Vibration isolation of dimple plate heat exchangers

Abstract

Dimple plate heat exchangers are a new type of welded compact plate heat exchangers. The dimple plates increase the turbulence of the fluid flowing over the plate, increasing the efficiency of the heat exchanger without increasing pressure drop over the heat exchanger. The compact design of the heat exchanger makes it possible to install the heat exchanger at the top of condenser columns, reducing the footprint area of the column by replacing standard shell and tube condensers at the foot of the column. After the implementation of these condensers in 2008, Sasol experienced failures of 12 column-top dimple plate condensers in unit 300. In these cases damage was observed at the weld between the plates and the bottom header. One possible reason for the damage was vibration caused by the flow over the dimple plates. The characteristics of flow-induced vibrations in dimple plate heat exchangers were studied in the scientific literature. It was, however, found that although the effect of dimples on channel walls had been well researched, the fluid-elastic vibration of the bluff body containing the dimples had not been sufficiently studied. A simple aerodynamic model was constructed to determine the characteristics of the combination of vibration caused by the bluff body (plate) and the flow over the dimples on the plate. The experiment showed the generation of two flow-induced vibration amplitudes. The structure of the heat exchanger was modelled by using mass, stiffness and damping elements. With certain assumptions the model was reduced to a two degrees of freedom system that simulated the most prominent vibration direction. This model was used to simulate the effect of design changes to the response of the structure to a range of forcing frequencies. An experimental model of the column-top condensers was constructed and the response due to different vibration frequencies was measured. The measured results were compared with the theoretically predicted values for cases with the current design and the cases where the vibration-control concept was implemented. This validated the theoretical model and the mathematical simulation as a tool to design vibration control systems for real heat exchangers. With the replacement of the very stiff mounts that are used in current designs with soft rubber mounts, the dynamic forces on the internal plates was reduced by up to 97.8% for certain forcing frequencies. The deflection of the internal plates is a main cause of stress in the plates and, more importantly, the weld fillets connecting the bottom of the plates to a common header. This repeated stress can easily cause fatigue failure in the welds. By therefore reducing the amplitude of vibration of the heat exchangers, the onset of fatigue failure will be substantially delayed, increasing the reliable lifetime of the column-top condenser. This concept is not only limited to dimple plate heat exchangers. The oscillating stress in any internal component can, therefore, be reduced by isolating the whole system with soft rubber mounts of a determined stiffness and layout.