01What coking really is: film temperature exceeding the limit
The layer of oil touching the heater element wall is always hotter than the bulk oil. The higher the wall heat flux and the lower the flow velocity, the bigger that gap becomes. Once the film temperature exceeds the oil's permitted film temperature, the oil cracks and polymerizes: first a dense varnish layer, then carbonized coke.
Coke then feeds itself. Its thermal conductivity is a tiny fraction of steel, so the same power drives wall temperatures even higher and coking accelerates. By the time an operator notices 'same power, lower outlet temperature', the coke layer is often past one millimetre.
02Three root causes, ranked by frequency
Excessive heat flux
Under-sized heating surface to save cost, or a standard circulation heater pushed into high-temperature thermal oil duty. Keep wall flux at or below ~2.5 W/cm² for thermal oil service, lower for high temperatures.
Insufficient velocity
Undersized pump, high system resistance or throttled valves drop the tube-side velocity below 1.5–2 m/s. Slow flow means a thick, hot boundary layer.
Local overheating / dry firing
Pump stopped while heaters stay on, low oil level, or temperature sensors placed where they never see the real wall temperature. Without flow and over-temperature interlocks, coking is only a matter of time.
03Prevention at selection
First, check the flux: total power divided by actual heated surface. Above the oil's permitted value you need larger element diameter, a longer heating section or lower per-element power. Second, confirm the velocity design: the manufacturer should state design flow and pressure drop, and your pump must deliver that velocity inside the heater — not just 'enough head'.
Third, control the ramp-up: cold thermal oil is viscous. Start with one heater group, circulate, de-aerate, then step up. Properly built systems use multi-stage staged energization for exactly this reason.
04Prevention in operation
Heaters off before pump off
Every shutdown sequence must keep circulation running after the heaters de-energize, to carry residual heat out. The trip logic should always cut heating first.
Regular oil analysis
Track carbon residue, acid number and kinematic viscosity. Carbon residue above ~1.5% or acid number above 0.5 mgKOH/g means it is time to replace oil — and hunt for the overheating source.
Watch the expansion tank
Falling expansion tank level and rising vent vapour often indicate internal cracking already underway. Don't write it off as normal losses.
05If coke is already there
Light fouling can be removed with circulating chemical cleaning — then re-verify velocity and temperature rise. If bundles are discoloured or deformed from overheating, replacement is the safer call: the overheated metal has lost its margin, and running it again risks burn-through. Fix the root cause before the new core goes in, or it will repeat the history.