GB2131877A - Rotary positive-displacement fluid-machines - Google Patents

Abstract

In a machine of the worm-and- wormwheel type that can be used as a compressor, an expander, or a pump depressions, or "cells", 15 are formed in both an annular surface 16 and the crest surfaces 25 of the helical lands, or "threads", 4 of a screw rotor 1 meshing with a pinion-wheel(s) 6. Portions of the lands between the cells lie on circles centred on the rotor axis. Alternatively, the cells are formed in the surface of the casing 3 co-operating with the rotor. These provisions permit a reduction in the radial clearance between the casing and the rotor. Instead of being generally cylindrical (as shown) the rotor may be of the disc, or "flat", type, Fig. 7 (not shown). The working fluid may be a refrigerant. <IMAGE>

Description

SPECIFICATION Avolumetric machine with screw and pinionwheel(s) This invention relates to a volumetric machine with screw and pinion-wheels for compressing, pumping or expanding a fluid.

The machines which are primarily contemplated in this specification comprise a screw adapted to cooperate with a casing in substantiallyfluid-tight manner by means of at least part of the screw-thread crests, at least one pinion-wheel which is disposed in a passageway within the casing and the teeth of which are adapted to engagewiththe screw th reads, at least one low-pressure port located at one end of the screw and at least one high-pressure port located at the other end of the screw and sepa rated from the pinion-wheel passageway by a casing portion of predetermined width.

Machines of this type are described in particular in U.S. patents No 3,133,695, No 3,180,565 and No 3,551,082.

In orderto achieve enhanced efficiency and to reduce leakages in these machines, it is a known practiceto inject into the machine a liquid (usually oil) which has a cooling function (when the machine works as a compressor) and which also serves to ensure fluid-tightness by forming a true liquid seal.

This has been described in particular in U.S. patent No 3,133,695 cited above.

This sealing function is no longer possible when injection of liquid is suppressed. In the field of application of refrigerating compressors, for example, it is only necessary to injectthe condensate ofthe liquefied gas into the compression chamber in order to cause vaporization of the condensate within the chamber and cooling of the different parts as well as the gas. As a consequence, there is no longerany available liquid for sealing-offthe clearances between the crests of the threads and the casing. However, the size of these clearances has a decisive influence on the efficiencyofthe machine.

Byway of example, it has accordingly been found in the case of a compressor operating with Refrigerant 22 and having a swept volume of approximately 2500 liters at 3000 rpm thatthe thermodynamic efficiency of the compressor decreases by approximately 1 % each time the clearance between screw and casing in creases by 10 microns.

Attempts have been made to reduce this clearance but all tests on radial clearances of less than approx- imately 70to 100 microns invariably result in seizure of the screw within the casing to an extent which is usually permanent, with the result that the two parts cannot be separated without destruction.

The object of the present invention is to provide a volumetric machine in which the clearances are of sufficiently small value to maintain high efficiency while atthesametime removing any potential danger of seizure.

In accordance with the invention, the machine corresponds to the specification cited in the foregoing and is distinguished bythe fact that one ofthe two cooperating surfaces respectively ofthe casing and of the screw is provided with a number of cells at least on part of the area of cooperation of said surface with the other surface, the lands comprising portions directed substantially circumferentially, some at least ofthe latter portions being of limited circumferential length and being to this effect limited by recesses at least one oftheircircumferential ends, and by the fact that the periphery of each cell carried by a thread crest or carried by the casing in a position to cooperate with thethread crests is atthe most cut by only one ofthe two edges of a thread crest carrying orfacing said cell.

It has been found that cells of the aforementioned type make it possible to reduce the screw-casing clearance to values of the order of 20 to 30 microns without inducing any seizure.

One explanation which can be given byway of indication without thereby implying any limitation of the scope of the invention is that seizure can be caused by impurities which have remained within the machine. Such impurities result in micro-seizure which is localized but has a tendency to spread to areas in which the thread crests are of substantial width and particularly to the high-pressure end since the seizure particle or chip undergoes a displacement in rolling motion without finding any escape route.

The presence of the cel Is serves to check spontaneous increase in size ofthe seizure chip which is thus being formed since itfalls into one ofthe cells, is thus prevented from rolling further and is subsequently removed.

If a seizure chip tends to form between a substantial lycircumferential land portion of limited length as defined hereabove, the chip also rapidly falls at one end of the circumferential land portion.

Preferably the recess limiting the length of each of at least some ofthe circumferential land portions is a cell. Some ofthe circumferential land portions may be limited bya cell at each oftheircircumferential ends.

If cells are carried by the screw, some of the circumferential land portions may be limited bya cell at one of their both circumferential ends and by a threadedgeattheirothercircumferential end.

In a preferred embodiment, any plane perpendicularto the axis of the screw and meeting the celled surface alternately cuts the surface ofthe lands and the bottom surfaces of cells or of recesses along its line of intersection with the celled surface.

Thus, the lands are nowhere continuously disposed on circles centred on the axis of rotation and any chip formed between the lands and the casing rapidly falls by rolling into a cell, or into a recess limiting a circumferential land portion, and is then stopped.

The depth ofthe cells is advantageously less than 0.2 mm in order to minimize any possible leakageflow bridge effects. In principle, the cells can be machined either on the casing or on the screw.

Shouldthecells be formed on the screw, the dimension of the cells in the direction oftheir displacement is preferably smallerthan the width of the casing zone located between the high-pressure portandthe pinion-wheel passageway in orderto guard againsttheformation of a leakage-flow bridge between said high-pressure port and the pinion-wheel passageway which is exposed to low pressure.

Other distinctive features and advantages ofthe invention will be more apparent upon consideration of thefollowing description and accompanying drawings, wherein: - Fig. is a longitudinal view of a compressor in accordance with the invention in which one halfcasing has been removed, this view being taken along line I-I offig. 2; - Fig. 2 is a plan view taken along line ll-ll of fig. 1; - Fig. 3 is a viewto a larger scale showing part of fig. 1; - Figs. 4 and 5 are sectional views respectively along lines IV-IV and V-V offig. 3; Fig.6 is a viewtaken aiong lineVl-Vloffig.2;; - Fig. 7 is a part-sectional plan view of another type of compressor in accordance with the invention; - Fig. 8 is an enlarged plan view of some cells; - Figs. 9,11 are views similarto fig. 3 but showing alternative embodiments; and -Figs. 10,12 are views similartofig. 8 but concerning the embodiments offig. 9 and 11 respectively.

Referring to Figs. 1 and 2, the compressor in accordance with the invention comprises a screw 1 rotatably mounted in bearings 2 within a casing 3 which is made up oftwo parts 3a, 3b. Said screw is adapted to cooperate with said casing in relatively fluid-tight manner by means of the crests 25 ofthe screw th reads 4.

A motor (not shown in the drawings) is coupled with the screw 1 in order to causethis latterto rotate in the direction ofthe arrow.

The threads 4 of the screw 1 are disposed in meshing engagement with the teeth 5 of pinionwheels 6 which are freely rotatable in bearings 7 fixed respectively on the half-casings 3a and 3b.

The pinion-wheels 6 are located within cavities 8 of the casing and project into a cylinder9 to a partial extent through passageways 11, said cylinder being used as a housing for the screw 1.

As shown in Fig. 6, a triangular port 12, one side of which is substantially parallel to the slope of the threads 4, is provided in the cylinder 9 and adapted to communicate with a discharge port 13.

During operation,thescrew 1 transmits motion to the pinion-wheels 6 which rotate in the direction of the arrows; each tooth 5 which comes into mesh with the screwtraps a predetermined quantity of gas which is admitted through the intake 14 into the space formed between the casing, the two threads ofthe screw with which said tooth is in mesh, and the face of said tooth.

The volume of the space aforementioned decreases progressively, thus compressing the gas which is trapped within said space. Dischargetakes place when the space comes into position opposite to the port 12.

Cells 15 are provided on the outer surface of the screw (as shown in Fig. 3) and essentially in that part of said screw in which theth read crests have the greatest width or in other words over a distance corresponding practically to the lower third and upper third of the length of the screw.

Said cells are formed not only on the crests 25 ofthe threads, but also on the circular strip 16 which is located between the end of the threads (on the high-pressure side) and the end of the screw.

The cells are recessesformed in the surface and separated from each other by lands 19.

In the example of Figures 1-6 and 8,the cells 15 are rectangular (nearly square) and are disposed in axial rows 31 a, 31b, 31 cand soon (Fig. 8) separated by lands 19 which are axially aligned.

Any two neighbouredcellsofa row are separated from each other by a land 19cwhich is directed circumferentially. Moreover, as far as arrangement of the cells in each row is concerned, each row (say 31 b) is shifted longitudinally of the length of one half cell with respectto both neighboured rows (31a,31c).

Accordingly, each land 19cis limited by cells ofthe neighboured rows at its both circumferential ends.

However, it will be seen from Fig. 3thatthe lands 19cadjacent a thread edge 21 or 22 are limited by said edge at one of their circumferential ends.

These features ensure that any plane 18 perpendicu larto the a2ís ofthe screw and meetingthecelled surface, alternately encounters lands 19e or 19a and cells 15 along its intersection line with the celled surface.

It is also essential to ensure that each cell is completely inscribed withinthethread crest on which it is formed or that said cell is cut by only one ofthetwo edges 21,22 of said crest in orderto preventformation of any leakage-flow bridge between the two sides of the thread. In the example herein described, which relates to a screw 140 mm in diameter, this condition is satisfied by adopting a pitch p of 2 mm.

In this example,the clearance J (as shown in Figs. 4 and 5) between the screw and the casing is very small, namely ofthe order of a few tens of microns. The depth of the cells is ofthe order of 0.1 to 0.15 mm and thetichness eofthe lands 19 is ofthe order of 0.5 mm.

The view of Fig. 6 showsthe cylindrical wall 9 ofthe casing which cooperates with the screw, in the region corresponding to the discharge port 12 and the pinion-wheel passageway 11.The screw cells 15 which pass in front ofthis portion ofthe casing are shown in chain-dotted lines. The operating principle ofthe compressor is such that the width Lofthe zone 23 between said two ports must have a minimum value, thereby ensuring that no volume of gaz is liable to be trapped within the screw thread and prevented from escaping. Furthermore, in order to prevent formation of a leakage-flow bridge between the port 12 which is at high pressure and the passageway 11 which is at low pressure, it is necessary to ensu rethat the dimension lox the cells in the direction F oftheir displacement is smallerthan the width L.

In order to avoid the need to machine very small cells since this would be a costly operation, a relatively substantial width L has been maintained over the greater part ofthe zone 23 whereas the port 12 has been enlarged by means of a groove 24 which extends over part of said zone. Any leakage which may subsequently develop is thus highly localized. Furthermore, such leakage is reduced to a very low value by the small depth ofthe cells, which also has the effect of reducing to a negligiblevaluethevolumeofhigh- pressure gas which is transferred by the cell as it undergoes rotational displacementfrom the highpressure porttothe pinion-wheel passageway.

The cell which has been described and illustrated in the accompanying drawings has the shape of a quadrilateral but, as will be seen hereinafter, other shapes such as circles or polygons, for example, can produce equivalent results. A quadrilateral is particularly easyto produce when the cells are formed by the electrical discharge machining process (EDM) since it is possible in this process to form the EDM electrode by simple operations of milling.

In another alternative embodiment, not shown, the lands 19a may be inclined with respect to the axial direction and/orthe lands 19e may be slightiy inclined with respect to the circumferential direction.

Profiles of greater complexity can beformed by means of other methods such as knurling or eec- trochemical deposition of a filler metal for building-up the lands, in which case the cells are obtained by preliminary deposition of a mask made of varnish, for example, which is subsequently removed.

The use of the cells has made it possible to provide verysmall clearance ofthe order ofafewtens of microns without giving rise to seizure, even when the screw and the casing are formed ofthesame metal such as cast-iron, for example.

Nowthis advantage is an essential condition forthe successful construction of air-conditioning and refrigeration compressors of the sing le-screw type without oil injection (with all the attendant advantages of lower cost and non-pollution of circuits due to suppression of oil) while achieving thermodynamic efficiencies which place this machine in the highest rank fro an efficiency standpoint.Thus in the case of a compressor equipped with a screw 140 mm in diameter which is rotatably mounted in a casing with a radial clearance of 30 microns at 3000 rpm, measure mentstakenwith Refrigerant 22 and with compression ratios of the order of 3 have shown isentropic efficiencies of the order of 77% which are on average 10to20% higherthanthebestcomparablemachines of known type and of similar swept volume.

With clearances of the order of 0.1 mm which have been adopted uptothe presenttime, efficiencies do not exceed 70%.

Referring now to Figs. 9-12, different embodiments of cells have been shown, which all meet the condition that lands 19c, directed circumferentially or substantially circumferentially are limited by cells (315,515 a, 515 c) at their both circumferential or respectively substantially circumferential ends, except adjacent theedges21 or 22 wherethe lands 19e are limited at one end by a cell and at the other end by one of said edges21 or22.

The embodiments of Figures 9-12 also all meetthe condition that any plane 18 perpendicularto the axis ofthe screw alternately meets cells and lands along its line of intersection with the surface provided with cells. This is ascertained by Fig. 10, 12, showing planes 18 in all the main possible positions in each case.

In Figs. Sand 10 the cells are circular, and as in Figure 1-6, they are disposed in axial rows each of which is shifted of the axial length of a half-cell with respect to both neighboured rows.

In Figs. 11 and 12, the cells are rectangular and are of two types having different lengths as measured parallel to the axis ofthe screw. The centres ofthe cells are aligned in axial direction and in circumferential direction. In both directions, there is alternately one large cell and one small cell.

In accordance with an alternative embodiment of the invention, the cells are formed in the casing wall which cooperates with the crest ofthe screwth reads.

Such an arrangement is particularly advantageous when the shape ofthe casing is suited to this form of construction as is the case, for example, with a flat or conical casing which readily permits the approach of an EDM electrode.

With reference to Fig. 7, a plane screw 101 is rotatably mounted within a casing 103 provided with a low-pressure intake 114. The use of the term "plane screw" is explained by the fact that the crests ofthe threads 104 are located in the same plane and cooperate with a flat portion ofthe casing.A pinionwheel (not shown in the drawings) is located in a plane at right angles to the plane ofthe screw and the pinion teeth mesh with the thread groove which is shaped accordingly. Acompressor of this type is described in greater detail in U.S. patent nO 3,180,565.

The clearance between the crests of the threads and the casing is of the same order as in the previous embodiment and the cooperating surface ofthe casing is provided with cells 115 limited by circumferential lands and by lands transverse, and more precisely perpendicularto the circumferential lands.

Thecircumferential landsarenotdisposedcon- tinuously on circles, butform portions of limited length, limited at their both ends by cells ofthe neighboured transverse rows. The cells are more precisely ofthe kind of Figures 1-6 and 8 but the kind of Figures 9-10 orl 1-12 orother kinds in accordancewith the invention could as well have been choosen.

In this embodiment, it is importantto ensure that the depth of the cells is reduced to a minimumandto ensure if possible that said depth does not exceed valuesoftheorderof0.10to 0.15 mm in the case of machines having a swept volume ofthe order of one litre per revolution.

Itshould infact be borne in mind that, during the displacement of the screw threads, each cell is filled with gas under pressure and the gasthen expands as the following thread arrives. The work output thus produced to no useful purpose is liable to attain considerable values in a very short time, thus removing all the benefit gained by the cells.

While the arrangement in accordance with the invention is advantageous in the case of machines without sealing liquid, said arrangement is also of considerableinterestwhenthesealing liquid (in a compressororexpansion machine) hasverylow viscosity, for example when oil is replaced by water which constitutes a seal of lower resistance, or in the event of utilization of the machine as a pump or hydraulic motorwhen the liquid employed also has low viscosity.

As will be readily appa rent, the invention is not limited to the examples hereinbefore described but extends to any alternative form or any application within the capacityofthose versed inthe art.

Claims (17)

1. Avolumetric machineforcompressing, pumping or expanding a fluid, comprising a screw adapted to cooperate with a casing in substantially fluid-tight manner by means of at least part of the screw-thread crests, at least one pinion-wheel which is disposed in a passageway within the casing and the teeth of which are adapted to engage with the screw threads, at least one low-pressure port located atone end ofthe screw and at least one high-pressure port located at the other end ofthescrewand separated from the pinion-wheel passageway by a casing zone of predetermined width, wherein one ofthe two cooperating surfaces respectively of the casing and ofthe screw is provided with a number of cells at least on part of the area of cooperation of said surface with the other, the lands comprising portions directed substantially circumferentially, some at least ofthe latter portions being of limited circumferential length and being to this effect limited by recesses at one at least oftheir circumferen- tial ends, and wherein the periphery of each cell carried by a thread crest or by the casing in a position to cooperate with the thread crests is at most cut by only one ofthe two edges of a thread crest carrying or facing said cell.

2. A machine according to claim 1, wherein the recesses are some ofthe cells.

3. A machine according to claim 2, wherein some ofthe circumferential land portions are limited by a cell at each of their both circumferential ends.

4. A machine according to claim 2, wherein the cells are carried by the screw and wherein some ofthe circumferential land portions are limited by a cell at one of their both circumferential ends and by a thread edge at their other circumferential end.

5. A machine according to one of claims 1 to 4, wherein any plane perpendicular to the axis of the screw and meeting the celled surface alternately cuts the suface ofthe lands and the bottom surfaces of cells or recesses along its line of intersection with said surface.

6. A machine according to one of claims 1 to 5, wherein the cells are arranged in rows transverse to the circumferential direction, each row being shifted longitudinally with respect to both neighboured rows.

7. A machine according to one of claims 1 to 5, wherein the cells are of two types having different axial lengths and their centres are aligned in substan- tially circumferential rows and in rows transverse to the circumferential rows, the cells being alternately of the large and the small type along both the axial and the transverse rows.

8. A machine according to one of claims 1 to 7, wherein the depth ofthe cells is less than 0.2 mm.

9. A machine according to one ofthe claims 1 to 8, wherein the cells are formed on the casing.

10. A machine according to claims 1 to 8, wherein the cells are formed on the screw.

11. A machine according to claim 10, wherein the dimension of the cells in the direction oftheir displacement is smallerthan the width of the casing zone located between the high-pressure port and the pinion-wheel passageway.

12. A machine according to claim 11, wherein the high-pressure port is enlarged by a groove extending over part of the zone located between the highpressure port and the pinion-wheel passageway.

13. Avolumetric machine, substantially as hereinefore described with reference to, and as shown in, Figures 1 to 6 and 8 ofthe accompanying drawings

14. Avolumetric machine, substantially as herein before described with reference to, and as shown in, Figures 7 and 8 ofthe accompanying drawings.

15. Avolumetric machine, substantially as hereinbefore described with reference to, and as shown in, Figures 9 and 10 of the accompanying drawings.

16. Avolumetric machine, substantially as hereinbefore described with reference to, and as shown in, Figures 11 and 12 ofthe accompanying drawings.

17. Any novel feature or combination offeatures described herein.