Thermal Fluid & Hot Oil
U-Tube Exchangers

Thermal Fluid & Hot Oil U-Tube Exchangers

High Temperature at Low System Pressure

Thermal fluids carry heat at high temperature and low pressure. That combination is why they exist: reaching the same process temperature with steam would require a pressure system heavy enough to change the economics of the whole plant.

This is the duty the U-tube was made for. Large differential temperature between the hot oil and the process, abrupt startup transients, and constant cycling would all require an expansion joint in a fixed tubesheet exchanger. In a U-tube bundle they require nothing at all, because each tube is anchored at one end and free at the bend. Tubes can even sit at different temperatures from one another without consequence.

Film temperature is the governing limit. Degradation happens at the tube wall, where the fluid is hottest and moving slowest, not in the bulk. A system can run entirely within its bulk temperature rating and still crack the oil if tube-side velocity is too low or heat flux too high. What follows is coke: a hard carbonaceous deposit bonded to the tube wall that insulates, raises wall temperature further, and accelerates the process that created it.

Coke cannot be cleaned off a U-bundle. Chemical cleaning barely touches it, and mechanical cleaning cannot pass a 180 degree bend. This makes film temperature control a design requirement rather than an operating preference, and it is why we ask for the fluid supplier's data sheet rather than just the operating temperature.

Oxidation and the expansion tank. Thermal oil oxidises rapidly wherever hot oil meets air, and the expansion tank is precisely where that contact happens. A nitrogen blanket is standard practice and inexpensive insurance. Oxidised fluid turns acidic and forms sludge, which fouls surfaces and attacks the system, and routine fluid analysis catches it long before the exchanger does.

Support design at temperature. A hot shell grows measurably. One saddle fixed and one sliding, with slots sized for the calculated growth, keeps that movement out of the nozzles and the connected piping. We state the expected growth and the required support arrangement on the drawing rather than leaving it to be inferred on site.

Related pages: high-temperature U-tube exchangers, tank and suction heating, horizontal mounting and supports and carbon steel construction.