01The three numbers that decide the system
Duty
kW = press platen mass × cp × ramp rate, plus the moulding or laminating load. A 3,000 kg steel platen heated 20°C in 30 minutes needs about 3,000 × 0.48 × 20 ÷ 0.5 h ÷ 3,412 conversion ≈ 85 kW before process load.
Flow
Volume flow = duty ÷ (ρ × cp × ΔT). For 100 kW at a 20°C oil-side rise and typical mineral oil (ρ 850, cp 2.1): 100 × 3,600 ÷ (850 × 2.1 × 20) ≈ 10.1 m³/h. Halve the ΔT and you double the pump.
Film temperature
The limiting number is not the bulk oil temperature but the oil film at the heating surface. Keep heat flux at or below about 2.5 W/cm² and tube-side velocity at 1.5-2 m/s or better; that is what keeps coking off the coil.
Cold start
Oil at 15°C can be ten times more viscous than at operating temperature. The pump and the strainer must survive the first minute, and the heater must not be allowed to energise before flow is proven.
Engineers routinely specify the duty and the temperature and leave flow to the supplier. That is the wrong order: flow determines the pipe size, the ΔT, the platen uniformity and the coking margin all at once.
02Circuit layout across a large platen
Series versus parallel
Parallel circuits from a manifold give the same temperature to every zone but need flow balancing valves. Series circuits self-balance on flow and get progressively hotter, which is sometimes exactly what a graduated press wants.
Plate thickness
A thick plate with closely spaced channels behaves like a heat spreader and forgives uneven inlet temperature. A thin plate with wide spacing shows the manifold temperature directly on the working face.
Dead legs
Any branch that stops flowing stops cooling the wall next to it. In an oil system the dead leg becomes a coking site within months, and the deposit then raises the local film temperature of everything nearby.
Instrumentation
Put a sensor at heater outlet, at platen inlet and at platen return. Three temperatures tell you the duty, the ΔT and whether a circuit is blocked; one sensor tells you nothing.
A common retrofit failure: the old system ran on steam or hot water and the channels were sized for a much larger ΔT. Switching to oil at high flow through the same channels raises the pressure drop sharply, and the pump that "worked fine" now cavitates at the hottest moment.
03Heater selection for the duty
Wall heat flux
Keep the electric heater at or below about 2.5 W/cm² of wetted surface. Above that, the oil film at the wall cracks faster than the bulk temperature suggests, and the carbon layer insulates the tube from behind.
Coil versus immersion
A coil-in-tank arrangement gives low velocity and needs a much lower flux; a circulation heater with the oil inside the tubes can hold 1.5-2 m/s and takes the higher load. For platens, the circulation type is the default.
Step control
Two or three contactor steps plus a modulating zone, or a power controller, depending on how tight the platen tolerance is. Rubber vulcanising presses tolerate ±5°C; optical laminating usually wants ±1-2°C.
Protection
High-limit on the heater outlet, low-flow trip on the pump side, low-level on the expansion tank, and a pump run-on that keeps oil moving while the coil cools down. The run-on is the cheapest life extension in the system.
Oversizing an oil heater is not like oversizing an air heater. The extra kW has to leave through the same coil surface, so a bigger machine at the same flow means a higher flux and a shorter oil life. If the process needs faster ramp, raise the flow as well as the power.
04Startup sequence worth writing into the PLC
The order that keeps oil alive: pump start, flow prove at above 70% of nominal for 60 seconds, heater enable with the first step only, then ramp the remaining steps as the return temperature rises. On shutdown, heater off, pump continues until the outlet falls below about 120°C, then the circulation fan or cooling loop stops.
Write it into the logic and interlock it, rather than leaving it in a manual that the night shift does not have open.