Analysis of Cavity Resonance in 5 HP Hermetic Reciprocating Compressor with Elliptical Shell

Hermetic reciprocating compressor with elliptical shell radiates noise whose frequency spectrum has a peak level at about 200 Hz and about 300 Hz. The 200 Hz and 300 Hz noise is mainly radiated from the top and the side of the compressor shell, respectively. For clarifying the noise radiation mechanism, experimental and theoretical analyses were performed. A noise source simulation test was set up where a speaker (driver unit) , connected to a compressor suction pipe, was driven by a white noise oscillator, and transfer function was measured by using four microphones. These tests clarified that the noise is cavity resonance in the compressor shell. By simulating a compressor mechanism and a shell to an annular cavity, resonance frequency can be calculated and it was clarified that 200 Hz and 300 Hz are cavity resonant frequencies in vertical derection and radius direction of compressor,respectivel~ By investigating the source using a compressor with some pressure and vibration acceleration transducers, it was clarified that the source is suction pressure pulsation in the compressor shell. INTRODUCTION Noise suppression is one of the most important requirements in a refrigeration compressor, outside of its compression function. With increasing air conditioner diffusion, demand for noise reduction is increasing year by year. In Japan, it is a fact that the noise reduction ratio for residential room air conditioner is 2 dB(A) every year. As noise reduction for frequencies below 500 Hz by noise absorbing method is difficult, countermeasures on the compressor itself are necessary. This paper describes experimentally and theoretically that the noise at about 200 338 Hz and about 300 Hz radiated from 5 ~ hermetic reciprocating compressor with elliptical shell is cavity resonance in compressor shell, and describes the investigation method for determining the exiting source of cavity resonance too. EXPERIMENTAL ANALYSIS Compressor sound measurements were made on a compressor testing refrigeration system in a semi-anechoic chamber. (Fig. 1) Three condenser microphones were set at a distance of 100 mm from the compressor shell. The microphones are set on the upper direction, long axis radius and short axis radius directions with regard to the elliptical compressor shell. Frequency spectrums were measured by onethird actave band frequency analyzer and narrow band frequency analyzer (Fourier analyzer) . Typical one-third actave band noise spectrums for three directions are shown in Fig. 2. 200 Hz band on the upper direction and 315 Hz band on the radius direction show a peak amplitude. For clarifying the noise radiation mechanism, the compressor, on which was mounted three pressure and three vibration acceleration transducers as shown in Fig. 3, was connected to a compressor testing refrigeration system and operated. When compressor revolution speed was changed by varying electric source frequency, the amplitude changes of 4th, 5th and 6th harmonics of compressor revolution are as shown in Fig. 4. Frequency spectrums of pressure pulsation in the compressor shell and in the motor cover used for suction gas entrance to the cylinder, are compared in Fig. 5. From these spectrums, the followings results are reported. (1) The cavity resonant frequencies in compressor shell are 2:.:'0 Hz and 275 ru 300 Hz. (2) The compressor shell vibration amplitude shows peak level at 220 Hz in the vertical direction and at 275 "-' 330 Hz in the radius direction for the compressor shell. (3) Exiting source of cavity resonance is suction gas pressure pulsation in the compressor shell. As next step, noise source simulation tests were performed. By driving the speaker ( dri ver unit) which was connected to a compressor suction pipe with a white noise oscillator, cavity resonant frequencies were measured. Four microphones were prepared, one was set in the suction pipe as the standard microphone for measuring the output sound pressure level from the driver unit. The others were set in the compressor shell for measuring the sound pressure level in the shell. (Fig. 6) As data for analysis, transfer function (Tril between the sound pressure level measured by standard microphone (Pref) and the level in the shell (Pi, i = 1,2 ,3) was measured. (i = 1,2,3) (l) where, i means microphone location, i = l, 2, 3 are, respectively, vertical direction, short axis radius direction and long axis radius direction. As this test was made in air, in which sound velocity is 340 m/s, and the velocity in refrigerant is 170 rn/s, measuring data were shown in corrected frequency value. Frequency spectrum on the long axis radius direction (i = 3) while closing some suction holes in the motor cover, is shown in Fig. 7. This data indicates that 170Hz and 290 Hz are cavity resonant frequencies defined by the cavity between compressor mechanism and shell. Furthermore, for making clear cavity resonance phenomenon, can of which dimension was 210 rnrn H x 130 rnrn L x 80 rnm D was assembled in the compressor shell, and cavity resonant frequency was measured. The same measurement was also made in the case of empty shell. Testing results are shown in Fig. 8. From these data, it is clarified that the larger the size of object assembled in the compressor shell, the lower the cavity resonant frequency becomes. Phase angles, in case of compressor assembled shell, are compared in Fig. 9. lst harmonic modes are the same phase at three points, but phase angle between point l and points 2 and 3 shifts 180 degrees in 2nd harmonic mode. Actual mode in the shell is presumed as shown in Fig. 10. From this phase shift, it is understood that the resonant frequencies of short axis and long axis rad;us direction are very close. THEORETICAL ANALYSIS (l) Cavity transfer function Consider the propriety of sound source simulation test. A suction pipe and a compressor shell are assumed to be a simple one dimensional acoustical tube as shown in Fig. 11. Sound pressure P1 at the tube entrance, P2 in cavity, and volumetric velocities u1 and Uz satisfy the following equation. Boundary condition is Uz = 0. Therefore, pl = AP2 Transfer function H(f) = Pz/Pl H(f) ;;g _sl. 1 S2 sin kt1 • Sl.n k£2 1/A: