The reader should know that there are different types of filter available on the market: each one can apply a different filtration technology, have a particular construction type, or integrate other accessories. Illustrating the details and the operating features of the different types of filters is beyond the scope of this chapter. A high‐level description of the available options for filters is provided in [11, 28].
2.8.2 Filter Placement
It is important to comment on the application of filters in hydraulic circuits, since there are different alternative options. In general, filters can be classified based on their location in the circuit:
1 Suction filters (Figure 2.15) are placed between the reservoir and the pump inlet, which is usually the first component of the hydraulic system crossed by the hydraulic fluid after the tank.
2 High‐pressure filters (Figure 2.16) are placed right after the pump outlet, before the working fluid can reach the control elements and the actuators of the hydraulic system.
3 Return filters (Figure 2.17) are placed on the return line, and they are the last component before the reservoir. These are often integrated in the reservoir.
4 Offline filters (Figure 2.18) are used within a separate circuit, created just for filtering the oil. In this case an auxiliary pump is used in a recirculation circuit including only the oil conditioning element(s). Often the filtration function is combined with thermal conditioning (oil cooling and or heating).
The reader could think that the best way to protect a system from external contamination is to put a filter right before the inlet of the pump (Figure 2.15). However, one should also consider that a real filter always induces a pressure drop, which, even if it is very small, can become very significant in suction lines. Here, the control of the pressure level is very important to avoid air release or vapor cavitation. Therefore, suction filters are rarely used, and most of the times they are just strainers, designed to catch large particles, usually above 60–75 μm. Suction strainers always have to be combined with other types of filters in the system.
Figure 2.15 Suction filtration circuit.
Figure 2.16 High pressure filtration circuit: (a) application to the whole circuit downstream the pump; (b) application to a specific branch of the circuit.
A high‐pressure filter (Figure 2.16) can be used for two possible purposes: protecting some or all of the components located downstream of the pump or ensuring that a failure of the pump does not damage the rest of the system. The use of a high‐pressure filter can be recommended when the hydraulic systems uses components with low internal clearances, such as servo valves. The drawback of high‐pressure filters lies in their cost and weight. In fact, these filters need to be able to withstand high pressures and therefore need a properly sized cast iron housing.
Figure 2.17 Return filtration circuit.
Figure 2.18 Offline filtration circuit.
