Heat Exchanger World Magazine March 2025 | Page 26

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Heat Transfer Enhancement

In this series of articles we will look at the idea of heat transfer enhancement . The benefits of enhancement are that your heat exchangers will provide the same performance at a lower cost or provide better performance at the same or smaller overall size and footprint .

By Himanshu Joshi , Heat Exchanger Specialist , and Lou Curcio , Heat Transfer Advisor

In this part we will present more information on the enhancement types which were listed in Part 1 , by elaborating on the following aspects : a more detailed description , which technique is applicable , and which side ( shell or tube ) is the best for enhancement . One important consideration in choosing the appropriate technique is the idea of “ controlling resistance ”, i . e . which component of the overall heat transfer coefficient is most beneficial to enhance .

Controlling resistance Repeating from Part 1 , the heat duty is represented by equation ( 1 ): Q = U * A * ∆T ( 1 ) Which can also be written as : A = Q / ( U * ∆T ) ( 2 )

Q = heat duty [ W ] U = overall heat transfer coefficient , OHTC [ W / m 2 – C ] A = heat transfer surface area [ m 2 ] ∆T = Temperature driving force for heat transfer [ C ]

Equation ( 1 ) shows that we can increase the heat duty by increasing the surface area and / or the OHTC , given the two flow rates and their incoming temperatures are fixed . Let ’ s begin by looking at the OHTC , and how it is calculated . The OHTC for a heat exchanger with two fluids , one hot and one cold , is expressed by the following equation :

R hot and R cold are determined based on the individual convective film heat transfer coefficients , which depend on the fluid properties , flow conditions , and the geometry of the heat exchanger . A thermal design is optimal when the R hot and R cold values are equal . This design may sometimes be infeasible due to constraints like pressure drop or mechanical limitations . In certain designs , one thermal resistance can be significantly greater than the others , and it is known as the Controlling Resistance . An example of this concept is the design of air-cooled heat exchangers . Often , the thermal resistance on the airside with bare tubes is the Controlling Resistance due to the low film heat transfer coefficient of air compared to that of the tube side fluid . When this situation arises , the thermal design will benefit from enhancing the airside with external , high fins . The fins significantly increase the outside surface area , which reduces the airside thermal resistance . In this example , the Overall Heat Transfer Coefficient ( OHTC ) decreases because it is based on the finned surface area instead of the bare tube area . The additional surface area from the fins overcomes this deficit , resulting in an increase in Q . There are two lessons from this example . First , the design is enhanced by reducing the Controlling Resistance using an appropriate enhancement technique , namely external high fins . Second , the combination of the OHTC multiplied by A must increase for the enhancement technique to boost Q .

Enhancement types Four types were listed in Part 1 , we will look at each in detail below .

Increased surface area Surface area can be increased simply by making the heat exchanger larger - more tubes and a larger shell diameter . This approach has many drawbacks - higher purchase cost , decrease in OHTC , lower velocities and higher fouling if the fouling is velocity dependent , and the possibility of poor flow distribution . Additionally , this approach cannot be used in debottlenecking situations where the shell size is already fixed . The most common technique to increase the surface area , without making the heat exchanger larger , is by forming or attaching fins on the outside of tubes . In shell-and-tube heat exchangers the fins are formed from the base tube material and such finned tubes are commonly referred to as Low Fin Tubes , here we will call them Integral Fin Tubes ( IFT ). A sample tube is shown in Figure 1 below . IFT provide 2.5-3.0 times the surface area compared to smooth tubes and are therefore ideal for use when the controlling resistance is on the shell side . In chemical process industries IFT can offer benefits for shell side condensing or gas flows , and occasionally with liquid flows .

Figure 1 . Integral fin tube . Photo courtesy of NEOTISS-HPT Fin Tube .

26 Heat Exchanger World March 2025 www . heat-exchanger-world . com