Filtration in the cooling lubricant circuit

The precoating agent can be added continuously or precoated before the actual filtration. A combination of precoating and continuous dosing is common in practice. [Luc04, p.191 ff], [Gas00, p.48 ff].

The filtrate and the dirt cake leave the filter. The filtrate remains in the coolant circuit. The dirt cake, however, is removed from the coolant circuit. The filtrate is composed of the material flows of the remaining dirt, the cooling lubricant and the filter aid. Filter aid should not normally be present in the filtrate, but this cannot be ruled out due to the technical conditions. [Gas00, p.48 ff.]

The dirt cake is also made up of the three components dirt, cooling lubricant and filter aid. The dirt cake should have a high dirt content and the lowest possible coolant content.

In the literature on general solid-liquid separation, there are different approaches to structuring the filtration processes. If one considers the filtration processes with regard to coolant filtration, Figure 3 provides a technically sensible and practice-related structure which allows the user a quick overview and orientation.

The filtration equipment practically used for coolant filtration can be divided into the four basic filtration principles:

•     filtration by sedimentation
•     filtration by centrifugal force
•     filtration by size differences
•     filtration by magnetic force action

In filtration by sedimentation, the dirt is separated from the cooling lubricant by different sedimentation behavior. Due to the difference in density between the dirt and the cooling lubricant, the dirt settles on the bottom of the sedimentation unit and forms a sediment, which can finally be discharged as a moist heap. [VDI 3397]

Size difference filtration, also known as sieve filtration, makes use of the difference in size between dirt particles and coolant particles.

A filter medium, which in principle acts like a sieve, retains the dirt particles and allows the coolant particles to pass through.

In the case of filtration by centrifugal force, similar to filtration by sedimentation, the different densities of the cooling lubricant and dirt are used to achieve solid-liquid separation. In this principle, the cooling lubricant and dirt rotate on a circular path and the resulting centrifugal force effect concentrates the dirt, which has a higher density than the cooling lubricant, on the outer circular path and then discharges it.

Filtration by magnetic force separates dirt particles from the cooling lubricant which react to magnetic attraction. The dirt particles to be separated should be attracted by a magnet. In the optimum case, they are ferromagnetic and can become magnets themselves.