A Simple Valve Model to Study the Performance of a Small Compressor

Reed valves in many small compressors arc usually activated automatically by pressure differences across the valve. These valves arc sometime called automatic valves. They have a very simple geometry and construction but they comprise one of the most complicated performance characteristics. The unsteady vibration of the valve reed during the operation requires a proper design. Generally too stiff the valve plate causes over~compression and too flexible causes unnecessaJy fluctuation of the valve plate. The paper presents a simulation model of a typically used reed valve in a small refrigeration roiling piston compressor. The narural fundamental frequency was obtained using the Rayleigh's quotient. The non-uniformity of the valve width can be conveniently accounted for by considering the deflected mode shapes of the valve reed using the actual valve geometry and dimensions. The mode shape can then be applied in the simulation model of the valve reed. The mode shapes for the deflected valve before and after hitting the valve stop arc taken into the consideration. Predicted results are presented for various compressor delivery pressurcs. Effects of various valve damping coefficient, valve thickness and sensitivity of the valve behaviour with respect 10 the valve force area coefficient arc also presented. NOMENCLATURE A, Valve flow area, m B effective vai'~e fon:e area, m2 c. Coefficient of discharge C1• fon:e area coefficient d valve body width, m E Young modulus, Nlm I moment of ineltia, m" h, upstream enthalpy, k.Jikg h!{ downstream entl)alpy assuming isentropic e»pansion, k./lkg I effective valve length, m m mass flowrate, kgls P pressure, Nld P.., indicated power, W P "'" over compression power loss, W q(x) mode participation factor r valve tip radius, m valve thickness, m v, specific volume, m1kg V flow velocity, mts x distance from valve free end, m y(x) deflection of valve m natural frequency ell(:. l valve shape function P, plate density, kglm C damping coefficient