The standard operation of a full sized shredder is not without risk of explosions. The high temperatures produced by the intense friction created during the shredding process will instantly igniting any flammables entering the hammer mill and could cause an explosion.

If the pre shredding process is omitted then inevitably partially filled or even empty car petrol tanks, gas tanks and LPG or BBQ gas tanks will enter the shredder chamber. As the mills hammers tear in to these tanks and release the gas or vapor an explosion will occur. These explosions can be quite sever and can cause damage to nearby buildings as well as being a major health and safety risk for the mill operator and other personnel.

 

During an overload situation the “press flap” returns to a neutral position to aid in freeing the material as both rotors spin in the opposite direction.

In this phase, the teeth of the low speed rotor tear the previously blocked material apart against the face of the “swinging-floor”. Grouser teeth in the swinging floor ensure the optimal positioning of the rotor teeth.

Once the blockage has been cleared, the rotors revert to their natural state of rotation to process the remaining material which was unable to be processed on the first pass.

In case of very dense scrap several passes may be required however this process can be repeated auto-matically without operator intervention until all the material has passed through the millfeeder (the number of cycles required to process heavy and dense scrap depends entirely on the torque and power available to the rotors).

At the end of the overload cycle, the “swinging-floor” returns to its normal operating position.

The operator can also choose to override the automatic cycle by adjusting the “swinging-floor” and the direction of rotation at his own discretion.

The contact surface of the “swinging-floor” is constructed of thick high strength wear resistant steel (HARDOX).

 

Grinding chamber of an Hammer Mill

 

Regardless of how much care is taken to sort hazar-dous from the scrap before feeding the mill, it is im-possible to completely remove all potential sources of explosion, especially if these sources are “ hidden” in auto bodies or bales.

 

By pre-shredding the material in a low speed shredder which uses teeth rather than hammers you can al-most entirely remove any explosion risks. Any petrol or gas tanks will be torn open at low speed and the po-tential explosive mix will harmlessly dissipate before reaching the hammer mill.

 

During normal operation of the pre shredder there is almost no chance of an explosion oc-curring due to the low speed of the rotors and the fact that the chamber of the pre-shredder is mainly open.

The different counter-rotating rotor’s speed, their position, number and shape of the teeth are all designed to harmlessly tear apart gasoline tanks and most gas containers.

 

Using a pre shredder such as the Bravo will give mill operators a distinctive economic advantage over shredder operators who chose not to pre shred their scrap. A quick return on investment and lower ope-rating costs of the main shredder are achieved due to:

 Lower energy consumption (even and optimal scrap fed to the hammer mill).

 Increased working life of the wear parts (hammers, grates, liners..)

 Lower maintenance costs due to the higher productive life of castings

 Lower service costs due to increased service intervals

 Less downtime associated with explosions and removal of “hidden” unshreddables.

 Allows smaller shredders operators to easily process bulky and dense material (such as bales) that otherwise could only be processed by larger more expensive machines.

 

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source http://ceg-balersshears.com/en/pre-shredder-mill-bravo-taurus/