Optimising shell and tube heat exchanger operation
Case studies show how inserts improve heat transfer coefficients, mitigate fouling and reduce end-of-run pressure drop, as demonstrated with the preheat train of a CDU
Nicolas Aubin Petroval
S everal studies published by Total Energies and Petroval have examined the improvements that can be obtained from tube inserts in heat exchangers. The benefits of using a combination of tube insert tech - nologies are manifested in extended run lengths between cleaning shutdowns, an increased heat transfer coefficient, a reduced fouling rate, and stability of pressure drop. From an economic viewpoint, the payback is achieved within a few months from four sources of improvements: the preheat train energy saved (by the increase in the heat transfer), the reduction in maintenance cost (reduced cleaning frequency), the increased throughput, and positive environmental impact stemming from the reduction in CO₂ emissions (as a consequence of the better heat transfer performance). Indeed, a very substantial benefit can be obtained if a unit is bottlenecked by a heat transfer limita- tion or the furnace. Technology limits Limiting the carbon footprint is now essential to achieve net- zero emissions in the oil industry by 2050. This ambitious target will require large investments in new technologies for heat transfer efficiency and carbon capture. However, the technologies that will be required in the future are not yet available at an industrial scale; time is needed for their maturation, investment, and timely operation. The oil industry relies mainly on preheat shell and tube heat exchangers to reduce the amount of firing required in fired furnaces. However, the performance of these exchangers is
oil preheat trains caused by asphaltene deposition and/or coke formation on hot surfaces. In these tests, heat exchangers forming part of preheat trains in three refineries were equipped with new inserts. Their performances were monitored over two to four years, depending on the circumstances, and compared to the durations of previous runs in similar process conditions. The improvements in heat transfer and the impact on CO 2 emissions will be further highlighted. Fouling reduction case studies Case A – Rotational effect The Turbotal rotating device is hooked onto a stationary head and installed at the inlet end of the heat exchanger tube (see Figure 1 ). This system is a continuous online cleaning device, the purpose of which is to reduce the foul- ing layer at the tube walls by means of a mechanical effect. The device uses the energy of the flowing medium in the tubes to achieve rotation at around 1,000 rpm during the whole run duration. This rotation speed is determined at the design stage by the mechanical design of the Turbotal and issued from correlations determined on experimental skids. The extra pressure drop generated is typically in the range of 100 millibar per pass at a flow velocity of 1.0 m/s, with a lifetime limited to three years due to mechanical erosion of the parts. The last two pairs of heat exchang- ers just before the furnace were suffering from severe fouling over a period of less than one year. All four heat
often limited by fouling and mechanical designs not upgraded to the required level of operation. There are tube insert technologies available on the market that offer a quick solution to enhance the performance of shell and tube exchangers. These provide immediate improvements in heat transfer from start of run (SOR) with no mod- ifications to the exchangers or the operating conditions. The benefits of using tube insert technologies were previously demonstrated in terms of an increased heat transfer coefficient1 , 3, reduced fouling rate,2 and stability of pressure drop. This current study will only consider fouling in crude
Heat exchangers used in Case A – design and operating conditions
Position in the train Number of bundles No. of tubes per bundle
Just before the furnace 2 branches of 2 bundles
626
Tube length
6,100mm
OD/BWG
1”/12
Product tube/shell side Flow rate (tube side) Flow velocity (tube side)
Crude/atmos residue
260/330/430 t/h 1.0 to 1.70 m/s
Tube inserts
Turbotal
Replacement frequency
Every 2 to 3 years
Figure 1 Turbotal on a tube bundle
Table 1
41
PTQ Q1 2025
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