What is the difference between a plate and frame filter press and a recessed plate filter press?
(Photos above are of plates and frames)
Plate and Frame Filter Press
The plate and frame filter press is the most fundamental design and the oldest type of filter press. The design is a skeleton made of metal that holds and clamps the filter plates together to form a pressure chamber. This clamping is done by either an old style wheel crank or more modernly with a hydraulic pump and cylinder (either manual or automatic). The plates are made out of a variety of metal such as cast iron, aluminum or plastics, with the most common being polypro.
The plate and frame design is to have a solid plate covered by a filter cloth and a hollow frame in between each plate. The slurry is pumped into one or more of the corner holes and it travels into each frame before filtering through the cloth. The clean filtered solution travels across the drainage service of the solid plate and into one or more of the corner holes that are not being used for the slurry feed. This clean solution travels to the discharge plumbing to customer’s tanks, drain or process. Once these hollow frames are full of solids, no more slurry feed can come into the filter press and it is time to open the press and knock out the solids.
The advantage of this design is that different thicknesses of frames can be used for a variety of cake thicknesses. This allows flexibility in the amount of filtered solids that can be produced per batch. One disadvantage is the fact that feeding the corner and flowing through a small opening into the frame can cause plugging leaving one frame full and another empty. The uneven pressure in the one frame can cause the solid plate to blow out it’s drainage surface and destroy the plate. The other disadvantage is this style of plate is what we call non-gasketed. The cloth that hangs over the plate serves as a gasket between the plates, but often the cloth wicks liquid under pressure or out and out leaks solution between the plates. Quite often these style of filter presses are mounted over grating or drainage pits to collect the leaked liquid.
Recessed Plate Filter Press:
The modern version of a filter press is to use plates that have recesses in them. When these plates are pushed together, they form the cavity (replacement for the hollow frames of the plate and frame design) for solids to collect in. The filter press skeleton is made the same way as a plate and frame where the plates hang on a steel frame and are clamped together by a hydraulic cylinder and pump (the smaller size can use a manual version while the larger ones are normally automatic).
These recessed plates are normally manufactured out of polypro and come in gasketed and non-gasketed types. The gaskteted plate is normally center feed with rubber gaskets around the corner holes and the solids cavity.
The feed slurry is fed through the center feed and the solids collect in the plate cavity, while the filtered liquid passes through the cloth to the drainage surface. The filtration cloth has a rope sewn in it and it pounds into the groove in the filtration area to hold it into place. From the drainage surface, the liquid flows to the corner holes where it goes into the discharge piping and then to the process tanks or drain.
The recessed plate is also manufactured in a non-gasketed design. The plates and it’s application are the same as a gasketed plate, except that it has no gaskets. The filtration cloths sticks out between the plates and often has a latex coating on the sealing surface to try and decrease or stop cloth wicking and leakage.
One of the advantages of a recessed plate is the ability to feed a large center hole which easily and quickly fills the cavities without clogging and damaging plates because of unequal pressure. The other advantage is the dry operation because of the gaskets or the latex edging on the cloth of a non-gasketed plate. Also, with the recessed plate, when you open the filter press and slide the plates, the cake falls out in most cases because it’s not held in by a 4-sided frame.
The disadvantages are the restrictions of the cake thickness based on the recess area. It can’t be changed by adding a thicker or thinner frame like in a plate and frame unit. The cloth change out is also a little more difficult than the simple drape over on a plate and frame.
In summary, most modern filter presses use recessed plates because of the advantages of dryer operations and the ability to feed the larger center hole. Plate and frame units are used more in production applications, like in the chemical industry, where different processes require different cake thicknesses and a frame change out easily accomplishes this.