Cool is relative. We humans enjoy a narrow bandwidth of ambient temperature, and we complain should we experience anything outside of a twenty degree span. To be fair, we can handle a few hundred degrees above absolute zero, but in the grand scheme of things, it’s nothing compared to the millions of degrees at the center of the sun, which is so abstract in my mind, you might as well start throwing around words like “bjillion.”
Because of our limited capacity to understand temperatures we can’t relate to, heat in a hydraulic system can be confusing as well. Because in our limited scope of thermal comprehension, we think anything hotter than a buck-fifty must be melting the tungsten wedding rings, but to be honest, a hydraulic system doesn’t even break a sweat until 150º F. Just because you can’t hold your hand on it, it doesn’t mean it’s hot, and to put it perspective, hydraulic oil viscosity is specified at a hundred degrees.
Regardless, 150° F isn’t ideal, and as temperature rises, oxidation rate accelerates and lubricity deteriorates. Of primary importance to a hydraulic system is ensuring your oil is within its optimal viscosity range, and depending on the fluid chosen, this could mean a temperature range anywhere between 100-140° F. Although choosing oil with higher viscosity will prevent it from thinning out “as the heat gets hot,” it can’t prevent oxidation or other issues such as varnish. It’s best to keep your oil cool, relatively speaking.
A well-designed, efficient hydraulic system will not require a hydraulic cooler. For example, if your system uses a load-sensing variable displacement piston pump, unless heat is being added through the environment, there is little need for an externally plumbed cooler. These systems will waste very little energy in providing your machine with the right amount of pressure and flow required at that moment. However, because of the cost and complexity of a load sensing system, these systems are sadly rare. Using an extra-large reservoir may reduce your requirement for cooler, but because of size and space limitations (especially on mobile machinery), giant tanks are rarely an option.
Sometimes it’s better to ask for forgiveness than ask for permission, and this is why we use hydraulic coolers. A cooler is any device which can take the heat generated by or added to a hydraulic system, and dissipate it to the environment. The two primary types are liquid-to-air and liquid-to-liquid. A basic liquid-to-air cooler is simply a radiator; essentially a spot heater with no fan, but conducts heat from the oil to its atmospheric surroundings. By adding the convection effect of forced-air, a fan blowing across the cooler increases its cooling capacity exponentially.
Forced-air coolers are the most common style, as they come in a wide range of sizes, and are available in DC voltage for mobile machinery, or high-voltage for industrial environments. The quality of the heat exchanger itself can vary vastly as well, ranging from the tube-and-fin type you see on the transmission oil cooler on your F150, to the high-flow welded aluminum types you find on a Spring Cup car. I can tell you its worth the money to buy the latter, because they can provide superior cooling in a smaller package, but those shiny rectangular outlet sections are also a great surface to put your #24 decals.
If you want to ramp up the cooling power in an even smaller plot of real estate, the most efficient cooler is the liquid-to-liquid. Water is vastly more efficient at transferring heat, so any cooler that allows oil heat to conduct to coolant heat will provide maximum cooling effectiveness. The downside, of course, is that you need continuous coolant or water flow. If you’re a steel mill sucking in metric tonnes of river water every second, you probably don’t mind. But if you’re a small machine shop, you probably don’t want to hook a garden hose up to your cooler just to keep your machine’s draw bar cylinder cool on a hot day, so liquid-to-liquid is just not an option.
And just as with forced-air coolers, the quality of a liquid-to-liquid cooler varies significantly. The basic shell-and-tube cooler is still highly efficient, because of the thermal conductivity of water, but you have to throw a lot more water at them to get them to do perform well. The most efficient coolers are the brazed plate type, which are so effective and transferring heat, you could use an i-Pod sized cooler to remove a couple hundred horsepower (disclaimer: this is an exaggeration, and your i-Pod will make a terrible hydraulic cooler. @FluidPowerTips is not responsible for any damage or harm to your KC and the Sunshine Band collection).
Regardless of your method in oil cooling, what is important is to keep it within a reasonable window of viscosity, while preventing the damage that occurs at high temperature. Cooling is the second of three critical fluid conditioning factors needing to be addressed, along with filtration. Since we’ve now covered contamination and cooling, you get no prize for guessing we’re on to moisture control next.
Also, check out “What is hydraulic fluid conditioning – part 1.”