Cavity Resonances in Hermetic Compressors: A Finite Element Approach

A tnan~:ular pnsm acoustic flnlte elem~nt was develnpP.d based on Craggs ( l) method. It has the advantage of representing shells of different shapes wlth change.s 111 cross sectlon along the len~:th. Two sample compressors were stud1ed for their resonant fr~quencies and mode shapes. Modifications to the shell or putr1p changed both the ,·esonant frequencies ~nd mode shapes. 1-D finlte elements were used to find resonant characteristiCS of a discharge tube c:tth aJCea changes. Mufflers used In cnmprP~Sors form a different study. However, the tubes used can be designed us1ng 1-0 finite elements. Finally, a Slmple cubical room whore the co>Hp>-essor/appliance js u,;;ed was aiM analysed to detP.ct lf any problen>anc frequencles can be removed. It was fc.und that acoustic dampins usin~:; absorbers and baffles may be the answer than chanli:lllg room d1mens10ns. INTRODUCTION" The no1se generated by a h~rmetic can1pres~or both "'"h1le 1t is 1·unn10g as ~Nell as dur1ng 1ts start and stop Is a very import~nt criterion to the customer ~nd often beco10es a selllng point. The vibratiOns of different parts of the pUII·oduction. If expenmental a11d an~lyt1cal analysi$ ls carr1Pd out ~t this stage, the reslilts are oft~11 VPry dlfflcul t to correlate to t.he cau~es. The avallable d.nalyt1cal technl•sonance~ that orcur 10 the cavity betwPen the shell and the pump, thP disrh~rge tube connecting the pump and the shell as in low stde compressors, and finally thP room 111 ~h1ch thP ~ppl1anc~ 1s ll~Pd. C<:tvlry l·esorJf,:i:llCeg, Jn ~-~frigerilnt compre::;$ors w.-1s reported by Johnson and Hamilton (2j. They have used an analytical approach on cylindrical shells ~;ith ~nnular pumps ~nd got re~sonable a~reement With the experimental values. Ho~ever, sinc:e actual compr-es:;or c:av1ty is not a true annulus the disagrP.~ment may be reduced by usin~ finlte element method. a study of noise reduction of refngerator compressors wac< dor1e by Kenji Tojo ~t a! : 3l. In this study analytlco.l n1ethods ~oere used to ~;et the canty re~rJnances. A 500 H2 noise 1.1a$ reI a ted to the co. vi ty resonance 4 A finJ te e 1 ement study was suggf'sted 1n th~1r paper for ease of ~·epres.e-nt1ng thP. cavity and accuracy of the re$ult5. MUnJal (4J has summari~~d the use cf f1n1te elemPnts for mufflers. HP pres,nted a hi~h order J2 cte~remall computl:'rs.