Heat exchangers used in Study B – design and operating conditions
Position in the train Number of bundles No. of tubes per bundle
Just before the furnace 2 bundles in parallel
600
Tube length
6,100mm
OD/BWG
1”/12
Product tube / shell side Flow rate (tube side) Flow velocity (tube side)
Crude/bottom P/A
431 t/h design
1.87 m/s
Tube inserts
Spirelf
Replacement frequency
Every 3 years
Table 2
Figure 2 Spirelf on a tube bundle Figure 3 Fixotal devices in the heat exchanger
exchangers were equipped with Turbotal and operated in the same range of process conditions as previously (see Table 1 ). The
monitoring of the performance was then compared to the previous data; the comparative trend of the outlet temper- ature will be presented in the results section. Case B – Vibrational effect The Spirelf vibrating device is fixed on both tube ends by a fixing wire (see Figure 2 ). This system also serves as a con- tinuous online cleaning device, reducing the fouling layer on the tube walls by means of a mechanical effect. The vibrating device uses the energy of the flowing medium in the tubes to convert it into vibrations of the device, both radial and longitudinal. The extra pressure drop generated by the device is typically in the range of 200 millibars per pass for a flow velocity of 1.0 m/s. The lifetime of the device is limited to six years since it must be removed and replaced at each turnaround for internal cleaning and inspection of the heat exchanger tubes. The last pair of heat exchangers, just before the furnace, suffered from severe fouling over a period of less than one year. The two heat exchangers were equipped with Spirelf and operated in the same range of process conditions as previously (see Table 2 ). The monitoring of the performance was then compared to the previous data. The comparative trends of the duty achieved and the flow rates will be pre - sented in the results section. Case C – Heat transfer effect To promote turbulence at the inside tube surface, the Fixotal
system significantly increases shear stress at the wall, pre - venting product stagnation in the boundary layer adjacent to the tube. The purpose of this fixed device is mainly to increase the rate of heat transfer by renewing the boundary layer at the tube wall, with an appreciable side effect on fouling mitigation, including certain types of fouling linked to wall temperature, such as polymerisation, paraffin solid - ification, scaling, and crystallisation. The extra pressure drop generated by the device is typically in the range of 200 millibars per pass for a flow velocity of 1.0 m/s. An example of Fixotal installed in a tube bundle is presented in Figure 3 to illustrate the device once installed. The chosen case study will review the performance of a complete preheat train of 12 heat exchangers that are all operated with the same fluids. Crude is flowing on the shell side from the desalter to the furnace, and atmospheric res- idue is flowing counter-current on the tube side from the tower towards the beginning of the hot train. Only the last three exchangers out of the 12 were equipped with Fixotal technology and operated in the same range of process conditions as previously (see Table 3 ). The monitoring of the performance was then compared with the previous data; the comparative trends of the overall heat transfer coefficient (OHTC) and duty will be presented in the results section. Due to a lack of instrumentation, only three temperature measurements points were available on each flow pass: at the inlet, in the middle (after six bundles), and at the outlet. Consequently, the improvements achieved in the last three heat exchangers were mitigated with the normal perfor- mance of the other three that were not equipped between the two temperature indicators. Case A results The trend presented in Figure 4 shows the OHTC of the four heat exchangers in operation on com- parative runs. The reference run in blue lasted only 183 days, with a significant loss of performance as the OHTC dropped from 230 kcal/h.m² °C at SOR to 87 kcal/h.m² °C within this six-month period. After this
Heat exchanger used in Study C – design and operating conditions
Position in the train No. of tubes per bundle
Just before the furnace
732
Tube length
5,000mm
OD/BWG
1”/12
Product on tube / shell side Flow rate (tube side) Flow velocity (tube side)
Reduced crude / crude
134.2 t/h 0.80 m/s
Tube inserts
Fixotal
Replacement frequency
Every 4 years
Table 3
42
PTQ Q1 2025
www.digitalrefining.com
Powered by FlippingBook