ONE of the critical conditioning requirements of hydraulic fluid is that it is maintained at an optimal operating temperature. As oil temperature drops, the viscosity of the fluid increases, making it more difficult to pump, creating higher pressure drop and increasing the chance of cavitation. As oil temperature increases, the viscosity of the fluid decreases, which reduces lubricity, increases oxidation rate and can cause the fluid to varnish.
Hydraulic systems use heat exchangers to control oil temperature—and therefore viscosity—within an optimal range, where the fluid has the best combination of properties useful to the components of the hydraulic system. Although a few hydraulic machines can make do without external cooling, such as small, low-duty or load-sensing systems, most require a device to keep oil in its ideal temperature range. This is where heat exchangers come in.
What a heat exchanger does is self-explanatory. It will use a fluid such as water or air to transfer heat into or away from hydraulic liquid; very simple. However, the nature in which heat exchangers transfer heat can vary vastly. Liquid-to-air and liquid-to-liquid are the two primary types of heat exchangers, and you can imagine they can use air and water, respectively, to remove heat from a hydraulic system.
Liquid-to-air coolers transfer the heat from the hydraulic fluid through radiation and convection. The simplest liquid-to-air coolers are radiators that count on the thermal difference between the hydraulic fluid and the ambient air. The rate in which a heat is removed from the oil is factored only by the temperature difference between the air and the oil (higher differential means more cooling) and by the existence of airflow (which is sometimes likely in a mobile application).
The basic tube and fin cooler is the most economical method of cooling hydraulic fluid, but is for light duty applications, such as low duty cycle or low horsepower applications. They are often very small, such as the type used in a vehicle’s transmission fluid cooler, but in hydraulic applications, they can be sometimes paired with light duty fans to improve efficiency. This type of cooler is small and light enough to be attached to the back of an electric motor to take advantage of the motor’s cooling fan. They’re constructed by forming a copper tube into a snaked web, and then aluminum or copper fins are added to surround the tubes. Heat energy is imparted from the oil, to the tube wall, to the fins and then to the air.
The more efficient liquid-to-air cooler is the plate and bar style. It will use thick and deep channels of rectangular aluminum with spans of aluminum fins separating them. The better coolers will come with a rough internal finish to those channels to add turbulence to the moving liquid. Although this technique adds pressure drop to the cooler, it removes heat with more efficiency. The fins on the air side of the cooler can also be crimped for a rough finish, increasing air turbulence and improving the rate of heat transfer to
Regardless of liquid-to-air cooler construction, any design will more rapidly remove heat with the addition of convection. Adding a fan to a cooler increases cooling exponentially. Cooling fans can be any size, from tiny dc fans used in the computer industry, to high horsepower ac motors used in industrial applications. Extreme duty mobile applications using dc fans can pull upwards of 40 A, which is the upper range of reasonable usage for 12 V applications, and is taxing on the electrical system in the best of cases. When mobile applications are severe, the fan motor can be hydraulic. With hydraulic energy, the fan can be high power while using absolutely no electrical current.
If massive amounts of heat must be removed from a hydraulic system, air coolers aren’t the most efficient option. Although some electric coolers can remove over 300 hp worth of heat, they can block a commercial doorway because of their unwieldy size. Liquid-to-liquid coolers use water or coolant to remove heat from the hydraulic fluid. Water transfers heat orders of magnitude more efficiently than air, and the same wall of air coolers could be matched in performance by a shell and tube cooler the size of a
The problem, of course, is liquid-to-liquid heat exchangers require water or coolant to do their job.
If you have unlimited supply of fresh water, you can simply use that water to continuously run through your cooler (or control its flow thermostatically). Tap water can also be used for cooling, although it can be expensive using the municipal water supply. In large plants or factories, a centralized cooling system can be put in place to supply coolant to the various machines of the plant. This is the most environmentally friendly option, but requires expensive infrastructure, such as complex plumbing and large chiller units exterior to the building.
The shell and tube cooler features a series of copper tubes installed into a larger tube shell. The copper tube can be a single bent tube, or can be many small tubes spanning two plates on either side of the shell. Either method will use a ported shell, itself just a metal barrel, with water flowing into one end and out the other. Water or coolant passes across the copper tubing, transferring heat from the hydraulic fluid to the cooling medium, and then discharging it to wastewater or back to the coolant system.
The most efficient method of cooling hydraulic fluid is with the brazed plate cooler. The same cooling capacity requiring a wall of air coolers or a bazooka tube can be achieved with a plate cooler the size of a textbook. It has long, wide plates brazed together to optimize surface area for heat transfer, and spaces the plates to alternate coolant with hydraulic fluid. Because they are so efficient, the brazed plate cooler can make do with half the water or coolant compared to shell and tube types, although they still need that fresh supply of water or coolant. Some large variants of the plate type cooler can remove a jet engine worth of heat, although those versions do require extraordinary amounts of cooling medium.
It should be noted that any liquid-to-liquid heat exchanger can be used to warm hydraulic fluid as well as cool it. By simply running hot water or coolant through the cooler instead of cold, heat can be transferred to the hydraulic oil. Most often, hydraulic oil is heated electrically, because using warm water requires a more complex cooling system circuit. Regardless, until hydraulic systems have nearly perfect efficiency, heat exchangers will always be a big part of hydraulic systems.