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With the introduction of the newest Euro VI trucks some sentiments of aban- doning EGR to more or less extent have been observed [6, 7]. Engines with no or low EGR tend to have lower fuel consumption, less heat rejection and a cost-effective design. When Waste Heat Recovery would be applied to those (European) configurations, only an exhaust evaporator would be present.
A robust and modular evaporator for Rankine Cycle applications has been developed at Bosal. The evaporator has low back-pressure in combination with a high heat exchanging surface per volume and thus can be used in an exhaust. Its design, validation and application will be discussed in this article. Furthermore some ongoing developments will be highlighted that aid the integration of the evaporator product in future applications.
2 Evaporator Design
As exhaust manufacturer, the company has broad experience in high temperature stainless steel applications. For Rankine Cycle applications an exhaust evaporator has been designed that is based on a vertical tube stack concept as depicted in Fig. 2. The stack is constructed from cells of parallel U-tubes. The flow from the U-tubes is distributed and gathered in collector caps before passing to neigh- bouring cells.
This design has the following desirable properties:
• Low backpressure: The vertical tube stack has considerable open frontal area that allows gas flow without excessive restriction.
• Effective heat transfer: The gas is in intensive contact with the tubes and the vertical tube orientation offers highest heat transfer to the boiling fluid. This allows compact evaporator design.
• Modular design: Variation of the number of cells and the tube length allows the evaporator core to be tailored to a desired performance specification.
• High pressure operation: The construction and material thickness have been optimised to withstand forces occurring at operating fluid pressures up to 60 bar.
• Capable of running dry: Withstands high temperatures in the absence of fluid
flow in case of less sophisticated controls and issues with the fluid loop.
• Built-in partial phase separation: Partial phase separation takes place in the collector caps. This ensures a constant output stream quality and prevents propagation of violent phenomena caused by the chaotic nature of boiling.
Fig. 2 Modular tube stack evaporator design
concept
• Robust: The design foresees expansion in vertical direction while offering a strong rigid structure in horizontal direction. This construction has to cope with severe temperature gradients and high pressures.
The tube stack is closely fitted with an outer plate such that intensive gas-tube contact remains. The complete core is fitted in a housing that is engineered for the specific application and ensures good flow uniformity (Fig. 3). Well thought material choice results in an evaporator that is able to operate in an aggressive exhaust environment and that is compatible with most working fluids.
3 Validation and Certification
The evaporator design has been validated through Finite Element Method (FEM) based Low Cycle Fatigue analysis in combination with in-house thermal cycling and vibration tests. This validation exercise has been input for obtaining European Pressure Equipment Directive and German AD2000 certification. Alongside design certification, also the production process has been proven compliant with certification rules which include traceability and welder certification (Fig. 4).