Dust cake

The term essentially a drag coefficient for the dust cake particles, should be a function of the median particle size and particle size distribution, the particle shape, and the packing density. Experimental data are the only reflable source for predicting cake resistance to flow. Bag filters are often selected for some desired maximum pressure drop (500—1750 Pa = 3.75-13 mm Hg) and the cleaning interval is then set to limit pressure drop to a chosen maximum value. 

K, Resistance coefficient of dust cake on filter fabric kPa in water ... 

Blinding A situation wherein fabric and dust cake permeability cannot be maintamed. The collected dust so adheres to or is so embedded m the fabric substrate that it cannot be removed by the cleaning method employed. This is evidenced by continually increasing pressure drop. 

Dust cake The dust layer that builds up on a fabric filter, initially improving its collection efficiency. 

Dust porosity The porosity of the dust cake, which has a direct influence on the filter pressure drop. 

Rapping A method used on bag or other filters to dislodge the dust cake from the fabric when the airflow resistance exceeds a certain value. 

One of the oldest, simplest, and most efficient methods for removing solid particulate contaminants from gas streams is by filtration through fabric media. The fabric filter is capable of providing high collection efficiencies for particles as small as 0.5 pm and will remove a substantial quantity of particles as small as 0.01 pm. In its simplest form, the industrial fabric filter consists of a woven or felted fabric through which dust-laden gases are forced. A combination of factors results in the collection of particles on the fabric filters. When woven fabrics arc used, a dust cake eventually forms. This, in turn, acts predominantly as a sieving mechanism. When felted fabrics are used, the dust cake is minimal or nonc.xistent. 

Two methods are used to remove the dust cake, both of which require interruption of the airflow. The difference in dust-cake removal conveniently divides filters into intermittent and continuous rating. In the intermittent type the pressure increases (with time) up to a pre-arranged level. The airflow is then stopped and the fabric is mechanically shaken. In the continuously rated filter the pressure drop rises to a low set point, after which it remains constant across the filter as a whole. The cleaning is done by isolating a part of the filter from the air stream and that section is cleaned. 

Because of the assumptions underlying its derivation, the Kozeny-Carman equation is not valid at void fractions greater than 0.7 to 0.8 (Billings and Wilder, op. cit.). In addition, in situ measurement of the void fraction of a dust layer on a filter fabric is extremely difficult and has seldom even been attempted. The structure of the layer is dependent on the character of the fabric surface as well as on tfie characteristics of the dust, whereas the application of Eq. (17-12) implicitly assumes that K2 is dependent only on the properties of the dust. A smooth fabric surface permits the dust to become closely packed, leading to a relatively high value of K2. If the surface is napped or has numerous extended fibrils, the dust cake formed will be more porous and have a lower value of K2 [Billings and Wilder, op. cit. Snyder and Pring, Ind. Eng. Chem., 47, 960 (1955) and K. T. Semrau, unpublished data, SRI International, Menlo Park, Calif., 1952-1953]. 

Leith and First [J. Air Pollut. Control Assoc., 27, 534 (1977) 27, 754 (1977)] studied the collection efficiency of reverse-pulse filters and concluded that once the dust cake has been established, straight-through penetration by dust particles that pass through the filter without being stopped is negligible by comparison with penetration by dust... 

P< pressure drop through the accumulated dust cake in a fabric bag filter Pa... 

Aramid—for high temperature applications Polytetrafluoroethylene—used to capture fine particles where an artificial dust cake is required... 

Based on observations by other researchers that formation of a dust cake is important to efficient dust collection for fixed bed granular filters, we have developed a new concept for a moving bed granular filter that makes use of this property. The goal is to establish a quasi-steady dust cake that is continuously or periodically renewed on the upstream side of the dust cake and swept away on the downstream side. 

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