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Q: What is the difference between nominal and absolute ratings and Beta Ratio?
A: Nominal Rating- Expressed as an average micron pore size and designed to retain some particles of that size.
Absolute Rating- Expressed as the maximum sized particle which the filter will pass. Provides consistent performance at rated micron.
Beta Ratio- A mathematical expression defining the number of particles of a given size upstream of a filter relative to the number of particles of the same size downstream of the filter.
Q: What is a filter's efficiency?
A: Efficiency Rating is the percentage of contaminant removed by a filter. It is determined by comparing amount of contaminant Before and After a filter.
Q: What is the difference between surface and depth filtration?
A: In Surface Filtration all particles retained lie on the upstream surface of the medium. Primarily uses mechanism employing direct interception or screening.
In Depth Filtration particles are removed throughout the thickness of the media matrix. Depth filtration uses both direct interception and inertial impaction and can involve physical-chemical mechanisms. It captures particles which are smaller than the pore size. Depth filtration can also use direct interception throughout the filter by graduating the density of the filter media.
Q: What is the pore size?
A: Pore size refers to the largest size of 'pore' in the structure. Filter sheets do not have discrete pores in the same way as membranes and rely upon tortuous depth filtration to achieve particle removal. Therefore pore size does not indicate what size of particle will be removed. Particles much lower in size will be removed by the filter sheet than its 'pore' size. Particle retention values are the correct way to assess the filtration performance of a depth filter.
Q: If I have a sterile filter sheet, why do I need a sterile cartridge after it?
A: Depth filters cannot be guaranteed to remove all particles below a certain size. They work on a tortuous path principle. In order to guarantee a sterile filtrate, it needs to be passed through a sterile 0.45um or 0.2um membrane cartridge. The depth filter takes the filtration load, the cartridge removes any small amounts that may be left and can be integrity tested to guarantee a sterile filtration performance. Both are required to have an economic solution.
Q: How long will my filter sheet last /how much product can I get through before it blocks?
A: While there are some guidelines for specific products at certain stages of filtration, basically we cannot advise the life of a filter sheet as it depends on the quantity of solids the filter sheet has to filter out. This normally varies from one batch to another with the same product.
Q: Why has my filter blinded early. Why has the differential pressure increased dramatically and the filter stopped?
A: This is due to the filter blinding. It is not a failure of the filter, but more so that the filter has done its work and taken the solids from the product. If this is earlier than normal for the same process conditions we would suggest the solids loadings were higher for this specific product.
Q: Why am I getting only foam out of the outlet?
A: For carbonated products some back pressure is required on the filter to ensure carbonation stays in the product.
Q: What is the maximum pressure I can run my filter at?
A: There are 2 pressures that we need to look at. The first is the inlet pressure and this is the maximum rated pressure of the filter press, say 4 bar. The second and more important is the differential pressure, the pressure drop across the filter sheet. In normal filtration we would start with a differential pressure of approximately 0.2bar. The filter sheets are clean and there is little resistance to flow. As the filter starts to load up with solids the differential pressure climbs slowly. We would normally expect to stop filtration when the differential pressure reaches 1.5bar. This is a guideline as some processes will stop at just 1bar, while others keep going to 2 bar. Normally once you reach 1.5bar differential pressure the pressure will increase quickly so very little gain in productivity can be achieved and flowrate will drop off dramatically.