56 Materials and Their Handling 



in chutet. If it is a liquid, it is run down in pipe 

 lines or is pumped. If the liquid is inflammable, 

 the delivery to the point needed should invariably 

 be against the action of gravity from a tank below 

 ground, and all the piping arranged so that it 

 will drain back into the tank. There should be 

 no check valves in the line or other valves which 

 can be shut off. Preferably the pump used should 

 be of the centrifugal type so that, when it is 

 stopped, the fluid will run back through it. This 

 is the only arrangement which obviates a he 

 fire risk. 



Grain, sawdust, seeds, and other light materials 

 may be moved in pipes pneumatically. It is gen- 

 erally desirable to do this by suction, rather than 

 by pressure, as the pipe lines and their connec- 

 tions are then dust proof. If the air is blown 

 through the tubes, it leaks out at the joints, scat- 

 tering the material being conveyed. The same 

 principle may be used for moving solids hydraulic- 

 ally. A familiar example of this is the discharge 

 of ashes from steamers, where a jet of water at 

 high velocity carries the ashes outboard. The 

 pneumatic or hydraulic method is convenient for 

 handling light materials, or for handling heavy 

 materials intermittently. It is not economical, 

 however, for handling heavy materials continu- 

 ously, as it is wasteful of power. 



Spiral Flight Conveyors. For the continuous 

 movement of light material, spiral conveyors 

 are well adapted. These consist of a spiral 

 flight mounted lengthwise around a rotating 



