The Rotary Drum Filter

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Drum Filter


The Rotary Vacuum Drum Filter belongs to the bottom feed group and is one of the oldest filters applied to the chemical process industry.

The filter consists of the following subassemblies:

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  • The Drum

The drum is supported by a large diameter trunion on the valve end and a bearing on the drive end. The drum  face is divided into circumferential sectors each forming a separate vacuum cell. The internal piping that is connected to each sector passes through the trunion and ends up with a wear plate having ports that correspond to the number of sectors.

  1. The Valve

    A valve with a bridge setting controls the sequence of the cycle so that each sector is subjected to vacuum, blow and a dead zone. When a sector enters submergence vacuum commences and continues through washing, if required, to a point that it is cut-off and blow takes place to assist in discharging the cake.

    The valve has on certain filters adjustable blocks and on others a fixed bridge ring. Adjustable bridge blocks enable the optimization of form to dry ratio within the filtration cycle as well as the "effective submergence" of the drum when the slurry level in the tank is at the maximum.

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Cake Formation
Cake Washing and Drying
Cake Blow Discharge

 Valve Connections

The majority of drum filters have a valve with three bridge blocks and a single row pipe plate as shown below and on the right. The duty of the bridges is: (please also refer to Operational Sequence)

  1. Vacuum and blow zones separating bridge. This bridge cuts off the vacuum so it is slightly wider than the internal pipe port.

  2. Dead zone bridge. This bridge opens to vacuum once a compartment submerges.

  3. Start-up assist bridge. At start-up the upper vacuum zone is open to atmosphere and a cake may be formed only when closing the valve that controls this zone. Once the cake starts to emerge from the tank the valve is gradually opened and fully opened when the entire drum face is wrapped with the cake. Since in continuous operation both lower and upper zones are under vacuum this bridge is slightly narrower than the internal pipe port so that the vacuum is continuous and the cake is held onto the drum.

However, there are also more complex drum filters such as lube oil dewaxers. These filters have a sophisticated valve that allows very quick evacuation of residual wash liquid from the descending compartments by purging inert gas through the internal piping manifold prior to cake discharge. The images below show the two different valves with their single and double row pipe plates:

Two Row Internal Piping

The exploded view below shows the assembly components of a typical "one row" set-up:

Pipe Plate
Wear Plate
Main Valve
Bridge Block
Cake Form Conn.
Cake Dry Conn.
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The internal piping manifold and the various leads and trail options discussed above are shown here:

Internal Drum Piping Leads and trails

The clip below shows the internal drum piping of the "two row" manifold. The trail pipes shown in red are normally handling the mother filtrate on the ascending side of the drum up to the 12 o’clock position and then the lead pipes shown in blue handle the wash filtrate on the descending side. The trail pipes are always connected to the outer row and have a bigger diameter than the lead pipes that are connected to the inner row. The reason for this arrangement is that the trail pipes handle more liquid than the lead pipes so require a bigger cross section to avoid vacuum losses.

The animation on the left shows a partial section of the cycle of a single compartment as it passes from cake washing down to cake formation on the descending side of the drum.

Note the point when the vacuum is cut-off and the trailing pipe opens to purge. At this point the leading pipe evacuates the filtrate that remains in the piping and compartment prior to blowing off the dry cake.


  • The Drum Deck

The drum deck is divided into separately isolated compartments each subjected to vacuum or blow while the drum is in rotation. The timing of vacuum or blow depends on the bridge setting of the main valve. The compartments are divided with grooved division strips along the drum face and around the circumference of the drum heads. These division strips are holding synthetic grids shown on the right that cover the entire drum and serve to support the filter cloth. The filter cloth itself is fastened to the drum face by inserting special caulking ropes into the grooves.

Drum Deck Grids
  • The Filter Cloth

The filter cloth retains the cake and is fastened to the drum face by inserting special caulking ropes into the grooved division strips. Nowadays, with some exceptions, they are made from synthetic materials such as polypropylene or polyester with monofilament or multifilament yarns and with sophisticated weaves and layers. The image on the right shows the method of joining the cloth ends with clippers and to retain the fines from passing through to the filtrate multifilament strings are threaded across the entire cloth width. Another option quite often used on belt discharge filters is to join the ends with a special sewing machine.

The entire subject of filter cloth and its selection will be discussed in a separate section that was not yet constructed.

Scraper Discharge Click on the thumbnails to maximize images  Belt Discharge Roll Discharge String Discharge


The cake discharge mechanism that can be either a scraper, belt, roll and in very rare cases a string discharge. Blow is applied only to filters with scraper and roll discharge mechanisms but not to filters with a belt or string discharge.

The images on the left illustrate the various mechanisms. 


Discharge Mechanisms

The selection of a suitable type of mechanism depends largely on the release characteristics of the cake from the filter media and will vary from process to process. Scraper discharge mechanisms will suit cakes that release readily and roller discharge mechanism are better for thixotropic cakes.

The drum filter has a drive with a variable speed that rotates the drum at cycle times that normally range from 1 to 10 MPR.

An agitator keeps gently the slurry in suspension and reciprocates between the drum face and tank bottom at 16 or so CPM.

The clip below shows a typical agitator:

The tank that houses the drum and agitator has baffled slurry feed connections, an adjustable overflow box to set a desired drum submergence and a drain connection. The tanks are normally designed for an "apparent submergence" of 33-35% however on certain applications 50% and more is possible. With these special designs the tank ends are higher in order to accommodate stuffing boxes on both the drive shaft and valve end trunnion.

On applications where cake washing is required, 2 or 3 manifolds with overlapping nozzles are mounted to a pair of splash guards bolted to the tank ends. The position of the manifolds and the quantity of wash liquid are adjustable depending on the wash characteristics of the cake.

The flow scheme of a Rotary Drum Filter Station will generally look like this:Flowscheme 


Operational Sequence

The entire filtration cycle on a rotary drum filter must be completed within a geometry of 360 degrees. Let us follow the cycle sequence of a single sector assuming that the drum rotates in a clockwise direction while viewing the valve end:


Cake Formation Zone

Cake Predrying Zone

Cake Washing Zone

Cake Final Drying Zone

Cake Discharge Zone

Dead Zone

All Zones

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Selection Criteria

In broad terms drum filters are suitable to the following process requirements:

  • Slurries with solids that do not tend to settle rapidly and will remain in a uniform suspension under gentle agitation.
  • Cakes when a single washing stage is sufficient to remove residual contaminants from the cake or yield maximum recovery of filtrate.
  • Cakes which do not require long drying times to reach asymptotic moisture values.
  • Filtrates that generally do not require a sharp separation between the mother and wash filtrates. Some complex valves, however, enable atmospheric purging of the sectors and internal piping to facilitate a sharp separation of filtrates.
  • Filtrates that are acceptable with a low quantity of fines that pass trough the filter cloth in the first few seconds of cake formation. Broadly, and depending on particle size and cloth permeability, the filtrate may contain 1000 to 5000 ppm insolubles.
  • For very corrosive applications plastic drum filters are available with up to 10-15 m2 filtration area.

Plastic Made Drum Filter



The slow rotation of the drum and reciprocation of the agitator reduce maintenance requirements to a minimum but the following should be inspected periodically: