This is a versatile filter, which can be directly welded into a reservoir’s suction and return line or conversely installed in pipes with a maximum pressure of 10 kg/cm2 (142 psi). This filter can remove particles as small as 3 u. It consists of an indicating element, which indicates the time when cleaning is required. That is why this filter is referred to as a telltale filter (Figure 7.8).
The operation of a tell-tale filter is dependent on the fluid passing through the porous media, which traps the contaminants. The tell-tale indicator monitors the pressure differential buildup due to dirt, which gives an indication of the condition of the filter element.
This is also referred to as a full flow filter, which means that all the oil in the system passes through it. Figure 7.7 illustrates an edge-type filter.
The edge-type filter consists of a stack of disks with holes in their center, like flat doughnuts, with very little separation between them. When entering the filter, fluid is guided from the bottom at the outer side of the stack. Before leaving the filter, it comes out of the center of the stack having passed through the disks. Impurities in the oil are left behind on the outer edge of the stack. A scraper blade moves over the outer surface of the stack, wiping off all the dirt collected. The blade is operated manually by means of a handle at the top of the filter housing.
Figure 7.6 shows a filter designed for either a suction line or a pressure line. This is a duplex filter.
A duplex filter, as the name suggests consists of two filters out of which only one is in use all the time. When the filter element gets clogged, the second filter is put to use. Dirty fluid comes into the middle section and passes down through the filter element. The filter element can be of fine gage nylon cloth or wire cloth or finely perforated stainless steel.
From the filter element, the fluid passes out of the unit and into the line. This unit has a ‘telltale’ indicator, which indicates when the filter element is excessively clogged and requires cleaning. If the filter is not cleaned after indication, the fluid bypasses the filter element and there is no filtering action. Such a bypass mechanism is important for a filter because, when the filter element is clogged heavily, the pump in line may get damaged due to starvation of hydraulic fluid.