Applications for extruded low-finned (LF) and externally and internally enhanced tubes are widespread in multiple industries, ranging from the air-conditioning and refrigeration, heating, automotive and power industries, as well as the hydrocarbon processing industry (HPI). A few selected examples are shown here:
Enhanced boiling and condensation tubes for packaged chillers
Inner grooved tubes for coil heat exchangers in the air-conditioning and refrigeration industry
Enhanced tubes for the hydrocarbon processing industry
Enhanced tubes for power steering oil cooling in the automotive industry.
For the HPI, distinct enhanced tubes for boiling and condensing, as well as single-phase heat transfer services have been derived from standard LF tubes (Fig. 1).1 The enhanced tubes are typically manufactured from plain tube-based material by an extrusion cold-rolling process. The fins on the outside, and on the inside are integrally connected to the tube wall.
| Fig. 1. Enhanced internal and external LF tubes. |
A wide range of proven references for both LF and enhanced tubes exists in the HPI from decades of refining, petrochemical, chemical and gas processing applications.24 Standard tube materials are copper-nickel carbon and low-temperature carbon steels. Now solutions are also available in low-alloy carbon steel, as well as stainless steel (SS) and titanium (Ti). These technologies resolve both capacity and plot-space limitations for existing plants and provide compact or most efficient solutions for new plant design. Very attractive applications are identified in liquefied natural gas (LNG) and ethylene plant due to the drastic recent increased plant capacity (Figs. 2 and 3).
| Fig. 2. Evolution of LNG plant capacity |
| Fig. 3. Evolution of ethylene plants capacity |
ENHANCED HEAT TRANSFER TECHNOLOGIES
The thermal advantage of the externally and internally enhanced tubes vs. a plain tube, in a shell-side propane boiling application is between a factor 2 and 3 of the boiling heat transfer coefficient. Together with an improved tube-side performance ranging between a factor 1.6 to 2.4, depending on single- and two-phase flow conditions, the overall heat transfer benefit leads to substantial overall benefits. At the same time, the tube-side pressure drop increases. However, it typically does not exceed the heat transfer improvement, allowing for enhanced shell-and-tube heat exchangers within allowable pressure drop limits.
The key benefit of the externally and internally enhanced tube is the capability of a superior operation for an external boiling application at low-temperature approachesdown to 2°C and below where standard plain or LF tubes are no longer efficient. In various schemes and applications, the benefits can be:
Reduced number of heat exchangers per unit
Capacity increase or energy consumption reduction resulting from improved efficiency.
A new enhanced condensing tube is available for industrial reboiler applications such as thermosiphon and kettle heat exchangers. Special attention is given both to the thermal and mechanical design, along with an improved kettle design, such as:7
Proper fluid distribution at inlet and outlet
Verification of liquid entrainment especially for suction line to compressor
State of the art thermal design tools from Heat Transfer Research Inc. (HTRI), as well as other advanced heat transfer are used as convenient thermal design tools for enhanced heat transfer solutions.8
In a similar way to the enhancement of reboilers, enhanced heat transfer solutions have been developed for horizontal condensers, with shell-side condensation of pure streams and tube-side cooling water. A typical solution is with an enhanced tube having an external LF structure in combination with an internal helical fin structure.
Both enhanced heat transfer technologies have been made available and qualified through local testing for horizontal shell-and-tube type reboilers and condensers. During two joint industry and academia research projects, JOULE III and AHEAD, funded by the EU, fundamental research and qualification have been conducted.5,6 Key activities have been the development and characterization of enhanced shell-side nucleate boiling structures, especially at low-temperature approaches, as well as tube-side enhancement structures for both single-phase gas and liquid and two-phase condensate heat transfer. The different steps from lab-scale testing to industrial application stretched over a period of almost one decade. The pre-requirement for the time being in these applications is for clean refrigerant and process fluids.
For base-load LNG plants, the enhanced heat exchanger technologies are highly attractive within the propane pre-cooling cycle. The major application for enhanced boiling tubes is for the main propane refrigerant chilling train with cooling/condensation of natural gas (NG) or mixed refrigerant (MR) on the tube side and propane refrigerant boiling on the shell side. The primary application of the enhanced condensation tube is for the propane refrigerant condenser with shell-side propane refrigerant condensing and tube-side cooling water.
For ethylene plants, enhanced heat exchanger solutions are available for the majority of reboiler and condenser heat exchangers in the cold section such as the C2 and C3 fractionation and splitting services as well as the refrigerant units (as summarized in Table 1).
Several studies have been done based on recent projects (FEED and EPC) allowing the technical and economic qualification of enhanced heat exchangers both in ethylene and LNG plants.
In ethylene plants, there are many heat exchangers, with very large heat transfer surface area representing around 20% of the total equipment cost of a plant. In parallel to the technical qualification of enhanced tubes in ethylene plants, a study was conducted to evaluate the economic interest of such solutions compared to the plain and LF tube solutions considering identical process conditions. This study concerns the C2H4 back-end (BE) hydroprocessing scheme of a typical ethylene plant, as shown in Figs. 4 and 5. The economic interest of enhanced boiling and condensation tubes is demonstrated using the key exchangers listed in Table 1.
| Fig. 4. Ethylene back-end hydrogenation process scheme |
based on ethane feedstock.
| Fig. 5. C3/MR liquefaction section. |
All design being conducted with maximal usage of the allowable tube-side pressure drop focus on two main goals:
Heat transfer surface area reduction
Shell number reduction.
For shell-side boiling services, Figs. 6 and 7 show the relative comparison of plain, LF and enhanced boiling tube design with the plain tube as reference. In conclusion, the average heat transfer surface area reduction is about 60%, and the average cost reduction is about 20% per equipment.
| Fig. 6. Relative heat transfer area for items |
equipped with enhanced boiling tubes.
| Fig. 7. Relative cost for items equipped with |
enhanced boiling tubes.
For shell-side condensing services, Figs. 8 and 9 show the relative comparison of plain, LF and enhanced condensing tube design with the plain tube as reference. The average heat transfer surface area reduction is about 75%, and the equipment cost average reduction is about 65% per equipment.
| Fig. 8. Relative heat transfer area for items |
equipped with enhanced condensing tubes.
| Fig. 9. Relative cost for items equipped with |
enhanced condensing tubes.
Additional savings come from:
Process optimization, considering the low temperature approach capabilities of externally and internally enhanced tubes and
Piping and structure reduction due to plot plan reduction, which are not included in this study.
The performance of enhanced boiling and condensing tubes is demonstrated in two representative cases both for a propane-refrigerant chiller and condenser in comparison to standard plain and LF tubes, as shown in Figs. 10 and 11. The cases are taken from a recent LNG project. In both cases, substantial size and weight reduction can be achieved by using externally and internally enhanced tubes. Especially for the large equipment units, the benefit becomes evident when considering the whole supply chain ranging from fabrication and transportation, as well as plant aspects covering installation, operation and maintenance.
| Fig. 10. LP/MR propane refrigerant chiller. |
Comparison of plain, LF and enhanced
| Fig. 11. Propane refrigerant condenser. |
Comparison of plain, LF and enhanced
A detailed techno-economic study of the two chilling trains for NG and MR showed very attractive savings in capital expense (CAPEX) and plot space, as well as capabilities for efficiency improvements or, vice versa, an attractive opportunity for capacity increase, as summarized in Table 2.
Both solutions with LF and internally and externally enhanced boiling tubes have been analyzed for the two chilling trains: propane/MR chilling train and propane/NG chilling train. Each train is operating at four propane levels. For the externally and internally enhanced boiling tube, a reduction of the cold approach to 2K is feasible and considered an improved LNG plant design. Other items have been considered for the CAPEX and include heat exchanger, piping, steel structure, piping and exchanger foundation.
For the standard cold approach of 3K, the externally and internally enhanced tube allows for a CAPEX reduction of 20% and 25% reduced plot space vs. a standard solution using LF tubes.
Considering a reduced cold approach of 2K, the compression power is reduced by approximately 2.2% translating into approximately 1% additional LNG capacity. The additional annual income, depending on the LNG price, is far superior compared to the total cost of the chilling train. Note that the case with the enhanced tube and 2K cold approach is with 13% plot space reduction, is still more compact with the same CAPEX and is not more expensive compared to the LF case with a 3K cold approach.
The first reference of the enhanced boiling tube dates from 2000 for a horizontal thermosiphon, C3 splitter reboiler as part of the capacity expansion of the Lyondell-Basell polypropylene plant in Knapsack, Germany (Fig. 12).8 The use of an enhanced boiling tube allowed an upgrade from 4 MW to 5 MW despite a substantial reduction of the LMTD. The cooling water return from the tubular polymerization reactor was able to be used for heating, thus avoiding the use of stream.
| Fig. 12. Installation of horizontal thermosiphon reboiler |
equipped with enhanced boiling tubes in a C3 splitter of
a Lyondell-Basell polypropylene plant in Knapsack, Germany.
In 2003, the enhanced boiling tube was applied for the first time in an LNG plant as part of the Qatargas debottlenecking project. The objective was to expand the capacity of the existing three trains from 2 million tpy (MMtpy) to 3 MMtpy per train. A new kettle-type chiller equipped with enhanced boiling tubes, with tube OD of 5⁄8 in., was successfully installed in each of the three trains.
Test runs following the startup of Train 2 in 2003, Train 3 in 2004 and Train 1 in 2005 confirmed the thermal and hydraulic tube performance. For Train 3, the performance was verified again in 2007, confirming stable performance. In addition, a very low cold approach temperature of 1.4 K between tube-side condensing MR and shell-side boiling propane is confirmed demonstrating the superior performance of the enhanced boiling tube. Qatargas is very satisfied with the overall performance of these chillers. In a joint venture, there are six trains at Ras Laffan, Qatar, with an annual LNG capacity of 7.8 MMtpy per train. All trains are in operation at full capacity.
Following the first successful application in the polypropylene plant in 2000, further applications followed with various expansion projects and new grassroots projects. Borealis Polymers in Finland, used enhanced boiling tubes in a C2 splitter reboiler/condenser in a heat pump scheme for an ethylene expansion project in 2002. The stable operation has been reviewed and confirmed in 2007, as shown in Fig. 13. Further applications with the enhanced boiling tube followed within the depropanizer and deethanizer condensers, both for the 10th olefin complex for JAM Petrochemical in Iran, and in Yansab, Saudi Arabia. HP
| Fig. 13. Kettle-type reboiler/condenser with enhanced |
boiling tubes in heat pump driven C2 splitter. Borealis
Polymers ethylene plant in Porvoo, Finland.
1 Technical Information on GEWA-PB and GEWA-KS Tubes, Wieland-Werke AG, Germany.
2 Webb, L. R., Principles of Enhanced Heat Transfer, John Wiley & Sons, 1994.
3 Thome, J., Heat Transfer Augmentation of Shell-and-Tube Heat Exchangers for the Chemical Processing Industry, 2nd European Thermal-Sciences and 14th UIT National Heat Transfer Conference, Rome, May 2931, 1994
4 Curcio, A., I. Louis, M. Fischer and T. M. Rudy, Field Applications of Double Enhanced Tubes in Shell-and-Tube and Air-Cooled Heat Exchangers, ASME 30th National Heat Transfer Conference, Portland, Oregon, Aug. 7, 1995
5 Thonon, B., Advanced and High-Performance Heat Exchangers for the Hydrocarbon Processing Industry, Heat Transfer Engineering, Vol. 28, No. 5, pp. 7384, 2005.
6 Thonon, B. and J. J. Delorme, Enhanced Reboilers for the Process Industry, Engineering Foundation Conference, Davos, July 2001.
7 Rabeau, P., H. Paradowski and J. Launois, How to reduce CO2 Emissions in the LNG Chain, LNG15 Conference, Barcelona, 2007.
8 Heat Transfer Research Inc., https://www.htri.net.
|The authors |
Brigitte Ploix is the manager of Heat Transfer Group, Process and Technology Division, Technip France, Paris, France. She has over 17 years of experience in the thermal design of all nonfired types of exchangers for oil refining and offshore oil production, as well as for the petrochemical, LNG and gas processing industries. Previously, M. Ploix worked as the lead discipline engineer for major international projects and joint ventures. She is a member of the TECHNIP WIELAND Steering Committee, French Association of Oil Industry Engineers and Technicians. Ms. Ploix has been a member of the HTRI Technical Committee since 2008; served as vice chair since 2011; served on the Communication CommitteeFrance since 2003 and chair from 20052006. She is a member of the HTRI Plate-Fin Exchanger Task Force. Ms. Ploix is a graduate engineer from the Institut National des Sciences Appliquées de Lyon (INSA,), Lyon, France.
Thomas Lang is the manager of business development for the Process Industry, Product Division High Performance Tubes of Wieland-Werke AG, Ulm, Germany. He has worked Wieland-Werke AG for 19 years. Mr. Lang is responsible for technical marketing and business development for enhanced heat transfer tubes and heat transfer engineering services for the process industry. His experience includes a wide range of enhanced heat transfer application primarily for shell and tube heat exchangers for the oil and gas industry, refining, petrochemical and chemical as well as power industry. Mr. Lang is a member of the HTRI technical committee since 2008 and a member of the HTRI communication committee Germany since 2002. He holds a diploma in mechanical engineering from the University of Stuttgart and an MSc degree from the University of Boulder.