Screw compressor

Abstract: The present invention provides a screw compressor having a wide operating range with respect to flow rate and pressure ratio, wherein a discharge opening portion (9) of a casing (7) is provided with a generally taperingly-cut notch portion (8) so that compressed gas is gradually discharged from a certain fixed point where a volume ratio of one groove defined by rotors (6, 6') and the casing (7) is smaller than an inherent built-in volume ratio. Since the present invention is arranged in the above-mentioned manner, rapid flow of the gas at the discharge opening portion is reduced by means of the notch portion (8) of the discharge opening portion (9) which is readily formed, whereby the variation range of the pressure (the amplitude of pulsation of the pressure) is decreased and the noise level is lowered. Further, a loss of pressure resulting from the rapid flow of the gas is restricted to thereby widen a range of preferable performance of the compressor.

Description: The present invention relates to a screw compressor according to the preamble of claim 1 and more particularly to a screw compressor for delivering or feeding gas such as air in a compressed condition. As shown in fig. 1, in a typical conventional screw compressor, gas sucked from a suction opening portion 1 is confined in a groove-like space defined by a pair of rotors 2, 2' and a casing 3 and when the rotation of the rotors 2, 2' is progressively advanced, the groove-like space is reduced to a volume which corresponds to a build-in volume ratio of the screw compressor. The gas is then compressed to a ratio corresponding to the built-in volume ratio and the compressed gas is then discharged from a discharge opening portion 4 of the casing. In the conventional screw compressor, however, the pressure at the discharge opening portion 4 normally varies due to a change in the flow rate during the discharging operation. The casing of the compressor is thus directly vibrated owing to the variation in the pressure of the discharged gas, so that a noise is generated by the casing. The variation in the pressure also causes vibration in the rotors of the compressor, and the vibrating force from the rotors is then transmitted to the casing via bearings. Further, noise is also generated by the gear and bearing portions due to the vibration of the rotors. On the other hand, the noise on the discharge side of the screw compressor is directly transmitted to the suction side therefrom through a solid member, i.e. the casing. Further, the vibration in the pressure on the discharge side is propagated to the suction side of the casing through leaking of the gas passing through the gaps defined between the two rotors and between the rotors and the casing. The inventor of the present application recognizes through his investigation that the noise at the suction opening portion 1 of the casing is mainly caused by the latter reasons. Tolerances of the gaps between the two rotors and between the rotors and the casing are determined in consideration of production accuracy (allowances in machining and assembling processes), heat deformation, torsional deformation of the rotors due to the axial torque and the like. The smaller the tolerance values of the gaps is, the lower the transmission of the pressure variation from the discharge side to the suction side through the leakage of the gas is. However, there are actually restrictions in accuracy due to the reasons stated above so that tolerance cannot be settled at less than a critical value. In general, in this type of compressor, the pressure ratio is essentially high and the using and operating range is normally limited to a high range. The uppermost efficiency point is designed as a target operating point. If a compressor is operated within a range outside of the designed operating point, the reduction in efficiency of the compressor and increase in the above-described noise caused by vibration are remarkable. On the contrary, there is proposed a compressor of the kind which includes a mechanism referred to as "slide vane" to thereby widen the operating range. The structure is however complicated, and accordingly, this type of compressor cannot give an essential measure for solving the problem previously described. A generic screw compressor is known from the JP-A-62 20 687. The screw compressor provides a casing enclosing rotors for defining groove-like spaces therebetween as well as suction and discharge openings. The discharge opening is formed in a V-shaped configuration gradually opening as viewed in flow direction. Either through-bores extending vertically with respect to the axis of the screw compressor or V-shaped notches are formed adjacent to the discharge opening to discharge overcompressed gas through the notches and thereby relieve the pressure pulsation due to overcompression. The screw compressor according to the JP-A-62 20 687 has a relatively narrow operating range with respect to flow rate and pressure ratio and is operated in specific conditions only. An object of the present invention is to further develop a screw compressor according to the preamble of claim 1 such that a simple construction is realized and a high degree of efficiency and noise reduction over a wide range can be realized. This object is achieved by the features of the characterizing portion of claim 1. Advantageous further developments are set out in the dependent claims. According to the invention the notch portions extend over the whole length of the V-shaped configuration of the discharge opening portion and have a generally tapered cross section. The design of the notch portions is further chosen such that start and end lines of the notch portion coincide with each other at least at one end thereof. It is preferable that the start line B is selected at a location where a later-mentioned volume ratio ViB/ViA is set within 70% - 90%. The volume V* of the taperingly cut notch portion is preferably set to about 1% - 6% of the suction volume V max of the one groove defined by the rotors and the casing. Further, the volume V* of the notch portion is preferably set to about 1% - 8% with respect to the volume VA of the one groove when the gas starts to be discharged where no notch portion is provided. Moreover, the volume V* of the notch portion is favorably set to about 5% - 50% with respect to the decrease amount of the volume VB-VA of the one groove between the start line B where the gas starts to gradually be discharged with the notch portion and the end line A where the gas starts to be discharged where no notch portion is provided. In addition, it is favourable that two or more seal lines defined by the lines of the tops of the rotors and the inner surface of the casing are optionally provided respectively on the male and female rotors. In the screw compressor arranged in the above-described manner, the gas in the groove-like space is gradually discharged through the generally tapered notch portion to the discharge opening portion and the rapid flow of the gas is more relaxed in comparison with a case where the notch portion is not provided. Accordingly, the variation range of the pressure (an amplitude of pulsation of the pressure) at the discharge opening portion caused by the discharged flow is decreased, the noise level is lowered, and further a loss of pressure owing to the rapid flow of the gas is reduced. The features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative examples. Fig. 1 is a longitudinal cross-sectional view illustrating a conventional compressor, Fig. 2 is a longitudinal cross-sectional view explaining a construction of the present invention; Fig. 3 is a cross-sectional view taken along the line III - III of Fig. 2; Figs. 4(a) to 4(c) are longitudinal cross-sectional views respectively showing different notch portions; Figs. 5)a) to 5)c) are respectively views showing different notch portions, as viewed from the inner side of a casing; Figs. 6 and 7 are a perspective view and a development diagram respectively indicating an inner surface of the casing; Fig. 8 is a graph illustrating a change in the volume of one groove during operation of a compressor; Figs. 9(a) and 9(b) are graphs showing experimental results of a variation in the pressure at a discharge opening portion; Fig. 10 is a graph of an experimental result of a relative noise level; and Figs. 11(a), 11(b), 12(a), 12(b), 13(a) and 13(b) are characteristic graphs of the noise level reduction amount and the variation rate of efficiency respectively, each showing results of the first, second and third experiments of a screw compressor according to the present invention. Preferred embodiments of the present invention will be described hereinafter with reference of the drawings. In Figs. 2 and 3, there are contained a male rotor 6 and a female rotor 6' in a casing 7, with gaps interposed between both the rotors and between the casing and the rotors. Gas sucked through a suction opening portion 5 is confined in a groove-like space defined by the rotors 6, 6' and the casing 7, compressed by rotation of the rotors 6, 6' and is then discharged from a discharge opening portion 9. A notch portion 8 cut out into a generally tapered configuration is formed at the discharge opening portion 9, as shown with a shaded portion. It is to be noted that the shaded portion shows a portion to be cut out. As shown in Figs. 4(a) to 4(c), this notch portion 8 having a generally tapered cross-section may be formed into a concave notch portion 8a, a convex notch portion 8b or a notch portion 8c which is curved on its side of the suction opening portion 5. As viewed from the inner side of the casing 7, the discharge opening portion 9 is formed into a V-shaped configuration in such a manner that the forward edge lines A of the notch portion may be parallel to seal lines C, C' formed between the rotors 6, 6' and the casing as shown in Fig. 5(c). Rearward edge lines B opposing to the forward edge lines A of the notch portion 8 are arranged so as to be parallel to the edge lines A in the embodiment shown in that figure. The volume V* of the notch portion 8 is set to about 1% - 6% with respect to the suction volume V max of one groove, i.e., the groove-like space, defined by the rotors 6, 6' and the casing 7. The various configurations of the notch portion 8 are shown in Figs. 5(a) to 5(c), as viewed from the inner side of the casing 7. Referring to these drawings, there is shown a notch portion 8A having edge lines A and B whose forward ends coincide with each other (Fig. 5a), a notch portion 8B having the edge lines A and B whose rear ends similarly coincide with each other (Fig. 5b), and a notch portion 8C having arcuate rear edge lines B which are in contact with the seal lines C, C' and whose both ends coincide with those of the forward edge lines A (Fig. 5c). As an alternative there is a notch portion provided on the side of the male rotor 6 alone. On the other hand, in a drawing of the inner surface of the casing 7 and a development diagram of the same respectively shown in Figs. 6 and 7, seal lines C formed by the lines at the tops of the rotors and the inner surface of the casing 7 are arranged to be two or more respectively with regard to the male rotor 6 and the female rotor 6'. The position and configuration of the rear edge line B of the notch portion is determined considering the following matter. A change in the volume V of one groove corresponding to the rotational angle of the male rotor 6 is shown in Fig. 8. The left half of the drawing designates a suction process and the right half of the same designates a compression process. The peak of the volume occurs at a joint point of both the processes. The volume at the joint portion is the suction volume V max of the one groove. A discharge of a gas to the discharge opening portion 9 from a confined groove starts from the fixed point B at the rear edge lines B of the notch portion 8, and a complete discharge starts from the fixed point A at the forward edge lines A of the discharge opening 9, and the discharge is completed at a position a shown in Fig. 7. Assuming that the volume ratio ViB at the fixed point B is V max /VB and the volume ratio ViA at the fixed point A is V max /VA, the position of the fixed point B is determined in such a manner that the volume ratio ViB/ViA may be within a range of 70% - 90%. A function of the notch portion 8 will be specifically explained here. Gas in the groove-like space is gradually discharged through the taperingly-cut notch portion 8 to the discharge opening portion 9 (or gas contraflows from the discharge opening portion 9 to the groove-like space), and the flow of the gas is less rapid, in comparison with a case where the notch portion 8 is not provided, whereby a variation range of the pressure (the amplitude of pulsation of the pressure) in the discharge opening portion 9 which results from the discharge flow is decreased and the noise level is lowered. Also, a loss of pressure caused by a rapid flow is reduced to thereby widen the preferable range of performance of the compressor. Propagation of the variation in the pressure from the discharge side to the suction side is decreased by providing at least two seal lines C, so that the noise level at the suction opening portion 9 is lowered. Next, experimental results of an embodiment in accordance with the present invention will be described with reference to Figs. 9 to 13. If the discharge pressure during actual operation, i.e., pressure for use is referred to as P2, the suction pressure at the actual operation is as P1, the pressure ratio π is P2/P1, and the built-in pressure ratio πi is (Vi) n (n: politropic index), it is obviously understood that the compression is excessive when π < πi and the compression is insufficient when π > πi. Figs. 9(a) and 9(b) show actual variations in the pressure at the discharge opening when π < πi and π > πi, respectively. In these figures, the conventional art wherein no notch portion is provided is illustrated with a broken line P O , and the same in the invention wherein a notch portion is provided is illustrated with a continuous line P, respectively. It is clearly understood from those figures that the pressure variation of the present invention is smaller than that of the conventional art in any cases. Referring to Fig. 10, a relative noise level in the present invention including the notch portion is shown with a continuous line S, whereas the same in the conventional art without the notch portion is illustrated with a broken line S O . In this figure, the minimum noise of the conventional art is set at level zero. From Fig. 10, it will also be understood that the relative noise level according to the invention is smaller than the same of the conventional art. Figs. 11(a) and 11(b) show relationships between a reduction in the noise level, a variation rate of efficiency and a ratio of the volume of the notch portion to the suction volume of one groove (V*/V max ) when π < πi and π > πi, respectively. In any case, it is recognized that the value of V*/V max within 1% - 6%, i.e. range R1, is the most effective for practical use. Fig. 12 shows experimental results of another test for the embodiment of the invention. In this test, it is confirmed that the value of a ratio (V*/VA) of the volume of the notch portion to the volume of the one groove at the fixed point A (Fig. 8), that is, the volume when the gas starts to be discharged from the one groove provided that the notch portion is not formed, within about 1% - 8% i.e. range R2 is the most effective for practical use in view of both a reduction in the noise level and a variation in the rate of efficiency. In the alternatives, Fig. 13 illustrates experimental results of a still further test for the embodiment according to the invention. In this test, it is confirmed that the value of a ratio [ V*/(VB - VA) ] of the volume of the notch portion to a difference between the volumes at the fixed points A and B, that is, the decreasing amount of the volume of the one groove between the position where the gas starts to gradually be discharged through the notch portion and the position where the gas starts to be discharged when no notch portion is provided, within about 5% - 50% i.e. range R3 is the most effective for practical use in view of both a reduction in the noise level and a variation in the rate of efficiency. Since the present invention is arranged in the above-described manner, the rapid flow of the gas at the discharge opening portion is reduced by the notch portion which simple and is readily formed, whereby the variation range of the pressure (the amplitude of pulsation of the pressure) is decreased and, therefore, the noise level is lowered. Further, a loss of pressure resulting from the rapid flow of the gas is restricted to thereby widen the preferable range of performance of the compressor.

Claims: A screw compressor comprising a casing (7), rotors (6, 6') rotatably supported within said casing (7), wherein at least one groove-like space is defined between said rotors (6, 6') and said casing (7), a suction opening portion (5) formed in said casing (7) at one end portion thereof and a discharge opening portion (9) formed at the other end portion of said casing (7) in a V-shaped configuration, wherein said discharge opening portion (9) is provided with a notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) so that compressed gas is gradually discharged beginning from a predetermined start line (B) where a volume ratio (ViB; V max /VB) of said groove-like space is smaller than a built-in volume ratio (ViA; V max /VA) of said groove-like space at an end line (A) where said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) ends, characterized in that said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) has a generally tapered cross section and extends continuously over the whole length of the V-shaped configuration of said discharge opening portion (9) wherein said start and end lines (A, B) of said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) coincide with each other at least at one end thereof. A screw compressor according to claim 1, characterized in that a volume (V*) of said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) is set to a value of about 1% to 6% with respect to a maximal suction volume (V max ) of said groove-like space. A screw compressor according to any of claims 1 or 2, characterized in that a volume (V*) of said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) is set to a value of about 1% to 8% with respect to a volume (VA) of said groove-like space at said end line (A). A screw compressor according to any of claims 1 to 3, characterized in that a volume (V*) of said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) is set to a value of about 5% to 50% with respect to the decrease in the amount of a volume of said groove-like space between said start line (B) where the gas gradually starts to be discharged through said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) and said end line (A) where the gas starts to be discharged where no notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) is provided. A screw compressor according to any of claims 1 to 4, characterized in that said predetermined start line (B) is selected at a location where a ratio of said volume ratio (ViB; V max /VB) to said built-in ratio (ViA; V max /VA) is within the range of 70 to 90%. A screw compressor according to any of claims 1 to 5, characterized in that the front ends of said start and end lines (A, B) coincide with each other. A screw compressor according to any of claims 1 to 5, characterized in that the rear ends of said start and end lines (A, B) coincide with each other. A screw compressor according to any of claims 1 to 5, characterized in that the front ends and rear ends of said start and end lines (A, B) coincide with each other, wherein the end line (B) is formed as an acuate edge line. A screw compressor according to any of claims 1 to 8, characterized in that said notch portion (8, 8a, 8b, 8c, 8A, 8B, 8C) is provided only on the side of one of the rotors (6).

Inventors: MAEDA MITSURU, MARUTA YOSHIYUKI, SASE TOSHITSUGU, YANAGISAWA KIYOSHI, MAEDA, MITSURU, MARUTA, YOSHIYUKI, SASE, TOSHITSUGU, YANAGISAWA, KIYOSHI
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1989
Main classification: F04C18
IPC codes: F04C18, F04C29