GB2233042A - Screw expander/compressor - Google Patents

Abstract

A multiple-throttle inlet port means 28, 29, 30 for the expander/compressor enables the expander section to be controlled in stepped stages of expansion whilst providing progressive control of gas flow through the expander/compressor between expansion steps. The throttles are formed by butterfly valves 25, 26, 27 which may be actuated through a lever and linkage system either manually or by a servo device. Such inlet port means may be associated with each screw of the intermeshing screw expander/compressor, in which case the butterfly valves are suitably paired. <IMAGE>

Description

MULTIPLE THROTTLE INLET PORT FOR A SCREW EXPANDER/COMPRESSOR This invention relates to a multiple throttle 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 d 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 angines 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 low 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 throttle inlet port for a screw expander/compressor having several butterfly throttle valves, each connected via its own duct to a specific position at the inlet end of the expander/compressor in such a way that as the throttle valves are closed they shut off the volumes formed by the 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 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, 26 and 27 are throttle valves connected to the inlet port of the machine via ducts 28, 29 and 30. For convenience the inlet port is situated mainly at the end of the male rotor 31.

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.

When throttle valves 25, 26 and 27 are open the machine works as a pure compressor.

In order to make the machine function as an expander, one or more of the throttle valves are closed. The control of these valves can be, for example, via a manually or servo actuated control lever attached to the valve spindles. With valve 25 closed, working gas is drawn in through inlet ducts 29 and 30 via open valves 26 and 27. As the ends of the rotor pass the final open inlet duct 29 the volume entrapped within the casing by the rotor lobes 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 manifests itself as a power contribution at the rotor shafts.If a greater degree of expansion is required then valves are closed at an earlier stage in the cycle, e.g. by closing both valves 25 and 26 a considerable expansion can be effected.

Since the flow of air to an engine must be controlled in a gradual manner and since closing off valves 25 and 26 gives stepped increases in expansion effect and also stepped reductions in air flow the valves 25, 26 and 27 must be capable of being closed in a modulating manner. Thus whilst any individual valve is being gradually closed, gas flow throttling losses will occur across that valve, these losses are not recoverable. However when the valve is completely closed there is a stepped increase in the expansion power recovery effect.

The amount of energy lost by throttling between each expansion step change can be reduced by increasing the number of throttle valves and interconnecting ducts but only at the expense of complexity.

In order that the expanding gas volumes contained within different meshes of the rotors are not interconnected it is important that the ducts between the throttle valves and the rotor chamber of the expander/compressor terminate at the end surface of the rotor chamber in such a shape and size that they are completely sealed by a portion of the rotor end face. If this is not achieved then the expanding effects in any one intermesh could be reduced by allowing the gas to recompress into an adjoining intermesh.

These ducts can be sealed more conveniently by the end surfaces of the male rotor 31 than by the female rotor 32 since the lands 34 of the male rotor 31 are larger than the lands 35 of the female rotor 32 and can thus seal off a larger duct. For this reason the inlet ducts are shown at the end of the male rotor 31.

However, inlet ducts could be accommodated at the end of the female rotor 32 provided the ducts, where they terminate at the end surface of the rotor chamber, have a cross-section narrow enough to be sealed off by the ends of the passing lands 35 of the female rotor 32. In this case the ducts to the male rotor and to the female rotor would be paired so that each pair was simultaneously connected to interconnecting meshes of the rotors.

Similarly the throttle valves in the duct pairs would be operated simultaneously.

Claims (6)

1. A multiple throttle inlet port for a screw expander/compressor having several butterfly throttle valves, each connected via its own duct to- a specific position at the inlet end of the expander/compressor in such a way that as the throttle valves are sequentially closed they shut off the volumes formed by the intermeshing rotors and the casing of the expander/compressor at progressively earlier sages in the expansion cycle.

2. A multiple throttle inlet port as in claim 1 whereby the ducts between the throttle valves and the inlet face of the compressor are cast integral with the casing of the machine.

3. A multiple throttle inlet port as in claim 1 whereby the ducts between the throttle valves and the inlet face of the compressor are paired, one half of each pair being connected to the male rotor inlet end area and the other half to the interconnecting and corresponding female rotor inlet end area.

4. A multiple throttle inlet port as in claims 1, 2 and 3 whereby the throttle valves are actuated by a linkage and lever system attached to the valve spindles.

5. A multiple throttle inlet port as in claim 4 whereby the valve spindle linkages are controlled manually.

6. A multiple throttle inlet port as in claim 4 whereby the valve spindle linkages are controlled by a servo device.