# Filters in Operation

9.2.4.1 Separation

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-

 Particle size (Jim) FIGURE 9.2 Efficiency of air filters vs. particle size. The figures should be the minimum efficiencies in an installation.
 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.