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.
Table 2.11 Typical pressure drops in filters depending on the filter installation choice.
| Filter type | Typical pressure drop | |
|---|---|---|
| Minimum [bar] | Maximum [bar] | |
| Suction filters | 0.02 | 0.1 |
| Medium‐/high‐pressure filters | 1 | 1.5 |
| Return filters | 0.3 | 0.5 |
Return filters (Figure 2.17) are often the preferred choice. This is because they are located in low‐pressure lines (where the pressure is usually lower than 5 bar) and do not require a heavy housing. Furthermore, return filters can be easily integrated in the reservoir. The evident drawback of a return filter is that it cleans the working fluid after it has passed through the hydraulic system before returning to the tank. Therefore, this type of filtration is not effective against contamination introduced in the reservoir.
Table 2.11 summarizes the typical pressure drops of filters depending on their installation.
In the further chapters of this book, for sake of brevity and to better focus on the actual concepts, most of the hydraulic circuits will be illustrated often without hydraulic filters. However, the reader should always keep in mind that a real hydraulic system always requires proper filtration to guarantee the correct operation.
2.9 Considerations on Hydraulic Reservoirs
Tanks, also referred to as reservoirs, are a very important element of the hydraulic system, not only because they hold the hydraulic fluid but also because they affect the thermal properties of the system and they can be designed to separate solid, liquid, and gaseous contaminants from the oil.
2.9.1 Tank Volume
A high volume of the tank improves both the thermal and contaminant separation functions. However, it also increases the cost and the weight of the hydraulic system. Mobile applications usually require smaller tanks compared with industrial applications. As a general rule, the volume of the tank can be related to the flow rate that it exchanges with the system, i.e. the pump flow rate:
Considering Qp expressed in liter per minute and Vtank in liter, τ is the resident time in minute of hydraulic fluid in the tank. In other words, during the operation of the hydraulic system, each fluid particle spends an amount of time τ minute inside the reservoir before being reintroduced into the hydraulic circuit. This time interval should allow the working fluid to cool down, thanks to the heat exchange between the tank surfaces and the environment. For this reason, it is a good practice to locate the reservoir in a properly vented region of the hydraulic system.
During the time τ, the fluid should also be able to release both the entrained air and the undissolved air so that only liquid reenters the hydraulic system.
For industrial application with intermittent operation, the volume of the tank should ensure a value of τ between 2 and 3 minutes. In case of continuous operation, this value can be increased by two or three times. This means that for a system requiring 100 l/min of flow rate in continuous operation, the tank can be up to 900 l.
The size of the tank, and consequently its weight, is much more of an issue in mobile applications, which are sensitive to payloads and where often the available space is very limited. For this reason, proper design strategies have been developed to promote both the heat exchange and impurity separation functions within the reservoirs for mobile applications. These strategies include the use of external HEs and filters, as well as sophisticated internal air separators, and in some cases pressurized tanks. As a result, it is nowadays common to achieve time constants τ below 1 minute particularly for aerospace applications.
The reader shall understand that Eq.