INDUSTRIAL FUMIGATION AGAINST INSECTS 35 



VACUUM FUMIGATION 



In vacuum fumigation commodities are placed in a gastight steel 

 chamber, and a large portion of the air is removed and replaced with 

 a gas lethal to insects. By this method immediate penetration of 

 commodities by the gas is obtained and the susceptibility of the 

 insects is increased by the reduction of the oxygen content of the 

 chamber. This permits smaller dosages and shorter exposures than 

 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. 27, 28, and 29) or 

 rectangular (fig. 30). 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 should 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 distribu- 



