A hydraulic power unit turns some input energy into heat through pump inefficiency, throttling, relief flow, piping and other losses. Excess oil temperature can change viscosity and shorten component life. Cooler selection should therefore begin with a heat-load estimate and a permitted oil-temperature range, not the motor nameplate power.
Find where heat is produced
List the pressure and flow during every stage of the machine cycle. Note long periods where a fixed pump may circulate against a valve or relief path. Include the duty of proportional controls, motors and accumulators where applicable. The electrical input and useful hydraulic work are different; their difference is a starting point for heat analysis, but not every loss enters the same oil volume at the same time.
Use measurements when the machine already exists
On a retrofit, oil temperature rise over a measured interval can help estimate the uncooled heat load, provided tank volume, fluid properties and operating state are known. HYDAC’s cooler selection tool explicitly offers a tank-temperature-rise method and a method using inlet/outlet temperatures of an existing cooler. A stable operating measurement under a representative worst-case duty is more useful than a brief idle reading.
Compare the cooling options
| Option | Site condition to confirm | Typical trade-off |
|---|---|---|
| Air-oil cooler | Maximum ambient air temperature, airflow space and fouling | No cooling-water supply, but performance falls as air temperature rises |
| Water-oil heat exchanger | Water inlet temperature, flow, quality and disposal | Compact heat transfer, but water circuit and fouling need management |
| Reservoir/passive rejection | Available surface area and duty cycle | Simple, but limited when continuous losses are high |
Cooler capacity depends on oil flow, viscosity and the temperature difference to the cooling medium. HYDAC’s product data makes these dependencies explicit. Confirm cold-start pressure drop, bypass provisions, fan power, cleaning access and the temperature sensor location before ordering.
Information for a cooler RFQ
- Measured or calculated continuous and peak heat load with the duty cycle.
- Fluid type, operating viscosity, target oil temperature and maximum ambient temperature.
- Available oil flow and pressure, water supply if applicable, and installation envelope.
- Contamination, corrosion, noise and maintenance conditions.
First establish the system load using our HPU duty-cycle sizing guide. Then size the cooler with the chosen manufacturer’s performance curves and confirm it in commissioning.
References: HYDAC CoolIT sizing method; HYDAC heat exchanger selection data. Cover: original schematic illustration.