01Why the reservoir needs heat at all
Viscosity grade
Most mobile and industrial hydraulic fluids are ISO VG 32 or 46. Their usable viscosity window is roughly 16-75 mm²/s; below about 20 mm²/s a vane or piston pump starts to lose its film, and above 100 mm²/s suction becomes the limit.
Cold-start damage
A machine that starts at 10°C oil is a machine that runs dry for its first minutes. Wear during cold start accounts for a large share of pump life consumed in service, and it is invisible until the teardown.
Contamination and water
Warm oil holds water and lets it pass through the filter; cold oil lets water settle and pool. Keeping the reservoir above the dew point of the shop air is part of the reason systems run heated even when idle.
Precision duty
Machines with tight dimensional tolerances — presses, injection moulding, CNC hydrostatic bearings — drift with oil temperature. A heated reservoir that holds ±2°C is a accuracy device as much as a wear device.
The sizing question therefore is not "how hot" but "how fast, from what temperature, and how well held". Those three answers give the kW, the control band and the element protection.
02Sizing the heater
Warm-up duty
A 400-litre reservoir of ISO VG 46 (about 350 kg, cp ≈ 2.0 kJ/kg·K) from 10°C to 40°C is 350 × 2.0 × 30 = 21,000 kJ ≈ 5.8 kWh. In two hours that is 2.9 kW delivered; add the tank loss and a 20% margin and 3.6-4 kW is the order of magnitude.
Standing loss
An uninsulated 400-litre tank with 2.5 m² of surface at ΔT 25 K and a natural-convection coefficient near 10 W/m²K loses about 0.6 kW continuously. Insulating the tank pays back in weeks on a machine that runs all day.
Watt density in static oil
1-1.5 W/cm². Hydraulic oil is not circulating at the element surface, and the film that sits against the sheath is the same film that carbonises. Local overheating also degrades the additive package, which shows up as varnish on servo valves months later.
Element count
Two 2 kW elements beat one 4 kW element: lower surface load each, and the machine still runs at reduced capacity if one fails. Flange size grows, which is usually the cheaper trade.
Pump heat is real and often forgotten: a system with a 15 kW pump motor rejecting 25% of its input as heat into the oil gains most of its warm-up from the pump itself once it is running. The heater is sized for cold start and idle, not for the running case.
03Where to put it, and how to control it
Location in the tank
In the return line zone, below the lowest working level, away from the pump suction strainer and clear of the tank bottom so sludge does not bury the sheath.
Mixing
A reservoir without a heater loop stratifies: hot at the element, cool at the suction. If the tank is large, put the element near the return entry, or add a small circulation loop through the cooler.
Thermostat and high-limit
One control thermostat plus an independent high-limit set a few degrees above normal maximum. Hydraulic fluid flashes and seals harden well above 60°C, and the control thermostat alone is the usual suspect after a stuck contactor.
Cooler interaction
If the system has an oil cooler with a thermostatic bypass, set the heater cut-out below the cooler cut-in so the two never fight. A heater and a cooler running together is pure wasted energy.
For machines that idle overnight, a hold-on control at 25-30°C gives a fast morning start without the energy of running at 45°C around the clock.
04The failure list
Burned element with an intact thermostat: usually a low level during a leak-down, or a machine whose oil level was dropped for a filter change and never topped up before power-on.
Varnish on servo valves: usually a local hot film at the element cracking the oil, not a high bulk temperature. Lower the watt density, add more elements, or move the element into better circulation.
Corroded sheath at the weld: water in the fuel or a chloride-bearing environment. Material selection is a question about what else is in the oil, not about the oil grade.
Contactor welded closed: the element runs to the high-limit or to destruction. The high-limit must cut the power at the contactor coil or the contactor feeder, not merely alarm.