Compressor with hermetically sealed casing

Abstract: A compressor comprises a motor-compressor unit of the volumetric type mounted by means of spring suspension assemblies within a hermetically sealed metal casing (5) composed of a casing body (6) and a cover (7). The casing (5) is of an asymmetric configuration having at any point thereof a double curvature of different magnitude relative to two ortogonal planes intersecting at the center axis (8) of the casing, with a substantially cambered lateral surface (9) of substantially eliptic cross-sectional shape. The audible noise emitted by the compressor is substantially reduced.

Description: The present invention relates to a compressor, particularly for domestic refrigerating appliances, comprsing a hermetically sealed casing enclosing a motor-compressor unit of the volumetric type as defined in the preamble of claim 1. Compressors of this type are well known and include a spring suspension system by means of which the motor-compressor unit is mounted within the hermetically sealed casing in a manner acting to reduce vibrations and noise generated therewithin. The hermetically sealed casing performs the double function of containing a refrigerant gas which is to be compressed, and of acting as a barrier for the noise generated in its interior to thereby reduce its transmission to the ambient space. Generally, and as described for example in DE-A-2 835 414, the hermetically sealed casing has a substantially cylindric configuration, with a lateral surface having mounted thereon a terminal assembly or the like for the required electric connections, a partially convex bottom wall, and a cover having a partially curvilinear sectional shape which is symmetric at least in relation to a vertical center plane. This configuration of the hermetically sealed casing is conducive to reconciling the mutually contradictory requirements of simplifying the construction of the compressor and of reducing the transmission of noise, particularly in the lower medium range of the audible frequencies. It is in fact known that noise in the low frequency range is particularly annoying, especially in the domestic environment. The overally result, however, is a compressor of relatively complex construction and substantially noisy operation, in contradiction to the actual requirements of containing noises within an acceptable range, specifically in a domestic environment. From GB-A-2 078 311 an hermetic compressor housing formed from metal sheet as two housing halves is known. The two halves form, when joined together, a generally ellipsoidal inner surface having a maximum radius of curvature of about one order of magnitude greater than the minimum values of curvature and deviating from the shape only when necessary for supporting the compressor motor assembly. The housing inner surface is substantially symmetric about each of three mutually perpendicular axes. From JP-A-53 134213 a housing for a motor compressor is known, having a spherical shape except for a projecting portion for encasing a protruding portion of the compressor body. To improve the rigidity of the casing, the central portion in the height direction has a straight line cylindrical-shape. Further, a portion connected to the straight line portion is formed of dummy spherical shape of a fixed radius ratio. It is therefor an object of the present invention to provide a hermetically enclosed compressor which is to be particularly silent in operation and to have a relatively simple construction of reduced dimensions. According to the invention, this object is attained in a compressor having a hermetically sealed casing as defined in claim 1. The characteristics and advantages of the invention will become more clearly evident from the following description, given by way of example with reference to the attached drawings, wherein: Fig. 1 shows a diagrammatic sideview of a compressor according to a preferred embodiment of the invention, fig. 2 shows a diagrammatic lateral view of the compressor of fig. 1 as rotated by 90°, fig. 3 shows a sectional view of the casing of fig. 1, taken along the line III-III and illustrating an electric connector attachment, and fig. 4 shows a histogram representing the performance of the casing of the compressor according to the invention in comparison to that of a conventional compressor. With reference to figs. 1 to 3, the compressor according to the invention comprises a hermetically sealed metal casing 5 having walls of substantially constant thickness and composed of a casing body 6 provided with a cover 7 secured thereto in a hermetically sealing fit. In the per se known manner, and therefore not particularly shown, casing 5 encloses a motor-compressor unit of the volumetric type of a construction as described for instance in DE-A-2 835 414, mounted therein by means of a suitable spring suspension system and operable to compress a gas contained in the casing. The operation of the motor-compressor unit results in the generation of noise which is transmitted to the environment through casing 5, the latter intrinsically responding in two distinct modes to the noise generated in its interior: 1) by acting as an acoustic enclosure reflecting the acoustic energy back to the interior, and 2) by acting as an acoustic transmitter or radiator transmitting the acoustic energy to ambient air, and transmitting vibrations to a structure connected thereto, for instance a refrigerator, these vibrations being subsequently converted into noise. The result of the combination of these two effects depends in the first place on the nature of the transmission of the noise to the casing 5, and on the characteristics of the dynamic response of the casing itself. As generally known, the dynamic response of the casing of a compressor of the type in question is characterized by its inherent vibration properties which may be identified in terms of modal parameters, i.e. natural frequencies, dampening coefficients, and relative deformation. In order to reduce the noise transmitted to the environment to a minimum, the casing should ideally be designed in such a manner that any excitation of acoustic or vibratory nature would act on the casing within the controlled range of its inertia. In other words, the casing should have a considerably mass which on the other hand is necessarily subject to limitations. As an alternative, the casing could be flexible, or at least of reduced rigidity, with natural frequencies of no more than 100 Hz and thus inaudible to the human ear, but this is obviously not possible in practice, because the casing, apart from being subjected to high pressures, is by its very nature rigid and of a limited mass. This results in that the most important natural frequencies of the casing are usually comprised between 1000 Hz and 5000 Hz. As generally known, the important natural frequencies are the prime frequencies which manifest themselves in the range of the lower frequencies (median frequencies of the audible), characterized by a greater wavelength, which are undesirably irradiated with greater facility. In conclusion, an ideal metal casing should have a generally spherical configuration, with two types of deformation during operation of the compressor, in the first place, the expansion and contraction type, and in the second place, deformations generated by flexional waves about the lateral surface of the casing. In any case, for a given volume a spherical casing would have relatively bulky dimensions and would, due to its having higher natural frequencies, insufficiently dampen the noise generated by the resonance of the assembly composed of the motor-compressor unit and the casing itself. According to an important aspect of the invention, a surprising condition of equilibrium between the above described mutually contradictive requirements has been obtained by designing the casing 5 with an asymmetric configuration having at any point thereof a double curvature of different magnitude relative to two ortogonal planes intersecting at the central axis 8 of the casing, and additionally having a lateral surface 9 of a substantially cambered shape, with a substantially elliptic cross-sectional shape. This configuration becomes evident from considering figs. 1 to 3 in combination. In particular, the vertical curvature of the lateral surface 9 is defined by a radius considerably greater than that of the associated horizontal curvature, to result in a reduction of the horizontal dimensions of the compressor as a whole. In particular, it has also been experimentally verified that the cambered shape of the lateral surface 9 in combination with the elliptic cross-sectional shape permits the natural prime frequencies of the hermetic casing 5 to be considerably dampened. In addition to the known fact that the elliptic cross-sectional shape is helpful to suppress the natural prime frequencies of the casing, this shape also permits the internal volume of the casing to be minimized (in relation to the dimensions of the enclosed motor-compressor unit), and to thus improve the dampening of the noise produced by the cavity resonance of the casing 5; in combination with the vertical curvature of the lateral surface 9, this effect permits to obtain a casing 5 the minimum natural frequency of which is particularly high and therefore much less disturbing. As shown in figs. 1 and 2, the cover 7 of the hermetic casing 5 is shaped, with respect to said two ortogonal planes, with respective sections of asymmetric curvature which merge with the lateral surface 9 of the casing through a conventional coupling flange 10. More specifically, each of these curvilinear sections comprises two lateral curvature zones 11 and 12 joined to one another by a central curvature zone 13; each of the curvature zones 11 to 13 is defined by a different radius, the central curvature zone 13 having a considerably greater radius than the other ones. This characteristic permits the vertical dimension of the casing 5 to be limited, and additionally results, as has been experimentally verified, in that the prime deformation of the cover 7 advantageously has a frequency of about 100 Hz higher as compared to that of a corresponding symmetric cover (at least with respect to one of said ortogonal planes). From figs. 1 and 2 it is also evident that the bottom 14 of the body 6 of the hermetic casing 5 is convex throughout, with sections of different curvature with respect to said ortogonal planes, and merging directly with the lateral surface 9 of the casing 5. As has also been experimentally verified, this configuration advantageously permits a prime deformation of the bottom 14 to be obtained about 2800 Hz higher as compared to a corresponding symmetric construction. According to another aspect of the invention (fig. 3), a per se known metal terminal bracket 15 for the electric connections of the compressor is secured, for instance welded, to the casing 5 at the location of a so-called antinode of the prime deformation of the casing. In this manner the casing 5 is rigidified in one of its most critical zones, as a result of which, and as has been experimentally verified, the casing is devoid of inherent deformations at frequencies lower than about 2700 Hz. The volumetric motor-compressor unit housed within the casing 5 is preferably mounted on spring suspension assemblies of a epr se known type, taking support on at least three pedestals 16 provided on the interior lateral surface of the casing. According to still another aspect of the invention as illustrated in fig. 3, the pedestals 16 are secured, preferably welded, to the interior lateral surface of the casing 5 adjacent respective nodal zones (i.e. zones subjected to minimum displacement) of three prime deformation areas of the casing. This is effective to minimize the transmission of vibrations and the resulting noise from the motor-compressor unit to the casing 5, and from the latter to the ambient environment. In summary, the compressor with hermetically sealed casing according to the invention is of a substantially simple and highly compact construction, in which the zones of potential generation and/or transmission of noise ar minimized, and in which noises still generated and/or transmitted are advantageously of relatively high frequencies and thus much less disturbing. In other words, the compressor according to the invention emits substantially reduced sonar energy, with an effective qualitative improvement of the noise still emitted. This is confirmed by fig. 4, which permits the comparison of the sonar energy at different frequencies emitted by the compressor according to the invention to the sonar energy emitted by a corresponding hermetically encased compressor of a conventional type. As will be noted, the values of the sonar energy emitted by the compressor according to the invention, as measured experimentally in a series of laboratory tests, are substantially lower, particularly at relatively lower frequencies.

Claims: A compressor comprising a motor-compressor unit of the volumetric type mounted by means of spring suspension assemblies within a hermetically sealed metal casing of substantially constant wall thickness and composed of a casing body and a cover, the casing (5) having at any point thereof a double curvature of different magnitude with respect to two vertical planes being orthogonal to each other and intersecting at the center axis (8) of said casing and having a lateral surface (9) of substantially cambered shape characterized in that said casing (5) is of an asymmetric shape with respect to both planes and said lateral surface (9) has a substantially elliptic cross-section. A compressor according to claim 1, characterized in that said cover (7) of said hermetically sealed casing (5) is configured, with respect to said orthogonal planes, with respective asymmetric curvilinear sections merging with said lateral surface (9) of the casing, each of said curvilinear sections comprising two lateral curvature zones (11,12) interconnected by a central curvature zone (13), each of said curvature zones (11,12,13) being defined by a different radius, said central curvature zone (13) having a radius substantially greater than that of the others (11,12). The compressor according to claim 1 or 2, characterized in that said body (6) of said casing (5) has a completely convex bottom (14) including different curvilinear sections with respect to said orthogonal planes, said sections merging directly with said lateral surface (9) of said casing (5). The compressor according to one of claims 1 to 3 characterized by , further comprising a metal terminal bracket for the electric connections of said compressor, said bracket (15) is secured to said casing (5) at the location of an antinode of the prime vibration deformation of the casing. The compressor according to one of claims 1 to 4, characterized in that said volumetric motor-compressor unit is mounted on spring suspension assemblies taking support on at least three pedestals provided on the interior lateral surface of said casing, said pedestals (16) being secured to the inner lateral surface of said casing (5) in proximity to respective nodal zones of three prime vibration deformation areas of said casing (5).

Inventors: BUCCIARELLI MAURIZIO, GIUSTO FABIO, BUCCIARELLI, MAURIZIO, GIUSTO, FABIO
Applicants: ZANUSSI ELETTROMECC, ZANUSSI ELETTROMECCANICA S.P.A.
Application year: 1993
Main classification: F04B39
IPC codes: F04B39