Optimizing Heat Exchangers with Twisted Tape Inserts: Key Findings of Truncation

Industries depend on tube heat exchangers to regulate temperatures in systems like HVAC, automotive cooling, and electronics. To optimize these systems, researchers are constantly exploring ways to increase efficiency without significant additional energy costs. This post highlights findings on using truncated twisted tape inserts in heat exchangers, showing exactly how specific tape configurations—like tape position, pitch, and truncation—impact heat transfer, pressure drop, and energy use.

 

Overview of the Study

This study specifically examined the effects of varying twisted tape designs in tube heat exchangers. The focus was on understanding how changes in pitch (spacing between twists), truncation (length of tape used relative to the tube length), and positioning impact both the heat transfer rate (measured by the Nusselt number) and pressure drop, which affects energy efficiency. Using computational fluid dynamics (CFD) simulations, the researchers measured the Performance Evaluation Criterion (PEC) for various configurations to balance the benefits of enhanced heat transfer with the energy cost associated with increased pressure drop.

 

Key Findings on Twisted Tape Insert Configurations

1. Pitch Variations and Optimal Performance

The pitch of a twisted tape refers to how tightly the turbulators is twisted. Four pitch values were analyzed in this study: full-length pitch (P = L), half-length (P = L/2), third-length (P = L/3), and quarter-length (P = L/4). Which is 4 times tighter than the full length pitch. Each configuration was tested under a range of Reynolds numbers to understand how fluid velocity interacts with the twists.

 

  • Lower Pitch, Higher Heat Transfer: The tightest pitch (P = L/4) produced the highest Nusselt numbers, resulting in an average heat transfer increase of 151% compared to a plain tube at a Reynolds number of 1000. This pitch configuration maximized the swirl and secondary flow effects, leading to superior thermal performance by continually mixing the cooler core fluid with the heated boundary layer.

  • Pressure Drop Trade-Off: While lower pitch values increased heat transfer, they also resulted in a significant friction factor due to greater flow disturbance. The tightest pitch (P = L/4) exhibited the highest pressure drop, requiring more energy to pump the fluid through the system.

     

  • Balancing Efficiency with PEC: Despite the higher pressure drop, the tightest pitch (P = L/4) provided the best overall PEC value of 1.76, making it the most efficient configuration for energy savings and heat transfer improvement in applications where some pressure increase is acceptable.

 

2. Impact of Twisted Tape Truncation

Using full-length twisted tapes can be effective but costly in terms of both material and the pressure drop penalty. This study tested truncated twisted tapes, examining truncations of 25%, 50%, and 75% of the tube length to see if shorter tapes could provide similar heat transfer improvements with reduced pressure losses.

 

  • Moderate Truncation Balances Heat Transfer and Pressure Drop: A 50% truncation yielded nearly optimal heat transfer, achieving 86.6% of the heat transfer benefit of a full-length twisted tape at a lower pressure drop. Truncated tapes at 50% or 75% reduced the pressure penalty while still inducing secondary flows, making them suitable for systems requiring moderate cooling improvements without extreme pressure increases.

  •  Best PEC for Varying Truncations: For different pitch values, optimal truncation percentages were identified to maximize PEC. For example:

    • At P = L, the ideal truncation was 75%, yielding a PEC of 1.08.

    • For P = L/2, 50% truncation provided a PEC of 1.24.

    • For P = L/3, the ideal truncation was again 50%, with a PEC of 1.4.

    • For the smallest pitch, P = L/4, a full-length twisted tape with no truncation was optimal, yielding the highest PEC of 1.76.

These results indicate that partial truncations can maintain performance gains without excessively increasing the friction factor, providing more cost-effective and adaptable solutions for different systems.

 

3. Positioning of Twisted Tapes in the Tube

The study also explored how the position of the twisted tape within the tube impacts performance, testing placements at the entrance, center, and exit of the tube.

 

  • Entrance Positioning for Maximum Cooling: Positioning the tape at the entrance of the tube consistently produced the highest Nusselt number and PEC values across truncation and pitch configurations. This positioning created a strong initial disturbance in the thermal boundary layer, which maintained enhanced heat transfer along the length of the tube. For example, a 25% truncated tape positioned at the entrance with P = L/4 pitch achieved a PEC close to the full-length configuration.

  • Center and Exit Positioning for Targeted Applications: Positioning the tape at the center or exit provided useful but slightly lower heat transfer improvements compared to the entrance. These positions might suit applications where cooling needs to be more evenly distributed across the tube length rather than concentrated at the entrance.

 

Practical Implications of the Findings

These findings provide concrete guidelines for engineers and designers aiming to maximize heat transfer efficiency in tube heat exchangers:

 

  • Choose Pitch Based on Application Constraints: If maximizing heat transfer is the primary objective and some pressure increase is tolerable, a tighter pitch (P = L/4) is ideal. However, for systems sensitive to pressure drops, using a moderate pitch (P = L or P = L/2) with partial truncation can offer a balanced solution.

  • Use Truncated Tapes to Balance Cost and Efficiency: Where budget constraints are in place or the energy cost of additional pumping power is a concern, using truncated tapes (especially around 50%) can achieve much of the heat transfer benefit with lower friction costs.

  • Optimize Tape Position for System Needs: For applications needing rapid cooling at the start of the tube, placing the tape at the entrance maximizes impact. For systems where heat transfer should be distributed, placing tapes at the center or exit is beneficial.

 

Conclusion

This study confirms that twisted tape inserts are a powerful tool for optimizing heat exchanger performance. By strategically adjusting pitch, truncation, and position, it’s possible to achieve significant heat transfer enhancements while controlling pressure drop and material costs.

*Ghalambaz, M., Mashayekhi, R., Arasteh, H., Ali, H. M., Talebizadehsardari, P., & Yaïci, W. (2020). Thermo-hydraulic performance analysis on the effects of truncated twisted tape inserts in a tube heat exchanger. Symmetry, 12(10), 1652. https://doi.org/10.3390/sym12101652