34 CIRCULAR 369, U. S. DEPT. OF AGRICULTURE 



under atmospheric conditions. With many commodities the period 

 of exposure may be reduced to as short a time as 1 hour, a feature 

 that has an appeal in industries where speed is essential, as in the 

 handling of foodstuffs and similar commodities. 



In addition to reducing the dosage and length of exposure, this 

 method of fumigation offers further advantages. At the end of a 

 fumigation the removal of the fumigant can be speeded by the 

 process known as air washing, which consists in drawing a high 

 vacuum and breaking it with air. Furthermore, workmen are not 

 exposed to such heavy concentrations of gas while unloading a 

 fumigation chamber. 



EQUIPMENT 



Vacuum chambers are either cylindrical (figs. 30, 31, and 32) or 

 rectangular (fig. 33). They are made in all sizes, from laboratory 

 outfits with a capacity of about 1 cubic foot up to those capable of 

 holding one or more carload lots. The size of the chamber will 

 depend on the type and quantity of material to be fumigated and 

 the space available for it in the factory. Some firms prefer to have 

 two chambers that can be operated with one set of equipment. While 

 one chamber is under fumigation, the other can be loaded or un- 

 loaded and the fumigation crew kept continuously busy. 



The tanks are usually installed so that small trucks or factory 

 skids can be run into them. If the vacuum chamber is equipped 

 with a door at each end, the trucks can be run in at one end and 

 out the other, so that the fumigated products will not interfere with 

 the next load. In some factories one end of the chamber opens 

 into the receiving room and the other end into the general storage 

 room. Raw materials likely to be infested are then fumigated before 

 they are placed in the general storage rooms. 



The doors of the vacuum tank shpuld be so balanced and hung 

 that they can be quickly opened and closed by one man. The gaskets 

 should be durable and at the same time provide a gastight seal. The 

 efficiency of a tank depends largely on its freedom from leaks. A 

 well-built tank should hold a vacuum without perceptible loss 

 throughout the average fumigation period. 



Each installation requires a vacuum pump capable of drawing a 

 28-inch vacuum in 10 minutes or less. 



Recent work has demonstrated that it is advantageous to intro- 

 duce the fumigant into all parts of the tank at once instead of 

 through one inlet. Rectangular tanks are now being supplied with 

 multiple gas inlets spaced evenly along the four longitudinal sides. 



It has also been demonstrated that by circulating the gas in a 

 tank the dosage can be reduced 25 percent by virtue of the better 

 distribution obtained. By means of a bypass from the exhaust to 

 the multiple gas-inlet system the vacuum pump can be utilized 

 to circulate the gas in the tank. This feature is now standard 

 equipment with some tanks. 



The use of a vaporizer in connection with a vacuum chamber in- 

 creases its efficiency by insuring the entrance of the fumigant in a 

 gaseous state. The most common type of vaporizer consists of a 

 steel tank in which steam is generated by means of electric heating 

 elements. A copper coil, through which the fumigant is conducted. 



