11.2 Filter plants

Filter plants are used for dedusting of the dust in the air generated e.g. by blasting. They are subject to many legal provisions and directives like for instance the German technical directive on air quality control (TA-Air), an administrative provision regarding the Law concerning the protection against harmful effects on the environment through air pollution, noise, vibrations and similar factors.

11.2.1 Bag filter plants

Bag filters are built in chambers whereas each chamber gets a multi bag. The multi bags hang in a rack element which can be changed easily and without any auxiliaries via an assembly door. The rack consists of a L-profile base with guide rods. Support frames are in the bags for the purpose of stabilization.

The filters are made of polyester needle felt. During a working day of 8 hours the filter bags have a durability of about 2 to 3 hours according to the amount of dust.
The filter is flown through from the outside to the inside. The dust that was filtered off forms the so-called filter cake on the outside of the filter bag. This cake causes an increase of the dedusting performance, but generates also a higher flow resistance so that cleaning becomes necessary at certain intervals. The condition of the bag can be assessed via visual control due to dedusting on the outside of the bag.

The bags cannot be used for permanent operation because dedusting of the bags is not possible by extraction of dust. Dedusting is performed with the help of a vibration engine that drives a vibration rod. The bags are guided in cogs which are mounted at the vibration rod. The cleaning interval varies according to the application. As a basic adjustment, cleaning is performed every 3 hours for about 30 seconds.

Bag filter

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11.2.2 Cartridge filter plants

Like bag filter plants, cartridge filter plants consist of one or several filter chambers according to the overall filter size of the plant. The size of the filter space can be varied either through the number of cartridges or through the dimensions of the cartridges. Filter spaces of 5 up to 20 m2 per cartridge are possible sizes of filter cartridges.

The filter cartridges have a star-shaped structure allowing achieving a relatively large filter size for compact plants. The cartridges have a nozzle opening on top and a closed cover at the bottom. Because of the star-shaped design filter cake is quickly formed at the surface of the cartridges.

The crude gas to be cleaned flows through the cartridge from the outside to the inside. During this process, the dust particles are filtered off on the outside of the filter material. By this it is also possible to asses the condition of the filter cartridge by visual inspection at the cartridge filter plant.

Filter cartridges are cleaned by a jet of compressed air. This impulse of compressed air is realised in opposite direction, i.e. from the inside to the outside. This causes the dust particles to be removed from the outside of the filter cartridge and then they fall into the receiver. The air being necessary for this process should have a pressure of 4 -6 b, and it should be dry and oil-free. Dedusting is performed continuously during the operation of the plant.

The impulses are adjusted at timely fixed intervals. The impulse should last between 0.1 s and 1 s. The interval of dedusting can be selected and fixed between 5 s and 50 s. Dedusting should take place every 25 s. As an example, for cartridges having a diameter of 325 mm and a length of 1,000 mm 25 – 30 l of compressed air is necessary per impulse.

During dedusting the compressed air flows from the top through a nozzle into the filter cartridge. By this, air is additionally sucked off from the clean gas chamber for cleaning based on the injector principle. The impulse the compressed-air stream applies on the filter removes the dust particles.

Cartridge filter

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