Overheated hydraulics can be caused by decomposing fluid, wear, or damaged seals and bearings. Heat exchangers can prevent overheating and extend the service life of your hydraulic system.
Smaller hydraulic systems with lower operating temperatures can often be cooled through natural convection. However, if natural convection is not enough, it becomes necessary to add a heat exchanger.
Heat exchangers 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 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 heat exchanger saves time, money, and maintenance.
Air oil coolers are ideal when water is not available. They pass hot oil through tubes where turbulators break up laminar flow to promote heat transfer from the fluid to the tube wall. A fan blows cooling air over the tubes and cooling finds.
Selecting the Best Heat Exchanger
Selecting a heat exchanger 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 heat exchangers, 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 heat exchanger 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. For example, some companies provide online sizing calculators and other interactive resources that let engineers plug in specifications to get an idea of what is needed.
To select the best heat exchanger, you’ll need to provide:
- Required heat dissipation in hp
- Oil heat load in BTU/hr
- Oil flow in gpm
- Maximum required oil temperature
- Maximum ambient air temperature during operation
- Environmental contaminants that can affect the system
- Maximum allowable oil pressure drop
If the required heat dissipation is not known, it can be estimated assuming 20% to 30% 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-cooled heat exchangers, you also need to know the cooling water’s inlet temperature and flow rate. Most manufacturers’ literature includes examples, steps, and simplified equations to properly size heat exchangers. Once the heat-load parameters and other key influencing factors are defined, the next step is choosing an air-cooled or water-cooled version.
Calorimeters, or component wind tunnels, can measure cooler performance and help optimize their design. Parker invested in the calorimeter pictured here, along with stat-of-the-art software, to test and optimize cooling solutions. These let them go from a “napkin sketch” concept to protype in 30 days or less.
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 the surrounding air.
In air-cooled heat exchangers, hot oil passes through tubes containing turbulators which break up laminar flow to promote heat transfer from the fluid to the tube wall. The tube is 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 connected to an array of external fins. The design is lightweight and offers low pressure drops. The tubes are typically welded aluminum with thin walls, making them susceptible to damage from extreme pressures and external debris that could be encountered in the application.
Bar-and-plate construction uses compact, efficient cores. They offer more cooling per cubic inch than tube-and-fin construction. They consist of finned chambers separated by flat plates which route fluids through alternating hot and cold passages. The design creates a honeycomb structure that resists vibrations and shocks. The core is usually made of aluminum and, regardless of what metal it is made of, the core is furnace brazed in a controlled atmosphere or high vacuum.
Bar-and-plate coolers offer engineers greater design flexibility. Finned passage sizes can be easily varied by changing fin type, height, and density. They can also be customized to fit a specific envelope.
Both types of air-based oil coolers typically have a fan driven by a hydraulic or electric motor. Mobile equipment such as in construction, forestry and material handling, use either hydraulic-driven or DC fan motors. Industrial equipment and HPUs have AC electric motors connected to the fans and pull air through the cooler core.
Heat exchanger manufacturers offer a lot of motor configurations, voltages and displacements, to fit various applications.
For more than 50 years, shell-and-tube, water-cooled heat exchangers have been the industry mainstay.
Water-Cooled Heat Exchangers
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. However, newer designs have recently been developed that increase effectiveness while providing an equivalent heat-transfer surface in a smaller package at a reduced cost.
Shell-and-tube heat exchangers have an outer flanged shell with end bonnets appropriately sealed to the shell ends. Inside, a precise pattern of tubing runs the length of the shell and terminates in the end plates. Tube ends are fastened to the end plates, which seal each end of the shell. Cool water flows through the tubes while hot hydraulic 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 flows. This pattern improves heat transfer by forcing the hot fluid to flow perpendicular to the tubes and promote laminar flow.
The shell-and-tube design adds fins to the external sides of tubes o increase heat transfer and efficiency. The fins add surface area and improve heat transfer, letting the overall size be smaller than standard shell-and-tube exchangers. However, due to increased heat transfer, the pressure drop is higher than in the older versions.
Brazed-Plate Heat Exchangers
A slightly newer type of heat exchanger is the brazed-plate style. In this heat exchanger, heat-transfer surfaces are a series of stainless-steel plates, each stamped with a corrugated pattern for strength, efficiency, and resistance to fouling. 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, end plates, and connections are 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 heat exchangers 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 exchanger. Their stainless-steel construction permits flow velocities up to 20 feet per second. Higher velocities, combined with turbulent flow, provide heat transfer at 3 to 5 times the rate of shell-and-tube exchangers. The higher heat transfer rate requires less heat-transferring surface area for a given capacity.
Due to their compact construction, brazed-plate heat exchangers are ideal when space and size are design considerations. 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 heat exchangers require chemical, rather than mechanical, cleaning.
Brazed-plate heat exchangers are compact, rugged, and provide high heat transfer capacity.
ROI of Heat Exchangers
When proper heat exchangers are included in hydraulic systems, they 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, it’s not uncommon to delay deciding on heat exchanger specifications until after seeing how a system performs and how much heat transfer is needed. When you have questions, the best advice is simply to directly contact a heat-exchanger supplier.
Manufacturers will have additional resources that you can use in the selection process, including specialized design software and testing equipment. This includes calorimeters, which are used to test and improve coolers. Rely on your manufacturer’s expertise and available resources such as training courses and digital calculation tools to ensure success when specifying heat exchangers. For example, Parker holds classes that educated engineers on cooler technologies and how to properly select them.
Francis Gradisher Jr. is the product marketing manager for coolers at Parker Hannifin’s Accumulator and Cooler Div. For a helpful checklist on selecting a heat exchanger, download the Essential Cooler Sizing Parameters booklet. For more information on heat sinks for hydraulics or Parker’s product school, Gradisher can be reached at (815) 636-4100 or by e-mail at [email protected]