How to use this calculator
- Enter pump flow and pressure. Use the average flow and working pressure for the hydraulic circuit condition you want to cool.
- Enter heat assumptions. Set pump efficiency, load heat fraction, duty cycle, additional heat and sizing margin.
- Enter cooler rating and ETD. Use the catalog cooler rating and the catalog oil-air temperature difference, then enter your operating oil-air temperature difference.
- Check utilization and oil rise. Compare the selected operating cooler capacity with the design heat load and check oil temperature rise through the cooler.
How it works
Hydraulic heat load starts with pump power. The useful fluid power is
P_hyd = Q · p / 600
when flow is in L/min and pressure is in bar. The pump shaft has to supply
P_shaft = P_hyd / η_t, so the pump loss heat is
P_shaft - P_hyd. The calculator then adds an entered fraction of
hydraulic power for valve, throttling, actuator and motor losses, averages the
result by duty cycle, adds any extra heat, and applies the sizing margin.
Cooler catalogs usually quote heat rejection at a stated oil-to-air temperature
difference, or ETD. For screening, the calculator scales that rating linearly:
P_cool = P_catalog · ETD_operating / ETD_catalog
and compares it with the design heat load. It also estimates cooler oil
temperature change from ΔT_oil = P_heat / (m_dot · c_p).
Worked example
Verified against the live calculator
A 40 L/min hydraulic circuit at 160 bar has
P_hyd = 40 × 160 / 600 = 10.7 kW. With 85% pump overall
efficiency, pump loss is about 1.88 kW. If 25% of useful hydraulic power returns
as load heat and the condition runs at 75% duty, estimated heat load is about
3.41 kW. With a 1.2 sizing margin the design heat load is about
4.09 kW. An 8 kW catalog cooler rated at 40°C ETD rejects about
6.0 kW at 30°C operating ETD, so utilization is about 68%.
Frequently asked questions
How do I calculate hydraulic system heat load?
Start with pump hydraulic power, P_hyd = Q(L/min) x p(bar) / 600. Pump loss heat is shaft power minus hydraulic power. Add the fraction of useful hydraulic power that returns as valve, throttling, actuator or motor losses, average it by duty cycle, then add any measured extra heat.
What is ETD on a hydraulic oil cooler?
ETD is the entering oil temperature minus entering air temperature. Oil coolers are often rated at a stated ETD, so this calculator scales the entered catalog rating by operating ETD/catalog ETD for a first-pass comparison.
Does this replace a cooler manufacturer curve?
No. It is a screening calculator. Final selection needs the cooler manufacturer curve or software for oil viscosity, oil flow, airflow, fan speed, fouling, shrouding, ambient temperature, altitude and installation constraints.
What fraction of hydraulic power becomes heat?
It depends on the circuit. Pump inefficiency becomes heat, relief bypass becomes almost all heat, and throttled pressure drops or motor/cylinder losses can return a large fraction of useful power as heat. The default load heat fraction is a starting screen, not a rule.
How is oil temperature rise through the cooler calculated?
Oil temperature rise is estimated from ΔT = P_heat / (m_dot x c_p), where m_dot is oil mass flow through the cooler and c_p is oil specific heat. This is the temperature drop the cooler must create at the entered heat load and oil flow.
Method & assumptions
- Hydraulic power uses the practical shortcut kW = Q(L/min) × p(bar) / 600.
- Pump inefficiency is treated as heat: shaft power minus hydraulic power.
- Load heat fraction is a user-entered screening allowance for throttling, valve drops, actuator losses, motor losses and relief flow.
- Cooler catalog rating is scaled linearly with oil-to-air ETD for a first pass only.
- Final cooler selection still needs the manufacturer's curve for oil viscosity, oil flow, air flow, fan speed, fouling, ambient temperature and mounting conditions.