Major Design Considerations

The ideal design of a bipolar plate should address the following key elements and requirements (a) a high-performance gas flow field for the efficient transport of reactant gases to gas diffusion layers of electrodes, removal of heat from the cell, and removal of water, specifically maintain adequate moisture level on both cathode and anode sides of a Nafion membrane  

One of the major challenges in the bipolar plate design is to house reduced-size and highly complex gas flow channels with complex patterns for both fuel and oxidant gas flows as well as house cooling/heating channels if required. Additionally, it has to integrate well with the internal supply and return manifolds. 

Metallic bipolar plate design with conventional channels and use of metal foams in the flow fields are also considered (Kumar and Reddy, 2004). Results show a superior performance of the fuel cell with the use of metal foam bipolar plates with lower permeability. Bipolar plates with conventional channels are restricted to higher permeability owing to machining limitations. 

The use of SS-316 bipolar plates with multiple-parallel straight channels is very common in the fabrication of PEM fuel cell stack. A porous bipolar plate design to transport liquid water from the cell to the coolant stream is also considered (Wheeler et al., 2001). Reviews of bipolar plate design and materials are given by Ajersch et al. (2003) and Kumar and Reddy (2004). 

There are two major considerations for the design of the bipolar plate (1) selection of materials for bipolar plafes and (2) gas flow-field design. A detailed description of fhese two aspecfs of the bipolar plate design is given in the following section. 

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In rotary devices, reradiation from the exposed shelf surface to the solids bed is a major design consideration. A treatise on furnaces, including radiative heat-transfer effects, is given by Ellwood and Danatos [Chem. Eng., 73(8), 174 (1966)]. For discussion of radiation heat-transfer computational methods, heat fliixes obtainable, and emissivity values, see Schornshort and Viskanta (ASME Paper 68-H 7-32), Sherman (ASME Paper 56-A-III), and the fohowing subsection. 

A significant number of developments have occurred over the past decade to address these important issues of pneumatic conveying (Wypych, 1995a). This chapter summarizes some of the major design considerations that have resulted from this work in relation to long-distance and pipe branching applications. 

We win consider ethylene oxide reactors in more detail in later chapters, because the cooling of these reactors is a major design consideration. 

Continuons emulsion polymerization is one of the few chemical processes in which major design considerations require the use of dynamic or unsteady-state models of the process. This need arises because of important problems associated with sustained oscillations or limit cycles in conversion, particle number and size, and molecular weight. These oscillations can occur in almost all commercial continuous emulsion polymerization processes such as styrene (Brooks et cl., 1978), styrene-butadiene and vinyl acetate (Greene et cl., 1976 Kiparissides et cl., 1980a), methyl methacrylate, and chloropene. In addition to the undesirable variations in the polymer and particle properties that will occur, these oscillations can lead to emulsifier concentrations too low to cover adequately the polymer particles, with the result that excessive agglomeration and fouling can occur. Furthermore, excursions to high conversions in polymer like vinyl acetate... 

All portable equipment and tools used in a sterile area must be thoroughly pre-washed with proper cleaning agents, final rinsed with WFI, and wrapped if required. These items are usually passed into the sterile area through a double door autoclave. Elimination of all particulate matter from any object entering the sterile area should be a major design consideration. 

Where fire is a major design consideration, testing may be required. The following ISO Standards consider the behaviour of the material itself but do not address the structural consequences ... 

For the interconnection of digital semiconductor devices, a major issue that must be considered is clock skew, which results from varying the length of clock lines and is a major design consideration for high-speed products. 

Hat-top chains are used almost exclusively on eonveyors. In praetiee, the flat-top chains covered here are basically special types of slat conveyors. Hat-top ehain eonveyor speeds are generally less than 300 ft./min and the maximum chain pull is usually much less than 1000 lb. The major design considerations for flat-top chains to be used in conveyors are several of the tensile loads, several types of wear, lubiieation, and environment. 

The other major design consideration in transfer systems again involves minimization of the heat inleak to the transfer fluid. Solutions to this specific heat-leak problem are similar to those noted for conventional dewars. 

PEFCs is 80°C), the saturation pressure is extremely sensitive to temperature. At 80°C and 1 atm total pressure, a 1 °C change is about a 5% change in the saturation pressure. Therefore, for low-temperature PEFCs, heat and water management are major design considerations. For higher temperature fuel cells, the water generated is generally in the vapor state or part of the electrolyte. 

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