Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 48

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:

(2.35)

Considering Q_p expressed in liter per minute and V_tank 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 c_p [kJ/kgK] is specific heat capacity (can be assumed to be 4.187 kJ/kgK), T_f is the temperature of the working fluid, T_amb 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.