In any discussion of oil purification the attention tends to settle entirely on the vacuum equipment. In a complete purification line, however, it is the plate and frame filter press that carries most of the dirty work. It does not remove water and it does not remove gas, and its efficiency does not look impressive, yet the moment it is taken away the operating condition of the vacuum equipment ahead of it deteriorates at once.
Its place in the line
The working principle of the plate and frame filter press is simple and direct: under pressure the oil passes through the filter cloth, the solid contaminants are held back on the cloth, and the filtrate leaves on the other side of the plate and frame. It is therefore good at two things. First, clarification, driving a large quantity of solids, sediment and sludge out of the oil. Second, high-precision filtration in combination with high-quality filter paper, where the accuracy depends on the medium rather than on the machine itself.
In a complete line the role usually falls ahead of the vacuum equipment: it holds back the dirtiest part first and protects the fine elements, the atomiser and the pump set behind it. The LY-200 has a capacity of 4800 L/h, 40 sets of filter plates of 300×300 mm and an 11 kW motor; the LY-50 uses 10 sets of 200×200 mm plates and frames, a 4-1.5-b3 motor and a kcb33.3 oil pump. The open cabinet design makes the filter frames easier to remove and fit, so that changing the cloth on site can be done faster.
Both the advantages and the limits have to be stated
- High dirt holding capacity: the cloth can absorb a large quantity of solids and a single run can handle oil with a fairly high level of contamination, which precision filter elements cannot do.
- Low consumable cost: filter cloth and filter paper cost far less per piece than deep-bed filter elements, and the cost per unit of throughput is clearly favourable at high flow rates.
- Backwashable and replaceable: the cloth can be washed and used again, and it can also be replaced at any time to raise the accuracy.
- It cannot run continuously unattended: clamping, changing the cloth and clearing the filter cake all depend on labour, and the pace of the work is limited by the manpower available.
- It does not deal with water and gas: filter pressing is a solid-liquid separation, which is an entirely different matter from vacuum degassing, and the two cannot replace each other.
There is a practical constraint as well: the filter area of a plate and frame filter press is built up by the number of plates and frames, so the footprint of the machine grows with the throughput. That is a different approach from the vacuum equipment, where the throughput is raised by the capacity of the pump set, and the way the capacity is extended makes the press more suitable for a fixed site than for frequent transfers.
The choice of medium matters more than the number of plates and frames
What decides the result of the filter pressing is the material, the accuracy and the oil-temperature resistance of the cloth and the filter paper. The cloth is oriented towards high-throughput clarification and the paper towards accuracy filtration; the higher the accuracy, the greater the filtration resistance and the lower the throughput at the same pressure. When the configuration is set, it should be done in two stages: the solids are brought down first with a cloth that favours throughput, and the work is finished off with a paper stage that favours accuracy. Choosing the wrong medium does not lead first to an incomplete filtration; it leads to the pressure rising too quickly, and a batch of media is used up very soon.
Treating the plate and frame filter press as a simplified version of a vacuum machine is the most expensive misconception there is; its value is that it keeps the solids away from the stages behind it, and only with its role correctly placed does the cost work out.