The Filterpress

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Filterpresses were introduced at the turn of the century and have been around for many years mainly dewatering waste sludges. They were considered labor intensive machines hence they did not find much acceptance in the sophisticated and highly automated process industries. It was not until sometime in the 60's that this image has changed by the introduction of advanced mechanisms that were oriented towards obtaining low moisture cakes that discharge automatically and enable the washing of the cloth at the end of the filtration cycle.

The Filterpress consists of a head and follower that contain in between a pack of vertical rectangular plates.

Each plate is dressed with filter cloth on both sides and, once pressed together, they form a series of chambers that depend on the number of plates. The entire  pack of plates is supported by side or overhead beams.

Feed Conn.
Filtrate Conn.
Filter Cloth
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The head serves as a fixed end to which the feed and filtrate pipes are connected and the follower moves along the beams and presses the plates together during the filtration cycle by a hydraulic or mechanical mechanism.  The plates have generally a centered feed port that passes through the entire length of the filterpress so that all the chambers of the plate pack are connected together. Likewise, four corner ports connect all the plates and collect the mother and wash filtrates in a "closed discharge" towards outlets that are located on the same side as the feed inlet. Some filterpresses have plates that are fitted with cocks at their lower side so that the filtrate flows in an "open discharge" to a trough and serve as "tell tales" on the condition of the filter cloth by the clarity of the filtrate that passes through each chamber. The disadvantage of this arrangement is that it cannot be used with filtrates that are toxic, flammable or volatile.

A typical flowscheme looks like this:



The present day Filterpresses, as mentioned previously, are equipped with features that enable fully automatic operation controlled by PLC's.

The main features are:




Precoating and Body-Aid

Often special measures are taken to ease cake discharge and enhance filtration.

The measures are:

Precoating the plates prior to introducing the feed is done only in the following cases:

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing.

When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal.

Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake.

Please refer to the section on Pressure Filters for further details on precoating and the addition of body-aid.

The Plates

For many years filterpresses, named Plate and Frame, have used flush plates with separate frames to contain the cake. These Plate and Frame Filterpresses had many sealing surfaces which were the main cause for leakages so the introduction of Recessed Plates has cut the number of surfaces in half and reduced the problem of drippings. The development of Recessed Plates has gone hand in hand with advances in cloth technology which enabled 3 dimensional stretching as opposed to Plate and Frame where the cloth remains in one plain.

Present recess depths are 16, 20 and 25 mm so the corresponding cake thicknesses are 32, 40 and 50 mm at maximum filling. Filterpresses are built for operating pressures of 7, 10 and 15 bar for cake squeezing and the largest available plates are 2 by 2 meters so the hydraulic pressure system that holds the closing force of the plates is designed accordingly. Filterpress plates are available in various materials of construction such as cast iron, aluminum alloys, high-density polypropylene and PVDF. The major area of development, apart from automation, was in the design of the plates since thermoplastics have enabled new structural concepts which were not possible with metallic plates.

Membrane plate

The special features are:

Most plates are extruded in polypropylene which withstands temperatures of 80-85C. Operating at higher temperatures will warp the plates and leakage or even squirts can be dangerous at such high temperatures.

Selection Criteria

Filterpresses are best selected in the following instances:

They should be selected with care:


Operational Sequence

The basic design of the following configurations may be viewed in both animations to the left:

Filterpress with conventional plates:

Pumping and discharging


Filterpress with membrane plates: 

Squeezing and discharging


Cake Disposal

Discharged cake

During cake squeezing drippings are collected in a drip tray underneath the plate stack to avoid them from reaching the dry cake collection bin. The drip tray consists of two hinged horizontal doors that span along the filterpress with a slight angle to drain the drippings towards the head or the follower. When the filtration cycle is completed the doors open with a pneumatic or hydraulic system, the plate stack starts shuttle shifting and the cake falls one by one into the bin. When all the cake is in the bin the doors close and are ready for another cycle.

The cake in the bin is then trucked away or transported with a belt conveyor. With very large filterpresses a well formed cake may weigh 200-300 Kg per chamber and when it falls into a bin or onto a belt conveyor in one solid piece the impact is very high. Hence, special measures are required to break and de-lump the sole hard cake and, for belt conveyors, it is also recommended to increase the number of belt support rollers below the discharge chute at the point of impact.

The clip below shows the operation of the drip pan:




The filterpress by itself requires little maintenance however the automation features that accompany modern filterpresses should be checked regularly and with particular attention to safety devices such as: