A manufacturer of a Thin Film Heat Exchanger (TFHE) – which is used in conjunction with other components in high temperature coating applications such as caramel coated popcorn – found cracks in their units.
The manufacturer had requested an evaluation of their TFHE units considering the recent design changes and the operational practices of their customer. We performed an engineering and finite element analysis to determine the root cause of the cracking, and make recommendations.
The 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 caused fatigue damage and ultimately a low cycle fatigue failure.
The addition of the 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 a corresponding increase in stresses at the sites of observed cracking. We provided design and operational changes to the units to ensure structural integrity.