Compressor Selection - Practical Issues

The same problem applies to the higher-pressure ratios, of above 1.6 up to 3, which corresponds to 0.6 to 2 bar gauge, or about 9 to 30psig. In this pressure range, the 

Fuel cells usually output warm or hot gases. Fuel reformers do likewise. This energy can be harnessed with a turbine. The turbine will frequently be used to turn a compressor to compress the incoming air or fuel gas. In a few cases, there may be excess power, and in such cases a generator can be fitted to the same shaft. 

There are only two types of turbines worth considering. The first is the centripetal or radial, which is essentially the inverse of the eentrifugal compressor considered above. 

The efficiency of the turbine is treated in a similar way to that for compressors, with the same assumptions. If the turbine worked isentropically, then the outlet temperature would fall to where, as with the compressor 

However, because some of the energy will not be output to the turbine shaft, but will stay with the gas, the actual output temperature will be higher than this. The actual work done will therefore be less than the isentropic work. (For the compressor it was more). We thus define the isentropic efficiency as 

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