INDUSTBIAL FUMIGATION AGAINST INSECTS 33 



VACUUM FUMIGATION 



Fumigation by vacuum (figs. 31, 32, and 33) consists in placing 

 the commodities to be fumigated in a gastight steel chamber, remov- 

 ing the air, and replacing it with a gas lethal to insects. By this 

 method there is immediate penetration of commodities by the gas, 

 and insects are killed with a rapidity that is not obtainable in an 

 atmospheric vault. The length of exposure required in vacuum fumi- 

 gation ranges from 1 to 3 hours, as compared with from 10 to 24 

 hours under atmospheric conditions. This process, therefore, has a 

 considerable appeal in industries where speed is essential, as in the 

 handling of foodstuffs and other commodities. 



In addition to reducing the time of exposure, vacuum fumigation 

 has several other advantages. At the end of a fumigation the gas 

 can be speedily removed from the treated commodities by a process 

 known as " air washing ", which consists in drawing a vacuum of 

 27 inches and breaking it with air. Furthermore, by this method 

 workmen are not exposed to the fumes of the gas. The disadvan- 

 tages are the original cost and upkeep of the equipment and the 

 larger quantity of fumigant needed. 



EQUIPMENT 



Vacuum chambers are usually cylindrical, although rectangular 

 tanks are increasing in popularity. 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 cham- 

 ber will depend on the type and quantity of material to be fumi- 

 gated and the space available for it in the factory. Some firms prefer 

 to have two tanks that can be operated with one set of equipment. 

 While one tank is under fumigation, the other can be loaded or 

 unloaded and the fumigation crew kept continuously busy. 



The tanks are usually installed so that small trucks or factory 

 skids on narrow-gage tracks 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 

 vacuum 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 the tightness with which the 

 doors fit. A well-built tank should hold a vacuum without perceptible 

 loss throughout the average fumigation period. 



Many tanks are equipped with steam coils or electric heating units 

 to maintain a favorable temperature during fumigation. A thermo- 

 stat should be used to maintain a constant temperature and to pre- 

 vent overheating. 



Each installation requires a vacuum pump capable of drawing a 

 28-inch vacuum in 10 minutes or less. 



