Normal heat generation
A thorough knowledge of how a system should operate is essential because all hydraulic systems generate heat. So knowing how much heat a system is supposed to generate allows determining when heat is excessive, and, therefore, leakage is occurring. Substantial wasted power is designed into many systems, and these systems use heat exchangers to remove the excess heat.
This wasted power is caused by internal leakage of components, pressure reducing valves, bypass flow at high pressure, and friction from normal motion to move a load. Most pumps and spool-type directional control valves operate using metal-to-metal clearances and depend on some leakage to form a lubricating oil film.
Leakage across the internal lands of directional-control valves is usually low — normally is measured in drops or in.3/min per land or per valve. This heat, from normal leakage, generally should not propagate more than about 2 ft from a component. This is why it is important to analyze system heat periodically, especially when a system is new or recently remanufactured. Storing heat distribution data of a new system establishes a baseline that can be used for comparison during subsequent studies.
Power loss from closed-loop control
The greatest heat loss designed into a system may come from the use of jet-pipe or flapper-nozzle servovalves and proportional valves in closed-loop motion- and pressure-control applications. These valves require a constant flow in their pilot sections directly relating to a constant power loss. Essentially, the valves need a source of constant pressure so they can react quickly to changing load demands. The potential must exist to move the load from zero to maximum velocity at a prescribed maximum acceleration. The problem is, during periods when the load demands less than full flow, the excess energy potential — which can be substantial — goes unused. This unused energy generates heat.
The primary source of heat generation generally occurs whenever system pressure is elevated beyond that required by the load. However, using a pressure-reducing or a pressure-reducing-and-relieving valve in a circuit theoretically does not generate any significant heat during a static flow mode of operation. Significant power is lost only during a fluid flow condition and is directly related to the set pressure drop of the valve.
Power loss in flow-control valves
There are three primary circuits for controlling the speed of a cylinder or other type of actuator: meter in, meter out, and bypass. All three methods restrict flow, which creates a pressure drop that wastes power and generates heat. Heat distribution for meter-in and meter-out is shown in the Figures 6 and 7.
Meter-in flow control causes a pressure drop from the system pressure to the moving cylinder load pressure, which generates heat downstream of the flow control valve. A meter-out circuit, on the other hand, generates heat past the flow control, toward the directional control valve, and, finally, to tank.
With a meter-in circuit, as the cylinder load decreases, the power lost will approach the power drawn. Although the power loss is significant, heat generation is relatively low because this is not a constant-flow condition. The power loss will, however, increase the surface temperature of the piping downstream of the flow control valve. The amount of temperature rise will relate directly to the number of function operations per unit of time and the differential between the fluid in the tubing and ambient.