Filters in Operation
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.
As the filter accumulates dust, the pressure loss increases, and the dust removed improves the normal separation. Another effect can be seen with electrostatically charged filter material. During operation, the impurities neutralize the material, and the filter’s capacity to separate is reduced. Figure 9.34 shows exam-
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Particle size (Jim) FIGURE 9.2 Efficiency of air filters vs. particle size. The figures should be the minimum efficiencies in an installation. |
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FIGURE 9.3 Example of efficiency changes in an installation with two F7 air filters. |
Pies of filters shown by laboratory tests to be in accordance with Class F7 (EU7). The efficiency drops dramatically from more than 80% to less than 20% after a few weeks’ operation in the case of the filter based on electrostatic charge. The effect varies much with fiber size and charge.
9.2A.2 Average Pressure Loss
The average pressure loss during operation is dependent on the characteristics of the plant and is often taken to be the average value of initial pressure loss and final pressure loss of the filter. With the lower energy requirement, more and more systems are being dimensioned for constant flow, and average pressure loss is the integrated value. Significant savings can thus be made using filters with a low pressure loss and small increase in pressure during the period of operation.
A filter’s energy consumption, E, based on average pressure loss, can be calculated as
^ 17IOOO’ ’
Where
Q is airflow (m3/s)
Ap is average pressure loss (Pa) t is operation time (hours)
77 is efficiency of fan
Over one year (8760 hours), a 1 m3/s filter with an average pressure loss of 100 Pa requires 1250 kW h if the fan’s efficiency is set at 70%. The energy cost is generally greater than the filter cost, and pressure loss reduction be-
Comes increasingly significant for energy reductions. Lower pressure loss by 10 Pa means 125 kW h less energy in the example above.
9.2.4.4 Lifetime
The lifetime of a filter is dependent on the concentration of dust, type of dust, airflow, and, of course, the selected final pressure loss. Filter material and filter construction are often a compromise or combination of filter effects and installation space. Low speed or large filter surface promotes efficiency, low pressure loss, but above all a longer lifetime.
9.2.4.5 Filter Replacement
Airflow changes in the plant have been the main criterion for changing filters, i. e., when pressure loss increases to the extent that the fan cannot maintain a specific minimum airflow. Reduction of maximum effect, energy consumption or economic evaluation, i. e., when the energy cost and filter cost reach a minimum, are becoming increasingly significant.
Considerations of hygiene are being applied more and more to filter re placement. Studies6 have shown that with RH (relative humidity) higher than 75% there is a risk of microbial growth in the filter and in the ventilation system. As it is in many cases difficult to avoid a high relative humidity in the air intake, filtering should take place in two steps. The first filter can often be ex posed to high humidity or to rain and snow. Organic impurities also become caught in the filters and could be released later. Particles and endotoxins from microorganisms can become loose in low-quality filters.
The first filtration step should thus be carried out using a filter of at least F7 (EU7) quality, which should be changed after a maximum period of one year’s continuous operation. The second filter of at least F7 (EU7) quality is not exposed to high RH, effectively stops microorganisms and particles, and can remain in place for about two years, provided the final pressure loss is not reached within this period.
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