How Cooling Works in Hydraulic Systems (Radiator Working Principle)
The purpose of cooling in a hydraulic system is to continuously carry the heat generated in the oil out of the system and keep the temperature in balance. In this article you will find, step by step, where the heat comes from, where the cooler connects in the circuit, and how fan-cooled, air-cooled models work.
Where does the heat come from?
No conversion in hydraulic power transmission is fully efficient. Inefficiency means lost energy, and that energy builds up in the oil as heat. The main sources are:
- Pressure drop: When oil passes through narrow openings such as a valve, an orifice or a relief valve and loses pressure, the lost pressure energy turns into heat.
- Internal leakage: Small leaks in pumps, motors and cylinders also generate heat.
- Friction: Flow resistance in the lines and friction in moving parts add heat.
The oil carries this heat to the reservoir. If the heat that dissipates naturally from the surface of the reservoir and pipes is not enough, the temperature rises.
Where does the cooler sit in the circuit?
In most applications, a hydraulic oil cooler is placed in the return line. The reason is simple: the oil in the return line has done its work and is hot, and its pressure is generally lower than in the main pressure line. The oil passes through the cooler, cools down and returns to the reservoir.
In some designs, the cooler runs in a separate circuit independent of the main flow. Which layout is suitable depends on the circuit diagram and the pressure limit of the cooler. For these limits, rely on the manufacturer's documentation.
Another advantage of the return-line layout is that the oil goes straight back to the reservoir after passing through the cooler. The oil in the reservoir therefore does not build up heat, and the pump draws in cooler oil. In addition, because the cooler is not exposed to high pressure, the housing and connections work under easier conditions.
In cold weather the oil is thick, so flow resistance and pressure in the return line can rise. For this reason, some circuits use a bypass that skips the cooler or a temperature-sensitive valve. Whether such an element is present depends on the machine manufacturer's circuit design.
What happens inside the cooler?
- Hot oil enters the cooler and passes through the channels or the coil.
- Heat is conducted through the metal wall to the fins.
- Air passing over the fins picks up the heat.
- The cooled oil returns to the reservoir.
In Safkan's fan-cooled series, hot oil passes through the channels and the finned coil, and the integrated fan blows air across it; the air never touches the oil.
Keeping the temperature in balance
The goal is not to cool the oil as much as possible, but to keep it within the manufacturer's range. When heat generation and heat rejection are equal, the temperature stabilizes. When load increases, heat generation increases; if the cooler capacity is sufficient, the temperature stays in balance.
In fan-cooled models, the fan can in most applications be set up to switch on depending on temperature, so the oil is not cooled more than necessary. This choice depends on the system design.
For the supply types and applications of fan-cooled models, see the article on fan-cooled oil cooler models, and for the product group see the fan-cooled radiators page. If you want to refresh the basic concept, the article what is oil cooling will help.
A simple heat balance model
It helps to think of the circuit as a balance. On one side is the heat entering the oil: it grows as load increases and as pressure drops and leakage get larger. On the other side is the heat rejected: through the reservoir surface, the pipes and the cooler. The temperature settles at the point where these two values are equal.
The cooler's job is to raise the heat rejected and so pull this balance point below the temperature the machine allows. If the selected cooler is too small, the balance temperature stays high; if it is sufficient, the oil temperature holds at a stable level.
Next step
Once you know the heat load, oil flow rate and working pressure of your system, selecting a cooler becomes much clearer. For product selection and technical information, reach us through the contact page.