Pressure Filters

Pressure Filters, with the exception of the Rotary Drum Pressure Filter, are semi-continuous type machines that enter a wash and cake discharge mode at the end of the filtration cycle. The filtration cycle may extend from 5-10 minutes on cake filtration applications and up to 8 or even more hours for the polishing of liquids. Since the operation is in batches, that are usually fed from and discharged to a continuous process, a surge tank is required upstream the filter and batch collection of cake downstream the filter. The collection of filtrate depends on the operating mode of the filter which can be constant flow rate, constant pressure or both with pressure rising and flow rate reducing as for a centrifugal pump.

Most Pressure Filters are batch operated but continuous filters are also available. However, owing to the difficulty in removing the cake they are mechanically complex and expensive so mainly applied in fine chemical processes where the added value to the product is high.

There are two constraining factors that determine the duration of the filtration cycle:

The filtration rate is influenced, in broad terms, by the properties of the slurry. The trend is that the rate goes up with increased pressure, coarser particles, particle distribution with high uniformity, non-slimy or non-gelatinous solids, non-compressible cakes, lower liquid viscosity and higher temperatures.


Body-Aid and Precoating

Body-aid (sometimes also called filter-aid) and precoating are often mentioned in connection with pressure filtration and the difference in their application is:

The following materials serve as body-aid or are used to form a precoat:

All the material above are produced to specification in a controlled process. However, for applications that do not require high quality any other solid that is readily available and has similar properties may be used as body-aid.


 The Types of Pressure Filters Pressure filters tree

Pressure filters may be grouped as shown in the tree to the right.

The sizes for standard equipment are as follows:

The block diagram for a typical filter station and its components may look like this:   Block diagram

The advantages and disadvantages of pressure filtration compared to other separation methods are: