The Tray Filter

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Indexing Filter


The Tray Filter, as opposed to the Horizontal Belt Filter, is relatively new to the chemical process industries and its main feature is that it lacks the traditional rubber belt. It belongs to the group of top feed filters and is primarily applied to the finer chemical compounds handling thin cakes although in recent years large machines suitable for thick cakes may be seen on the bulkier processes.

Sizes may vary from 0.25 to 3.0 meter wide units and lengths of 2 meters for pilot units and up to 25 meters for industrial machines, hence, the range of effective filtration areas is from 0.5 to 75 m2.

There are basically two design concepts both of which use indexing with the "vacuum on" during filtration and "vacuum off" for a very short time to enable a new "vacuum on" cycle:


The Reciprocating Trays Type

This filter consists of a series of trays which are allocated for cake formation, washing and drying. The trays are each connected through a flexible hose to a manifold that runs along the filter and collects the filtrate to vacuum receivers. To meet such requirements the manifold is separated by blind flanges which can be set at different positions to suit any specific application such as separating mother from wash filtrates or counter-current washing in several stages. The number of receivers is determined by the process requirements with one unit collecting the mother filtrate and others the wash filtrate and the cake drying.

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Filter Cloth
Reciprocating Trays
Flexible Hoses
Filtrate Manifold
Filtrate Valve
Air Purge Valve
Feed Box
Discharge Roll
Cloth Form Roll
Take-up Roll

The filter consists of the following subassemblies:

The reciprocating trays are designed so that, while under vacuum, they can move freely in the forward direction together with the cloth and at the same velocity . When they reach the end of stroke point the vacuum is cut-off, the trays are purged by an atmospheric release and pulled back by an air driven pneumatic cylinder. During the backstroke the filter cloth keeps moving forwards since there is no vacuum between the retracting trays and the backside of the cloth which holds them together.

The vacuum and purge modes are controlled by 3-way solenoid actuated valves with one valve located at the end of the manifold and the others on the inlets to the vacuum receivers. Just before the manifold valve opens to purge the trays the valves on the receivers get closed so that there is no interruption in the vacuum level downstream the filtrate outlets. The trays then enter backstroke and when they reach their end position the manifold valve closes and the receiver valves opens so that the trays are again under vacuum and ready for a new cycle. With efficiency in mind the backward stroke is fast so that the ratio of filtration time to downtime will be as high as possible.

A typical flow scheme with two co-current washing stages is shown on the right. The trays are fixed and the cloth moves forwards as explained below.


Fixed trays filter


The Fixed Trays Type

The trays of this filter are fixed to the frame and the cloth moves forwards in short increments during "vacuum off" and remains stationary at the "vacuum on" mode. The vacuum modes are controlled by solenoid actuated valves arranged in a similar method described in the section on the Reciprocating Trays Filter.

Cake discharge is achieved by one of two systems:

  • Retracting roll with pneumatic actuator

The discharge roll moves forwards during "vacuum on" and retracts to allow the cake to drop to the discharge chute. This type is shown in the flowsheet above, in the image on the right and the animation below.

For further description of the operational sequence please click here.

This machine, having pneumatic activation, has no major electric components except for limit switches or sensors for the belt aligning mechanism. Therefore it is suitable for operation in hazardous environments that would otherwise require explosion proof devices.

The picture on the  right shows a 1m2 pilot filter and its pneumatic cloth drive actuator.


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Cake Discharge Roll
Cloth Locking Roll
Cloth Tensioning Roll
Fixed Trays
3-Way Valves
Cloth Wash Manifold
Pneumatic Cylinder

Play the movie to watch how the cake forms and dries under the stationary cloth
with vacuum "on" and discharges from the moving cloth when vacuum is "off"



  • Fixed roll with "on-off" motor actuator

In this system the discharge roll is mounted on the frame in a fixed position and a timer controlled pulley moves the cloth at "vacuum off" to allow cake discharge and "vacuum on" during filtration. A special tilting feed box pours the slurry onto the filter deck when the cloth moves at "vacuum off".



Tilting Slurry Feed Box
Fixed Cake Discharge Roll
Cloth Drive Pulley
Cloth Take-up Roll
Cloth Aligning Mechanism
Wash Boxes
Mother Filtrate Outlet
Wash Filtrate Outlets
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Selection Criteria

Tray type filters are generally selected in the following cases:

Operational Sequence

The Reciprocating Trays Type

The indexing sequence is done in two stages:

The Fixed Trays Type

The indexing sequence for the type with the moving discharge roll (shown in the animation above) is done in two stages:


As opposed to rubber belt filters, there are practically no sealing problems with tray type filters.

The main assemblies that require inspection for preventive maintenance are: