Heat Exchanger Troubleshooting - Root Cause Analysis Reveals Shortcoming of Standard Design Methodologies

Heat exchangers are remarkably complex pieces of equipment, with most units encompassing a combination of turbulent, two-phase flows with boiling/condensation occurring in convoluted geometries. All Design and Rating methods include significant simplifications to facilitate performance prediction of these industry work-horses - including assumptions of pure cross or longitudinal flow and well-mixed liquid-vapor phases. In this talk, we will go over two real industry case studies.

The first one involves multiple steam generator operating in parallel suffering from repeated tube failures. These steam generators are thermosiphon reboiler units connected to a common steam drum, and each exchanger is a TEMA AES type with vertical cut double segmental baffles. A visual inspection of the extracted bundle and shell interior reveal erosion damage on the outside surface of tubes in the upper rows and at the shell outlet nozzle connection. The failure led to multiple plant shutdown periods, leading to significant loss of throughput and revenue. During root case analysis, the following topics will be discussed: piping flow loop calculations around the heat exchanger, tube vibration, erosion/ corrosion prediction guidelines and phase separation. The heat exchanger rating software Xist will be utilized in this analysis, and hand calculations methods will be discussed. Insights gained from research performed at Heat Transfer Research Inc. will also be discussed, including high speed videos of phase separation inside horizontal shell-and-tube heat exchangers.

The second case study involves a vacuum overhead condenser, where de-superheating, condensation and subcooling are all expected to occur in the same unit. Such a design has multiple vulnerabilities due to uncertainty in design methods for desuperheating and subcooling, as well transition between different mechanisms. In particular, the use of a single bulk temperature profile can lead to divergence between prediction and performance when significant flooding is expected in the condenser. This unit has never provided the required output, despite being significantly over surfaced. During troubleshooting of this unit, various fundamental condenser design issues will be discussed: removal or venting of non-condensables, condensate drainage and removal of tubes. The effect of condenser design on its performance, with respect to non-idealities like vapor bypass will also be discussed.

Hopefully, the talk will impress upon the attendees the importance of sound engineering tools and engineering judgement in designing heat exchangers, and how research can help simplify some of the mystery often associated with heat exchanger troubleshooting.