but 

 also as a boiler, producing heated vapor or steam for boiling in the succeeding 

 pan, and as the condenser for the pan immediately preceding. 



In the multiple-effect pans the steam from the boiler plant, which has a 

 temperature slightly above 212° F (100° C), enters the steam belt of the first 

 vacuum pan and is condensed there. The heat from this steam is transmitted 

 through the tubes and absorbed by the evaporating brine, which is under a 

 vacuum of about 15 inches. As this brine boils, it gives off heated vapor which 

 passes to the steam belt of the second vacuum pan. The temperature of this 

 vapor, which is the same as that of the boiling point of the brine at a 15-inch 

 vacuum, is approximately 175° F (79.4° C). If the liquor in the second effect 

 is under a vacuum of 24 inches, the vapor at a temperature of 175° F (79.4° C) 

 in the steam belt causes the liquor to boil. The vapor from the second effect is in 

 turn led into the steam belt of the third pan, and so on. Triple-effect vacuum 

 pans are commonly used for the recrystallization of salt since a fourth effect is 

 seldom profitable. Inasmuch as the design and operation of the multiple-effect 

 vacuum evaporators are highly specialized phases of salt manufacture, the details 

 will not be discussed here. The size and design of a simple vacuum pan will be 

 briefly discussed, instead. 



The body, or shell, of the larger vacuum pans, which are greater than ten to 

 twelve feet in diameter, is usually made of cast iron; in smaller pans steel is 

 used. The tubes are made of either steel or copper. Steel, copper, or tobin-bronze 

 are used for the construction of the tube sheets. The customary dimensions for 

 the tubes in the smaller pans are 2/2 inches (o.d.) by 48 inches. In the larger pans 

 where the tubes are sometimes as long as 72 inches 2J2-inch tubes are also used. 



The capacity of a single-effect salt pan is, roughly, one 280-pound barrel of 

 salt in twenty-four hours for each 2J2 X 48-inch tube, or one 280-pound barrel of 

 salt for each 2/2 square feet of heating surface in twenty-four hours. In other 

 words, the capacity of a single effect pan in barrels per twenty-four hours is ap- 

 proximately equal to the number of four-foot tubes that it contains. This figure 

 can be considerably exceeded if the pan is clean and artificial circulation is re- 

 sorted to. 



The efficiency of salt pans is generally 70 to 80 per cent. Usually, the salt made 

 for single effect is 12 to 14 barrels of salt per ton of coal; for double effect, 21 to 

 24 barrels of salt per ton of coal; for triple effect, 33 to 36 barrels oi salt per ton 

 of coal; and for quadruple effect, 42 to 46 barrels of salt per ton of coal. These 

 figures depend upon the grade of coal used. 



The chief difficulty encountered in the crystallization of salt in vacuum pans 

 is the formation of a hard porcelain-like scale of gypsum on the inside of the 

 vacuum pan. This scale, a very poor conductor of heat, greatly reduces the 

 efficiency of the apparatus. However, once formed, its removal is a difficult opera- 



