What to check on a Thermal Oil Heater (for tankers).
A thermal oil heater does not raise steam. It heats a mineral or synthetic heat transfer oil in a closed loop and pumps that hot oil through coils welded or clamped inside the cargo tanks, through cargo line trace heating, and back to the furnace. Because the oil stays liquid at temperatures where water would need very high pressure to remain as saturated steam, the whole system runs close to atmospheric pressure even at oil temperatures of 250-300°C. On a tanker carrying viscous cargo — heavy fuel oil, bitumen, molasses…
What sets a thermal oil heater apart
A thermal oil heater does not raise steam. It heats a mineral or synthetic heat transfer oil in a closed loop and pumps that hot oil through coils welded or clamped inside the cargo tanks, through cargo line trace heating, and back to the furnace. Because the oil stays liquid at temperatures where water would need very high pressure to remain as saturated steam, the whole system runs close to atmospheric pressure even at oil temperatures of 250-300°C. On a tanker carrying viscous cargo — heavy fuel oil, bitumen, molasses, tallow, some vegetable oils — that combination of high temperature and low pressure is the whole point: it keeps cargo pumpable for discharge without the pressure-vessel risk of a live steam boiler sitting a few frames away from cargo tanks. Unlike an exhaust gas economiser or an auxiliary boiler, a thermal oil heater is a single-fluid, single-purpose system built around one furnace and one oil inventory, not a steam header feeding multiple consumers.
Main components
Furnace and coil
A fired furnace, usually a single coiled tube run rather than a shell-and-tube arrangement, transfers combustion heat directly into the flowing oil. Firing is by diesel oil or heavy fuel oil burners with forced draught, sized to the maximum heat duty required for the fastest planned cargo heat-up.
Expansion tank and nitrogen blanket
An elevated expansion tank absorbs the oil's thermal expansion and keeps the loop full. It is nitrogen-blanketed to keep oxygen away from the hot oil, because oxidation is what turns thermal oil into sludge and coke deposits.
Circulation pumps
Duty and standby centrifugal pumps keep oil moving through the coil at all times the furnace is fired — stopping flow to a hot coil for even a short period causes local overheating and coking at the burner end.
Cargo heating coils and control
Coils in each tank, isolating valves so unused tanks can be shut out of the circuit, and a temperature control loop that throttles firing rate or bypasses oil around the furnace to hold a set cargo temperature without overheating the oil film at the coil wall.
Selection and sizing
The heater is sized on heat duty, not on tank volume alone. The relevant figures are:
- Required heat-up time and target cargo temperature, which set the peak duty in kW or kcal/h
- Maximum oil film temperature the chosen oil grade tolerates before thermal cracking begins, usually well below its stated maximum bulk temperature
- Oil flow rate needed to keep the film-to-bulk temperature difference small at full firing
- Turndown ratio, since most of a voyage needs only trim heating, not full cargo heat-up duty
- Compatibility of the coil material and cargo heating temperature with the cargoes the ship is built to carry
Regulations and class
SOLAS Chapter II-2 governs fire protection for the furnace space and its separation from cargo tanks, since this is an open-flame heat source operating close to flammable or combustible cargo. Classification societies treat the furnace, coil and expansion tank as pressure and fire-risk equipment subject to periodic survey, including internal coil inspection for coking and wall thinning. MARPOL Annex I applies if a coil leak lets cargo or oil residues reach machinery space bilges or the sea.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Coking inside the coil | Local overheating from low flow or oxidised oil | Reduced heat transfer, eventually a blocked coil section |
| Expansion tank oil loss | Vent or level instrumentation fault, undetected leak | Air ingress, accelerated oxidation, fire risk near the furnace top |
| Pump seal failure | Dry running or seal wear at high oil temperature | Loss of circulation and rapid coil overheating if not tripped in time |
| Cargo coil leak into tank | Corrosion or mechanical damage to coil | Cargo contamination and loss of heating oil inventory |
What to look for in a supplier
- Documented oil film temperature at full duty, not just bulk oil temperature, against the oil grade specified
- A chemical cleaning or flushing connection built into the coil circuit for removing coke deposits without a full strip-down
- Coil material and thickness matched to the cargo range the ship will actually trade in, with margin for corrosion
- Spare parts commonality for pumps and burners with equipment already on board
Track oil film temperature trend at the coil outlet over successive voyages: a slow rise at constant duty is the earliest sign of coking, long before flow or bulk temperature readings show anything wrong.