Globoid-worm Machines For Varying The Pressure Of A Fluid

Abstract

The invention deals with a globoid-worm machine such as a compressor or expansion machine for varying the pressure of a fluid, which comprises a globoid worm having a plurality of threads in meshing relation with at least one transverse pinion and rotatably mounted within a stationary casing which is applied in leak-tight manner against at least part of the external surface of the worm and which has main ports for connecting the interior of said casing respectively to a fluid admission chamber and to a fluid discharge chamber. The casing is provided with a plurality of auxiliary ports which connect the interior of said casing to said discharge chamber and are fitted with movable means for leak-tight closure.

Description

This invention relates to globoid-worm machines such as compressors or expansion machines which are intended to produce a variation in the pressure of a fluid.

It is known that these machines comprise a globoid worm having a number of threads in meshing relation with at least one transverse pinion and rotatably mounted within a stationary casing which is applied in leak-tight manner against at least part of the peripheral surface of the worm. The casing is provided with ports for connecting the interior of this latter respectively to an admission chamber and to a discharge chamber. The teeth of the pinion can be located either on a flat surface or on a curved surface of revolution as described in French patent Application of Feb. 8, 1968 in the name of Bernard Zimmern in respect of "Improvements to globoid-worm compressors."

In these machines, the compression or expansion ratio is determined by the geometrical characteristics of the machine and especially by the number of threads of the worm which are simultaneously in mesh with the teeth of the pinion. Said ratio is therefore invariable in respect of a given machine. The fact that it proves impossible to vary this ratio is attended by disadvantages in numerous applications.

Thus, in the case in which the machine is employed as a vacuum pump for drawing air from a chamber which is initially at atmospheric pressure and discharging said air to the atmosphere, the pressure within the admission chamber is equal to the atmospheric pressure at the beginning of the pumping operation and progressively decreases under the action of the machine. Thus, during the initial stage of operation, the machine is caused by its fixed compression ratio to compress the gas to a pressure which is higher than atmospheric and then to discharge the gas to the atmosphere, unnecessary work being consequently done. Under these conditions, the power rating of the motor which drives the machine must be between 50 and 100 percent higher than the power rating which would be necessary if the gas were not compressed to a pressure above atmospheric.

Similarly, the compressors for refrigeration units must discharge the fluid at pressures which vary according to the temperature difference between the condenser and the evaporator. A machine having a fixed compression ratio consequently has low thermodynamic efficiency as soon as the environmental conditions deviate from the theoretical conditions for which it has been designed.

Finally, air compressors which are required to provide a variable delivery are employed in industry and especially in public works. During more or less long periods of time, the quantity delivered must be zero even if the drive motor of the compressor is not stopped. It is then customary during these periods to short-circuit the compressor on itself, which means that the air at the output is fed back to the suction side after expansion. However, by reason of the invariable compression ratio, the air is compressed each time it passes through the machine and this results in an unnecessary consumption of power which can attain one-half the power on full load as well as heating of the air which makes it necessary to provide means for removing the heat produced.

In the case of machines of the Lysholm type with parallel rotors, it has already been proposed to vary the size of the discharge port in order to adjust the compression ratio. However, the range of variation which can be achieved in practice is very limited and automatic adjustment can be obtained only by making use of control systems. These costly and complex systems are not to be compared with the simplicity of piston-type compressors in which adjustment of the compression produced to the discharge pressure is obtained very simply by means of the operation of the discharge valves.

It has also been proposed to vary the compression ratio of machines in which leak-tightness between worm and casing is ensured by means of an injection of liquid, a variation being produced in the flow rate of injected liquid. However, only a relatively small range of variation is again obtained in this case.

The present invention has for its object to overcome the disadvantages mentioned in the foregoing. The primary aim of the invention is to permit a variation in the compression or expansion ratio of a globoid-worm machine over a wide range from a zero value to its maximum value and either automatically or under direct control, this being achieved by having recourse solely to simple and rugged means. The invention thus makes it possible to endow globoid-worm machines with the same possibilities of variable compression as piston-type machines by adopting means which are as simple as those employed in these latter.

In accordance with the invention, the globoid-worm machine such as a compressor or expansion machine for varying the pressure of a fluid, which comprises a globoid worm having a plurality of threads in meshing relation with at least one transverse pinion and rotatably mounted within a casing which is applied in leak-tight manner against at least part of the external surface of the worm and which has main ports for connecting the interior of said casing respectively to a fluid admission chamber and to a fluid discharge chamber, is characterized in that the casing is provided with a plurality of auxiliary ports which connect the interior of said casing to the discharge chamber and are fitted with movable means for leak-tight closure.

In a preferred embodiment of the invention, the line of intersection of the auxiliary ports with the internal face of the casing is substantially inscribed within the crest surface of the worm threads.

The compression ratio can thus be varied by producing action on the means for closing-off the auxiliary ports. Moreover, even if the end of the closing-off means is not level with the internal face of the casing or does not conform exactly to the shape of this latter, there is no communication between two adjacent compression or expansion chambers since the worm thread which separates said chambers closes-off the port as it passes in front of this latter.

In an advantageous embodiment of the invention, the auxiliary ports open into the interior of the casing in the vicinity of the discharge side in that zone in which the worm threads have a crest surface of maximum width.

The auxiliary ports can thus be given sufficient dimensions so as to permit of high delivery without any appreciable pressure drop while preventing any communication between adjacent worm-thread chambers.

In a preferred version of the invention, the means for closing-off said auxiliary ports are valves arranged so as to be maintained in the position of closure by the pressure of the discharge chamber or by the pressure of the expansion chambers according as the machine is either a compressor or an expansion machine and by ancillary restoring means such as springs.

The maximum compression pressure or minimum expansion pressure within the machine can thus be adjusted automatically as a function of the discharge pressure and just as simply as in a piston-type machine. Opening of the valves can also be controlled at will, depending on the application which is contemplated.

Further properties of the invention will become apparent from the detailed description which follows hereinafter.

A number of embodiments of the invention are illustrated in the accompanying drawings which are given by way of non-limitative example and in which:

FIG. 1 is a plan view taken along line I--I of FIG. 2 and showing a compressor in accordance with the invention;

FIG. 2 is a sectional view taken along line II--II of FIG. 1;

FIG. 3 is a part-sectional view showing an auxiliary port of the compressor of FIG. 1;

FIG. 4 is a part-sectional view showing a second embodiment of an auxiliary port of the same compressor;

FIG. 5 is a sectional view taken along line V--V of FIG. 4;

FIG. 6 is a view in elevation of the globoid worm of the compressor of FIG. 1 and shows the position of an auxiliary port;

FIG. 7 is a partial view showing another embodiment of an auxiliary port;

FIG. 8 is a developed diagrammatic view of the globoid worm showing a particular arrangement of the auxiliary ports;

FIG. 9 is a diagram of a compressor in accordance with the invention with a device for short-circuiting the compressor during periods of zero delivery .

There is illustrated in FIGS. 1 and 2 a compressor consisting in a manner known per se of a globoid worm as represented diagrammatically at 1 and having a cylindrical external profile. The worm 1 is mounted on a shaft 2 and cooperates with pinions 3 which are two in the illustrative embodiment shown in the drawings. Said worm is mounted within a casing 4, the internal face of which is applied against the external surface of the threads of the worm 1. The casing 4 is adapted to carry end-plates 5 and 6 on which are mounted bearings 7 and 8 which support the worm shaft. The end-plate 5 is provided with ports 9 which connect the internal volume of the casing to a suction chamber 11 which is fitted with a duct 12 for the admission of fluid to be compressed. The casing 4 is provided with ports 13 of substantially triangular section which open into the interior of the casing in the vicinity of the pinions 3. Said ports 13 are connected to passageways 14 which are substantially parallel to the axis of the worm 1 and open into a fluid discharge chamber 15. The chamber 15 is delimited by a cover 17 provided with a fluid outlet 16 and fitted with seals 18.

Provision is made in accordance with the invention for auxiliary ports 19 which are pierced in the casing so as to extend in a direction parallel to the axis of the worm 1 and are angularly spaced in accordance with a predetermined law which depends on the particular application considered. Said ports open into the discharge chamber 15 and are also connected to radial ducts 21 which open into the interior of the casing 4.

Movable means for closing-off the ducts 21 in leak-tight manner are mounted within the auxiliary ports 19. In the embodiment which is illustrated in FIG. 3, said closing-off means are constituted by valves 22 slidably mounted with a very small clearance within the ports 19. Said valves are each pierced by an axial bore 23 and have a terminal cone 24 which is brought to bear on a conical seat 25 formed at the top of the port 19. Each valve 22 is applied against its seat by a spring 26.

In a preferred embodiment (shown in FIGS. 6 and 7), the ducts 21 are so dimensioned that the line of intersection 33 of said ducts with the internal face of the casing 4 can be inscribed within the surface of the crest 34 of a worm thread. In particular, in the case of a globoid worm having thread crests located on a surface having a generator-line which is a straight line, the ducts 21 open into the interior of the casing in the vicinity of the discharge side in that zone in which the worm threads have a crest surface area of maximum value. Worms of this type can have either a cylindrical, conical or flat external profile and have been described in French patent No. 1,331,998 of May 8, 1962 as filed in the name of the present Applicant. In these worms, the width of surface of the thread crests increases towards the ends of said threads and has a maximum value at the point at which the height of the threads becomes zero. It is in this zone of maximum width of the thread-crest surface that the ducts 21 have their openings.

The curve 23 of intersection of the ducts 21 with the internal face of the casing can be a circle (as shown in FIG. 6) or an ellipse (as shown in FIG. 7). It is readily apparent that other shapes such as a rectangle can also be contemplated.

The volume of the axial duct 21 which is comprised between the valve 22 and the internal face of the casing 4 is limited at a maximum to twenty per cent of the volume of a compression or expansion chamber which is delimited by two contiguous threads of the worm and is preferably chosen equal to a few per cent of the volume of this worm-thread chamber.

During operation, when the pressure within the worm-thread chamber which is located opposite to a duct 21 exceeds the pressure of the discharge chamber 15, the valve 22 is subjected to an axial thrust and thus displaced from its seat, with the result that the fluid of the worm-thread chamber escapes into the discharge chamber 15. The maximum compression pressure within the worm-thread chamber is thus limited automatically to the discharge pressure which prevails within the chamber 15. If the machine comprises a device which is known per se for injecting a sealing liquid between the casing and the worm, part of this liquid can also escape into the discharge chamber 15.

When one thread of the worm comes into position opposite to a duct 21, said thread completely masks the entrance of said duct, thereby preventing any internal leakage which could arise from a temporary communication between adjacent worm-thread chambers if the apparent contour of the duct 21 were not inscribed within the width of a thread crest.

It has also been found that, by providing an outlet for the ducts 21 in the zone of maximum width of the thread crests, said ducts could be given a sufficient cross-sectional area to permit of substantial delivery without any appreciable pressure drop. It is thus wholly ensured that the pressure within the worm-thread chamber is equalized with the pressure which exists in the discharge chamber.

When the valve 22 is closed, a certain quantity of gas remains trapped within the duct 21. This gas is compressed and then suddenly expands when the following worm-thread chamber comes into position in front of the duct 21. It is for this reason that the volume of said duct is limited as has been stated earlier.

There is shown in FIGS. 4 and 5 one arrangement of the valves which prevents intercommunication between worm-thread chambers even if the duct 21 is not completely masked as a worm thread passes in front of said duct. The valve 27 slides within the duct 21 with a very small clearance and is provided with an annular shoulder 28 which is applied against the internal wall of the port 19. The valve 27 carries a rear guide rod 29 which is slidably fitted in a bore 31 formed in the casing 4 and given a shape such that the valve 27 cannot rotate about its axis. A spring 32 applies the valve 27 against the wall and maintains this latter in the closed position. In this position, the front valve-face 33 (shown in FIG. 5) is flush with the internal face of the casing and has the same curvature as said face. Thus the cross-sectional area of the auxiliary ports can be so determined as to prevent any pressure drops and there cannot be any communication between worm-thread chambers since the end face of the valve 27 is located exactly in the line of extension of the internal face of the casing 4. However, in order to ensure good operation, the valves aforesaid entail the need for more accurate machining and more careful guiding than in the embodiment of FIG. 3.

The arrangement which is represented diagrammatically in FIG. 8 is applicable to the case of a vacuum pump or of the embodiment illustrated in FIG. 9. There is shown in a developed view that portion of a globoid worm 35 which is comprised between two adjacent pinions 36. The ports 37 or openings of the auxiliary ducts are spaced on the internal face of the casing at angular intervals which are substantially equal to the intervals determined by two consecutive threads 38 of the worm 35. Thus, the entire compression chamber which is closed-off by one tooth of a pinion 36 is in relation with one and only one port 37, with the result that the pressure within the worm-thread chambers is limited to the value of the discharge pressure, that is to say in practice to atmospheric pressure. Any unnecessary compression work is accordingly avoided.

The arrangement of FIG. 9 concerns a compressor of the variable delivery type, the operation of which involves periods of zero delivery. This is the case with air compressors which are employed in public works, for example. The compressor 39 of the type illustrated in FIGS. 1 and 2 is driven by a continuously-rotating motor 41. The discharge chamber of said compressor is connected to a utilization pipe 42 in which is fitted a check valve 43. Upstream of said valve, the pipe 42 is connected to a short-circuit pipe 44 which opens at the other end into the admission duct 45. A valve 46 actuated by a solenoid 47 is mounted in said auxiliary pipe and is closed during normal periods of use.

When the required delivery is zero, the valve 46 is opened so that the discharge side of the compressor is connected directly to the suction side. The pressure drop within the pipe 42 results in closure of the valve 43, thereby preventing the return of the gas under pressure. As explained earlier, the valves such as 22 open, with the result that the compressor delivers in short-circuit on itself without compressing the air. The power consumed is reduced to a value of less than 20 percent of the energy on full load instead of 40 percent to 50 percent in the usual design solutions. In point of fact, there is no need to overcome the mechanical resistance to motion and the very low resistance which is set up as the gas is circulated through the machine without compression. Provision can be made in the return pipe 44 for a heat exchanger (not illustrated) in order to remove the power referred-to above.

If said compressor operates with an injection of sealing liquid, means (not shown) can be provided for stopping the injection of liquid at the same time as the operation which consists in opening the valve 46.

It must be clearly understood that the invention is not limited to the embodiments which have just been described and that a number of alternative forms of execution of these latter can be contemplated without thereby departing from the broad purview of this invention. The machine can thus operate either as an expansion machine or as a motor, the valves being intended to open when the pressure within the expansion chambers falls below the low pressure at the discharge outlet. The loss of power which corresponds to the production of a relative vacuum within the worm-thread chambers is thus prevented. The globoid worm can have either a conical or flat external profile instead of being cylindrical whilst the pinions can be flat or cylindrical and preferably provided in this case with teeth of trapezoidal shape so as to increase the maximum crest area on the discharge side.

Claims

1. A globoid-worm machine such as a compressor or expansion machine for varying the pressure of a fluid, which comprises a globoid worm having a plurality of threads in meshing relation with at least one transverse pinion and rotatably mounted within a stationary casing which is applied in leak-tight manner against at least part of the external surface of the worm and which has main ports for connecting the interior of said casing respectively to a fluid admission chamber and to a fluid discharge chamber, wherein the casing is provided with a plurality of auxiliary ports which connect the interior of said casing to said discharge chamber

2. A machine according to claim 1, wherein the line of intersection of the auxiliary ports with the internal face of the casing is substantially

3. A machine according to claim 2, wherein the auxiliary ports open into the interior of the casing in the vicinity of the discharge side in that

4. A machine according to claim 1, wherein the means for closing-off the auxiliary ports are valves actuated as a result of the difference between the pressure existing within the chambers delimited by adjacent threads of the worm and the pressure existing within the discharge chamber, said

5. A machine according to claim 4 and intended to operate as a compressor, wherein the valves are arranged so as to be maintained in the closed

6. A machine according to claim 4 and intended to operate as an expansion machine, wherein the valves are arranged so as to be maintained in the closed position by the pressure of the expansion chambers which are

7. A machine according to claim 4, wherein the ancillary restoring means

8. A machine according to claim 4, wherein the residual volume comprised between the internal face of the casing and the end face of a valve is smaller than twenty per cent of the volume of a chamber which is delimited

9. A machine according to claim 4, wherein the end face of the valve is brought level with the internal face of the casing in the closed position

10. A machine according to claim 1, wherein the discharge ports and the auxiliary ports are spaced around the periphery of the internal face of the casing at angular intervals substantially equal to the angular

11. A machine according to claim 1, wherein the discharge chamber is connected to a utilization pipe fitted with a check valve and to an auxiliary pipe which is connected to the admission chamber through a

12. A machine according to claim 11, and comprising means for injecting a sealing liquid in the vicinity of the pinions, wherein said machine comprises means for stopping the injection of liquid and simultaneously opening the controlled valve.