11.2.3 Residual dust content

The maximum admissible residual dust content contained in the clean gas is indicated in TA-Air; inorganic dusts are divided in three special categories because of their toxicity.

Bag filters can have a residual dust content of 7 mg/m³ at most. The seams of the individual bags are a weak point as regards bag filtering. When these seams are agglutinated with polyester felt a value of 2 mg/m³ can be realised.

The residual dust content of a cartridge filter that can be achieved amounts to some 2 mg/m³.

11.2.4 Comparison of the filter plants

Compared the other filter systems the compact construction is an advantage of the cartridge filter plant. The filter space of a filter cartridge is, for example, bigger by a factor of 7.5 compared to a filter hose of the same diameter. This spatial advantage is reduced by the lower filter area loading resulting form the star shape.

The filter area loading results from the ratio of crude gas volume flow to filter area. For example: At a volume flow of 5,000 m³/h and a filter are of 46 m² (bag filter) or 70 m² (cartridge filter) the following filter area loadings result in a comparably big filter chamber:

f_sB = 108.7 m³/h m² (bag filter)
f_sC = 71.4 m³/h m² (cartridge filter)

Compared to bag filters the continuous dedusting of the cartridge filters is a great advantage. The filter conditions can be maintained relatively constantly by regular dedusting during operation. When dedusting the filter bags, operation must, however, be interrupted during the cleaning process. Furthermore, the filter conditions will change by the formation of a respective filter cake in case of longer filtering without dedusting.

The pneumatic dedusting of the cartridges is more material-gentle than mechanical dedusting of the filter bags. Reinforcements can be attached in the area of the cogs to protect the filter bags against damages during dedusting.


11.2.5 Filter monitoring

The filter element clogs with increasing use resulting in an increase of the pressure difference at the filter. If the resistance exceeds a limit value and if sufficient cleaning is not possible any longer, then the filter element will have to be replaced. This value is controlled by measuring the pressure difference between filter chamber entry and exit.

11.2.6 Pressure losses

For sucking off dust-laden air both smooth pipes as well as air ducts can be used to set up connection with the filter plant.

Air ducts are plastic hoses that are reinforced by a spiral made of spring steel. According to the respective spiral distance the air duct can be used for very different vacuums.
As a standard, air ducts are available at diameters ranging from 250 – 2400 mm and in single lengths of 5 and 10 m.

The pressure loss amounts to about 50 Pa per m/air duct (NW 300), as regards a smooth pipe this value is only about 10 Pa per m/pipe (air velocity about 20 m/s).

The total pressure loss in a pipe transporting air should not exceed the value of ventilator compression, if possible. If the pressure loss of the plant is higher than the ventilator compression the air volume flow decreases.