Fluid enters the inlet at the bottom of the tube, passes from the inside to the outside through the filter element and exits at the side outlet. When a new and recently cleaned filter element is fitted in the housing, the tell-tale indicator will indicate that it is ‘clean’.
As dirt deposits on the surface of the element, the pressure differential across the inlet and outlet of the element rises. The bypass piston senses this difference in pressure. The piston is held seated by a spring. When the element requires cleaning, the pressure differential is high enough to compress the spring, forcing the piston off its seat. The piston movement causes the tell-tale indicator to point towards the ‘needs cleaning’ position.
When the filter element is not cleaned when this signal comes on, the pressure differential continues to rise, causing the piston to uncover a bypass passage in the cover. This action limits the rise in the pressure differential to a value equal to the spring tension and the fluid bypasses the filter element. A cutaway view of a tell-tale filter is shown in Figure 7.9.
The following schematics show a few of the typical methods used for filtration. Figure 7.10(a) shows the location of a proportional flow filter. As the name implies, proportional flow filters are exposed to only a percentage of the total flow in the system. The primary disadvantage of this type of filtration arrangement is that, there is no positive protection of any specific component within the system and there is no way to know that the filter is dirty.
Figures 7.10(b)-(d) show full flow filtration in which all the flow from the pump is accepted.