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TWISTED TUBE HEAT EXCHANGER TECHNOLOGY

R. Donald Morgan Brown Fintube Company12602 FM 529 RoadHouston, Texas 77240-0082 USAPhone: 1 (713) 849-8197Fax: 1 (713) 466-3701e-mail: morganOd@kochind.com

ABSTRACT

Over 85% of all new heat exchanger applications in oil refining, chemical, petro-chemical, and powergeneration are accommodated through the use of conventional shell and tube type heat exchangers.The fundamental basis for this statistic is shell and tube technology is a cost effective, proven solutionfor a wide variety of heat transfer requirements. However, there are limitations associated with thetechnology which include inefficient usage of shell side pressure drop, dead or low flow zones aroundthe baffles where fouling and corrosion can occur, and flow induced tube vibration, which canultimately result in equipment failure. This paper presents a recent innovation and development of anew technology, known as Twisted Tube technology, which has been able to overcome the limitationsof the conventional technology, and in addition, provide superior overall heat transfer coefficientsthrough tube side enhancement. This paper compares the construction, performance, and economics ofTwisted Tube exchangers against conventional designs for various materials of construction includingreactive metals.

KEYWORDS

heat exchanger, twisted tube technology, heat transfer, corrosion resistance

CONVENTIONAL SHELL AND TUBE DESIGN

Conventional TEMA (Tubular Exchanger Manufacturers Association) type shell and tube type heatexchangers consist of a number of round tubes attached to a tubesheet inside a cylindrical vessel, withtube sizes, tube lengths, and shell diameters varying depending on the requirements of the application.Heat transfer surface areas can vary from a few square feet to over 25,000 square feet. The tube bundlenormally contains a number of baffles to accomplish the dual objectives of providing a supportstructure for the tubes, and to direct the shell-side flow across the tubes rather than along the tubes(Fig 1). The resulting back and forth shell-side flow will yield a higher than expected pressure dropper unit of heat transfer because energy is used to reverse the flow rather than to enhance heat transfer.Also, the energy consumed in reversing the flow will tend to force the shell-side fluid through baffle-to-tube and baffle-to-shell clearances yielding lower cross flow and lower heat transfer coefficients.Finally, fluid flow around the baffles is non-uniform resulting in areas of low flow and dead spots,which are prone to fouling accumulation, corrosion, and poor heat transfer.

The thermal effectiveness (�), of a shell and tube exchanger is normally calculated assuming perfectradial and no axial mixing of the shell side stream. In practice however, there is considerable axialmixing within a baffle compartment, and further, the stream is in cross-flow for part of the time ratherthan axial flow. These effects are further complicated by leakage of flow that occurs at the baffle-to-tube

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and baffle-to-shell joints that does not take full part in the heat transfer in the bundle. The overalleffect of these limitations is the actual thermal effectiveness (�) will be lower than the theoreticalvalue, and it will be lower than the values obtained for other types of heat exchangers that do notsuffer from these limitations. Typically, thermal effectiveness of a conventional shell and tube typeexchanger will be in the range of 60% to 80%

The Twisted Tube Heat Exchanger

The Twisted Tube heat exchanger originated in Eastern Europe and became commercially available inScandinavia in the mid 1980’s. It was developed primarily to

Figure 1. Conventional Heat Exchanger

overcome the limitations inherent with conventional shell and tube technology. Applications ofTwisted Tube technology were primarily in single phase and condensing duties in pulp and paper anddistrict heating with limited exposure in the process industries. In 1991, Koch licensed the technologyand in 1995 subsequently acquired the technology outright.

Construction

The Twisted Tube exchanger consists of a bundle of uniquely formed tubes assembled in a bundlewithout the use of baffles (Fig 2). The tubes have been subjected to a unique forming process whichresults in an oval cross section with a superimposed helix

Figure 2. Twisted Tube Heat Exchanger Bundle

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providing a helical tube-side flow path (Fig 3). The forming process ensures that tube wall thicknessremains constant and the material yield point is not exceeded thereby retaining mechanical integrity.The tube ends are round to allow conventional tube to tubesheet joints.

Figure 3. Tube-side Flow Path

A wide range of tube materials can be used including carbon and stainless steels, Cr-Mo alloys, duplexand super duplex alloys as well as titanium, zirconium and tantalum. Tube sizes may vary from ½ inchto 1 inch.

Tubes are assembled into a bundle on a triangular pitch one row at a time with each tube being turnedto align the twists at every plane along the bundle length. This alignment results in tubes contactingadjacent tubes at many points along the length of the tube in the bundle (Fig 4). The completed bundleis then tightly strapped circumferentially to ensure no tube movement and a robust bundle is the endresult. Bundles can be constructed with more than 5000 tubes and up to 6 feet in diameter with tubelengths up to 80 feet (Fig 5).

Figure 4. Tube Alignment and Support

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Figure 5. Completed Twisted Tube Bundle

The shell-side flow path is complex and predominantly axial in nature (Fig 6). Typically, the shell sideflow area is approximately equal to the tube side flow area. The bundle is often shrouded to ensureshell side flow remains in the bundle and minimizes bypassing. Paths are available to allow the fluidto flow into and out of the bundle at each end. When high inlet and outlet velocities must be avoided,“vapor belts” may be used as with conventional designs. The Twisted Tube design imparts a swirlflow to the tube-side fluid enhancing the tube-side heat transfer coefficient.

Figure 6. Shell-side Interrupted Swirl Flow

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Advantages

Thermal and Hydraulic PerformanceElimination of the shell-side back and forth flow path with a more unidirectional flow yields a muchhigher heat transfer coefficient per unit of pressure drop. Typically, heat transfer coefficients are 40%higher for the same pressure drop or, conversely, pressure drops are halved for the same heat transfercoefficient. Moreover, the tube-side swirl induced flow enhances the coefficients by an amount similarto that of twisted tape or turbulator inserts in a plain round tube. The overall effect of this is asubstantial reduction of heat transfer area for a twisted tube exchanger compared with a conventionalexchanger for the same duty. Alternatively, significant improvements in the performance of anexisting exchanger can be achieved by replacing a conventional bundle with a Twisted Tube bundle.

Higher Thermal EffectivenessThe closer approach to pure plug flow on the shell-side means that designs achieving higher thermaleffectiveness, more typical of plate type exchangers, are possible with Twisted Tube exchangers

Lower Fouling and Cleanability The elimination of dead spots on the shell-side and the increased turbulence, both on the shell-side andthe tube-side results in reduced fouling. Particulate fouling is reduced by the scouring action. Othertypes of fouling such as scaling and chemical reaction products are prevented by the removal of hotspots. Fouling characteristics are therefore, more typical of those found in plate exchangers ratherthan shell and tube type exchangers. The lower shell side pressure drop for a given flow means thathigher velocities are possible, thereby reducing clogging and plugging with fibrous materials. Shouldfouling occur, the twist alignment in the twisted tube exchanger provides cleaning lanes even thoughthe bundle is constructed using triangular pitch tube layout. Hence, the cleanability of a conventionalsquare pitch layout is combined with heat transfer area density of a triangular layout.

Vibration EliminationFlow induced vibration can occur in conventional exchangers although special precautions such as “notubes in window” are available to overcome the problem by providing more tube support. The mostdamaging vibration arises from fluid-elastic instability that can lead to damage within a few hours ofoperation. The possibility of such vibration in twisted tube exchangers is completely eliminated byaxial flow and because the tubes are supported approximately every two inches along the tube length.Clearly, there is some cross-flow at the inlet and outlet regions but good tube support effectivelymitigates this potential for failure. Further, the cleaning lanes provide additional smooth paths with aflow entering and exiting the bundle.

Codes and MembershipsTwisted tube heat exchangers are manufactured to most codes including A D Merkblatter, ASME, B,BS, CODAP, HPGCL, ISPSEL, STOOMWEZEN, and TEMA. Brown Fintube is a member of ASM,ASME, AWS, AQS, HTFS, HTRI, ILTA, NACE, and SME, and manufactures Twisted Tubeexchangers in Houston TX, Luxembourg and in Asia through strategic alliance.

ApplicationsOver 400 Twisted Tube heat exchangers have been designed, built and delivered. A partial list ofapplications can be found in Table 1. Figure 7 shows a Twisted Tube exchanger bundle being installedin an existing shell in a North American facility. Table 2 contains a comparison of Twisted Tubeexchangers and conventional shell and tube exchangers for actual applications including heat transfersurface area and cost savings. Data presented in table 2 are for units constructed with carbon steel,however, in general, in the correct application, cost savings through the use of twisted tube will vary

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directly with the material cost and the surface area of the heat exchanger. Stated differently, greatersavings can be realized as the cost of the material increases.

Figure 7. Installation of Twisted Tube Bundle

Table I. Applications of Twisted Tube Heat Exchangers

Industry Application

Chemical Sulfuric acid coolingAmmonia preheatingHydrogen peroxide heating / cooling

Petroleum High pressure gas heating / coolingCrude oil heatingBitumen heatingLNG heating

Pulp & Paper Black liquor heating / coolingWhite water coolingOil heating / coolingEffluent cooling

Power Turbine steam condensingBoiler feed water heatingLube oil cooling

Steel Quench oil coolingCompressed gas coolingLube oil cooling

Mining / Mineral Processing Liquor coolingEffluent cooling

District Heating Closed loop water heating

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Steam heating

Table II. Comparison of Twisted Tube and Conventional Heat Exchangers

Service Feed / Bottoms Lean / Rich Crude Oil MVGOExchanger DEA Cooler Product Cooler

Shell-sideFluid Stripped Water Lean DEA Crude Oil MVGO ProductTemp In/Out deg F 250 / 138 244 / 134 122 / 97 260 / 180

Tube-sideFluid Sour Water Rich DEA Sour Water WaterTemp In/Out deg F 100/201 97 / 200 64 / 73 125 / 174

Surface Area Conv / Twisted Conv / Twisted Conv / Twisted Conv / TwistedSquare feet 9612 / 4746 1151 / 764 8966 / 5511 2163 / 1097

Cost Conv / Twisted Conv / Twisted Conv / Twisted Conv / Twisted $, 000 $130 / $90 $35 / $25 $215 / $170 $40 / $30

CONCLUSIONS

The construction, thermal characteristics, performance, and use of Twisted Tube type heat exchangershave been reviewed. It has been shown that this type of exchanger offers a number of advantages overthe conventional shell and tube exchanger with segmental baffles. In suitable applications, TwistedTube heat exchangers offers superior economic performance as defined by cost per unit heat loadwhen compared to the alternative of conventional shell and tube type equipment.

REFERENCES

1. TEMA, 1988 Standards of the Tubular Exchanger Manufacturers’ Association, New York 7th

ed.

2. Butterworth, D., Guy, A. R., and Welkey, J. J., Design and Application of Twisted Tube HeatExchangers.

3. Small, W. M., and Young, R. K. 1979 Heat Transfer Engineering, Vol. 1

4. Gentry, C. C., Chem. Engng. Progress, Vol. 86, No. 7 pp 48-57

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