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.