Dust collectors or filter systems are used in the pharmaceutical and chemical industries to bind the dust that is generated during production. In the filter cells installed in it, the particles are separated from the air, so that in the end clean air is created and the danger from the dust is eliminated.
The type of dust or the composition of the air mixture as well as the requirements from the process result in the selection criteria for the filter cells.
Filter cells are available in various types, designs and materials. The following article is intended to give a brief overview of the construction methods and classifications of filters.
In order to be able to compare the individual filter cells in their various designs with each other or to compare them with the requirements from the process, there is a classification system that is defined on the basis of standards.
Classification of the filter cells
In order to be able to correctly assign the various filter classes or filter groups, an overview of the air filters considered should first follow in order to show when which classification applies.
Air filter overview
Air filters are classified according to their ability to separate particles of different sizes.
There are “dust filters” that are used for high dust loads and coarser particles in the air. These are further subdivided into “coarse dust filters” and “fine dust filters” – depending on the particle size. For these filters, classification is based on the EN ISO 16890 standard.
If finer or finest particles are to be filtered, “HEPA filters” are used. Here, a further subdivision is made into “standard HEPA filters” and “high-efficiency HEPA filters”. Here, too, the classification is based on the actual particle size. Classification is based on the EN 1822-1 standard.
Classification according to EN ISO 16890
The basis of the classification is the filtration efficiency of the filter cell. This indicates how many particles are retained by the filter medium and how many particles can still be found in the exhaust air after passing through the filter. The value that is of importance here is the separation efficiency of particles with sizes from 0.3 µm to 10 µm.
Based on this value, the corresponding filter group can be assigned in accordance with the EN ISO 16890 standard.
The standard provides for 3 levels according to which the filter cells are classified:
- PM1: Separation efficiency for particles up to 1 µm (0.3-1 µm)
- PM2.5: Separation efficiency for particles up to 2.5 µm (0.3-2.5 µm)
- PM10: Separation efficiency for particles up to 10 µm (0.3-10 µm)
If at least 50% of the specified particles are separated, the filter cell has reached the corresponding group. In this case, one speaks of the “minimum efficiency”, which is determined on a new filter cell. In parallel, the “average efficiency” is also measured over the entire filter cell.
Thus, in the procedure for determining the filter group, the minimum efficiencies for the 3 particle sizes are first determined together with the average efficiencies. At the step where at least 50% of the deposition is achieved, the average efficiencies are taken and rounded down to the nearest 5 step. These values can then be used to specify the classification
be
Example: ISO ePM2.5 80%
If it happens that a filter cell fulfills the conditions for several stages, it is up to the manufacturer of the filter cells to decide which indication he makes on the product.
Classification according to EN 1822-1
The DIN EN 1822-1 standard refers to the classification of high-performance particulate and high-performance HEPA filters used in cleanroom technology or the pharmaceutical industry. In addition to the classification of the filter cells, the production and testing are also specified in the standard. It should be noted here that Part 1 of the DIN EN 1822 standard is valid for the classification of filters – the specifications for production and testing, however, can be found in Parts 2-5 of ISO 29463.
The DIN EN 1822-1 standard distinguishes between filter groups and filter classes:
| Filter group | Filter class | Integral value separation efficiency in MPPS | Integral value transmittance in MPPS | Local value separation efficiency in MPPS | Local value transmittance in MPPS |
|---|---|---|---|---|---|
| EPA = high performanceparticulate filter | E10 E11 E12 | ≥ 85 % ≥ 95 % ≥ 99,5 % | ≤ 15 % ≤5% ≤ 0,5 % | - - - | - - - |
| HEPA = HEPA filter | H13 H14 | ≥ 99,95 % ≥ 99,995 % | ≤ 0,05 % ≤ 0,005 % | ≥ 99,75 % ≥ 99,975 % | ≤ 0,25 % ≤ 0,025 % |
| ULPA = High performance HEPA filter | U15 U16 U17 | ≥ 99,999 5 % ≥ 99,999 95 % ≥ 99,999 995 % | ≤ 0,000 5 % ≤ 0,000 05 % ≤ 0,000 005 % | ≥ 99,997 5 % ≥ 99,999 75 % ≥ 99,9999 9 % | ≤ 0,002 5 % ≤ 0,000 25 % ≤ 0,000 1 % |
The basis of the classification here is the minimum filtration efficiency of the filter cell for particles of size 0.1 – 0.3 µm, the “MPPS”, as explained in the paper Filter Influence Factor Principle.
Classification proceeds in three steps: first, the particle size at the separation minimum is determined on the unfolded filter medium (MPPS). In the second step, the absence of leaks (= local separation efficiency) is tested. Finally, the integral filtration efficiency of the filter element is determined using a test aerosol for the particle size in the separation minimum.
Based on the values obtained, the filter element can be assigned to the appropriate filter class with the help of the overview in the standard.
The following figure shows in which areas the different filter classes or filter groups can be used.
Filter cell construction methods
Various designs are available for filter elements, depending on the manufacturer, application and requirements.
Various factors play a role in the selection: from process requirements to the installation situation, resulting from the design of the filter system, to the question of cost, there is a lot to consider.
In this article, we will take a look at the “cartridge filter”, “lamella filter” and “cassette filter” designs – i.e. the filters that are used in the plants of HET Filter GmbH.
The three filter designs have different characteristics, which are described below:
Cartridge filter
Structure:
In the cartridge filter, the pleated filter medium (filter fleece) is located inside a cylindrical carrier basket. This design allows for cost-effective production.
Deployment:
for air with high dust loadings
Classification:
Coarse , fine filter according to EN ISO 16890
Special features:
Due to the design, the filter cannot be changed under safe-change conditions, i.e. only minor containment requirements can be met with this filter. These are described in the blog articles on the topic of “Containment”.
Lamellar filter
Structure:
Lamellar filters consist of a
folded fleece, which is either horizontally
or can be installed vertically. The
respective filter stage of a dust extractor sets
from several of these lamellar filters
together. The design allows the
individual lamella filters by means of pulsed
Compressed air can be used for cleaning. The compressed air
sets the elements in vibration, so
that the particles dissolve and enter the
Dust discharge fall. Due to the simple
structure, this type of filter is comparatively
favorable.
Deployment:
in air with high dust loadings,
for difficult dusts
Classification:
Coarse , fine filter according to EN ISO 16890
Special features:
Due to the design, the filter cannot be changed under safe-change conditions, i.e. only minor containment requirements can be met with this filter. These are described in the blog articles on the topic of “Containment”.
Cassette filter
Structure:
In the case of the cassette filter, the
Medium in a robust, conductive
Galvanized steel housing or
Stainless steel. When used in non
potentially explosive atmospheres, the
frame can also be made of wood (MDF).
The filter fleece is layered or V-shaped
folded and bonded by means of potting compound to the
Housing potted. Between the individual
layers are spacers, which
make sure that the layers are not damaged during the
suction of the air remain open and the
Air flows unhindered through the filter
can. In addition, they are dissipative to
prevent electrostatic charges.
Deployment:
for finest dusts of the pharmaceutical industry,
Dust loadings up to 2,000 mg/m3
Flow rates up to 2,000 m3/h
due to the standardized dimensions in
various plants can be used.
Classification:
HEPA filter according to EN 1822;
Dust filter according to EN ISO 16890
Special features:
the change of the filter is described under Safe-
Change conditions possible, i.e. that
with this filter, among others, containment-
Requirements up to OEB6 are met
can.
The filter cells can be cleaned, which means that
the service life can be increased.
Conclusion
Filter cells are available in a wide range of features and design options. The selection depends on the properties of the dust as well as the requirements of the process. Therefore, it is indispensable for the design of the filter systems to know the dust exactly and to transmit the data to the manufacturers of the filter systems. Only in this way can the right filter elements be selected, so that in the end their task is fulfilled – providing clean air, without danger to people and the environment.
