Dynamic Behavior of In-Cylinder Pressure Causing Fatigue Failure of Reed Valves

Numerous studies considering interaction between refrigerant and reed valve motion in positive displacement compressors have been cited in literature. CFD and FEA simulation tools have allowed modeling of fully coupled interaction of fluids and moving parts [1]. The present paper describes a simplified model of a multi-cylinder reciprocating piston compressor and estimation of pressure surge at high compressor speeds. The results show that the delayed discharge valve opening and closing causes surge in pressures due to formation of pressure waves. For the chosen geometry and operating conditions in the present paper, the characteristic travel time of such waves is much shorter (~ 0.2ms) as compared to longer response time of reed valves (> 1ms) owing to stiffness and exhibit delayed opening due to others factors too like stiction effect. These pressure surges may exceed the fatigue limit of reed valves and cause failures. The bending fatigue limit of up to 720 N/mm2 for the stainless steel (SANDVIK 7C27Mo2 or equivalent) is considered in present study [2]. These also influence the performance of compressors by causing noise and vibrations across AC subsystems thereby lowering the COP of compressor. FEA simulations were conducted and results compare well with experiments explaining the location and nature of failures. The simplified model analysis is sufficient to provide initial design guidelines for most effective reed design and minimize such failures by dampening such surges.