By Doug Stelling
An analysis of a helical internal cooling coil in a fermentor vessel for a leading pharmaceutical company has just been completed. The 2 inch diameter stainless steel cooling coils had been suffering from excessive wear at their U-bolt support points. The wear problems had plagued the plant for years with coil failures and resulting product contamination. Previous "fixes" appeared to only prolong the time to failure without treating the root cause of the problem.
The Caesar II piping analysis program was used to analyze the system and determined that the helical pipe coil's natural frequency was close to the blade passing frequency of an internal mixer. Modifications to the support system were then made to increase the coil natural frequency to be well above the mixer blade passing frequency, and thus greatly reduce the potential for wear. Thermal flexibility and weight analyses were then made to confirm that the design modifications were also acceptable from a thermal fatigue and weight stress standpoint.