Hydraulic Circuit Oil Coolers
The basic idea of an oil cooler in an hydraulic circuit to dissipate the energy losses that occur in the system is simple. The selection of an appropriate cooler, its sizing and position within the circuit are not so straightforward.
High temperatures affect the composition of an hydraulic oil, its properties and life. The chemical stability of an oil is measured mainly in terms of its resistance to oxidation. When oxidation occurs, oxygen combines with oil molecules to set off a chain of chemical reactions that create soluble and insoluble products of degradation. As a result the oil becomes darker in colour, its viscosity and acidity increases and gums and sludge’s are deposited in the hydraulic system. The tendency of the oil to oxidise is greatly increased by high temperatures, and by the effects of air, water and certain wear materials.
Although slight oxidation is not harmful, problems are encountered when the level becomes excessive. For example, the service life of the oil would be shortened, the system would be exposed to corrosive attack, the system components liable to sluggish operation, and the ability of the oil to separate from water and air reduced.
In industrial hydraulic applications it is usual for systems to operate with bulk oil temperatures of around 50 – 60 degrees C. In mobile duties rather higher temperatures may be encountered, of 70 degrees C and above. Temperature has a very major effect upon lubrication through the change that it produces in fluid viscosity. For an increase in temperature of 20 degrees C the viscosity of an oil may typically halve. Manufactures of hydraulic components specify a recommended viscosity range for the oils which may be used with their products. Fairly representative figures for the common hydraulic oils would involve a minimum viscosity of around 10cs and a maximum of several hundred cs. Normal working levels would typically be in the range of 20cs to 40cs.
Guidance about probable life of oils is very difficult to give, since it is affected both by the condition of the hydraulic system and the harshness of the working environment. Under good operating conditions an oil may last for tens of thousands of hours, whilst under bad conditions its life may be a matter of a few thousands of hours or less.
Although this blog is concerned with oil coolers, sometimes a neglected consideration concerns the operation of hydraulic systems from a cold start. In adverse conditions where low initial temperatures may be encountered it is usual to provide a heater in the oil reservoir.
Energy Losses
Energy is dissipated as heat within an hydraulic system principally as a result of the pressure losses associated with the fluid flow. Losses occur through pipe friction, the effect of control elements such as pressure relief valves, pressure reducing valves, orifices and flow control valves, and through leakage and mechanical friction. The latter factors being particularly important in pumps and motors.
Hydraulic Circuit Considerations
Correct positioning of an oil cooler within an hydraulic circuit requires identification of the major sources of heat generation and, within the bounds of physical constraints, the most advantageous location to dissipate the heat.
Types of Oil Cooler
The two principal types of oil cooler employ either water or air as the cooling medium. Where an adequate water supply is available the water type is commonly used. It has the advantages of compactness and of being less susceptible to changes in ambient air temperature. Water flows through the tubes and oil across the tubes, the latter guided in its flow path through the shell by baffle plates. The maximum oil pressure which the cooler can be subjected to is limited by the shell, a fairly typical figure would lie in the range 15 – 30 bar. The pressure drop associated with the oil flow through the cooler is usually small, of the order 1 bar.
Sometimes overlooked is the practical detail of fitting a strainer to the suction of the pump providing the water flow. Debris restricting the water flow through the cooler is one of the most common problems that arise in operation.
The air blast cooler in contrast is of lighter construction, and for the same heat dissipation is larger. Oil passes through the tubes of the cooler, which are usually finned to aid heat dissipation, and air blown over the tubes by a fan. The operation of the air cooler is sensitive to changes in the ambient air temperature and care must be taken in its selection to allow for this effect. The maximum allowable oil pressure is typically lower than the water type, a figure of 7 bar being representative. One of the most important applications of the air blast cooler is in the mobile industry.
For both types of cooler automatic temperature controls may be used where it is desirable to maintain the working temperature of the oil within prescribed limits. For a water cooler a commonly used technique is to employ a thermostatic valve to control the water flow, whilst for an air cooler temperature regulation can be achieved either through the intermittent use of the fan or through variation of its speed.
Here at Hydraproducts we have a wealth of experience in selecting the correct cooler for a system, for more information on this subject please call the sales team on 01452 523352.