Plate and frame filter presses have been used for many years, and predate the recessed chamber filter press, which is the type that most people think of today when they hear the term “filter press”. Although recessed chamber presses are now more of the norm, the plate and frame design has some advantages with certain applications and is still used today.
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The plate and frame design utilizes a flat, solid plate (known as a “flush plate”) covered by a filter press cloth, with a hollow frame between each plate. A metal skeleton holds and clamps the filter plates together to create a chamber, which is formed by the flush plate on each side of a frame. Filter cake is collected within the frame. Historically, plates were made of wood or cast iron. Today, the most common material is polypropylene.
The slurry is pumped into a corner hole and flows into each frame, allowing solid particles to accumulate on the filter cloths. The remaining filtered liquid (also known as filtrate) then moves to a drainage port in the flush plate and into a corner hole that is not being used for feeding the slurry. The filtrate then travels to discharge piping and is directed to the next step in the process.
After a period of time, the frames become filled with solids, the slurry feed pump turns off, and the filter press is ready to open. Each frame should now contain a filter cake, which is the end result of the solids forming on the filter cloths. The filter cakes are then scraped out of the frames using a spatula, ideally falling into a cake hopper placed below the press.
The primary advantages of plate and frame presses are the ability to quickly change the filter cloths as well as the ability to use a wider range of materials for the filtration medium, including paper, which is often used for fine filtration applications.
For example, in a process where immediate particle retention is important, filter paper may be required as fine solids would either blind off the filter cloth or simply pass through it. Note that filter cloths are still in place to support the filter paper. The filter cloths are reused, but the filter paper is replaced after every filtration cycle.
Plate and frame presses are best suited for projects where filtration area is more important than volume. This is why plates and frames have their greatest prominence in polishing applications (as opposed to dewatering projects), where the feed solids concentration is often well below 1% by weight.
Polishing applications are sized for a particular filtration area based on the process flow rate and the known flux rate (volume per area per unit of time). To avoid dips in achievable filtration flow as the press fills with solids, polishing applications usually include a redundant filter press that is also sized to accept 100% of the desired process flow.
One disadvantage of the plate and frame design is the potential for building uneven pressure between frames, which can lead to plate damage. This issue stems from the fact that the frame is being fed through a small opening in the corner, which can lead to blockage. This clogging can cause one frame to become full while the next frame in-line remains empty.
Another disadvantage is the potential for leakage. The filter plates used in plate and frame presses are non-gasketed, which allows them to wick liquid under pressure. This type of filter press is usually mounted on a raised platform or positioned over a pit where any leaked liquids can be collected more conveniently.
A recessed chamber filter press utilizes filter plates that form a series of cavities between them when they are clamped together. Suspended particulates collect on the filter cloths and build up inside each cavity space to form a filter cake. Compare this to the hollow frames of the plate and frame design where solids are collected.
The skeleton of a recessed chamber filter press is constructed in a similar fashion as a plate and frame press where the plates hang on a steel frame, and the frame acts as a clamping device for the filter plates. Both types of presses also utilize a hydraulic cylinder and pump for opening, closing, and clamping the press.
As with plate and frame filter plates, recessed chamber plates are most commonly made from polypropylene. However, recessed chamber plates can be either gasketed or non-gasketed. Gasketed plates offer greater protection from leakage, but can also require a more time-intensive cloth changing process. Non-gasketed cloths or plate and frame cloths can have latex applied to the edges to help minimize (but not eliminate) wicking.
Slurry is pumped into the filter press through the feed (most commonly in the center of the plate) and the solids are distributed evenly in each chamber throughout the press (1). Solids begin to build on the filter cloths. Most of the solid/liquid separation is accomplished by the particulates building on the cloths. At first, some fines pass through, but eventually the solids begin to form a layer on the cloths (2). This layer traps the fine particles and eventually forms a filter cake. The filter cake then serves as a depth filter as the dewatering process continues.
The filtrate (liquid) usually exits the filter pack (plates) through corner ports into a manifold. When the correct valves in the manifold are open, the filtrate exits the press through a single point, the filtrate outlet. The filtrate is then directed via discharge piping to the next step in the process.
As the filter press builds pressure, the solids build within the chambers until they are completely full of filter cake (3). Once the chambers are full, the fill cycle is complete, and the filter press is ready to be emptied (4). The press is then opened, and the plates are moved apart. As the plates are shifted, the filter cake falls from each chamber into a cake hopper or drum below the press. A spatula can be used to scrape the cake from the cloths if needed.
One of the main advantages of the recessed chamber plate design is the ability to feed a large center hole versus a small opening in the corner of the plate. The large hole in the center of the plates allows the chambers to fill faster and more evenly, with less risk of clogging and causing plate damage due to unequal pressure. This is especially important in applications with high feed solids.
However, it’s also important to note that a larger feed inlet (combined with gaining one more filtrate outlet in a center feed configuration) is important in large-scale polishing applications with high flux rates (for example, a Merrill-Crowe process). If the filter plate inlets, outlets, and internal porting are not large enough, the calculated press flow rate will not be achievable.
Another advantage is the ability to achieve higher feed pressure with the recessed chamber design. The “frame” or sealing surface of the recessed chamber plate is all connected to the center web. This keeps the “frame” from bowing out under high pressures.
Also, filter cake release is generally easier with a recessed plate. When the press is opened, and the plates are shifted, the cake either falls out or is quickly scraped off the cloths using a spatula. Cake release may not be as fast with a plate and frame press, as the filter cake is held by a non-tapered frame and may require more manipulation for removal.
Although recessed chamber presses have several advantages over plate and frame, one disadvantage is the limitation on cake thickness, which is dictated by the depth of the chamber. Recessed chamber plates cannot be easily swapped out for a wider or narrower frame as in a plate and frame press. This makes the plate and frame design better suited for applications where testing variations in filter cake thickness is important.
Another disadvantage can be seen in changing filter cloths. The plate and frame design allows for a simple drape-over cloth, whereas recessed chamber either requires pounding filter cloths into the plates (gasketed) or securing cloths to the plates using cloth dogs and zip ties or hook-and-loop fasteners (non-gasketed).
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Air operated diaphragm pumps are used with filter presses to pump sludge from the bottom of the sludge holding tank into the filter press. These pumps are ideal because they can handle the slurry which is generally about the consistency of a milkshake.
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The diaphragm pump usually starts out slowly and gradually ramps up. Once the pump starts to slow back down, (30-60 seconds between thrusts), it is usually an indicator that the filter press is full. At this point, the feed pump should be turned off and the valves on the filter press closed as the operator prepares to operate the air-blow-down process.
The diaphragm pump can be manually turned up or an Automatic Pump Control system may be added. This starts the pump at 25 psi, then moves up to 50, 75 and finally 100 psi which is standard operating pressure for the pump.
Met-Chem provides diaphragm pumps in the following sizes: 1”, 1.5”, 2” and 3”. The material of construction ranges from cast iron, aluminum, polypro or stainless steel.
With the purchase of a new Met-Chem filter press, Met-Chem can mount your air operated diaphragm pump to the legs of your filter press. This option also includes a flex hose that connects the outlet of the diaphragm pump to the center feed pipe, an air filter, a regulator, and an oiler. We can also provide a mounted diaphragm pump for your existing press.
Automatic Filter Press Pump Control SystemThe Met-Chem Automatic Pump Control System is used in conjunction with an air operated diaphragm pump to automatically increase the feed pump pressure at predetermined intervals during the filter press filling cycle. If the pump runs at a higher initial pressure, the filter cake will become a packed layer of solids on the filter cloth. This will restrict the flow of the slurry thereby creating a very dense coating of the filter cake. This can cause the cloths to blind off and not allow further filtration. Starting at a lower feed pressure allows a soft layer of slurry particles to be deposited on the filter cloth. This initial layer becomes the filtering media which will enhance the filterability of the incoming slurry.
The automatic pump control system automatically and gradually increases the feed pressure in four stages during the filling cycle. These stages are adjustable to suit your slurry and solids concentration. However, it is commonly set at increments of 25 psi. (25, 50, 75, 100). By monitoring the feed pump flow sensor, during the final pressure stage, the operator will be able to determine the cycle completion.
The Automatic Pump Control System also incorporates a low hydraulic pressure safety shut down the device. If there is not enough hydraulic clamping pressure present, the system will completely shut down, eliminating any possible leaking from between the filter plates.
The pump control system allows your filter press to run at its optimum efficiency creating dryer filter cakes, reducing disposal costs, reducing operator time and increasing the life of your filter cloths and feed pumps.
This system can be incorporated with a new filter press from Met-Chem, a used unit or even your own existing press.
Spare Parts KitMet-Chem is pleased to offer our spare parts kit which includes our 5 piece center feed pipe assembly, a head, tail, and 2 intermediate filter press cloths, and 100 foot of gasketing material. These are essential items for the operation of the filter press. A cracked pipe, ripped cloth, or torn gasket can happen anytime and having these items in your stock will ensure you will be back up and running in no time.
Center Feed and Manifold Filter Press UpgradesA center feed pipe assembly on a filter press should be able to last many years the same way any other type of plumbing will. However, if the material of the center feed pipe assembly is not strong enough to handle the corrosiveness of the solution running through the press then the center feed pipe assembly can corrode.
For cases of corrosive material or high temperatures, Met-Chem is pleased to offer upgrades from our standard center feed pipe and manifold. We are able to upgrade to CPVC, Carbon Steel, 304 or 316SS Stainless Steel, and Titanium. Met-Chem can also provide additional manifold upgrades such as automatic valves
The center feed pipe is what connects the filter press to the product that needs to be filtered. The center feed pipe assembly consists of five different parts. The first part is the lock nut which fastens on the center feed pipe and connects to the outside of the press. The pipe, which is the second part, goes through the head of the filter and all the way through the head plate of the filter press. The third part of the center feed pipe assembly is a gasket which ensures the assembly is watertight. Next, is the clip nut seat which fastens the assembly to the head plate. This part of the assembly will always stay in place unless you are replacing the assembly or one of its parts. The final part, the cloth clip nut, will be removed often as it is screwed into the cloth clip nut seat after the head cloth is put on the head plate. The cloth clip nut needs to be tightened in order to keep the cloth in place. Met-Chem provides a tool to make this process easier. The Met-Chem Spanner Wrench is a specially designed tool that can be used to loosen and tighten the cloth clip nut when changing the head cloth on a filter press. This tool can be very helpful as the cloth clip, but it has a design that requires a unique tool to tighten and loosen it.
Longer Legs and Filter Press Sludge Disposal ChuteMet-Chem can provide longer legs and a disposal chute on your new filter press. This is especially useful if you are releasing the sludge from the press into a sludge bag or a drum. The chute will direct the sludge directly into your container. If you plan to use a sludge disposal bag, we can add hooks to the chute so that the sludge bag can be hung from the frame of the press.
For some larger presses, we can build a catwalk along with the disposal chute in order to accommodate a roll-off container.
Sludge DumpsterSludge Dumpsters are used directly under the filter press to catch the sludge that is released from the filter press during cleaning. The dumpster is on casters for easy mobility. They are also equipped with forklift slats for convenient use with a tow motor. The operator can move the dumpster to a roll-off container with the forklift and then pull the release pin for easy emptying. The dumpster can then be snapped back to the upright position and put back under the filter press.
Backer Plate for Smaller Filter Press BatchesIf you produce varying size batches of wastewater and subsequently less sludge, or if the overall volume of waste that is generated at your facility has been reduced, you can still utilize your existing filter press. With the purchase of a backer plate, you can alter the holding capacity of your press to meet your individual batch needs.
A backer plate provides support to the tail plate so that it can be moved anywhere in the plate pack in order to isolate the chambers between the feed end and the tail plate. If you try to use the tail plate in a different position in the plate pack without the backer plate, you run the risk of damaging the chamber of the tail plate.
There are different kinds of backing plates. Some are made of plastic and look much like a standard plate; we prefer to use ~3/8” thick flat stock steel and cut an outline of a plate. This is clearly visible when placed in the stack so that all around know that the filter press is not at full capacity. This steel backing plate is then painted with the same two-part epoxy coating as the frame of the filter press.
Filter Press Plate ShiftersThe start-up and installation options are available with all Met-Chem waste treatment equipment. We can provide full support as you get your system up and running. We have installation technicians and technical representatives that we can send to your facility to make sure the system is running as it should based on the engineering and design.
Our services include but are not limited to:
All units and systems are complete with manuals and support. However, if you need someone to come to your facility, we will be happy to quote you on that as well.
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