PTQ Q3 2026 Issue

Film temperature

% Vaporisation

360. 380. 400. 420. 440. 460. 480.

10 15 20 25 30 35 40

1 wt% steam 0.5 wt% steam 1.5 wt% steam Dry vacuum

1 wt% steam 0.5 wt% steam

1.5 wt% steam Dry vacuum

5 0 Tube number from shock towards radiant outlet 15 10

-5 0 5

Figure 2 Impact on film temperature

Tube number from shock to radiant outlet

was taken up for analysis. The horizontal coil vacuum heater had a relatively common coil size of 6 inches (Sch40 tubes expanding to 8 inches Sch40), which further expanded to 10 inches at the last tube. In very deep vacuum units, the coil size can expand to a larger tube, reaching up to 16 inches; however, this is not common. Two different aspects were checked. First, what is the optimum steam injection rate for this particular vacuum heater operating case and second, at what location is the steam injection most optimised. What happens when rate of steam injection is varied? To investigate the optimum rate of steam injection, three dif - ferent scenarios were analysed. The base case corresponds to steam injection at 1 wt% of the RCO mass flow rate. The second case was considered as a 0.5 wt% steam injection rate. The third case was considered as a 1.5 wt% steam injection rate. The definition of the cases covers the entire +/-50% steam injection rate over the base case of 1.0 wt%. As shown in Figure 1 , significant vaporisation is induced as soon as steam is injected into the vacuum heater RCO. Furnace crossover (convection to radiant) has been taken as the common steam injection location across all three sce- narios, varying the steam injection rate. It can also be seen that, for dry vacuum units, vaporisation starts deep in the radiant section, and approximately in only three to four tubes upstream of the radiant outlet. Thus, in a dry vacuum unit, the linear velocity of the fluid remains relatively low in most of the radiant section, which also has the highest heat flux in the heater. Owing to a very low degree of vaporisation resulting in low velocity, the residence time is very high. This effectively means that the high-temperature fluid prone to cracking spends a very long residence time in the high flux zone, which is not good in terms of coking. Now, compare this to 1.0 wt% steam injected into the RCO at the crossover location. It can be seen that vaporisa - tion is induced immediately, almost one or two tubes down - stream of the steam injection at the crossover location, thus increasing the mixed fluid velocity multifold. The resulting lower residence time reduces the probability of coking. Another important point that can be inferred from Figure 1 is the degree of vaporisation between the various steam rates. It can be noted that the induced vaporisation per - centage is appreciable when the rate of steam injection

• The higher the tube metal temperature or the fluid film temperature in contact with the tube metal, the higher the rate of coking. • Coking rates increase when the fluid residence time above cracking temperature increases. These three promoting factors of coking, therefore, indi - cate that a high fluid velocity can reduce the rate of coking. Whereas the first point is defined by the mass velocity of the fluid, the next two points are better defined by the linear or volumetric velocity of the fluid inside the coil. To elaborate, any Fluid A having the same mass velocity as Fluid B, but with a certain higher percentage of vaporisation, will have a higher linear velocity, a higher heat transfer coefficient, and, as a result, a lower residence time. Fluid A will also have a lower film temperature for the same heat flux and lower tube metal temperature, but will have a higher pressure drop. This introduces the concept of inducing vaporisation by lowering the partial pressure of the hydrocarbon fluid. The injection of velocity steam plays a critical role here. Steam injection in the vacuum furnace coil impacts heater perfor - mance in two ways: • The steam reduces the partial pressure of the RCO, which promotes higher vaporisation at sub-atmospheric pressures. The outlet of the vacuum furnace is generally at a pressure of 250-350 mmHgA. Thus, reduced partial pressure at a particular coil outlet temperature (COT) will promote higher vapour production. The same amount of vapour at the same pressure, without any steam injection, will call for a much higher COT. Due to the inherent nature of fluid cracking/cok - ing tendency, this higher COT might not be prudent, as it can induce accelerated coking rates. • Due to higher vaporisation achieved by steam injection, the linear velocity of the fluid is accelerated. This helps reduce coking by increasing the shear effect and reducing the residence time inside the coil. This answers the important ‘why’ of steam injection in VDU furnaces. Scenarios To illustrate the impact of steam injection, a conventional vacuum heater operating in a 180,000 BPSD CDU/VDU Figure 1 RCO vaporised versus steam injected (percent - ages are approximate figures only)

PTQ Q3 2026