Cells safety factor

In the operation of the nitrification unit, the nitrifiers and the heterotrophs are coexisting. This means that there are two mean cell retention times that must be satisfied. The nitrifiers are slow growers compared to the heterotrophs, therefore, their mean cell retention time, is greater. Therefore, in the design of the reactor, G c must be made equal to Gc . In this case, the retention time for the heterotrophs is satisfied as well as the retention time for the nitrifiers. If the reverse is made equal, that is, if G is made equal to G the time will be very short and the nitrifiers wiU not have sufficient time to grow and get established and will simply be washed out into the effluent. In addition, a safety factor, SF, should be applied to obtain the design G G c = G ). A value for the safety factor of 2.0 may be used. In equation form, this is written as... 

Mechanical Requirements. The pottant material should have a relatively low modulus (< 2000 psl at 25°C). The maximum tolerable modulus depends on the difference In expansion coefficients of the cells and the rigid member and on the thickness of the layer between them. Relatively high modulus rubbers could be used but would require inordinately thick and thus expensive layers to damp out the expansion differences. For example, with an 1/8-in.-thick glass superstrata and silicon cells, which will take 5000 psl maximum linear stress, a pottant of 1000 psl modulus needs to be a minimum of only 1.5 mils each side for a 1 1 safety factor in service with a 2500 psl modulus the minimum is - 3.5 mils, etc. A safety factor higher than 1 1 Is highly desirable. 

The assembly of these cells into a battery must take account of electrical engineering considerations, thermal control problems, questions of accessibility and maintenance and safety factors. With as many as 500-1000 cells involved it is evident that a series/parallel network will be required the design of this network poses particular problems for Na/S cells because of two characteristics they possess ... 

In order to improve durability and cost-efficiency of the cells, the stacks, and the system, much of the development has in the past focused on lower operating temperature, increased power density, and material savings based on reduced cell and stack component thickness. Current SOFC technology is relying upon cells made of ceramics with their inherent, somewhat unrehable properties in addition to their intolerance toward thermo-mechanical stresses [28-32]. Therefore, industrialization of SOFC concepts based on the current materials requires implication of large safety factors, which leaves further cost reduction as well as reliability improvements challenging. 

The low value of fracture toughness makes ceramic cells sensitive to material defects and unsuspected flaws. The consequence is that the ultimate strength of ceramic cells deviates from their nominal mean value, and significant safety factors (maximum tolerated intrinsic load/applied extrinsic load) have to be used in order to... 

Safety factor for gas emission fj (includes 10% faulty cells and aging) 5 5 5... 

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