Optimal grinding

Published on: 6/14/2018
Author/s : Gabriela Chiric / La Meccanica.

An optimal grinding requires the right amount of intake of air, which is cleaned in the nozzle filters. This also guarantees an economic use of the hammer mill. The air is necessary to quickly transport the fine product from the area of the hammers through the grinding sieve and, at the same time, to cool the product. 

By the ventilator, mounted in front of the filter a vacuum is generated and the air is sucked into the air inlet funnel of the hammer mill infeed device, passes through the milling chamber and milling screen, thus keeping the screen perforation clear. On its way to the filter, which is of automatic self-cleaning type, the aspiration air carries dust along which settles at the outer surfaces of the filter pockets. 

The compressed air burst is injected into the inside of the filter pockets thus reversing the airflow for a split second and removing the settled dust from the filter cloth. The intervals of these are controlled by the rotary air distributor which releases the compressed air from the pressure tank via air valves to the injector nozzles mounted in front of the open filter pockets. The dust is collected in a hopper with an airlock or directly dumped into following bin or conveying system. 

• Sleeve material type antistatic polyester 
•  Sleeve extractions from the front door 
•  Compressed air with air pressure 5-6 bar 
•  Compressed air distribution system complete with a tank for compressed air complete with the air valves (high-speed action) and a connector for condensate drain 
•  Pipes distributors with nozzles that can be easily removed for the maintenance of the sleeves 
•  Pre-chamber for removal of heavier particles 
•  Series of filter bags in polyester, resistant up to 130 ° C (antistatic treatment) 
•  Series of "venturi" in two extended cones 
•  Series of cages made in 3 mm steel rod coated with thermosetting resin designed to be attached to the sleeves 
•  Main body made of carbon steel 
•  Manometer 
•  Electronic control unit for the automatic sequential control of solenoid valves 
•  IP56 protection 

The filtering surface must be evaluated considering a filtering ratio = 2 

This means that the airflow in m3 /min is twice the filtering surface in m2. For example, in order to filter 90 m3/min of air aspirated from the mill, we'll need a 45 m2 filter. 
The volume of the air used to clean the sleeves in Nl/min is equal to 5-8 times the filtering surface in m2 

According to the ATEX rules, these are the solutions adopted for the filters for hammer mills: 

•  In case of an explosion inside the building, there must be installed a flame catcher. 
•  Sometimes they add also a bottle with CO2. This bottle opens automatically very quickly. Another possibility is to build a channel and guide the explosion gas out of the building.

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