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June 07, 2012

GFMT Article: Grinding using proven concepts


Today we've got a very informative article on grinding by Willem de Vaan, General Director, Wynveen International BV.

Full text follows or read online here 
Grinding using proven concepts simplifies choices
by Willem de Vaan, General Director, Wynveen International BV, The Netherlands

Together with mixing and pelleting, grinding is one of the important processes when producing animal feed.

Traditionally, hammer mills are used in the feed industry for grinding raw materials, which is mainly caused by the relatively simple construction and high variation in particle reduction compared to other techniques used at roller mills and crackers.

Roller mills and crackers showed better results when it comes to energy consumption, but the new designed mills feature adjustable pre-breaker plates, variable speed drives and automatic screen change systems.

This results in an energy saving which brings the hammer mill again in a favoured position, certainly when taking in consideration the relatively high costs for wearing parts on roller mills and crackers. Given the fact that a hammer mill also allows grinding of mixed products, it becomes clear that hammer mills still are the majority.

Wynveen International BV has a rich tradition in manufacturing machines and equipment for the animal feed industry, petfood industry and fish industry.




Recently the company has moved into its new premises, which means a doubling of the office and production floor compared to the old building. With this new office Wynveen International BV is prepared of the future and has made the logical next step to meet the growth targets it is aiming for. With its new website, introduced during the opening of the new building, one can see in a glance what capabilities can be offered.

Besides the design and delivery of complete turnkey feed mills, Wynveen International BV is manufacturing the main equipment like hammer mills, mixers, coater and sifters and conveying equipment.

 Wynveen International BV is manufacturing hammer mills in a wide range.
The features of the of hammer mills that can be delivered are:
Big diameter 1200mm - 1500rpm
Diameter 650mm - 3000rpm
Capacities from 2t/h up to 60t/h
Frequency controlled drives
Manual or automatic screen exchange
With or without adjustable breaker plates
Two directions of rotation possible
Motor capacities up to 400kW
Open rotor design
Temperature control on main bearings and chamber
Feeder with magnet and stone trap
Ergonomic design

Already, five years ago, the Wynveen hammer mill underwent a complete makeover and there was the idea to develop a hammer mill that will be prepared for the future and can deal with the challenges of the future. This means minimal energy consumption and the possibility to answer to the need of the various applications required in nowadays-feed mills.

Most animal feed mills are looking for a grinding concept that can produce the particle size that fits with the specific required coarseness of ingredients needed for the feed to produce.

The latest version of the Wynveen hammer mills type GHM has a grinding chamber diameter of 1200mm, which gives at a speed of only 1500rpm a hammer tip speed at the ideal level of approximately 90m/seconds. The length of the chamber can vary from 750 to 1250mm, depending on the required capacity. The low speed gives a lower noise level, less wear and tear and lower bearing load compared to a 3000-rpm hammer mill.

To suit the various demands in feed mill capacities, Wynveen International BV can deliver hammer mills that can deal with different capacities from two tones/hour up to 60 tones/hour. For the smaller capacities the 3000-rpm versions are still used.
In the enclosed table you will find the various possibilities. By introducing frequency control on the motor, it became possible to grind ideal structures and particle size. In fact you can create a new screen set-up but still using the same screen perforation. When using a high speed you are able to create fine grinding, while with a lower speed a coarser grinding is achieved.

Frequent changing
To answer the growing need for producing a great number of recipes, the screens need to be changed frequently, for which reason Wynveen has designed the GHM version with an optional automatic screen change system. This gives more possibilities to grind on various perforations without the need to stop the motor during production and loose a lot of production time. Wynveen has chosen to make use of the ergonomic design already available at the hammer mills with manual screen exchange, which means a horizontal movable screen cassette in which 4 screens (so also four different perforations selectable) are mounted.

The big advantage versus vertical screen exchange is the space needed.
Often an upper hopper is positioned above the hammer mill, which reduces the possibilities for vertical screen exchange. To avoid dust and waste of product during exchange of the screen, while the motor is still running, it is important to maintain, an under pressure in the grinding chamber of the hammer mill. Special inflatable sealing’s with labyrinth ensure a correct tightness of the sealing in operation and stop’s the product leaving the grinding chamber.

To minimise energy consumption adjustable breaker plates are introduced.
Tests have proven that hardly any grinding takes place below the level of the main shaft. By designing two adjustable breaker plates in the top of the machine, an energy saving of approximately 30–35 percent compared to the old designs has been achieved.

The lower part of the hammer mill is executed with screens; by making use of the gravity force a good discharge of product is possible. The idea of adjustable breaker plates was to create an asymmetric form (ellipse instead of circle) in the grinding chamber. Because of this the product is forced to leave their natural flow and, because of the centrifugal force the product will hit the breaker plate, as a result it is bounced back and then again gets a hit of the hammer.

So a more frequent frontal collision is achieved between hammers, breaker plates and product. By making the breaker plates adjustable we can create this ellipse in both directions.

The hammer mills of Wynveen International BV allow a drive motor up to 400kW.
Furthermore, a so-called open rotor has been designed with two functions. One is to create sufficient stiffness and balance and secondly to minimise the temperature increase while grinding.

For monitoring purposes the main bearings and the chamber are fitted with temperature sensors. All components can also be delivered in an Atex execution. To make use of all four corners of the hammers and reduce the downtime of the machine, the direction of rotation of the hammer mill can be changed and the rotor has double holes for the hammer axles. To ensure a good product entrance to the mill, a feeder is developed which contains an integrated, (automatically cleaned) magnet and a stone trap.

This will make sure that possible metal parts and/or stones are prevented from ending up in the feed mix. The combination of the two makes the feeder unique.

Risk analyses
To meet the latest stringent guidelines of CE regulations, Wynveen International BV has conducted a risk analyses for its hammer mills.

This has been done for both non-hazardous environments and ATEX environments. To meet both as a standard, the hammer mills are equipped with proper earthing and sensors to detect possible increase of heat. Where required we make use of a pressure release membrane and/or flame arrestors.

For reducing the noise and vibration our hammer mills are provided with shock absorbers.

Much attention was spent to the ergonomic aspect. By designing two big doors, the hammer mills are very easy accessible for screen and hammer exchange. Both doors easily roll sideways via a rail and wheels and are equipped with safety switches to make sure that no mistakes can be made during maintenance.
Other important points to keep in mind when designing a grinding plant are not only the right choice of hammer mill, but also the correct energy efficient motor type and very important, an adequate aspiration set-up.

Especially the last item needs special attention.
The capacity of the aspiration fan should not only be in the right relation to the screen area of the mill but also depends on products, which have to be ground. Depending on the air volume, the necessary filter surface can be determined.
In general, reverse air cleaned filters are used, which allow for an air load on the filter cloth of approximately two m3/minute of air per m2 of filtering area.
Finally, it is important that the filter unit is not directly positioned beside the outlet product stream of the hammer mill to make sure that a good airflow and aspiration is safeguarded.





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