The pressure-leaf filters are used for the filtration of vegetable oils in various processing stages (press oil purification, winterization, bleaching…).
In general all liquids that exhibit rheology similar to vegetable oils can be filtered by the pressure sheet filter.
1. Filter container
2. Container lid
3. Pivot screws
4. Hydr. hoist
5. Lifting arm
6. Filter leafs
7. Pneum. vibrator
9. Cover flap
10. Pneum. cylinder cover flap
1. Cake bloating
The operation of the filter is based on the principle of cake filtration.
The filter elements (filter leafs) consist of a framework which in several layers is covered with fine-meshed metal net. Several filter sheets are arranged in the filter box on the discharge pipe. The filter box is filled under pressure with cloudy oil. The oil flows through the surface of the filter elements inside the framework and leaves via the discharge pipe. The cake and the filter aids are collected at the surface of the filter sheets. If a sufficiently thick layer of filtered cake has formed on the surface of the filter sheets the filtered oil can be rerouted into the pure oil tank. This phase is called cake bloating. During cake bloating the oil is led back into the cloudy oil tank. This process can be accelerated by the use of filter aids.
Sufficient formation of cake at the filter elements becomes apparent with the increase of pressure and this can be visually observed. The duration of the filtration phase and the quantity of the obtained oil are determined by several measured variables:
- Oil temperature, which is critical for the kinematic viscosity,
- Proportion of cake in the cloudy oil,
- Particle size distribution in the cake – a value which is strongly dependent on the operating mode of the oil press
- The type and quantity of the filter aids used (sawdust, cellulose…)
- Filter surface
In this phase the advantages of the pressure leaf filter become apparent in comparison to the chamber filter presses. This procedure, and therefore the entire process, can be completely automated. In addition the metal fibre filters have a longer lifetime than the filter cloths of the chamber filter press.
If a certain pressure is reached, the supply of cloudy oil is interrupted. The cloudy oil from the full filter is discharged into the cloudy oil tank. Subsequently the sticky cake on the filter elements are dried by pressurised air. During this stage, air exhausted through the discharge pipe is very oily and must be first led through an oil separator.
The dry filter cake can be easily released from the filter elements by the pneumatic shaker. The filter cake from the filter is discharged through a pneumatic flap. The flap is again closed, the valves return to the positions for cake bloating, and a new filter cycle can begin.