Oil and Chemical Immersion Heater

Watt density is the whole design, and it is set by the liquid

In water an element can carry ten watts on every square centimetre. In thermal oil the same element must be derated to one to three, and in heavy fuel oil to the bottom of that band. Nothing about the element changes — what changes is how fast the liquid takes heat off the sheath.

That means a heater for oil is long, or it is several elements, or both. If a supplier fits the same physical element that would go in a water tank and just labels it for oil, the tank will char the sheath within weeks. The way to size it is to work backwards: pick the safe density for the liquid, then find the heated length that the power needs at that density, then design the bundle to fit it into the tank.

Charring is a runaway, not wear

Once carbon deposits on the sheath, it insulates. The sheath then runs hotter to push the same heat through the deposit, which chars more oil, which insulates more. It accelerates, and it ends with a failed element and an oil charge that has to be dumped. This is why the safe watt density figure is not conservative advice — it is the line the process stays stable below.

Moving oil, or a heater that cannot deliver its rating

Still oil against a hot sheath forms a stagnant film that carries heat badly. Circulation is what breaks that film. A pumped loop is best; failing that, place the element low with a clear path above it so natural convection can run. What does not work is an element sitting in a corner behind a baffle where the oil never moves.

On mazut and heavy fuel oil there is a second reason: the fuel is thick or solid when cold. Bring it up slowly, and do not run the heater at full power against a solid charge — the oil next to the sheath melts, gets very hot and chars while the bulk of the tank is still cold.

The chemical, not the category, chooses the sheath

“Acid tank” is not enough to specify a heater. Sulphuric at one concentration and temperature behaves very differently from the same acid at another, and hydrochloric is a different problem again. Titanium covers a wide range of plating and pickling work. Strong acids that attack every metal need a PTFE-coated element, which brings its own limit — the coating sets the maximum temperature, well below the sheath’s own.

Send the solution name, the concentration, the working temperature and anything added to it. That is the information the choice actually rests on.

Protect for the case where the liquid is gone

Two independent things must be true for a fuel or oil heater to be safe: the control sensor works, and the element cannot be energised when it is not covered. The first is the thermostat. The second is a level switch wired to break the supply, not to sound a warning. Oil that has been overheated is a fire, not an inconvenience, and this is the one place in the catalogue where that has to be said plainly.

Coated elements need handling rules of their own

A PTFE-coated element is only as good as its coating. A chip from a spanner, a knock against a tank wall on the way in, or a scrape on the boss exposes bare metal, and the acid then attacks that one spot. Fit them carefully, never lever them, and check the coating whenever the tank is opened.