March 2021

Heat Transfer

The use of tube inserts in fired heaters

In 1896, Whitham1 reported the successful use of twisted tapes (originally called retarders, and now also called turbulators) to increase heat transfer in boiler fire tubes—their effectiveness in increasing heat transfer is well known.

Martin, M., XRG Technologies

In 1896, Whitham1 reported the successful use of twisted tapes (originally called retarders, and now also called turbulators) to increase heat transfer in boiler fire tubes—their effectiveness in increasing heat transfer is well known. Engineers usually use twisted tapes to improve heat transfer in laminar flows, but academics and industry have extensively studied their use in turbulent flows.2

Refining and petrochemical plants commonly use fired heaters where the process requires high-intensity heat. State-of-the-art fired heaters are some of the most fuel-efficient devices in use, with efficiencies over 92%. It is not readily apparent how the use of turbulators might benefit such highly efficient systems. However, using the knowledge that the process flow in many fired heaters at least partially vaporizes, one can show that properly applied tube inserts that outwardly resemble turbulators can improve fired heater performance.

Fired heaters, heat transfer and heat flux uniformity

FIG. 1 shows a typical fired heater. Burners fire into a “radiant section,” transferring heat from the flue gas to the process fluid, which flows through pipes commonly called “tubes.” The process fluid typically receives 60%–70% of the heat within the radiant section. The flue gas then flows into the “convection section,” which transfers 15%–20% of the remaining heat to the process fluid. The principal mode of heat transfer in the radiant section is thermal irradiation, while the principal mode of heat transfer in the convection section is convective.

FIG. 1. A typical fired heater. Burners fire into a radiant section, generating hot flue gas. Radiation transfers 60%–70% of the heat in this section, after which the flue gas flows through the convection section where the process absorbs 15%–20% of the remaining heat.

The process flow in practical fired heaters is turbulent, with a Reynolds number on the order of 106. Most of the heat transferred to the process occurs within the radiant section. The convection section compensates for small reductions in radiant section heat transfer due to fouling and non-ideal flames; a higher radiant section exit temperature results in more heat transfer within the convection section. Fired heater designers have had little incentive to increase heat transfer intensity using turbulators. The process flow is turbulent, resulting in a high tube-side convection heat transfer coefficient, and the convection section design already results in the desired flue gas exit temperature. However, the benefit of in-tube mixing enhancement goes far beyond the benefits of increased heat transfer.

Reformers and pyrolysis heaters fall among a class of heaters where intentional and valuable reactions take place inside the heater tubes. In some reactor charge heater tubes, undesirable chemical reactions cause in-heater feed conversion, which reduces the ultimate yield. Other heaters produce unintended reactions that reduce the value of the product. Heaters used in certain services, such as crude distillation, vacuum distillation and delayed coking, have both unintended reactions and phase change within the heater tubes. Similar devices to heaters, such as once-through steam generators (OTSGs), do not have chemical reactions within the tubes but do exhibit phase change. In all cases, not only is the total absorbed heat important, but also the location of the absorbed heat.

To see why the variation in temperature along the outside coil surface is critical in heaters with multiphase process flow, consider the idealized graph of heat transfer coefficient vs. temperature difference between wall and fluid, illustrated in FIG. 2. Beneath the chart is a representative picture of the liquid/vapor composition within the process coil corresponding to the heat transfer coefficient. The highest temperature at any point in the flow occurs at the boundary between the process coil wall and the process flow. When the combustion heats the process flow, vapor first forms at the interior wall of the pipe. Gases have a significantly lower overall convection heat transfer coefficient when compared to liquids, so the process flow transfers less heat away from the pipe wall. In this way, adverse feedback ensues wherein the high wall temperature begets more boiling that, in turn, reduces the inside convection heat transfer coefficient, thereby increasing the wall temperature and the resulting boiling. In this way, “hotspots” can form on the heater tubes given an initial slight difference in heat transfer.

FIG. 2. An example heat transfer coefficient vs. temperature differential for a multiphase horizontal pipe and flow regime.

Gravity further exacerbates phase non-uniformity inside horizontal sections of process coils. Gravity pulls liquid—being denser than vapor—to the bottom of the pipe, resulting in a strong tendency toward stratified flow with resulting higher temperature on the upper pipe surface.

The major sources of temperature non-uniformity result from an unavoidable combination of geometry and physics. The burner flames and hot flue gas transfer more radiant heat to the flame-facing surface of the coil. Heaters with burners mounted on one or both sides of the coil exhibit this non-uniformity. The measure of this non-uniformity is known as the circumferential flux factor and has been well-characterized. As combustion releases heat from the burner flame, an additional longitudinal flux factor exists.

Engineers have been unable to characterize the longitudinal flux factor in a general sense because it is heater and burner dependent. FIG. 3 shows an example of single-fired tubes with a high-longitudinal variation in flux and the resulting peak-to-average flux ratio of 2.3. This flux ratio translates to a 16.1°C (61°F) higher film temperature and a 37.8°C (100°F) higher tube metal temperature in example calculations.

FIG. 3. Flux factors used in heater tube temperature calculations.

Variation in heat transfer to the process fluid comes from both within and without the process coil. So, what is to be done? Variation in the physical properties of the fluid that are driven by vaporization inside the coil plays a strong role in the local tube metal temperature. It is also apparent that physics and geometry of the flue gas side drive non-uniformity from the outside the coil. A common method of increasing homogeneity in a process is to increase the mixing of the constituents. One can show through simulation that properly designed tube inserts can improve process homogeneity. This improvement can, in turn, be used to reduce tube metal temperatures, increase yield and increase heater capacity.

Simulation results: Improved area goodness factor for tube inserts

Increasing the mixing inside a process coil comes at a cost: the pressure drop through the system also increases. The process fluid diffusion inside the heater tube can be estimated by using heat transfer as a surrogate measure. To measure the relative effectiveness of various inserts, step-change designs were simulated in combination with automated optimization for thousands of combinations of geometric parameters. The relative merit of each design was judged by comparing the area goodness factor, or the Colburn factor (j) divided by the Fanning friction factor (f).

The final optimized design has a 30% increase in area goodness factor vs. traditional twisted tapes. The practical implication is that by using an optimized design, more process mixing with lower pressure drop can be achieved. A key parameter in twisted tape design is the twist-pitch, or the number of pipe diameters required for the twisted tape to make a helical revolution within a length of the tape. FIG. 4 shows a comparison in the simulated area goodness factor for both traditional twisted tapes and the optimized design vs. increasing twist pitch.

FIG. 4. A comparison of optimized tube insert design to a traditional twisted tape shows an increase in goodness factor of 30%.

Simulation results: Multiphase flow through a horizontal return

Simulations comparing the optimized design to an empty tube demonstrate that the in-coil mixing translates to increased process homogeneity for multiphase flows. FIG. 5 shows the predicted convection heat transfer coefficient over the entire tube surface and liquid volume fraction at the inlet and outlet of the tube for a simulation with 80% liquid and 20% vapor by volume. There are inserts both before and after the return, but in this case, the flow requires one straight section of tube to establish the mixing motion provided by the insert. With the tube insert in place, the convection heat transfer coefficient is more uniform when compared to the empty tube.

FIG. 5. Comparison of convection heat transfer coefficient and liquid volume fraction for a tube without (left) and with (right) tube inserts.

Near the inlet, at Point A, gravity stratifies both flows. At Point B, the return has temporarily changed the stratified flow to annular flow, producing a more uniform heat transfer coefficient at the tube surface for both cases. At Point C, the stratified flow returns in the empty tube, but the liquid remains adhered to the tube wall when using the insert.

On the outlet leg of the coil section, the tube insert increases the area-weighted average heat transfer coefficient by 20% when one compares the empty tube. More importantly, the minimum heat transfer coefficient over the same section of the coil using inserts is 50 times higher than the minimum heat transfer coefficient of the same section of the empty tube. Tube failures occur at specific, often initially small, locations. The use of the insert eliminates the point of minimum heat transfer coefficient where this failure would likely occur. By limiting the use of inserts to the tubes that are prone to failure or that need the most process improvement, their benefit can be maximized while reducing the additional pressure drop.

Estimates of economic impact

The variability in causes of shutdowns will impact any economic assessment for the use of in-tube inserts. If a plant shuts down often due to tube failures, the value for preventing those failures is far greater than incremental increases in run length. However, the value of using the inserts for normal operation without special cause failures can be estimated.

To evaluate the effect on run length, a model was created for a coker unit furnace. The computational fluid dynamics (CFD) model of the coil included sub-models for phase change of the vacuum residual oil, as well as the propensity for coking based on the work of Ebert and Panchal.3 The predicted coking rate and daily temperature rise for coils with the insert in place is three times less than of that of an empty tube. The predicted run length of the furnace is increased by 35%. Using validated assumptions for the coker spread, shutdown days and current energy costs, the 2-yr net value addition of inserts is $1.96 MM for a typical 30,000-bpd coker unit.

Practical considerations and takeaway

The use of tube inserts does raise the practical concern of how to remove the inserts when the coil requires pigging. A mechanical solution has been developed by the author’s company that uses flanged connections outside the heat-affected zone. This solution removes the potential for leakage of the process fluid, which is a problem for traditional plug headers. Physical testing was performed to ensure that the insert can be removed from a tube even if coke or scale has completely seized it in place.

Tube inserts are a well-proven method to increase heat transfer. They also increase process homogeneity. The proper application of in-tube mixing should increase coil longevity, run length and unit profitability. These improvements should be more pronounced for processes with in-tube phase change. Properly optimized mixing elements provide both relatively low pressure drop and the increased mixing required to bring heaters to a new level of performance. HP

ACKNOWLEDGEMENT

The research results discussed in this publication were made possible in total or in part by funding through the award for project number AR18-015, from the Oklahoma Center for the Advancement of Science and Technology.

LITERATURE CITED

  1. Whitham, J. M., The effect of retarders in fire tubes of steam boilers, American Society of Mechanical Engineers (ASME XVII), Philadelphia, Pennsylvania, 1896.
  2. Manglik, R. M. and A. E. Bergles, “Heat transfer and pressure drop correlations for twisted-tape inserts in isothermal tubes: Part II, transition and turbulent flows,” Journal of Heat Transfer, American Society of Mechanical Engineers, 1992.
  3. Ebert, W. and C. B. Panchal, Analysis of Exxon crude oil slip stream coking data in fouling mitigation of industrial heat exchange equipment, Begell House, New York, 1997.

The Author

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