GB2233041A - Screw expander/compressor - Google Patents

Abstract

A multiple slot inlet port 27, 28 for the expander/compressor enables the expander section to be controlled by adjustable segment plates 29, 30 in an effectively infinitely variable way to provide progressive control of gas flow through the expander/compressor. The multiple slots may instead be of honeycomb form. The segment plates may be actuated manually or by a servo-mechanism. <IMAGE>

Description

MULTIPLE SLOT INLET PORT FOR A SCREW EXPANDER/COMPRESSOR This invention relates to a multiple slot inlet port for a screw expander/compressor.

Screw rotor positive-displacement machines for an elastic working fluid are well known both as compressors and expanders. In such machines there is a casing comprising two intersecting bores having parallel axes and forming a boundary wall which encloses the working space, a high pressure and a low pressure end wall disposed perpendicularly to the axes of the bores and high pressure and low pressure ports. Each bore contains one rotor of a pair of intermeshing rotors of so-called male and female form with helical lands and intervening grooves. Each rotor has a wrap angle of less than 3600. The male rotor is formed with convex lands and usually has the major portions of each land outside the pitch circle. The female rotor, formed with concave lands, usually has the major portion of each land inside the pitch circle.

It is an advantage in compression or expansion machines of this type that when running dry (i.e. with a gas medium devoid of liquid lubricant) that the two rotors are kept apart by timing gears in order to avoid rubbing and seizure of the dry rotor surfaces. The timing gears normally run in a sealed case where they can be lubricated.

Alternatively lubricant may be injected into the gas compression or expansion space, in which case the rotors may be run one against the other.

In this case the need for accurate and expensive timing gears is dispensed with. However the oil must be separated from the gas after compression.

When operated as a gas expander unit it is usually inconvenient to use lubricant in the gas medium and timing gears must be used.

A typical example of a screw machine is shown in Figs 1, 2, 3 and 4 and the operating principles and the methods by which the machine can operate both as a compressor and as an expander will be described with reference to the figures.

Fig 1 shows the rotors 4 & 5 contained within their casing 1, as viewed from the inlet end of the machine. 2 is the inlet port and 3 is the inlet side of the machine, 8 and 9 are synchronisation gears typically used in compressors where there is no lubricant within the compressor working chamber. Fig 2 shows the rotors 4 and 5 and timing gears 8 and 9 without the casing for clarity. Arrows 6 and 7 show the rotation of the rotors.

The rotors in Fig 2 are shown in the same configuration as Fig 1 (i.e. with the inlet end and inlet side towards the viewer).

Fig 3 shows the same machine viewed from the outlet side of the machine, again with the inlet end towards the viewer. Fig 3 is thus a view from the underside of Fig 1.

In Fig 3, 10 is the outlet port. Fig 4 shows the rotors 4 and 5 without the casing and in the same configuration as Fig 3 (i.e. with the outlet side and the inlet end towards the viewer).

Considering Figs 1 and 2, as rotors 4 and 5 rotate according to arrows 6 and 7, the voids formed by the surfaces of the grooves of the rotors and the casing expand. This action sucks gas into the voids. When the leading edges 11 and 12 of the rotor profiles pass the edges 13 and 14 of inlet port 2, the voids are effectively closed off. This closing point is usually chosen to occur when the voids have been expanded to or near to their maximum volume by the rotation of the rotors (e.g. the voids formed by grooves 15 and 16 of the rotors in Fig 2 are shown in this condition).

If a view is now taken on the outlet side of the rotors as in Fig 4, grooves 15 and 16 can now be seen to be rotating towards one another and if the rotors are rotated further the cooperating meshing of the rotors causes the voids formed by the surfaces of the rotor grooves and the inner surfaces of the casing bores to interconnect and to decrease in volume causing a compression of the gas contained therein. In Fig 4 the voids created by grooves 17 and 18 and the casing inner surfaces can be-seen to have been much reduced in volume compared with the voids created by the grooves 15 and 16.

The position of the edges of outlet port 10 can be chosen so that the voids described above are not exposed to the outlet port until the volumes of the said voids have been reduced to any required degree. The degree of compression which occurs within the machine can thus be pre-determined.

Referring again to Figs 1 and 2 it can be seen that if the inlet port of the machine is closed prior to the voids formed by the grooves of the rotors and the inner surfaces of the casing having reached their maximum volume then as the rotors continue to rotate and the voids consequently increase in volume, an expansion is effected of the gas trapped therein.

With judicious choice of inlet port closing position and outlet port opening position the machine thus has the ability to act simultaneously as an expander and a compressor.

One application where this is of potential benefit is in the supercharging of engines. In particular for spark ignition engines where the amount of fuel and air drawn into the engine per induction stroke has to be varied and regulated according to the power output required of the engine. This part-load regulation is normally achieved by throttling the flow of air into the engine. However since throttling is a non-reversible process there is an associated power loss at the engine pistons during the induction stroke.

A more efficient method for part-load engine operation would be to use the expansion ability of a screw supercharger to reduce the charge air density instead of throttling.. By so doing, some of the engine piston work associated with the induction stroke would be recovered.

However it has hitherto been difficult to close down the inlet port of a screw expander/compressor in an effective and controlled manner.

According to the present invention there is provided a multiple slot inlet port for a screw expander/compressor having a multitude of small slots connecting specific positions at the inlet end of the rotor chamber of an expander/compressor to a control surface having a means provided for closing off the slots in such a way that as the slots are sequentially closed they shut off the expanding volumes formed by the rotating and intermeshing rotors and the casing of the expander/compressor at progressively earlier stages in the expansion cycle.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which Fig 5 shows in perspective the expander/compressor with the rotors 31 and 32 contained in their casing 20 as viewed from the inlet end and inlet side of the machine.

21 is the inlet end and 22 is the inlet side of the machine. 23 and 24 are timing gears.

25 are slots allowing gas to enter the machine at the inlet end of the male rotor 31 and 26 are slots allowing gas to enter the machine at the inlet end of the female rotor 32. The slots 25 may be of the same length as 26 or of longer or shorter length. The slots may be of any shape with, for example, further subdividing walls to produce, for example, a honeycomb configuration as shown in Fig 10 where 37 is a honeycomb structure with the honeycomb openings forming the gas transfer slots. 27 and 28 are the control surfaces and 29 and 30 are the control means here shown as segment plates which can be rotated to cover more or less of the slots where they adjoin with the control surfaces 27 and 28.

The control segment plates can be operated independently or geared together so as to operate in a pre-determined manner.

Fig 6 shows the rotors 31 and 32 in the same configuration as Fig 5 i.e.

with the inlet end and inlet side towards the viewer but with the casing removed for clarity. Arrows 41 and 42 show the directions of rotation of the rotors.

Fig 7 shows an end elevation of the inlet end of the machine as viewed on arrow 40 showing the ends of the slots as they would appear at the control surfaces. The control segment plates are shown in the open position.

Fig 8 is a port cross-section through AA showing the slots and control segment plates.

Fig 9 is an end elevation similar to Fig 7 but with the control segment plates shown in a position whereby they cover some of the slots 25 and 26 which communicate between the inlet end of the working chamber and the control surface.

When control segments 29 and 30 are in a position such as to expose all of the slots, the machine works as a pure compressor.

In order to make the machine function as an expander, both of the control segments are rotated such as to cover and close off some of the slots (e.g.

in Fig 9). As the edges 33 and 34 of the rotor lands pass the last remaining open slots 35 and 36 the volumes entrapped within the casing by the rotor lands and the casing walls is closed off prior to this volume having reached its maximum. As the rotors continue to rotate, this volume increases thus effecting an expansion of the gas contained therein. This expansion will manifest itself as a power contribution at the rotor shafts.

If a greater degree of expansion is required then the control segments are rotated to cover and close off more slots.

Since the flow of air to an engine must be controlled in a gradual manner the control segment plates must be capable of being moved in a modulating manner.

As each slot is closed off there is a stepped increase in expansion effectand as the segment plate progressively closes off the next slot a gradual throttling of flow occurs across that slot until it is finally closed.

However by having a large number of slots the amount of throttling occurring at any one time is very small thus reducing throttling losses to a small and insignificant amount.

In order that the expanding gas volumes contained within different meshes of the rotors are not interconnected it is important that the slots between the control surface and the rotor chamber terminate at the end surface of the rotor chamber in such a shape and size that any one slot is capable of being sealed off by the end surface of the rotor land.

Claims (7)

1. A multiple slot inlet port for a screw expander/compressor having a multitude of small slots connecting specific positions at the inlet end of the rotor chamber of an expander/compressor to a control surface having a means provided for closing off the slots in such a way that as the slots are sequentially closed they shut off the expanding volumes formed by the rotating and intermeshing rotors and the casing of the expander/compressor at progressively earlier stages in the expansion cycle.

2. A multiple slot inlet port as in claim 1 whereby the means for closing off the slots at the control surface is a rotating segment plate.

3. A multiple slot inlet.port as in claim 1 whereby the means for closing off the slots at the control surface is 2 rotating segment plates, one to close off the slots leading to the end of the male rotor and the other to close off the slots leading to the end of the female rotor.

4. A multiple slot inlet port as in claim 3 whereby the 2 rotating segment plates are interconnected.

5. A multiple slot inlet port as in claims 1, 3 and 4 whereby the slots disposed about the axis of the male rotor, connecting the inlet end of the male rotor chamber to the control surfaces are of different lengths from the slots disposed about the axis of the female rotor, connecting the inlet end of the female rotor chamber to the control surfaces.

6. A multiple slot inlet port as in claims 1 to 4 whereby the slots are configured in honeycomb fashion to optimise the structural stiffness of the slot matrix.

7. A multiple slot inlet port as in claims 2 to 5 whereby the movement of the segment plates is actuated by manual or servo mechanism means.