Structural Validation of Asymmetric Tubesheets for 80/20 Refrigerant Systems: A Comparative Study of Autodesk FEA and Multi-Stage Pressure Testing

Abstract

This research presents a structural integrity validation of a specialized asymmetric tubesheet designed for high-pressure HVAC applications using 80/20 refrigerant blends. The study investigates the mechanical response of an assembly comprising an SA 516 Grade 70 shell and SB359 C12200 copper tubes. A hybrid methodology is adopted, integrating numerical simulation via Autodesk Finite Element Analysis (FEA) with physical multi-stage pressure testing. The asymmetric tube layout is engineered to optimize flow distribution during liquid-vapor phase transitions, addressing the limitations of conventional symmetric designs. A 7-stage incremental loading protocol is executed, scaling from a 4.15 Bar initial leak test to a 27.2 Bar proof load, as mandated by ASME Section VIII Division 1 (UG- 100). The results indicate a maximum Von Mises stress of 218.45 MPa at peak pressure, remaining within the material\'s elastic limit. A linear correlation between numerical and experimental data is achieved with a minimal deviation of 0.10%, validated the FEA model as a reliable digital twin for non-standard pressure vessel validation.

Introduction

Shell-and-tube heat exchangers are widely used in industrial and HVAC systems, but modern applications involving high-pressure, multi-phase refrigerants (such as 80/20 blends) introduce complex flow behavior. These conditions cause phase changes, uneven expansion, and flow-induced vibrations that traditional symmetric tubesheet designs cannot handle effectively. To address this, engineers are increasingly adopting asymmetric tubesheet configurations, which improve vapor flow distribution and reduce erosion, but introduce greater mechanical complexity and variable stress distribution.

Because of this complexity, Finite Element Analysis (FEA) is essential for validating structural performance. The study develops a high-fidelity digital twin using Autodesk Inventor Nastran to simulate stress behavior in an asymmetric heat exchanger made of SA 516 Grade 70 steel (shell/tubesheet) and SB359 C12200 copper tubes. The model uses refined meshing to accurately capture stress concentrations in critical ligament regions. Since operating conditions involve two-phase flow, structural validation is performed using single-phase pneumatic and hydrostatic tests in accordance with ASME standards.

Methodology

A 7-stage incremental loading protocol is used to validate the design, ranging from leak detection tests to full ASME compliance proof tests:

• Pneumatic testing for shell-side pressure validation
• Hydrostatic testing for tube-side strength verification
• Gradual pressure increases up to 31.85 Bar

FEA results are compared with physical measurements at each stage to verify accuracy and elastic behavior.

Literature Review Highlights

Previous studies show that:

• FEA is highly effective for analyzing complex tubesheet stress patterns.
• Asymmetric designs require finer meshing due to localized stress concentrations.
• Two-phase flow improves the need for asymmetric layouts but increases mechanical challenges.
• Material pairing (SA 516 steel and copper tubes) is standard but requires careful thermal expansion management.
• Multi-stage proof testing is essential for non-standard geometries under ASME Section VIII guidelines.

Results and Discussion

The comparison between numerical simulation and experimental testing shows excellent agreement, with stress deviations as low as 0.10%, confirming the accuracy of the FEA model as a reliable digital twin. Maximum stresses remained below the material yield strength (260 MPa), even at peak loading conditions (27.2 Bar proof pressure). Stress concentrations were primarily observed in tubesheet ligaments but remained within safe elastic limits.

Conclusion

This research successfully validated the structural integrity of an asymmetric tubesheet design for high-pressure HVAC applications. The integration of Autodesk Finite Element Analysis (FEA) with a physical 7-stage pneumatic loading protocol provided a comprehensive framework for verifying non-standard pressure vessel geometries. The results demonstrated that while asymmetry introduces complex stress gradients, the maximum Von Mises stress of 218.45 MPa remained well within the elastic limits of the SA 516 Grade 70 material. Furthermore, the minimal 0.10% deviation between numerical predictions and empirical shop-floor data confirms the reliability of using refined mesh models as digital twins for manufacturing validation. Ultimately, this study proves that asymmetric configurations can safely optimize flow characteristics for multi-phase refrigerants while maintaining strict compliance with ASME Section VIII Division 1 safety standards.

References

[1] Patil, S. J., et al., \"Structural Analysis of Horizontal Tube Sheet Filter Using FEA,\" International Journal of Engineering Research & Technology, vol. 6, no. 5, 2017. [2] Shinde, S. B., and Payaghan, N. S., \"Design and Analysis of Shell and Tube Heat Exchanger,\" Journal of Mechanical Design and Analysis, vol. 10, no. 2, 2021. [3] ASME Boiler and Pressure Vessel Code, Section VIII, Division 1: Rules for Construction of Pressure Vessels, American Society of Mechanical Engineers, 2023. [4] TEMA Standards, Standards of the Tubular Exchanger Manufacturers Association, 10th Edition, 2019. [5] Kirloskar, R., \"Volumetric Expansion and Phase Change Analysis of 80/20 Refrigerant Blends in Industrial Coolers,\" HVAC Manufacturing Review, 2024.

Copyright

Copyright © 2026 Ganesh Balasaheb Pawar, Dr. A. D. Desai, Mr. P. G. Sarasambi, Dr S. D. Shinde. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.