Filtration devices have been used for the express purpose of protecting equipment and machinery for decades. Typical systems use a filtration system in-line with the equipment that uses the lubricating fluid. This type of system is usually called a full-flow filtration device. Full-flow filters are designed to remove contaminants from the lubricating oil, but these filters are capable of removing only particles larger than a certain size. Removal of smaller particles requires finer filtration.
In the past few years, use of by-pass filtration for the purpose of removing smaller particles (less than five microns) has gained widespread acceptance in many applications. In engine applications, and specifically diesel engines, centrifugal separators have been used for this purpose. Centrifuges have also been used in industrial (nonengine) applications to protect equipment and machinery. These applications include a wide range of industries such as water filtration, food processing and machining centers. Though centrifuges of various designs have been used in different applications (such as liquid-liquid separation), the focus of this article is on designs that remove solid contaminants from the fluid in a system, and more particularly, the removal of soot and other contaminants in engine oil systems.
The introduction of centrifugal filtration into various applications has been helped by advances in technology and manufacturing methods. Initial centrifuges, used for cream separation, included separation cones stamped out of metal. The units were hand-cranked, until electric units were introduced later. In recent years, centrifuge manufacturing technology has advanced to take advantage of new materials. Cones and other separation means are now made of engineered polymers, which makes it possible to manufacture these products into configurations that could not be stamped. Fleetguard Inc. has been a pioneer of such designs in its Centriguard™ family of products.
Typical centrifuge designs use the pressure of the fluid going through the system to spin the rotor at speeds high enough to separate contaminant particles from the fluid. In the case of engine applications, the lube oil (typically at 200ºF and between 40 and 70 psi) is directed to the centrifuge in a bypass loop, which uses from 5 to 15 percent of the pump’s flow, depending on the application. This pressurized flow travels through the centrifuge rotor, and as it exits the rotor, the oil goes through two small nozzles. The reaction force created by the exiting fluid creates the torque necessary to overcome any friction, and the rotor begins to spin. Typical speeds between 5,000 and 8,000 rpm can be achieved in most applications (dependent on oil temperature, oil pressure and available flow).
Centrifuges used in engine applications do not affect the oil additive package. However, if various components of the additive package precipitate out of the oil over time, then the centrifuge can remove them. One example of this is calcium, which can be found in the contaminant collected in a centrifuge after it has been in service for some time. Though the composition of the contaminant collected in a centrifuge can vary by percentage, the three main categories include: volatiles (oil), soot and inorganic contaminants (wear metals, precipitated components of the additive package, dust introduced into the system, chemicals introduced from coolant leaks, etc.). Typical percentages for newer engines meeting EPA 2002 regulations are 45 to 50 percent volatiles, 45 to 50 percent soot, and the remainder is inorganic contaminants.
Centriguard™ products use cones or spiral vanes to enhance the separation efficiency of the centrifuge. Laboratory and field testing have proven this technology to be effective in removing soot from diesel engines. This test was conducted using sooty oil from an engine in an on-highway trucking application.
Centrifuge manufacturers continue to develop new designs to meet the needs of their customers. These needs vary based on customer, industry segment, regional requirements, and the end-users’ requirements of the filtration systems and devices. Pictures of two different technologies offered by Fleetguard Inc. are shown in Figure 1.