In this work, a numerical analysis for observing the proficiency of reciprocating compressors used in CNG (Compressed Natural Gas) stations is studied thermodynamically for ideal model and real one. AGA8 state equation is intended for modeling real gas. The analysis is carried out on the basis of the entropy concept. To investigate effects of important design factors on entropy generation, isentropic and exergy efficiencies, a numerical method is developed. The considered design factors are clearance, angular speed, pressure ratio and input and output valve areas. In this analysis, the compressor cylinder is presumed as control volume, and methane is also chosen as the natural gas and modeled as both an ideal gas and real one. The results highlight that with increasing angular speed, entropy generating increases and consequently exergy and isentropic efficiencies decrease, while increase in clearance acquires the inverse results. Furthermore, it will reach to higher efficiency, if input valve area is considered more than output valve area. For instance, with choosing a suitable proportion, efficiency can increase as much as 6.47%.

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