Introduction
Filter Presses are used in many product manufacturing processing systems as well as to efficiently manage waste material separations. They may be needed to recover water or other solutions from the process or waste stream, and/or to produce or recover dewatered solid materials. Because Filter Presses are used so broadly, many equipment options are available.
Here are the most common options to consider when selecting a Filter Press system.
1. Filter Press plate design
When Filter Presses were first utilized many years ago, they were composed of a series of simple rectangular frames sandwiched between flat solid plates with channels or grooves on the plate surfaces to allow liquid to be removed. This is where the term “plate and frame Filter Press” originated. Solid material accumulated on filtration cloths held between the frame and flat plates. When no more material could be pumped into the frame area, the plates were separated, allowing the solid material to fall out of the frames.
The plate and frame style press design is plagued with problems when it comes to removing the dewatered solid material from the frame. Although plate and frame press designs are still used in some industries/applications, today most Filter Press systems employ recessed plate or membrane plate designs.
Recessed filter plates
Recessed plates have depressed surfaces inside the perimeter area of the plate face, eliminating the need for a frame. Filter cloths are installed on the plate surfaces. When a series of these plates are held tightly together, void spaces are created between them. A pump is used to force the solid and liquid slurry material between the plates, where solid material is captured in the recessed void spaces and the liquid is passed through the cloth and discharged through ports in the plate body.
Membrane Filter Press
Similar to recessed plates, membrane plates offer an additional means of separating solid and liquid materials that are compressible. The membrane plate is recessed, but the plate face surfaces are flexible, allowing them to be expanded into the void space created between the plates.
Similar to recessed plates, dewatering occurs initially by pumping solid and liquid material between the plates. Once no more material can be forced into the plate’s void spaces, the pump is stopped. The flexible membrane plate surface is then expanded with water or compressed air, forcing the flexible plate surface into the dewatered material. As the plate surface expands, the accumulated solid material is compressed, resulting in additional liquid removal.
2. Filter Press plate support beam designs
Another basic Filter Press option involves how the filter plates are supported. There are two basic designs: overhead beam and side beam.
Overhead Beam Filter Presses support the filter plates from an overhead beam(s). This design allows complete access to both sides and the bottom of the filter plates. It eliminates any interferences with the dewatered solid cakes dropping out of the press and allows the best access for cleaning and maintaining the press. In addition, the mechanism that separates the filter plates in an overhead beam press is typically housed above the Filter Press. This minimizes equipment contamination from the materials being dewatered, optimizing equipment maintenance cost and equipment life.
In a side beam design, the filter plates are supported with long beams on the sides of the Filter Press. With this design, the plates opening mechanism is either accomplished by the stroke of the main hydraulic compression cylinder or by a separate plate opening mechanism mounted on the side beams. In either case, the plates are attached to one another with short chains or mechanical linkages, allowing multiple plates to be opened in a single movement. The side beam design is typically used where smaller Filter Presses have sufficient capacity and where space constraints limit the equipment size. The side beam design also allows open access above the plate stack for cloth changes and plate maintenance.
3. Filtrate water discharge options
The liquid (filtrate) removed during dewatering on a Filter Press passes through the filter cloths mounted on the plate surfaces and exits the plate via internal ports or channels. On plates with closed filtrate discharge, the liquid passes from the plate surface into small ports in two or more plate corners.
From there, every plate has a corner hole milled through the entire plate thickness. With the plates clamped together, these corner holes act as a pipe pathway for the filtrate water to exit on one or both ends of the plate stack through the Filter Press’s structural plates. Piping mounted on the structural stationary and/or mobile plates directs the filtrate to a filtrate collection tank or to a thickener/clarifier tank.
Another option for removing liquid from the press is called open filtrate discharge. In this case, each individual filter plate has an internal port on both bottom corners, allowing liquid to discharge directly from each plate into a collection trough or directly onto a drip tray below the Filter Press.
Open Filtrate Discharge allows the operator to visually observe the quantity and cleanliness of the filtrate being discharged from each plate. When a filter cloth is damaged or worn excessively, very dirty filtrate will exit the press, and Open Filtrate Discharge allows the operator to immediately determine the location of the damaged cloth.
4. Filter Press feed pumps
All Filter Presses are fed by some type of slurry feed pump to force the solids and liquid into the spaces between the filter plates. The pressure created by the pumping action creates the force necessary to separate the liquid and solids. The most common slurry feed pump is a centrifugal pump. One or more single-stage centrifugal pump(s) in series can be used or a single multi-stage centrifugal pump can be used.
Positive displacement pumps are also commonly used to feed Filter Presses. An individual positive displacement pump can be used, or a positive displacement pump can be used in series with a centrifugal pump. The pump equipment used will determine the maximum pressure and flow of the slurry being fed to the filter press. Multi-stage centrifugal pumps or positive displacement pumps can generate +400 psi and several thousand gallons per minute of flow rate. Higher pressures and flows will typically increase the capacity of the Filter Press.
5. Dual feed and core blow features
Filter Presses can be fed from one or two ends with appropriately designed feed piping. Larger Filter Presses with more than 50-100 filter plates require a large flow of slurry during the initial filtration period. Feeding the press from both ends allows the press filtration time to decrease so the press can be filled faster. The dual feed reduces the amount of wear on the filter cloths, extending the life of the cloth.
When a press is designed to feed both ends, it is also possible to complete a “core blow.” At the end of the filtration stage when the feed pump is stopped, wet slurry still remains in the core hole of each plate. A core blow sequence is completed to remove the remaining core slurry before the filter plates are opened, preventing wet slurry from being discharged with the dewatered cakes.
The core blow is accomplished using a set of automatic valves connected to the slurry feed piping. The slurry is forced out of the core hole with pressurized water and/or compressed air. The water and/or compressed air are fed into one feed end of the press, and the slurry is forced out of the opposite end of the press, typically back to the slurry tank.
An animation of the core blow.
6. Filter cloth cleaning and drip trays
The filter cloth media allows solid particles to be captured between the filter plates while the liquid passes through the filter media. All cloths eventually become contaminated with some amount of solid material on the surface as well as in the spaces between the cloth weave. Eventually, the accumulation of material on the cloths will impact the filtration rate and the ability to seal the plates together to a point where the cloth will need to be replaced.
How quickly the contamination occurs is very dependent on the characteristics (particle size distribution, shape, makeup, abrasiveness, tackiness, etc.) of the material being dewatered. The cloth life can be extended by some amount of routine cleaning.
Manual cloth cleaning with water is the simplest and most typical way to prolong the efficient life of the filter cloths. Simple handheld force hoses with medium pressure and volumes are used to remove the excessive buildups on the filter cloth surfaces, especially in the sealing and stay boss areas. Fully automated cloth washing systems, shown in the video below, can also be used to more effectively clean the entire cloth surface as well as to remove some of the contamination material within the cloth weave. These washing systems use high-pressure cleaning showers that are automatically positioned for each filter plate and can be programmed to routinely clean the cloths at defined intervals of times or cycles.
Fully automated cloth washing system.
Drip trays are large moveable panels installed below the Filter Press to capture the cleaning water during cloth washing. The movement of the drip trays into the raised or lowered position is typically accomplished using hydraulic cylinders. With the drip trays in the raised position, the cleaning shower water is directed into a collection trough where it can be recycled. When the drip trays are in the lowered or in the open position, the dewatered cakes can discharge out of the press into a storage area or onto a conveyor.
7. Cake dry and cake wash
Certain Filter Press applications require as much liquid removal as possible or require very dry cakes to be discharged. Similar to a membrane plate squeeze, cake dry is used to remove more liquid from the solid material after the filtration stage has been completed using the feed pump action. After the feed pump is stopped, the plates are held tightly together while compressed air is forced between the filter plates. The compressed air passes through the solid material captured between the plates and sweeps additional liquid out of the press.
Recessed plates and membrane plates can be used to complete the cake dry stage, but the plates will have additional ports to direct the compressed air through the cake material.
Some filtration applications require additional removal of soluble material out of the solid dewatered material captured between the plates. In this case, a liquid such as clean water or other solvents are pumped into the Filter Press to “wash” additional material out of the cakes. Similar to cake dry, the cake wash is accomplished by using recessed or membrane plates with ports that direct the liquid through the cakes after they have been formed by the pumping filtration action. Once the cake dry or cake wash has been completed, the Filter Press is opened to allow the cakes to discharge before another cycle is started.
Considering these seven Filter Press options ahead of time can help you have a better idea of what will work best for your application. If you need assistance, contact McLanahan.
