Heat Transfer Innovators

8/13/2019 Heat Transfer Innovators

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Since the properties of the hot and cold fluids are fixed, the two factors affecting the heat transfer rate are the fluidconditions, and applied fouling factors. These factors are most influenced by the heat exchanger designer andprocess engineer.

For new applications, the designer maximizes the heat transfer rates by increasing the turbulence of each fluidwithout exceeding the allowable pressure drop (the exceptions are transfer rates controlled by phase changecoefficients). Numerous devices aid in this endeavor.

Shellside enhancement devices include fintubes, an array of baffle types such as segmentals and Low-Pressure Drop(LPD) tube supports, and Twisted Tubes :

Longitudinal Fintube with cut & twisted fins Longitudinal Fintube with peripheral baffle

Segmental baffle Multitube hairpin bundle with segmental baffles

Low Pressure Drop tube support Twisted Tube bundle with interrupted swirl flow


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Tubeside enhancements include multiple tube passes, various tube inserts, and Twisted Tubes:

Turbulator or twisted tape tube inserts

Cored Helix Heli-Core (Twisted Tube tube insert)

Twisted Tube Looking through a Twisted Tube


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Twisted Tubes and tube inserts can be used to retrofit existing applications to increase available surface areaand/or heat transfer rates.

Twisted Tube replacement bundle Installation of a retrofit Twisted Tube bundle

Fouling Effects on Heat Transfer Rates

Fouling factors are assigned by the process engineer and are used to de-rate the heat transfer coefficients to accountfor anticipated fouling of a given fluid, thus increasing the required surface area to extend the runtime of a heatexchanger between cleaning.

Depending on the clean heat transfer coefficient, the applied fouling factor can have a negligible or significantimpact on the overall heat transfer coefficient. For example, if a 0.003 fouling factor is applied to a clean transferrate of 700 (common for fluids with water-like properties), the actual rate is reduced to 215, resulting in 300%excess surface area for fouling. However, when a 0.003 fouling factor is applied to a clean rate of 90 (common forhydrocarbons), the resulting fouled rate is only reduced to 71, providing only 30% excess surface area for fouling.

Compounding the effect of a reduced heat transfer coefficient are restrictions on heat exchanger length, requiringlarger shell diameters to accommodate increased surface area requirements, thus reducing velocities and clean heattransfer coefficients even further. Lower velocities can actually contribute to increased fouling, resulting in a self-fulfilling prophecy.

Design for Reduced Fouling

Rather than simply applying fouling factors to cover all types of fouling, we should take steps to understand thedifferent types of fouling and what design considerations can be used to minimize their occurrence. Two commontypes of fouling are sedimentary, where small particles fall out of a fluid, and high temperature related, such ascoking or polymerization where the cold fluid contacts the hot metal surface heated by the hot fluid.

Sedimentary type fouling is elevated in areas of low velocity or dead spots. Turbulent flow and the elimination ofdead spots are needed to minimize this type of fouling. On the tubeside higher velocities can be obtained withmultiple tube passes and devices that enhance swirl flow to keep particles in suspension, such as Twisted Tubes.On the shellside, elimination of dead spots is most effective.


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With traditional segmental baffles, fouling occurs in the dead zones on each side of a baffle up against the shell.Bundles using Twisted Tube have no such dead spots that reduce the effectiveness of any heat transfer area.

ConventionalShell & Tube

Twisted Tube

For fluids containing large particulates, double-pipe heat exchangers provide constant velocities and large inner pipesizes (1-1/2 IPS 4 IPS) for solids to pass and for ease of cleaning. Conventional shell & tube designs usesmaller tube diameters and have channels where deceleration occurs and particulates can fall out of the fluidultimately blocking tube flow.

Double Pipe hairpin sections connected in series for slurryservice.

Temperature related fouling can be minimized by designing for reduced metal temperatures. Temperaturedifference and differences in heat transfer rates between the cold and hot fluids affect metal temperature.

For temperature related fouling where high temperature steam is the heating medium, increasing the capacity of an

existing heat exchanger by retrofitting with a Twisted Tube bundle can allow a reduction in steam temperature tohelp minimize such fouling.

For new applications, the use of longitudinal fintubes can be used to reduce the metal temperature for heat sensitivefluids on the shellside. With the heating medium on the tubeside and fins on the outside of the tube, there is atemperature gradient between the base of the fin and the fin tip resulting in an overall reduction in metaltemperature.This reduction in temperature difference results in the need for more surface area, which in turn reduces the overallheat transfer rate. The reduction in efficiency due to the lower metal temperature is made up by the significantincrease in surface area fintubes offer over bare tubes.


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Variations in metal temperature and fin efficiency (E) at different fin heightsbased on 0.035 thick carbon steel fins and a finside coefficient (hf) of 200.

Further information is available in the paper: A New Perspective on the Impact of Fouling on the Performance ofFinned Heat Exchangers in comparison to Heat Exchangers Without Extended Surface by Richard L. Shilling, P.E.of the Brown Fintube Company.

Maximizing Temperature Difference and Reducing Utility Fluid Usage with True Countercurrent Flow

True counter current flow is the most effective means to maximize temperature difference and reduce the size of aheat exchanger. It is used to efficiently obtain closer temperature approaches, and to introduce a temperature crossin the process.

A temperature cross is when the outlet temperature of the cold fluid is higher than the outlet temperature of the hotfluid. A very common application is the feed-to-effluent heat exchanger where effluent is used to preheat feed to

reduce energy costs.

Hairpin heat exchangers provide true counter current flow while providing the benefits of a removable tube bundle.Furthermore, a temperature cross can be handled with a single hairpin heat exchanger.


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Surface Area The Last Part of the Equation

While the designer works to minimize the needed surface area for a given heat exchanger, he/she is also working tomaximize the surface area in the smallest shell diameter for better economics and higher heat transfer coefficients.Longitudinal and/or helical low-fin fintubes can be used to pack a lot more surface area into the same size shellcompared to bare round tubes.

For example, a twenty foot (20) long hairpin heat exchanger with an 8 sch 40 shell diameter and 55 bare o.d.tubes will have 450 ft2 of surface area. Using only 19 longitudinally finned tubes in the same size and length shellyields 852 ft2 of surface area, a 90% increase!

There are times when a fintube is not the solution and surface area and tube count are limited by the tube pitchutilized. For high fouling services, a square tube pitch is often employed to aid in cleanability; thus the tube countsuffers. A solution to this is Twisted Tube where a triangular tube pitch is utilized and still provides cleanability.

Twisted tube bundle construction Hydroblasting through a Twisted Tube bundle

For example, in a 35 i.d. BEM S&T exchanger using O.D. round tubes on a 15/16 rotated square tube pitch,the tube count is 1020 tubes. Using Twisted Tubes, the tube count is 1285, an increase of 26%. This difference ismore pronounced when the O.D. round tubes are on a 1 rotated square tube pitch.

This significant increase in tube count makes Twisted Tube a viable option for bundle replacement or replacement inkind as a solution to increase capacity without the need to modify piping or foundations for a larger heat exchanger.

Pressure Drop Conservation & Vibration Elimination

Maximum use of available pressure drop ensures the smallest heat exchanger design by maximizing velocities forhigher heat transfer coefficients. There are some applications, however, where pressure drop can be costly and canlimit throughput. In these cases a reduction in pressure drop is desirable. Pressure drop sensitive fluids are bestplaced on the shellside and several devices exist that can minimize shellside pressure drop. Various segmental baffleconfigurations are available; however, these introduce a cross flow component that adds unnecessary pressure dropand risk of vibration.

Longitudinal fintubes is an option that provides a large amount of surface area for heat transfer along with axialshellside flow for minimized pressure drop. Another device is the Low-Pressure Drop (LPD) tube support used byBrown Fintube. The LPD is similar to the Rod Baffle; however, the LPD can be used with a triangular tube pitchresulting in a higher tube count than the Rod Baffle, which is limited to square tube layouts.

Perhaps the best solution for a combination of low shellside pressure drop and elimination of vibration concerns isthe Twisted Tube heat exchanger. Since Twisted Tubes are self-supporting, baffles are eliminated altogether. Thecombination of axial flow and unprecedented tube support guarantees this design against any possibility ofvibration-induced failure.


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Summary

There are numerous tools available that allow the Process Engineer, Maintenance Engineer, or Operator to eliminatevibration, increase capacity, reduce pressure drop, enhance heat transfer coefficients, gain closer temperatureapproaches, or reduce fouling with their tubular heat exchangers.

These tools include such things as Twisted Tubes, Hairpin Heat Exchangers, counter-current flow, fintubes, tube

inserts, and Low Pressure Drop baffles.

The Process Engineer should take care when specifying fouling factors to avoid a self-fulfilling prophecy. Moreemphasis should be given to design features that help to minimize fouling such as minimum velocities, reducedmetal temperatures, and the avoidance of dead spots in heat exchanger designs.

Brown Fintube Company has over 60 years of experience in providing innovative-engineered solutions for your heattransfer needs.

Heat Transfer Innovators

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