Field

This disclosure is concerned with improvements relating to rotating screw machines, particularly but not exclusively to helical screw machines.

In the detailed description provided below, aspects of the present disclosure are described with particular reference to a helical twin-screw compressor. However, it will be appreciated by persons of ordinary skill in the art that the arrangements disclosed herein are equally applicable to other types of rotating screw machines, such as internally geared compressors, twin-screw pumps (including vacuum pumps, liquid pumps, and pumps for a mixture of liquids and solids), and internally geared pumps (including vacuum pumps, liquid pumps, and pumps for a mixture of liquids and solids).

In light of the foregoing, it should be remembered that the scope of the following disclosure should not be read as being limited solely to helical twin-screw compressors.

It has been noted that the regulation of the internal volume ratio ¹ (also known as the volume index) of rotating screw machines is important for efficiency and reliability of such machines.

If the internal volume ratio is correctly set then the internal pressure rise in the machine is matched with system pressures, and the machine tends to be efficient and reliable. If, on the other hand, the internal volume ratio is incorrectly set, then the machine either over-compresses or under-compresses. This over- or undercompression requires additional energy which increases the gas temperature inside the machine, leading to a reduction in efficiency and/or reliability.

It has previously been proposed to try and automatically regulate the internal volume ratio of rotating screw machines, but previously proposed arrangements have been overly complex and have tended to reduce the reliability of such machines.

Chinese Utility Model no. 204961294U discloses a device for automatically adjusting the internal volume ratio of a screw compressor in air conditioning equipment. The device comprises a plate with a plurality of valves that is provided in a platen groove formed in the exhaust seat of the screw compressor. One issue with the device disclosed in this Chinese Utility Model is that as it requires a groove to be formed in the exhaust seat of the screw compressor to accommodate the plate, the device cannot

¹ The internal volume ratio or volume index, Vi, is the ratio of the chamber volume after the suction port is closed to the chamber volume just before the discharge port is opened. readily be retrofitted to existing screw compressors (or, indeed, other rotating screw machines) without significantly modifying the exhaust seat. Another issue associated with the device disclosed in this Chinese Utility Model is that the groove formed in the exhaust seat which accommodates the device defines a discharge port that is always open, and as a result the maximum volume ratio is defined by the size of the discharge port. This means that the port has a relatively low volume index and if the pressure in the system to which the compressor is connected is high, the compressor will always tend to exhibit under-compression and a discharge loss as fluid will tend to be sucked back into the compressor.

Aspects of the disclosure provided herein have been devised with the foregoing in mind.

Summary

In accordance with a presently preferred implementation of the teachings of this disclosure, there is provided a discharge plate for a rotating screw machine, said rotating screw machine comprising: a rotor housing defining an internal cavity in which a plurality of rotatable screw rotors are located, and an outlet casing defining a discharge chamber, wherein: said discharge plate is configured to define a plurality of pressure balancing ports that each provide a passageway through the plate, and said discharge plate further comprises a plurality of valves coupled to a face of the plate, each said valve being moveable between a position where the valve closes one or more of the pressure balancing ports and a position where the valve opens one or more of the pressure balancing ports; wherein the ports and valves are arranged so that the valves can open into the discharge chamber to open one or more of said ports when the discharge plate is coupled between the rotor and outlet housings of the screw machine and thereby provide one or more passageways between the rotor housing and the discharge chamber that enable pressure balancing to occur between the internal cavity of the rotor housing and the discharge chamber; the discharge plate being configured so that when compression pockets between the rotors align with said pressure balancing ports, fluid can pass from the rotor housing to the discharge chamber only via one or more of said passageways.

An advantage of this arrangement, vis-a-vis the aforementioned Chinese Utility Model, is that it is not necessary to modify the exhaust seat of a screw compressor or other rotating screw machine. Another advantage is that by configuring the plate so that fluid can pass from the rotor housing to the discharge chamber only via one or more of the passageways when compression pockets between the rotors align with said pressure balancing ports, the above-described issues associated with undercompression can be avoided.

A yet further advantage of this arrangement over that disclosed in the aforementioned Chinese Utility Model is that as pressure balancing automatically occurs via the ports, it is no longer necessary to design a platen groove with an appropriately shaped discharge port.

The discharge plate may further comprise a shaped opening defining a passageway through the plate, the shaped opening having a size and shape that is configured to define a maximum allowable internal volume ratio in the screw machine. In this implementation the shaped opening enables any remaining fluid in the compression pockets to be evacuated once the compression pockets are no longer aligned with the pressure balancing ports.

The discharge plate may further comprise a plurality of circular openings, each configured to accommodate a rotor shaft of a said rotor of said plurality of rotors.

The discharge plate may comprise two circular openings, a first circular opening for accommodating the rotor shaft of a male screw rotor and a second circular opening for accommodating the rotor shaft of a female screw rotor, the male and females screw rotors being mechanically meshed together so that rotation of one screw rotor in a first direction causes the other to rotate in a second direction opposite to the first direction.

In one implementation the pressure balancing ports may comprise slots.

The discharge plate may comprise a first array of discharge slots associated with said first circular opening, and a second array of discharge slots associated with said second circular opening.

In one implementation the slots of each array may extend outwardly along notional radial lines emanating from the respective centres of each circular opening.

In one implementation the first array of slots may extend from a first radial position outside of the first circular opening to a second radial position further from the centre of said first circular opening than said first radial position.

In one implementation said first radial position may correspond - at least approximately - to a minimum diameter of the male screw rotor, and said second radial position may correspond - at least approximately - to a maximum diameter of the male screw rotor.

In one implementation the second array of slots may extend from a first radial position outside of the second circular opening to a second radial position further from the centre of said second circular opening than said first radial position.

The first radial position may correspond - at least approximately - to a minimum diameter of the female screw rotor and said second radial position may correspond - at least approximately - to a maximum diameter of the female screw rotor.

In one implementation each array of slots may comprise one or more spaced groups of slots, and each group comprises one or more slots.

In one arrangement, each valve may be associated with a group of slots and be configured to be operable to move to open all of the slots in that group. Each valve may comprise a reed valve.

In one implementation the discharge plate may comprise a first array of discharge ports associated with said first circular opening and a second array of discharge ports associated with said second circular opening.

The first array of ports may be regularly spaced around a portion of the circumference of the first circular opening and the second array of ports may be regularly spaced around a portion of the circumference of the second circular opening.

In one implementation each valve may be associated with a said port and may comprise a valve body movable to open or close the port with which it is associated. Each valve may lie within a said port when the port is closed.

In accordance with another presently preferred embodiment of the invention, there is provided a screw machine comprising a discharge plate of the type described herein, the screw machine comprising a rotor housing and an outlet housing, said discharge plate being provided between the rotor housing and the outlet housing and arranged so that the valves can open into a discharge chamber of said outlet housing in operation of said screw machine.

The screw machine may be selected from the group consisting of: a screw compressor, a helical twin-screw compressor, an internally geared compressors, twin- screw pumps, twin screw vacuum pumps, twin screw liquid pumps, twin screw pumps for a mixture of liquids and solids, internally geared pumps, internally geared vacuum pumps, internally geared liquid pumps, and internally geared pumps for a mixture of liquids and solids.

In yet another presently preferred embodiment of the present invention, there is provided a discharge plate configured for retrofitting between an outlet housing and a rotor housing of a rotating screw machine, the plate includes a plurality of pressure balancing ports and a plurality of valves each operable to open and close one or more of said ports to automatically vary the screw machine's volume ratio.

In a preferred implementation, the discharge plate disclosed herein may be retrofittable to existing screw machines. The discharge plate may be configured and arranged to be sandwiched and clamped between the rotor casing and outlet casing of the screw machine in use.

Other features, advantages and embodiments of the present invention will be apparent to persons of ordinary skill in the art, inter alia from the detailed description provided below.

Brief Description of the Drawings

Various aspects of the teachings of this disclosure, and arrangements embodying those teachings, will hereafter be described by way of illustrative example with reference to the accompanying drawings, in which:

Fig 1 is a part cut-away perspective view of a typical rotating screw machine, in this instance a compressor, that includes a discharge plate (shown without being cutaway);

Fig 2 is a cross-sectional view through the screw machine of Fig. 1 along the longitudinal axis of the screw machine's main rotor;

Fig 3 is a perspective view of a side of a discharge plate that faces the screw machine's compression chamber when the discharge plate is installed in a screw machine;

Fig 4 is a perspective view of the other side of the discharge plate to that depicted in Fig.3, namely the side that faces the screw machine's discharge chamber when the discharge plate is installed in a screw machine;

Fig 5 is an elevation of the side of the discharge plate depicted in Fig. 4;

Fig. 6 is a perspective view of another discharge plate showing the side of the plate that faces the screw machine's discharge chamber when the discharge plate is installed in a screw machine;

Fig 7 is part cut-away perspective view of the screw machine from the compression chamber side of the discharge plate towards the discharge chamber, and

Fig 8 is a part cut-away perspective view of the screw machine from the discharge chamber side of the discharge plate towards the compression chamber.

Detailed Description

As mentioned above, Fig 1 is a part cut-away perspective view of a typical rotating screw machine, in this instance a compressor, that includes a discharge plate (shown without being cut-away); and Fig 2 is a cross-sectional view through the screw machine of Fig. 1 along the longitudinal axis of the screw machine's main rotor. In an envisaged implementation, the discharge plate is retrofitted into an existing screw machine. Referring now to Figs. 1 and 2, the screw machine is in this particular example is a rotary screw compressor 10. The compressor 10 comprises a rotor casing 12, an outlet casing 14 and a discharge plate 16 sandwiched and clamped between the rotor casing 12 and outlet casing 14. The rotor casing 12, outlet casing 14 and discharge plate 16 are fastened together to define an internal rotor cavity 18 in which a male screw rotor 20 and a female screw rotor 22 are sealed for meshed rotation with one another, as is best shown in Fig. 8. The discharge plate is generally planar, as shown, and is preferably relatively thin so that it may be retrofitted into existing screw machines without major modifications being required to the rotor or outlet casings.

In Fig. 1, the rotor casing 12 and the outlet casing 14 are partially cut-away to show the male screw rotor 20 and the female screw rotor 22. Fig. 2 is an axial crosssection of the compressor 10 at approximately the cusp between the meshed male and female rotors 20 and 22, and for clarity the female screw rotor 22 has been omitted. Male screw rotor 20 includes a shaft 24 which, when rotated, causes the male screw rotor 20 to rotate and, by virtue of geared engagement therewith, female screw rotor 22 to rotate.

As shown in Fig. 2, the rotor casing 16 includes an intake port 26 that allows working fluid (which may be a liquid, a gas or a mixture thereof) to enter the screw compressor 10. The outlet casing 14 includes a discharge chamber 28 that is in fluid communication with the internal rotor cavity 18, and which opens to a discharge port 30 that enables working fluid to exit the compressor.

As will be appreciated by persons skilled in the art, in operation of the compressor 10 working fluid at a low pressure (relative to the pressure of the working fluid leaving the compressor) enters the screw compressor 10 at intake port 26 and travels into the internal rotor cavity 18 in which the male and female screw rotors 20, 22 are located. The low pressure working fluid enters a compression pocket defined between lobes 32 of the rotors and the wall of the internal rotor cavity 18. As the shaft 24 rotates the male screw rotor 20 and - by virtue of geared engagement - the female screw rotor 22, the volume of the compression pocket reduces and the working fluid is compressed as the pocket translates between the lobes 32 towards the outlet casing 14. As the compression pocket opens to the outlet casing 14, working fluid at a higher pressure (relative to the pressure of the working fluid entering the compressor) discharges from the compression pocket into the discharge chamber 28, through discharge port 30 and into the system that the compressor forms part of. Since the discharge chamber 28 is in open communication with high pressure fluid and the discharge pressure of the system in which the compressor 10 is used, the pressure in the discharge chamber 28 reflects changes in the operation of the compressor 10.

As aforementioned, a discharge plate 16 is sandwiched and clamped between the rotor casing 12 and the outlet casing 14. The discharge plate is configured to enable automatic volume ratio variation, and hence enhance compression efficiency over a range of operating conditions for the compressor 10 by balancing the pressure in the compression pocket (typically, just before it comes into communication with the discharge chamber 28) with the pressure in the discharge chamber 28. In oil free compressors the discharge plate 28 also functions to mitigate problems associated with over-compression of the working fluid, which over-compression can cause the compressor to overheat, and may cause it to seize.

Fig 3 is a perspective view of a side of the discharge plate 16 that faces the internal rotor cavity 18 of the screw machine's rotor casing 12 when the discharge plate is installed in a screw machine. Fig 4 is a perspective view of the other side of the discharge plate to that depicted in Fig.3, namely the side that faces the screw machine's outlet casing 14 when the discharge plate 16 is installed in a screw machine and clamped between the rotor and outlet casings. Fig 5 is an elevation of the side of the discharge plate depicted in Fig. 4.

Referring now to Fig. 3, the discharge plate 16 includes first and second circular openings 34, 36 that are configured to accommodate rotor shafts of the male and female screw rotors, respectively, which shafts pass through the discharge plate and engage with bearings provided within the outlet casing 14 so that the rotors can be rotated. The plate also includes a first array 38 of pressure balancing ports 39 that are associated with the first circular opening, and a second array 40 of pressure balancing ports 39 that are associated with the second circular opening. In this particular embodiment the pressure balancing ports comprise slots, but ports with other shapes may be utilised if desired. The arrays of ports provide passageways through the plate between the faces shown in Figs. 3 and 4. The slots of this embodiment extend in a radial direction from the centre of the circular opening that they are associated with. The slots are generally of the same length, but could be of different lengths if desired.

The first array of slots 38 extend from a first radial position 42 outside of the first circular opening 34 (which first radial position 42 corresponds - at least approximately - to the minimum diameter of the male screw rotor 20) to a second radial position 44 (which second radial position 44 corresponds - at least approximately - to the maximum diameter of the male screw rotor 20). In a similar manner, the second array of slots 40 extend from a first radial position 46 outside of the second circular opening 36 (which first radial position 46 corresponds - at least approximately - to the minimum diameter of the female screw rotor 22) to a second radial position 48 (which second radial position 48 corresponds - at least approximately - to the maximum diameter of the female screw rotor 22).

The discharge plate also comprises a number of apertures 70 that are each located in the vicinity of the periphery of the plate. The apertures 70 are arranged to allow the fasteners, for example bolts or screws, that couple the outlet and rotor casings together to pass through the plate when the plate is retrofitted into a screw machine and sandwiched between the outlet and rotor casings. In one envisaged implementation the apertures each comprise a smooth bore through the plate. In another envisaged implementation the apertures could be internally threaded for engagement with the fasteners that couple the rotor and outlet casings together.

As shown in Fig. 3, each array of slots may comprise one or more spaced groups of slots, and each group may comprise one or more slots. For example, in the particular example shown in Fig. 3, the first array of slots 38 comprises four groups of slots, with adjacent groups being spaced from one another in a circumferential direction; and each of the four groups that make up the first array is comprised of seven slots. The second array of slots 40, on the other hand, is comprised of four spaced groups of slots, with each group being comprised of three slots.

In addition to the aforementioned arrays of slots, the discharge plate 16 further comprises a shaped opening 50 that extends through the plate 16. The shaped opening 50 has a size and shape that is configured to define the maximum allowable internal volume ratio, Vi, in the compressor 10. In this implementation the shaped opening allows any fluid that remains in the compression pockets between the rotors to be evacuated once those compression pockets have started to move past, and out of alignment with, the pressure balancing ports.

Referring now to Figs. 4 and 5, the opposite face of the discharge plate 16 to that depicted in Fig. 3 is provided with a plurality of discharge valves 52, some of which are shown in their open position in Fig. 4. The discharge valves 52 are arranged, in this particular embodiment, so that each valve is capable of closing or opening one group of slots in the discharge plate. In other envisaged arrangements a valve may be provided for each slot, or for two or more groups of slots. Of particular note is that the shaped opening 50 that functions to limit the internal volume ratio, Vi, is not covered with a valve, but is instead always open.

In this particular implementation, the discharge valves 52 each comprise reed valves that are fixedly coupled, for example riveted, to the discharge plate. In other envisaged implementations the discharge valves may be coupled to the discharge plate by removable fixings, such as screws.

As will be understood by persons skilled in the art, in operation the pressure in the compression pocket will gradually increase as the screw rotors 20 and 22 rotate in opposite directions (indicated by the arrows in Fig. 3) towards one another and reduce the size of the compression pocket between the lobes thereof. As a consequence, the pressure in the compression pocket gradually increases from the outside pressure slots (i.e. the slots of one array that are furthest from the other array of slots) 43 to the inside pressure slots 45 (i.e. the slots of that one array that are closest to the other array of slots).

If the pressure in the compression pocket is higher than the pressure in the discharge chamber 28, the discharge valve 52 that is associated with the slot or slots between the higher pressure compression pocket and the discharge chamber 28 will open to equalise that pressure. The valves will tend to gradually open from the outside slots 43 to the inside slots 45 until the pressure between the compression pocket and the discharge chamber equalizes. Those valves 52 that are associated with slots where there is no appreciable pressure difference will not open, and as a consequence the discharge plate disclosed herein enables automatic pressure balancing between the compression pocket and the discharge chamber 38.

If the pressure in the discharge chamber 28 should be higher than the pressure in the compression pocket, the discharge valves 52 will remain closed and the shaped valve-less opening 50 will allow the compressed working fluid in the compression pocket to escape to the discharge chamber 28 to avoid pressure build up in the compressor. As aforementioned, the position and shape of the shaped opening 50 will depend on the desired maximum pressure in the compression pocket.

In the embodiment depicted in Figs. 3 to 5, the discharge valves for the first and second slot arrays comprise four generally trapezoidal moveable elements that are coupled together by a common connecting element 54. When the plate is assembled within a compressor, the connecting element is sandwiched between the rotor casing and the discharge casing to help resist movement of the discharge valves relative to the discharge plate.

As mentioned above, reed valves are merely one contemplated type of valve for the discharge plate. In the arrangement depicted in Fig. 6, the discharge plate includes a plurality of through-holes 56 that are each closed by an associated valve body 58 which is pivotally attached (for example hinged) at one end to the discharge plate.

Figs. 7 and 8 are further part-sectional views of the screw compressor 10. As can be seen in Figs. 7 and 8, lobes 32 of the male rotor 20 and the female rotor 22 co- operate with the rotor casing to form compression pockets 60, 62, 64, 66. The pressure of the working fluid in these pockets gradually increases from pocket 60, which is closest to the inlet port, to pocket 66 which is exposed to the discharge chamber. The pressure in pocket 64 will be lower than the pressure in pocket 66. If the pressure in the discharge chamber 28 shown in FIG 1 is lower than the pressure reached in pocket 64 (which is just exposed to the slots), the valve closing the slot or slots between this pocket and the discharge chamber will open and pressures in pocket 64 and discharge chamber 28 will tend to equalise. As the rotors further rotate, pocket 64 will eventually come in the position 66 and the volume of that pocket will reduce. Valves 52 which cover that pocket will remain open. If, however, pressure in the discharge chamber 28 is higher than the pressure in pocket 64, corresponding valves will remain closed. As the rotors rotate into position 66, the volume reduces and pressure increases until the pressure in pocket 66 becomes larger than pressure in the discharge chamber 28, when the corresponding valve will open.

It will be apparent from the foregoing that the teachings of the invention disclose a discharge plate that is capable of automatically varying compressor volume ratio in a rotary screw machine so that it more closely matches the final compression pocket pressure. Since the plate is relatively thin, it is likely that it can advantageously be embedded (retrofitted) in any existing machine with only small modifications to the outlet casing being required. It is also eminently possible for the teachings of the present disclosure to be incorporated in new screw machines.

It will be apparent to persons skilled in the art that, for oil injected screw machines, the teachings of the arrangements disclosed herein improve efficiency by automatically adjusting volume index to the required pressure ratio.

In oil-free screw machines, the teachings of the disclosure provided herein allow close matching of Vi to the required pressure whilst reducing the likelihood of overcompression occurring. By avoiding over-compression, the likelihood of overheating can be reduced, which is important as excessive heating of machine elements could result in rotating and stationary elements of the machine coming into contact, and the eventual failure of the machine.

In oil injected compressors that are not operating at extreme temperatures, the teachings of the disclosure will reduce the likelihood of over- or under-compression occurring, which ultimately improves efficiency (in some cases quite significantly). In oil injected vacuum pumps where suction is at sub-atmospheric pressure and discharge is at atmosphere, the teachings of the present disclosure can significantly improve efficiency by enabling appropriate internal volume ratios for both low pressure ratios and high pressure ratios. Lastly, in multiphase pumps the teachings of the present disclosure enable the automatic release of liquid in the discharge chamber, which allows such systems to operate reliably.

It will be appreciated that whilst various aspects and embodiments of the present disclosure have heretofore been described, the scope of the disclosure is not limited to the particular arrangements set out herein and instead extends to encompass all arrangements, and modifications and alterations thereto, which fall within the scope of the appended claims. For example, it will be appreciated by persons of skill in the art that the pressure balancing ports need not be configured as slots, and that other shapes of ports are possible. It will also be appreciated that the number of groups and slots per group may be varied to adjust the way that the discharge plate responds to pressure differences between the compression chamber and the discharge chamber 28. It is also the case that whilst the discharge plate disclosed herein is well suited for retrofitting to existing screw machines, it may also be included in new machines.

It should also be noted that whilst the accompanying claims set out particular combinations of features described herein, the scope of the present disclosure is not limited to the particular combinations hereafter claimed, but instead extends to encompass any combination of features herein disclosed.

Finally, it should be noted that any element in a claim that does not explicitly state "means for" performing a specified function, or "steps for" performing a specific function, is not to be interpreted as a "means" or "step" clause as specified in 35 U.S.C. Sec. 112, par. 6. In particular, the use of "step of" in the claims appended hereto is not intended to invoke the provisions of 35 U.S.C. Sec. 112, par. 6.