Maintaining a normal oil temperature in all hydraulic systems is important for successful system operation. Normal operating temperatures for hydraulic systems is 110 to 130° F (unless specified by the equipment manufacturer). At high temperatures, oxidation of the oil is accelerated. This oxidation shortens the fluid’s useful life by producing acids and sludge, which corrode metal parts. These acids and sludge clog valve orifices and cause rapid deterioration of moving components. The chemical properties of many hydraulic fluids can change dramatically by repeated heating/cooling cycles to extreme temperatures. This change or breakdown of the hydraulic media can be extremely detrimental to hydraulic components, especially pumping equipment.
Overheated hydraulics can be caused by decomposing fluid, wear, or damaged seals and bearings. Coolers can prevent overheating and extend the service life of your hydraulic system. However, smaller hydraulic systems with lower operating temperatures can often be cooled through natural convection. If natural convection is not enough, it becomes necessary to add a cooler.
Coolers are also crucial for systems with temperature requirements such as needing to stabilize the hydraulic fluid’s viscosity by keeping it at a specific temperature, or equipment with a history of hot oil problems that shortened seal life and break down the fluid. Hot fluid is always a concern with large mobile equipment, as well as commercial and industrial processing equipment. Specifying a properly sized cooler saves time, money and maintenance.
The process to select a cooler is driven by the type of system that needs to be cooled. Parameters to consider include heat load, power source, noise, operating costs, space available, environmental conditions, and more.
Actual heat generation varies throughout the machine’s cycles, as well as changing environmental factors and ambient temperatures. This can make it challenging to accurately define your cooling needs. When considering the application and sizing of coolers, the hydraulic fluid’s ideal operating temperature and the time it takes to arrive at that temperature must be used.
For new designs and retrofits, the first step in selecting the right cooler is identifying the challenges and performing the necessary calculations. Virtual design and sizing tools are available from most manufacturers to help determine the best fit for your application. Some companies provide online sizing calculators and other interactive resources that let engineers plug in specifications to get an idea of what is needed. Parker offers a comprehensive suite of online cooler sizing software. For instance, Parker offers an online cooler selector tool for each of the different types of coolers including brazed plate, shell and tube and air-oil coolers.
To select the best air-oil coolers, you’ll need as much information about the application as possible, including, but not limited to the following:
Oil Heat Load in BTU/Hr or HP
Oil Flow Rate (GPM)
Maximum Inlet Oil Temperature (°F)
Maximum Ambient Air Temperature During Operation (°F)
Environmental contaminants that can affect the system
Maximum Allowable Pressure Drop Across the Cooler (PSI)
If the required heat dissipation is not known, it can be estimated assuming 20-30 percent of the installed horsepower will be converted into heat load. The most accurate way to calculate the heat load is to record the time it takes the oil to get up to temperature without a cooler in the system.
For water-oil coolers, which include Parker’s ST and OAW series coolers, you also need to know the inlet temperature and flow rate of the cooling water. Most manufacturers’ literature includes examples, steps and simplified equations to properly size coolers. For instance, Parker provides engineering specifications for their ST series water-oil coolers, such as cooling capacity, flow rate, working pressure, sizing, and connection thread in their online tools to enhance the specifying process. Once the heat-load parameters and other key influencing factors are defined, the next step is choosing an air-oil (air-cooled) or water-oil (water-cooled) cooler.
Air-oil coolers remove heat from the oil in a cooler by using the ambient air around the cooler. Air-oil coolers convect heat, which makes them ideal when no water source is available or when the preference is to remove heat from the oil by using ambient air. In air-oil coolers, hot oil passes through channels that contain turbulators to prevent laminar flow from developing in an effort to promote heat transfer from the fluid to the channel wall. The channel wall is always constructed of metals with high thermal conductivities.
The cores of air-oil coolers are constructed in two different styles: tube-and-fin or bar-and-plate construction. Tube-and-fin construction consists of round or oval tubes mechanically connected to an array of external fins. The tube-and-fin design is lightweight and offers low pressure drop across the core. The tubes in a tube-and-fin design can be susceptible to damage from pressure spikes and external debris that can be encountered in any application. Bar-and-plate construction uses compact and efficient cores that offer more cooling per cubic-inch than a tube-and-fin design. They consist of finned chambers separated by flat plates which route fluids through alternating hot and cold passages. The bar-and-plate design creates a honeycomb structure that resists vibrations and shocks. This core is usually made of aluminum and is furnace brazed in a controlled atmosphere or high vacuum. With all the bar-and-plate design characteristics that provide certain benefits over the tube-and-fin design, it can be seen that bar-and-plate coolers can offer design engineers greater system design flexibility.
Both types of air-oil coolers typically have a fan driven by a hydraulic or electric motor. Off-road or mobile equipment used in construction, forestry, or material handling typically use either a hydraulic-driven or DC electric-driven fan motors. Industrial equipment such as Hydraulic Power Units (HPU) use AC electric-driven motors to drive the fans. Cooler manufacturers offer a lot of motor configurations, voltages, and displacements to fit various applications. For instance, Parker offers a variety of air-oil coolers with AC, DC, hydraulic fluid, and engine driven fans.
Cooler manufacturers offer a lot of motor configurations, voltages and displacements to fit various applications. For instance, Parker offers a variety of air-oil coolers with AC, DC, hydraulic fluid and engine driven fans. Parker’s two most popular air-oil coolers are the ULDC Series (DC fan motor) and the ULAC Series (AC fan motor).
Water-oil coolers remove heat from oil by using a second fluid (typically water). For more than 50 years, shell-and-tube oil coolers have been an industry mainstay when considering water-oil coolers. However, newer designs have been developed that increase efficiency while providing an equivalent heat-transfer surface in a smaller package at a reduced cost.
Shell-and-tube (bare tube) coolers have an outer flanged shell with end bonnets appropriately sealed to each shell end. Inside, a precise pattern of tubing runs the length of the shell and terminates in the endplates. Tube ends are fastened to the endplates, which seal each end of the shell. Cool water flows through the tubes while hot oil flows around the tubes within the shell. The tubes run through several baffle plates that provide structural rigidity and create a maze through which the hot fluid must traverse. This maze created by the baffles lengthens the path the hot oil must flow through. This elongated path increases the amount of heat transfer from the hot fluid to the water by forcing the hot fluid to travel around the tubes for a longer period of time.
As mentioned above, there are shell-and-tube designs in the market now that mechanically add fins to the external surface area of the internal tubes, which increases heat transfer and efficiency. Parker does offer this type of high-efficiency “hybrid” design in the ST Cooler Series. The way the hybrid design works is that the fins add surface area and improve heat transfer, letting the overall size be smaller than standard shell-and-tube exchangers without fins on the tubes (bare-tube). However, due to the increased time the hot fluid has to traverse the interior of the cooler, the pressure drop can be higher than shell-and-tube coolers without the increased flow path created by the baffles.
Another type of water-oil cooler is the brazed-plate style. In this cooler design, heat-transfer surfaces are a series of stainless-steel plates, each stamped with a corrugated pattern for strength, efficiency, and resistance to fouling by creating turbulence in the flow of both fluids. The number and design of the plates varies depending on the desired heat-transfer capacity.
Plates are stacked with thin sheets of copper or nickel between each plate. The plate pack, endplates, and connections are then brazed in a vacuum furnace to join the plates at the edges and all contact points. This design can be used with several different types of inlet and outlet connections.
Brazed-plate coolers are compact, rugged and provide high-heat transfer capacities. They hold approximately one-eighth of the liquid volume of a thermally comparable shell-and-tube cooler. Their stainless-steel construction permits flow velocities up to 20 feet per second. Higher velocities, combined with turbulent flow, provide heat transfer at three to five times the rate of shell-and-tube coolers. A good example of the increased horsepower is Parker’s OAW Series that offers up to 275 horsepower of cooling at an entering temperature difference of 60°F, based on a 2:1 water flow. The higher heat transfer rate requires less heat-transferring surface area for a given capacity.
Due to their compact construction, brazed-plate coolers are ideal when space and size are design requirements. One drawback of using a brazed plate cooler is that there can be higher pressure drops when compared to an equivalent shell-and-tube design. In addition, tests prove brazed-plate designs handle particles up to 1-mm in diameter without issue. Filters or strainers should be used if larger particles will be encountered. Due to their construction, brazed-plate coolers require chemical, rather than mechanical, cleaning.
When properly specifying the right cooler into a hydraulic system, a system will maintain the correct working temperature, which yields numerous economic and environmental benefits, including:
Extending the hydraulic system’s service life
Lengthening the oil’s useful life
Improving operating time and decreasing shutdowns
Reducing service and repair costs
Delivering high efficiency for continuous operation
Resources that simplify the choice
Given the many variables involved when specifying coolers, it is always best to directly contact a cooler supplier such as Parker, with any questions you have. Manufacturers will have additional resources that you can use in the selection process, including specialized sizing software and testing equipment like wind tunnels and cooler design simulation software. Lastly, it is important to take advantage of and rely on your manufacturer’s expertise and available resources to ensure you successfully size and implement the best cooler for your application. To view Parker’s wide range of coolers, visit www.parker.com/ACD.
This article was contributed by Francis C. Gradisher Jr., product marketing manager - KleenVent & Coolers, Parker Hannifin's Accumulator, and Cooler Division.