Diesel Particulate Filters: How Filters Work
Nett S-Series Diesel Particulate Filters
Nett® diesel filters utilize cordierite or silicon carbide wall-flow monoliths to trap the soot produced by heavy-duty diesel engines. The cylindrical filter element consists of many square parallel channels running in the axial direction, separated by thin porous walls. The channels are open at one end, but plugged at the other. This way the particle laden exhaust gases are forced to flow through the walls. Gas is able to escape through the pores in the wall material. Particulates, however, are too large to escape and are trapped in the filter walls.
A proprietary catalyst is coated onto the inside surface of the filter monolith. The catalyst lowers the soot combustion temperature allowing the filter to regenerate. The accumulated soot is oxidized in the filter during regular operation of the engine. Periods with exhaust temperatures of 325-400°C (615-750°F), which are necessary for proper filter regeneration, occur on most heavy-duty diesel engine applications.
The catalyzed filter monolith is wrapped in a fiber mat and packaged into a stainless steel housing, which is installed in the vehicle’s exhaust system.
The soot filtration efficiency of the Nett® S-Series catalytic diesel filter increases with the soot loading in the unit. Even at low soot loads the filter efficiency exceeds 90% (blue line in the graph). The visible smoke is completely eliminated by the filter. Due to the presence of the catalyst, reductions in carbon monoxide and hydrocarbon emissions are also observed.
Typical exhaust gas pressure drop on a properly regenerating filter (red line) is between 5 and 10 kPa (20-40" H2O). Applications with higher exhaust temperatures regenerate better, accumulate less soot in the filter, and experience lower pressure drop.
Nett F-Series Flow-Through Filters
Nett® F-Series flow-through filters utilize a metal foam monolith to trap the soot produced by diesel engines. The cylindrical filter element is made of a rigid open-cell metallic foam which allows exhaust gases to flow through the element but heavier particulate matter becomes trapped in its complex structure. The pores in the filter element are large enough to allow exhaust gases and particulates to pass through the filter in the event that the soot storage capacity is reached. In this case, the engine and equipment are able to operate normally, even though the filter is full.
A proprietary catalyst is coated onto the inside surface of the filter monolith. The catalyst lowers the soot combustion temperature allowing the filter to regenerate. The accumulated soot is oxidized in the filter during regular operation of the engine. Periods with exhaust temperatures of 325-400°C (615-750°F), are necessary for proper filter regeneration, and may not occur on all diesel engine applications.
The catalyzed filter monolith is wrapped in a fiber mat and packaged into a stainless steel housing, which is installed in the vehicle's exhaust system.
The soot filtration efficiency of the Nett® F-Series flow-through filter increases with the soot loading in the unit until it reaches capacity. Once the filter is at capacity, soot begins to pass through the filter and filtration efficiency drops off. The visible smoke is greatly reduced and may be completely eliminated by the filter in some applications. Due to the presence of the catalyst, reductions in carbon monoxide and hydrocarbon emissions are also observed.