Minimum filter efficiency

Minimum filter efficiency The value of a filter s efficiency relating to its performance classification under specified operating conditions. See also Maximum penetrating particle size. 

Table 3 compares the minimum filter efficiencies cited in the guidance documents on aseptic manufacturing and in the Clean Room Standards. It is curious that the specification for filter efficiency in the Orange Guide" is tighter than the specifications given in the two Clean Room Standards it refer-... 

Table 3 Comparison of Minimum Filter Efficiencies Referenced in Guides to Aseptic Pharmaceutical Manufacture and Clean Room Standards...

It is noticed that for such a filter containing fibres of the same size, it is unavoidable to have the minimum filter efficiency in filtering particles of certain sizes, as shown in Fig. 10.2. For very small particles less than dpi in diameter, the primary filtration mechanism is diffusion. For particles between dpi and dp2, the filter is less efficient, as the particles are too large for a great diffusion effect and too small for a large interception effect. For particles of diameter above dp2, the filter is very efficient again because the interception, along with inertial impaction, effects is predominant in the filtration. 

Maximum penetrating particle size The particulate size for which a filter has minimum removal efficiency under test conditions. 

Final Filter Efficiency (as determined by BS 3928)1 Recommended Minimum Air Max. permitted number of particles per cubic metre eaual to or above 2 Max. permitted No. of viable Nearest Equivalent Standard Classification ... 

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

An ideal filter of highest value of filter quality coefficient means that it has the maximum separation capacity with minimum pressure loss, and these two parameters remain constant during the service life of filter. The major criteria of the performance of a filter include filter efficiency, pressure drop, and the filter quality performance. ... 

Filter group Filter class Minimum MPPS efficiencies (%)... 

The main advantage of HGMS is high efficiency of separation even at relatively high dow rates and minimum pressure drops across the filter. The capital cost is very high, and only large installations are attractive economically because capacity increases with the square of the diameter of the canister while the weight of copper conductor increases linearly with diameter. 

All filters require a filter medium to retain solids, whether the filter is for cake filtration or for filter-medium or depth filtration. Specification of a medium is based on retention of some minimum parficle size at good removal efficiency and on acceptable hfe of the medium in the environment of the filter. The selection of the type of filter medium is often the most important decision in success of the operation. For cake filtration, medium selection involves an optimization of the following factors ... 

Fibrous or particulate filters are not important anymore because membrane filters are relatively compac t and perform veiy well. For filtration by straining, there is an intermediate air velocity at which filtration efficiency is a minimum because different collec tion mechanisms predominate at different ranges of velocity. At low velocities, diffusional and elec trostatic forces on the particle are important, and increased velocity shortens the time for them to operate. At high velocities, inertial forces that increase with air velocity come into play below a certain air velocity, their effect on collection is zero. Surges or brief power failures could change velocity and collection efficiency. 

It is important to be aware of the filter s properties in different environments. Figure 9.2 shows how, in the case of new filters, separation varies with particle size and filter class. The filter class is based on the average efficiency, and a new filter normally has much lower initial efficiency. In the case of electrostatically charged filters, separation may be significantly higher for new filters. The figure should be seen as an indication of minimum separation during actual operation. 

Efficiency, minimum The minimum efficiency obtained during the performance classification of a filter. 

Filter aids are widely used in die fermentation industry to improve the efficiency of filtration. It is a pre-coated filter medium to prevent blockage or blinding of the filter by solids, which would otherwise wedge diemselves into the pores of the cloth. Filter aid can be added to the fermentation broth to increase the porosity of the cake as it formed. This is only recommended when fermentation product is extracellular. Filter aid adds to the cost of filtration. The minimum quantity needed to achieve the desired result must be established experimentally. Fermentation broths can be pretreated to improve filtration characteristics. Heating to denature proteins enhances the filterability of mycelial broths such as in penicillin production. Alternatively, electrolytes may be added to promote coagulation of colloids into larger, denser particles, which are easier to filter. The filtration process is affected by the viscosity and composition of the broth, and the cell cake.5... 

This first step makes necessary a correction of the atmosphere aberrations by means of an adaptive optics or at the minimum a tip tilt device. If the turbulence induces high aberrations the coupling efficiency is decreased by a factor VN where N is the number of spatial modes of the input beam. Note that tilt correction is also mandatory in a space mission as long as instabilities of the mission platform may induce pointing errors. Figure 10 (left) illustrates the spatial filtering operation. This function allows a very good calibration of... 

Air handling unit has been provided with prefilters two inches thick at a minimum and medium efficiency cartridge or bag filters. 

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