GB2145159A - Rotary positive-displacement fluid-machine - Google Patents

Abstract

The machine has a stationary casing containing a pair of blades 11 that revolve about fixed axes parallel to and in a common plane with the axis of the casing. Stationary spacing-pieces 13 are located on a circle intersecting the axes of rotation of the blades. A rotor disc 1 secured to and co-axial with a rotating power input, or output, shaft, Fig. 2, separates a fluid-intake chamber (3) from an axially-adjacent working space (2) for the blades. The rotor disc carries a curved wall 5, which divides the working space into intake and delivery zones 7, 8 and has a radially inner and outer lobes 9, 19. Drive-transmission means e.g. gears (10), between the blades and the shaft turn the blades continuously at half the speed of the rotor disc. The machine may be used as an air compressor or a pneumatic motor. <IMAGE>

Description

SPECIFICATION Improvements in and Relating to Rotary Positive Displacement Machines This invention relates to rotary positive displacement machines, known as Blade type Blowers, examples of which are described in British Patents 804602 and 1370353 and has been devised with the object of decreasing size and cost whilst increasing the volemetric efficiency of such machines.

Machines of the kind with which the invention is concerned comprise a stationary casing containing a series of equally spaced blades, usually four in number, centrally pivoted for rotation about fixed axes parallel to the axis of the casing. Within the casing there is concentrically mounted a rotor comprising an inner sealing hub supporting or itself forming a radially inner lobe and a radial bar which supports a radially outer lobe, and radially inner and radially outer sealing plates which seal the ends of the blades farthest from a backplate when they contain between them the "trapped air volume" of a sector shaped working space. The diameter of these blades is such that there is always an effective seal between a delivery space and an intake space.In order to pass between the radially inner and radially outer lobes with a constant clearance at the various angles of rotation the blades are caused to rotate at half the rate of rotation of the rotor and also the thickness of the blades is reduced from the parallel towards the ends.

The low cost referred to above results from using two blades and these are made of large diameter such that the blade tips pass close to or beyond the rotor axis, when the blades are radial to this axis. By this proportioning practically the whole of the working space is swept by the lobes, apart from the annulus of radial width equal to the gap between the lobes, which is the thickness of the blades. The result is the smallest machine possible for a given swept volume, and using the fewest and smallest gears.

To maintain a seal at the lobes when the thickness of the blade is not within the lobe gap, spacing blocks are used, which extend from blade tip to blade tip from the point where the tips pass the blade pitch circle diameter.

The major factor influencing efficiency is the leakage through the sealing clearances. This is minimised in three ways.

Firstly by a novel design of gap between the rotor lobes which is in the form of a short arcuate channel with closely defined bevelled ends, such that the sides of the blades seal on these bevels, whilst the spacer and blade hub seal when passing within the channel.

Secondly leakage is reduced by having, for a given swept volume, the minimum lengths and leakage gap along those sealing lengths. This comes about from the large swept volume per unit size of the machine, as working tolerances are usually proportional to the dimensions to which they apply.

Lastly, it is the seal around the rotor periphery that forms the greater part of the leakage path, so a high ratio of rotor length to diameter is chosen.

To obtain a high ratio of rotor length to diameter, requires blades also of great length.

This is achieved by having them thick enough to withstand bending caused by the air pressure acting on one side at a time, and of a diameter such that the ratio of the useful blade area to its useful thickness area is large. It is a property of the proposed rotor lobe gap that it forms a good seal on both spacers and thick blades, without the trapping of pockets of air which would be transferred from the delivery zone to the intake zone. Previous designs have relied on a knife edge shaped lobe, subject to damage and wear, with resultant high leakage.

Having achieved a large swept volume for a given rotor diameter the air flow passage areas have to be made commensurately larger, which cannot be achieved with the previous known layouts. It is a feature of the use of blade diameters greater than the pitch circle diameter (P.C.D.) of the blade axis that the outer rotor lobe does much more of the displacement than the inner one, e.g. over 80 per cent of the total.

Very effective air flow is achieved by using the whole of the area in front of the rotor disc as an air intake chamber and the disc of the rotor cut away over the whole of the available intake section of the working space to form the intake port.

This space is defined by the edge of a blade when its tips are in sealing contact, with the inner rotor lobe and the scroll shaped wall projecting axially from the rotor disc which also forms the outer rotor lobe; from here the space follows the scroll wall extending round to where it meets again the sealing blade.

Together with another sealing blade a trapping zone is formed momentarily during the cycle which separates the intake and delivery zones.

For the delivery zone which is in front of the rotating lobe there is no axially projecting wall from the rotor disc. Thus all the circumference, from the edge of the second sealing blade to the point where the scroll shape of the outer lobe is met, is available for the radially outward flow of air into a delivery duct which is also a scroll.

Because of the relative directions of thito that of the lobe scroll adequate delivery flow area can be obtained, by the scroll covering only about 300 degrees of the circumference. This not only reduces the overall size but also brings one side, where the scroll is smallest, close to the drive shaft centre line, to provide a compact mounting.

The rotor is further simplified by the addition of a more effective balancing weight which is positioned within the unbounded air delivery zone thereby permitting the use of a simple casting.

A preferred embodiment of machine incorporating the above mentioned improvement is hereinafter described by reference to the accompanying drawings, in which: Figure 1 is a cross-section through the rotor and blades at right angles to the rotor centre line.

Figure 2 is a longitudinal cross-section, along the rotor axis.

Figure 3 composed of Figures 3a, 3b and 3c shows the sealing of the blade tipes within the lobes.

The machine will be described as an air compressor or blower but it could be used as an air motor. The duties of the inlet and delivery can also be reversed by changing the direction of rotation. In Figures 1 and 2 there is shown a rotor disc 1, which separates the working space 2 from the intake chamber 3 and has, projecting from it an axially extending wall 5 which, coming within working clearance of the stationary back-plate 6, divides the working space into intake and delivery zones 7 and 8. The wall forms an approximation to a spiral about the centre of rotation, such that it is both a lobe and boundary to the intake zone. A radially inner lobe 9 forms the hub and is part of the through shaft which drives a gear train 10.

The radially inner lobe 9 and the radially outer lobe 1 9 do the work on the air, delivering it against the delivery pressure. The portion of the axial wall 5 at near maximum distance from the centre of rotation is so formed that the intake and delivery zones are further separated by one of the blades 11, which rotates on its own axis, parallel to the rotor axis, at half rotor speed and forms one of a sequence of a similar blade to form a continuity of seal between intake and delivery.

These blades, of which two is the minimum number, airways point to the gap 12 between the inner and outer lobes 9, 19, such that they pass through that gap with a sealing clearance.

Between the blades are spacing blocks 13, which maintain the continuity of seal at the gap. Thus at all times the intake zone is completely enclosed within the working space and as the lobes move away from the sealing blade the intake volume increases and air is drawn in through a port 14, in the rotor disc 1, from the intake chamber 3. On the other side of the lobes 9, 19, the volume between them and the other sealing blade is continually reducing and air is delivered out to a scroll 15, surrounding the rotor. Momentarily as one blade takes over from the other the duty of sealing, there is a volume trapped between them and the sealing section 1 7 of the rotor disc at one end and the casing backplate at the other. The air in this space is compressed by back-flow compression, to delivery pressure before being displaced from the working space.This causes a momentary break in the continuity of air delivery with a frequency of the number of blades.

Leakage of air from the delivery zone to the intake zone is a major source of loss and it is kept low by the high quality of seal at the arcuate channel 12 between the lobes and the arcuate spacers. This is particularly good in this invention because of the high ratio of the distance through the seal to the leakage gap and is a preferred feature of using few blades so that the spacers then occupy a large portion of the total revolution. To avoid unnecessary overloading of the machine an air unloading valve 1 6 can be accommodated between delivery and inlet ducts to recirculate air and provide a control between the air delivery pressure and intake pressure as desired.

Sequence of Operations The air enters the intake chamber 3, by a conveniently situated opening 1 8. From here it passes through the port 14 which gives access to the intake section of the working space 7. This space is bounded by the rotor lobes 9, 19, the outer roll wall which extends round in the form of a scroll and a point where it meets one of the blades 11 which has turned to a position when its tips begin sealing simultaneously with the rotor wall and the centre hub. As the lobe rotates away from the sealing blade 11 air is drawn in. This in turn becomes trapped between two blades as a sealed off section 1 7 is formed momentarily between blades and end faces.Further rotation then causes the blade on the delivery side to break the seal from the rotor wall, and back flow compression from zone 8 raises the pressure of the trapped air to that of the delivery. Further rotation then decreases the volume to the second blade and the air is displaced from the rotor. This is collected by a delivery scroll 15, and fed out without loss of the velocity created by the lobe.

Thus not only is the delivery port area very large but also it is situated where most of the air displacement is taking place and the velocity energy of the displacing lobes is not lost.

Air leakage losses are low because the small rotor diameter, for a given swept volume, not only reduces the total length to be sealed but the clearance gaps may also be proportionately reduced. The quality of the seal at the lobe gap is high when the spacers are passing through (Figure 3a), and also when the blade hubs pass through as shown in Figure 3b. During these periods, therefore, there does not have to be an allowance for gear phase error.

Figure 3c shows that with the correct taper to the blades they can make a seal with the ends of the lobe's arcuate channel. The ends of the lobes have bevelled corners, to give a better seal against the blade sides and to make these ends less subject to wear, or damage.

The small number of blades result in a small pitch circle diameter, and the gears are smaller and less expensive. The rotor is of a shape which can easily be cast, and it is within the scope of the invention to taper the blades to suit the casting draught angle.

It is an advantageous feature to have the blade diameter larger than the pitch circle diameter so that the inner lobe 9 has a reduced length and, therefore, slightly reduced displacement, whilst the same length added to the outer lobe 19 adds greatly to the displacement, and also adds greatly to the intake port area by putting a greater fraction of the area outside the spacers and blade ends.

As a result of using large diameter blades there can be no driving shaft actually on the rotor axis.

For this reason the drive to the rotor is at the front of the rotor disc, and the central part of the shaft that forms the inner lobe 9 transmits only the very small amount of power required to drive the blade phasing gears at the rear of the machine. A main bearing 20 is situated close to the front of the rotor to axially locate the rotor and to accept the heavy loads on the belt drive.

It is a unique feature of this design that a balance weight 21 can be fitted within the working space delivery zone 8. A weight in this position, extending axially, can balance each section of rotor along its length, so giving dynamic balance without complication and the addition of more than the minimum of extra material to the rotor.

It is a characteristic of the invention to provide a blade type blower in which there is an arcuate channel between the lobes which co-operates with the spacers to provide a seal of great length to gap ratio and with the blades thus to form a continuity of seal over the whole revolution of rotation this without the carry-over of trapped volumes of air, from the delivery to the intake side.

It may be a further feature of the invention that the walls of the arcuate channel should have bevelled ends which for part of the time, are parallel with and close to the surface of the blade it is passing.

It may be a further feature of the invention that the rotor should have a balance weight within the working space, and extending along the axial length to give both static and dynamic balance.

It may be a further feature of the invention to provide an air scroll which has a radius substantially greater than the rotor radius for no more than 2700 (degrees).

It may be a further feature of the invention to provide a rotor having port means extending radially over more than half the radius, of the rotor disc and used for one air flow, the other flow being directly to or from the lobes via a scroll casing.

It may be a further feature of the invention to provide a substantially greater part of the working space outside the pitch circle diameter of the blades.

It may be a further feature of the invention that the ends of the blades pass over the rotor centre line.

Claims (7)

1. A fluid compressing, displacing or expanding machine having a stationary cylindrical casing with a plane end surface and containing a pair of like blades mounted for rotation about fixed axes parallel to and in a common plane with the axis of the casing and positioned intermediate the inside surface of the casing and a shaft extending axially through the casing, arcuate spacing pieces intermediate the blades on the same circle as the axes of rotation of the latter, a rotor disc co-axial with said shaft and separating a fluid intake chamber in said casing from an axially adjacent working space therein, the disc having a curved wall which, in conjunction with a casing end, divides the working space into intake and delivery zones, said rotor disc having a radially outer lobe constituted by a terminal part of the wall, and a radially inner lobe which forms a hub part of the rotor disc, the lobes being spaced apart for the widthwise passage of each blade and each spacing piece as the rotor disc is rotated, and drive transmission means between the blades and the shaft for turning the blades continuously at half the speed of the rotor disc from a radial direction on one side of the casing to a direction at right angles to the radius on the diametrically opposite side of the casing as the rotor disc rotates so as to provide a sealing portion between the inner surface of the cylindrical casing and the hub part of the rotor disc in the former direction and to allow the blades to pass between the lobes in the latter.

2. A machine in accordance with Claim 1 wherein the blades are of such large diameter that the blade tips pass close to or beyond the rotor disc axis when the blades are radial thereto.

3. A machine in accordance with Claim 1 or Claim 2 wherein the gap between the inner and outer rotor lobes is of arcuate shape and the lobes have bevelled ends such that the sides of the blades seal thereon.

4. A machine in accordance with any of Claims 1 to 3 wherein the wall of the rotor disc is of scroll shape.

5. A machine in accordance with any of Claims 1 to 4 wherein the blade diameter is larger than the pitch circle diameter.

6. A machine in accordance with any of Claims 1 to 5 wherein a balance weight is fitted to the rotor disc within the working space delivery zone.

7. A fluid compressing, displacing or expanding machine constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.