Average powder loss from a normal high-efficient cyclone should not exceed 0.5% (≈ 250 mg/Nm3) when spray drying skim milk. However, today authorities conclude that 0.5% is too high a figure. From 2007 the EU requires a powder loss of ≤10 mg/Nm3. Final cleaning of the air is therefore necessary. This is usually done in bag filters consisting of numerous bags or filters arranged so that each bag receives equal quantities of air. The direction of the air is from outside in through the filter material to the inner part of the bag from where the cleaned air enters an exhaust manifold. With a correct selection of filter material high efficiencies can be obtained and collection of 1 micron particles is reported from the manufacturers. The collected powder is automatically shaken off by blowing compressed air via a Coanda venturi nozzle positioned inside the top part of each bag. The powder is collected at the bottom via a rotary valve. See Fig. 56 and 57.

Fig. 56 Bag filter
Fig. 57 Spray dryer with bag filter

The bag filter may also replace the cyclones, a solution often chosen for one-stage dryers for whey protein powder or egg white. To prevent condensation, especially on the conical part of the filter housing, warm air or heat tracing is established.

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