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. (2.35) represents only one of the design requirements that has to be satisfied for the tanks. In fact, during the operation of the system, the volume of hydraulic fluid in the tank can significantly vary due to the presence of accumulators or differential cylinders. These elements store variable amounts of fluid: for this reason, the reservoir has to accommodate for such variation while also allowing for a sufficient air cushion. Therefore, the designer needs to ensure that the value provided by Eq. (2.35) is well above the maximum volume variation ΔV that can be observed during the operation of the system. Otherwise, the volume of the tank has to be increased accordingly.
In case the tank is used as primary source of thermal conditioning of the fluid, to guarantee that the temperature of the working fluid remains stable during the operation of the hydraulic system, the volume of the tank needs also to allow a minimum heat exchange surface to the environment. An empirical formula, valid for parallelepiped‐shaped tanks, is the following [11]:
(2.36)
where cp [kJ/kgK] is specific heat capacity (can be assumed to be 4.187 kJ/kgK), Tf is the temperature of the working fluid, Tamb is the ambient temperature, H [kJ] is the heat to dissipate, and c is a constant that, with the above units, is equal to 1.25.
2.9.2 Basic Design of a Tank
The basic design of a hydraulic reservoir consists of a parallelepiped made of steel sheets. Small tanks (usually below 70 l) can be made of metal alloy or plastic. In a simplified cross‐sectional view, the tank appears as shown in Figure 2.19. The suction duct and the discharge duct are usually placed at the opposite ends to increase the residence time of the working fluid inside the tank. Separator septs or baffles are used to prevent a straight flow path between the discharge and the suction pipe. In this way the fluid is forced to travel a longer path with reduced speed. In many cases, the septs force the fluid to flow vertically within the reservoir so that the release of the undissolved air is promoted by gravity. The discharge pipe is usually well below the free surface level of the fluid. This helps in preventing splashing of the fluid and whirls that would increase foam and entrained air. The pipes are often cut with angles greater than 45°. At the discharge pipe this further promotes a smooth entering of the fluid in the tank; but most importantly, at the suction pipe, it allows for a gradual acceleration of the fluid within the pipe, thus reducing the entrance losses.
The figure also shows an air breather element, which should always be present to allow the air to enter or leave the tank as the level of the fluid varies to accommodate volume variations driven by accumulators or differential cylinders.
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