Cylindrical element filters

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Cylindrical element filters, often referred to as candle filters , have cylindrical elements or sleeves mounted vertically and suspended from a header sheet, which divides the filter vessel into two separate compartments. The filtration takes place on the outside of the sleeves. The inlet is usually in the bottom section of the vessel and the filtrate outlet in the top section above the header sheet. A less usual design is to locate the filtrate outlet at the bottom of the elements and thus allow the top chamber to be opened for easy inspection of the elements during operation. 

Cylindrical element filters find wider use where cake washing is not required. Their inherent advantage is that, as the cake grows on the tubular elements, the filtration area increases and the thickness of a given volume of cake is therefore less than it would be on a flat element. This is of course only of importance where a thick cake is being formed and the rate of increase in the pressure drop is less with tubular elements in such cases. 

As observed from Figure 27, the cake removal by fluid shear is also aided by centrifugal force. Other arrangements include stationary filtration media and rotating disks to create the shear effects, and rotating cylindrical elements it has also been shown how such filters can be used for cake washing. 

The axial filter (Oak Ridge National Laboratory) (30) is remarkably similar to the dynamic filter in that both the rotating filter element and the outer shell are also cylindrical. An ultrafiltration module based on the same principle has also been described (31). Unlike the disk-type European dynamic filters described above, the cylindrical element models are not so suitable for scale-up because they utilize the space inside the pressure vessel poorly. 

A candle filter contains a number of cylindrical elements in a vertical tank, using materials of construction similar to those used in leaf filters. The elements can be built up, in modules similar to those used in mist eliminators (Section 9.1.5), or they can be solid cylinders made of carbon or a ceramic. The latter are porous, with walls about... 

Figure 12.5 A cylindrical element (candle) pressure filter...

The American version of the dynamic filter, known as the Artisan continuous filter (Fig. 30), uses such nonfiltering rotors in the form of turbine-type elements. The cylindrical vessel is divided into a series of disk-type compartments, each housing one rotor, and the stationary surfaces are covered with filter cloth. The feed is pumped in at one end of the vessel, forced to pass through the compartments in series, and discharged as a thick paste at the other end. At low rotor speeds the cake thickness is controlled by the clearance between the scraper and the filter medium on the stationary plate, while at higher speeds part of the cake is swept away and only a thin layer remains and acts as the actual medium. 

Pressure Leaf Filters Sometimes called tank filters, they consist of flat filtering elements (leaves) supported in a pressure shell. The leaves are circ ilar, arc-sided, or rectangular, and they have filtering surfaces on both faces. The shell is a cylindrical or conical tank. Its axis may be horizontal or vertical, and the filter type is described by its shell axis orientation. 

Filters are available in several constructions, effective filtration areas, and configurations. Depending on the individual process, the filter construction and setup will be chosen to fit its purpose best. Most commonly used for RO filters are tubular devices, so-called spiral wound modules due to the spiral configuration of the membrane within the support construction of such device. UF systems can be found as a spiral wound module, a hollow fiber, or a cassette device. The choice of the individual construction depends on the requirements and purposes towards the UF device. Similar to the different membrane materials, UF device construction has to be evaluated in the specific applications to reach an optimal functioning of the unit. Microfilters and depth filters can be lenticular modules or sheets but are mainly cylindrical filter elements of various sizes and filtration areas, from very small scale of 300 cm to large scale devices of 36 m. A 10-inch high cylindrical filter element can be seen in Fig. 6. 

Element Any structural member in a filter on which the septum is supported may be round, rectangular, or cylindrical. 

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