Heat is generated in the fluid in hydraulic systems because of pressure losses in pipes, fittings and, particularly, in control valves where the rate of heat dissipa­tion can be of the same order of the power being produced at the system output. The temperature created in the fluid will depend on the system duty cycle and its environment, as natural heat convection from pipes and reservoirs is not at a very significant level.

In industrial systems the fluid temperature is usually around 50 to 60°C and in mobile equipment this can be as high as 80°C. The condition of most hydraulic oils is significantly affected by operation at high temperatures, which will shorten the life of the oil and reduce its viscosity to unacceptable levels.

Hydraulic component manufacturers specify the viscosity range to be used and in the main these will call for a minimum value of around 20 cSt although some will operate satisfactorily at 10 cSt and less. It is important to realise that the volumetric efficiency of pumps and motors is significantly affected by operation with low viscosity fluids, which will cause a further increase in the heat load.

It is therefore necessary to estimate the amount of heat generation in order to establish the size of cooler that will be required to maintain a satisfactory fluid temperature.

Cooler types

Coolers use either air or water as the cooling fluid. In water coolers the water flows through the tubes and the oil across the tubes, the latter guided in its flow path through the shell by baffle plates. There are two common constructions; in the first the tubes are arranged in a U-bundle with a single tube sheet, in the second two tube-sheets are used in a straight tubing arrangement.

The maximum oil pressure that the cooler can be subjected to is limited by the shell, a typical figure would lie in the range 15 to 30 bar. The pressure drop associated with the oil flow through the cooler is usually small, of the order of 1 bar. Water coolers are more compact than those using air and, providing an adequate supply of cool water is available, they are less sensitive to environmen­tal conditions. In some cases it may be necessary to fit a strainer at the cooler inlet in order to prevent blockage of the water flow.

Air coolers using fans to create the necessary airflow are of a lighter construction than water coolers but are larger and sensitive to the environ­mental conditions, which needs to be considered. Air coolers are mostly used for mobile applications and usually can only work with oil pressures up to around 7 bar.

For both types of coolers automatic temperature controls are available using thermostats to either control the water inlet flow or the speed of the fan.

Thermodynamic aspects

The cooling characteristics are usually presented in the form shown in Figure 1, which will apply for a particular value of inlet temperature difference. For different values a correction factor is applied to suit the application.

 Figure 1. Cooler Performance