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PRESCAD / Technical notes: Differences Between Nominal and Absolute Filtration

PRESCAD / Technical notes


In the filtration of liquids there are two criteria that are used to define the efficiency with which a filter can retain particles with a certain micration. These criteria are called Nominal Filtration (NF) and Absolute Filtration (AF).


Differences Between Nominal and Absolute Filtration

Each filter is designed to remove particles of a certain micron size. There are situations in which it is required that absolutely all particles greater than a certain micron have to be retained by this filter, because if they were not caught, the consequences would be serious. And there are situations in which it is not so necessary that all these particles are retained and if they are not retained the consequences will not be serious.

In order to differentiate the efficiency with which each type of filter works, two classifications have been created: The Nominal Grade filters and the Absolute Grade filters.


Nominal Degree Filtration

The filters that can be classified in Nominal Filtration (FN) catch a large percentage of particles of a certain size but never 100%, this will always oscillate between 50% and 95%. So for example if we have an application in which we install a filter of 5 microns Nominal grade, at most we will catch 95% of the particles equal to or greater than 5 microns and 5% of the particles of 5 microns will not be retained and will still be present in the fluid. The filtration efficiency will be greater with the larger particles that are present in this fluid. For example, this same filter will retain 97% of the particles of 10 microns and 100% of those of 50 microns.


It seems that a nominal grade filter is an imperfect filter, but given the economy advantages offered by these filters, there are many situations that justify installing them. Nominal grade filters are installed in applications where the economic factor is important and where the consequences of not catching all the particles are not serious nor will cause great damage later


Absolute Grade Filters

Filters classified as Absolute Grade Filters are much more expensive than Nominal Grade Filters but offer the highest filtration efficiency by retaining most of the particles for which they are designed. For example, an absolute degree filter for 5 microns will retain at least 99% of all particles of 5 microns or larger.


GA filters are used when it is extremely important to catch all particles of a certain size because if any of them were not caught there could be serious consequences. A typical market that uses GA filters is the pharmaceutical industry, since many of the solutions are for human consumption. On the other hand there are applications where the consequences of not catching all the particles of a certain size are not life or death but it is often more economical to use GA filters to protect equipment that are much more expensive than GA filters.


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Source: Prescad / Elblogdefiltración