Failure Analysis – Cracked Thin Film Heat Exchanger (TFHE)

failure analysis on a cracked Thin Film Heat Exchanger (TFHE) using FEA

Analysis and Design Recommendations


Failure analysis on a cracked Thin Film Heat Exchanger (TFHE) – which is used in conjunction with other components in high temperature coating applications as caramel coated popcorn.

A client had requested an evaluation of their TFHE units due to recent design changes and the operational practices of their customer. One such design change was adding 2 inches (thick) insulation and a cover on the outside of the unit. We performed finite element analysis on the unit to determine the root cause of the cracking, and make possible subsequent design recommendations.

The FEA model was developed using the following assumptions:

1) The TFHE is essentially a construction of two concentric cylinders, between which product flows to be heated. The outer cylinder has a steam jacket and the inner cylinder contains steam. Product flowing between the cylinders is heated to a desired temperature. Thus, the FEA model was created assuming an axisymmetric structure, not considering the various inlet and outlet nozzles.

2) Two types of stainless steel are used in the construction of the TFHE: ASTM A316L and ASTM A304. Temperature-dependent Elastic Modulus and Mean Coefficient of Thermal Expansion data are the same for these two materials, as is the 2004 ASME Boiler and Pressure Vessel Code fatigue design curve (for < 1 M cycles). Slight differences exist in thermal conductivity and material strength.

3) Two thermal analyses were performed. The first examined normal operating conditions and the second examined end-of-day shutdown procedures. The results of these analyses were used as input for the structural analysis.

4) All analyses were assumed to be steady state. No transient analyses were performed. The shutdown condition approximates a sudden temperature drop in the inner cylinder while the remaining components essentially maintain their temperature.

5) Fatigue analysis was performed in accordance with the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, 2004 Edition.

Stresses are presented for pressure loading only, as well as for pressure plus thermal loading. From the results, it is clear that thermal loading induces significantly greater stresses than pressure loading.

The FEA results determined that the use of cold flush water when shutting down the TFHE (either at the end of the day or due to a problem in the process line) created high stresses in susceptible regions of the outer cylinder assembly structure. Repeated application and removal of these high stresses causes fatigue, and ultimately a low cycle fatigue failure.

The addition of insulation on the outside of the TFHEs (relative to previous designs) increased the temperature of the outer steam jacket wall, which in turn created a larger temperature difference between the inner and outer walls and thus, a corresponding increase in stresses at the sites of observed cracking. We provided design and operational changes to the units to ensure structural integrity.


O’Donnell Consulting Performs Design, Analysis and Failure Analysis Services for Clients in Industries including Pharmaceutical and Food. 

(412) 835-5007

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