IMPROVEMENTS IN AND RELATING TO VALVES

FIELD OF THE INVENTION

This invention relates to valves and to apparatus for creating a reversible flow of fluid, which apparatus includes such a valve.

BACKGROUND TO THE INVENTION

The invention is particularly, but not exclusively, applicable to flow reversing valves. These valves are typically used in flow control systems, in between a fluid pump (such as a fan or hydraulic pump) which only runs in one direction and a load, circuit or actuator, through which the fluid needs to be passed in either of two opposite directions at different times. Flow reversing valves are used in many fields of application, such as those of heat exchangers, refrigerators, air conditioners, hydraulic and pneumatic systems. The valves are also used in apparatus for wind testing various articles, for example full-scale wind testing apparatus for testing buildings or building panels or fatigue testing apparatus for testing fatigue of aircraft components. In these cases, a pressure cell or wind tunnel is connected to a fan via a flow reversing valve which is operable to modulate the pressure of air in the testing cell/wind tunnel and to alternate the direction of flow through the tunnel. Such systems need to be able to deliver large volumes of air at high pressures, and to be able to vary the pressure and flow direction very rapidly.

One known example of a flow reversing valve which is used in this way is the BREWULF valve used by the United Kingdom Buildings Research Establishment. The valve is a five port valve which has two arcuate valve members each of which selectively seals either of a respective set of two ports, the unsealed member of which can communicate through the body of the valve with the fifth port. One member of each pair of ports (which can be selectively sealed) is connected to the atmosphere (the other member being connected to a fan intake or outlet). The fifth port is connected to the pressure cell. The two valve members are connected by mechanical push rod linkage which co-ordinates the operation of the valve members so that either the fan inlet or fan outlet is connected to the fifth port, the other side of

the fan being connected to the port that is open to atmosphere. The valve also has a neutral position in which neither the inlet nor the outlet is connected to the fifth port, and in which approximately half the flow through the fan is directed to and from atmosphere, and the other half recirculates through the valve. It has been found that this type of valve cannot produce rapid, controlled alterations in the flow of air in the pressure cell, causes large pressure drops in fluid being fed through the valve (thus reducing the efficiency) and can cause problems of overheating in the fan, partly because of the lack of efficiency of the valve and also because the fan recirculates air when the valve is in the neutral condition.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a valve comprising a housing having a plurality of ports, each connected to a respective opening within the housing, the openings being arranged in two opposed sets, separated by a rotary valve member for at least partially occluding openings whilst allowing selected pairs of openings in the sets, and hence the corresponding ports, to be connected through the valve, the alteration of the selection of ports to be connected being achieved by changing the angular position of the valve member, wherein each set comprises two or more respective openings. Since these sets of openings are arranged in an opposed relationship, the invention enables a large cross-sectional area of openings to be accommodated in a relatively small housing, thus providing a compact design of valve capable of conveying large volumes of fluid at high flow rates. The pathways of fluid through the housing can also be relatively short, thus reducing pressure loss, and the rotary valve member can have a low moment of inertia, so that relatively little torque is needed to rotate it, thereby facilitating rapid adjustments of the valve member.

Preferably, the rotary valve member is rotatable about a single axis of rotation.

Preferably, the openings of at least one set are separated by partitions which extend radially relative to the axis of rotation.

Preferably, the openings of both sets are separated by such partitions.

Preferably, the openings in the sets are at least partially angularly aligned, relative to the axis of rotation, so that the path of fluid flow from one opening in one set to a selected opening in another set is substantially parallel to said axis.

Consequently, the pressure loading on the rotary valve member is perpendicular to the directions of movement of the latter. As a result, there is no (or very little) component force from the pressure loading that will interact with the actuation torque applied to the member, so that there is little or no adverse built-in feedback to interfere with control of the valve.

Preferably, each set of openings is contained within a respective one of two initially separate parts of the housing.

Preferably, the rotary member has two opposed faces through which said axis passes substantially perpendicularly, the faces being substantially parallel to each other.

Preferably, the surfaces are substantially planar, and the rotary member comprises a disk apertured or sectored to provide areas for connecting the opposed openings.

A sectored disk may comprise at least one sector or a plurality of sectors which are angularly spaced from each other, and are preferably coaxial and have the same radius. The rotary member is conveniently attached to a shaft which is coaxial with said axis of rotation and operative to transmit rotational movement to the member.

The shaft may to advantage be connected to a drive such as a servo motor for rotating the member and thus changing the state of the valve.

The low moment of inertia, and perpendicular pressure loading as described above enable a relatively simple drive arrangement to be used, without the need for, for example, a bulky or costly transmission to couple the motor to the shaft. Indeed a direct connection between the motor and shaft may be used.

Preferably, the ports are connected to the openings via passageways at least some of which have a short smooth bend so that their corresponding ports are aligned substantially perpendicular to the axis of rotation. This arrangement provides space to accommodate the servo motor body in close proximity to the valve disk without increasing the disk diameter (and thus inertia). It also allows the

servo motor to be connected to the disk with a relatively short and rigid shaft that is substantially supported by the bearings in the servo motor. This avoids the need for a pair of bearings in the valve and a flexible shaft coupling to cope with any misalignment between a pair of independently mounted shafts, and thus simplifies the manufacture and assembly of the valve.

Preferably, the member comprises an apertured disk, preferably the valve is a five port valve having at least three positions (i.e. connection conditions).

The first set of openings may comprise two openings, the second set having three openings. Preferably, the valve member has a first position in which one port of the first set is connected to a first port of the second set, a second position in which the other port of the first set is connected to said first port of the second set and a third position in which the two ports of the first set are each connected to a respective one of the other two ports of the second set. If the two ports of the first set are respectively connected to the inlet and outlet of a fan, the first port of the second set is connectable, through the valve, to either the inlet or the outlet, so as to provide a reversible flow control for air through said first port. Furthermore, the other two ports may be open to atmosphere so that the third position corresponds to a neutral position in which there is little or no flow through the first port, but the fan is in an open circuit condition, in which the fan can be kept running and does not recirculate air (thus avoiding or mitigating problems of heat accumulation).

Preferably, when the valve is in the first or second position, the port of the first set not connected to the first port of the second set is connected to said respective other port of the second set.

Thus, in the situation where the ports of the first set are connected to a fan, part of the first set not blowing air into or sucking air from said first port of the second set will be connected to atmosphere.

This helps smooth the transition from one state to another by preventing strangling of the flow of air to or from the fan inlet/outlet connected to the respective other port (of the second set) through the valve.

Preferably, the shapes and sizes of the openings and of the rotary member are such that, during movement of the valve member from the third to the first or second positions, a connection is maintained between the port of the first set not connected to said first port and the respective other port of the second set. Preferably, the openings and/or the rotary member are so shaped that the ratio of change in area of an opening occluded by the member to the angular movement of the member varies with the angular position of the member.

Preferably, that ratio, for a given opening, is directly related to the amount of opening that is exposed, so that the ratio decreases as the valve member occludes the opening.

Thus, as the valve member approaches a position in which an opening is substantially fully occluded, the rate of occlusion for a given angular speed of movement of the member, will reduce, preferably progressively.

This feature further improves the transitional characteristics of the valve, preferably such that the relationship between valve member angle and pressure at said first opening is linear.

Preferably, this characteristic is achieved by providing non-radial lands in or in the region of a partition for said opening.

Preferably, the valve member is spaced from the partitions by an amount sufficiently small to prevent substantial leakage of air across the partitions for either set of openings.

This feature avoids the need for the valve to include rubbing seats between the valve member and the partitions, and hence reduces friction and improves responsiveness of the valve. According to a second aspect of the invention, there is provided a valve having five ports and one or more or valve members moveable so as to connect selected ports through the valve, wherein either port of a first set of two of the ports is individually connectable to a first port in a second set of the remaining three ports, and when either of said ports of the first set is so connected, the other port of the first set is connected to a respective one of the other two ports of the second set, the valve also having a condition in which neither of the ports of the first set is

connected to said port of the second set, each first set port being connected to the respective other port of the second set.

Preferably, the valve is so arranged that the change from the condition in which neither port of the first set is connected to the first port of the second set to the condition in which one of the ports of the first set is so connected is achieved without interrupting or restricting the connection between the other port of the first set and the respective other second set port.

The valve may to advantage include a rotary valve member for blocking connections between ports whilst allowing selected ports to be connected, each condition of the valve corresponding to a respective angular position of the valve member.

The invention also lies in apparatus for creating a reversible flow of fluid, the apparatus comprising a fluid pump having an inlet and outlet; a valve having two ports, each connected to a respective one of the inlet and outlet, and three further ports, wherein the valve is so arranged that the pump inlet is connectable to the first of the further ports, the outlet being connected to one of the remaining further ports, a second condition in which said outlet is connected to the first of said further ports, the inlet being connected to the other of the remaining ports, and a third condition in which the inlet and outlet are each connected to a respective one of the remaining ports.

Preferably, when the valve is in its third condition, each of the inlet and outlet is connected to the respective further port to which it is connected when the valve is in the first or second condition.

Preferably, the valve has a rotary member for partially occluding passages through the valve to the ports, thereby to enable said selected connections to be made, wherein the condition of the valve is determined by the angular position of the rotary member.

Where said lands are provided in the valve housing, these are preferably detachably mounted on the housing, so that they can be removed and replaced with other, differently shaped lands. This enables the transitional characteristics of the valve to be easily tuned, i.e. without requiring a redesign of the other housing parts.

A further understanding of the functional and advantageous aspects of the invention can be realized by reference to the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is an exploded isometric view of a first embodiment of valve in accordance with the invention; Figure 2 is a similar view of the valve, from a different angle;

Figure 3 is an isometric view of part of the valve housing (the part having three ports) and a valve member therein;

Figure 4 is a corresponding view to Figure 3, with the valve member removed; Figure 5 shows the valve when connected to a fan;

Figure 6 is a diagrammatic view of apparatus for creating a reversible flow of fluid incorporating the valve and fan of Figure 5;

Figure 7 diagrammatically represents a second embodiment of valve in accordance with the invention, and illustrates the openings in the two housing parts (when viewed in the direction of the axis of rotation of the valve member) and also shows the valve member, in plan elevation;

Figure 8 shows the valve member and openings of Figure 7 superimposed on each other so as to illustrate the relative positions of the openings and valve member for the various modes of operation (i.e. connection conditions) of the valve, the modes also being illustrated by connection diagrams included in Figure 8;

Figure 9 is a sequence of views, each corresponding to the superimposed views of Figure 8, but showing the movement of the valve member from a position corresponding to one condition of the valve to a position corresponding to another condition; Figures 10-12 are views, respectively corresponding to Figures 7-9, of a third embodiment of the valve;

Figures 13 and 14 are views, respectively corresponding to Figures 7 and 9 of the first embodiment of the valve;

Figures 15 and 16 are views, also respectively corresponding to Figures 7 and 9, of a fourth embodiment of the valve; Figure 17 is an isometric view, corresponding to Figure 3 of part of the valve housing of the first embodiment of valve when the valve is in a neutral condition; and

Figure 18 is a plan view of the part shown in Figure 17, also with the valve in a neutral condition.

DETAILED DESCRIPTION OF THE INVENTION Generally speaking, the systems described herein are directed to improvements in and relating to valves. As required, embodiments of the present invention are disclosed herein. However, the disclosed embodiments are merely exemplary, and it should be understood that the invention may be embodied in many various and alternative forms. The Figures are not to scale and some features may be exaggerated or minimized to show details of particular elements while related elements may have been eliminated to prevent obscuring novel aspects. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. For purposes of teaching and not limitation, the illustrated embodiments are directed to improvements in and relating to valves.

As used herein, the term "about", when used in conjunction with ranges of dimensions or other physical properties or characteristics, is meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where on average most of the dimensions are satisfied but where statistically dimensions may exist outside this region. It is not the intention to exclude embodiments such as these from the present invention. The first embodiment of valve is a five port three position valve for use in a full scale wind testing apparatus to vary the air pressure, and flow direction, in a

pressure box which is connected to a fan, in this case a Regenerative Blower, through the valve.

With reference to Figures 1 and 2, the valve comprises a housing in the form of a cast aluminium alloy casing having two opposed parts 1 and 2. The casing part 1 has a part domed body portion 4 in which there are formed two ports 6 and 8. The ports 6 and 8 lead on to semi-circular openings o and i in the body of the Part 1 , the openings being diametrically separated from each other by a pair of radial partitions 10 and 12 extending from opposite sides of a central cylindrical passage 14. Each of the ports 6 and 8 communicates with its respective opening via a respective one of two passages which are isolated from each other, by the partitions 10 and 12 and inner walls (not shown) in the domed portion.

The domed portion 4 terminates at a circular outwardly directed flange 16 having a number of axial through bores spaced at regular intervals, via which the flange can be fixed to a similar flange 18 of the second portion via nuts and bolts. Thus, for example, the bolt 20 passes through the hole 22 in the flange 16, and through the corresponding hole 24 in the flange 18, the free end of the bolt being secured by means of a nut and washer arrangement 26. The other arrangements of the other bolts, apertures, nuts and washers are identical, and are therefore not referenced. Three curved connectors 28, 30 and 32 extend from the flange 18, and define three corresponding passages, each of which is terminated at one end by a respective one of three ports 34, 36 and 38. The other end of each passage terminates at a respective one of three openings, shown at a, b and c in Figure 2. More specifically, the port 34 is connected to the passage b, the port 30 to the passage c and the port 36 to the passage a.

The openings a, b and c are separated from each other by means of partitions 40, 42 and 44 which radiate out from a central cylindrical hub 46 (having an outer diameter of approximately 80mm) to the inner periphery of the flange 18. As can be seen from the drawings, the passages which connect the ports to the openings curve substantially through 90 degrees. The openings o and i in the Part 1 form a first set of openings which, in the assembled valve, are opposite the second set of openings constituted by openings a, b and c of the Part 2 of the valve.

The internal diameter of the housing, where the Parts 1 and 2 meet, is approximately 202mm and each of the ports is of a diameter of approximately 80mm.

At the outer periphery of each of the openings a and c there is provided a respective mounting plate 48 or 50 via which a respective land 52 or 54 is screwed into position in the opening. The shape of the lands 52 and 54 can more clearly be seen in Figure 4. Each land has an arced outer edge having a radius of curvature corresponding to that of the inner periphery of the flange 18, so that the land sits closely against said inner periphery. In Figure 4, the outer portion of the land 50 is referenced 55, and leads to a radial portion 56 that abuts the partition 44. This surface projects approximately halfway along the length of the partition 44, and leads to an arced, convex portion 58 having a radius of curvature of approximately 180mm. The land, in this example, subtends about 45 degrees of the sector defined by the opening c, (although different shaped lands subtending angles between 30 and 60 degrees may be used). The land 52 is identical to that land 50. The lands 50 and 52 are substantially flush with the top face of the flange 18 and hence with the edges of the partitions 40, 42 and 44 as viewed in Figure 4. However, as can be seen from Figure 1, the lower end of the passage 14 (as viewed in Figure 1) and the lower edges of the partitions 10 and 12 stop short of the flange 16. Consequently, those edges are spaced from the partitions and hub of the second portion 2, and that space accommodates a sectored disk 60 machined from a stock plate aluminium alloy.

The lands 50 and 52 are independently interchangeable with other lands having different profiles. Accordingly the land 52 may, for example, have a similar, but not identical, profile to that of the land 50.

The disk 60 has two identical, diametrically opposed sectors 62 and 64, each of which subtends an angle of about 120 degrees, corresponding to the angle subtended by each of the openings a, b and c. These sectors 62 and 64 meet at a hub 66 of a diameter of approximately 80mm. When seen in plan view the edges of the sectors may be radial to the axis of rotation, or offset but still parallel to a line radial to the axis of rotation, or profiled with respect to such a radial line. All such shapes are readily achieved by profile machining the disk from flat sheet material.

Some portion of the edge profiles of the sectors may additionally be radiused, chamfered or otherwise profiled in cross-section to fine tune the transitional flow characteristics of the valve as the edges pass over the partitions.

In the described embodiment, the sides of each sector are radial to an axis which is parallel to the axis of rotation, but displaced towards the respective sector. The arrangement is such that, when a given sector is symmetrically placed over any of the three openings a, b or c, its edges are parallel to the radial edges of that opening (defined by the edges of the partition facing into the opening) but are spaced in the plane of the disk from the edges of the opening so as to define a gap of approximately one millimetre, as shown at G1 and G2 in the detailed portions of Figures 17 and 18. In those figures, the sector 64 is aligned with the opening b, so that the radial edges of the sector are spaced one millimetre from, but parallel to the edges of the partitions 40 and 56 bordering the opening b.

The disk 60 is attached to a circular head 68 of a steel drive shaft 70 by means of nuts and bolts or countersunk screws passing through registering apertures in the head and in the hub portion 66 of the disk 60. A dowel 72 is a press fit in aperture 74 in the head 68 and a close clearance-fit in aperture 76 in the disk 60. The dowel facilitates angular alignment of the disk 60 with the head 68 so that the bolt apertures of those two components are in register prior to the components being fastened together by the nuts and bolts as mentioned above.

The shaft 72 includes an axial slot 78 (Figure 2) in which a key 80 is secured. When secured in the slot 78, the key 80 stands proud of the shaft 72. Before the key 80 is secured in the slot 78, the shaft 72 is inserted into a plain bush bearing 82 which is, in turn, passed into a bush housing 84. In the assembled valve, the bearing 82 is accommodated within the housing

84, and both these components sit against the opposite face of the head 68 from the disk 60.

The portion of the shaft 70 protruding beyond the housing 84 extends into a steel, cylindrical shaft coupling 86 having a hollow, axial bore 88 into which the shaft 70 extends. The bore 88 includes an axial keyway in the form of a slot 90 which accommodates the key 80 so as to provide angular location of the shaft 70 relative to the coupling 86. The coupling 86 also includes an axial split 92 on one side,

across which four pinchbolts (one of which is reference 94) extend. The bolts can be tightened to close the slit, thereby reducing the diameter of the bore 92 and fixing the shaft 70 within the coupling 86.

The coupling 86, in turn, extends through the cylindrical passage 14, and onto the output shaft 96 of a servo motor 98. The shaft 96 is retained in the bore 88 in a similar fashion to the shaft 70, and includes a key 100 which extends into the opposite end of the slot 90 from the key 80, thus also providing a rotational key between the shaft 96 and the shaft 70, via the coupling 86.

In some variants of the valve design, the disk is non-symmetrical. In such cases, the combination of using countersunk screws and the dowel 72 provides a unique and convenient means of correct assembly of the disk on the shaft. Since the slot 78 in the shaft is aligned via the shaft coupling to the key 100 on the servo motor shaft, this establishes a unique angular alignment between the disk and the servo motor encoder which in turn facilitates valve control. The motor 98 includes a square face plate 102 by which the motor 98 is mounted on the housing part 1 by means of threaded studs, such as stud 104 passing through registering holes (for example 106 and 108) in the plate 102 and housing part 1. As can be seen from Figures 1 and 2, there are four fixing studs, for example, 104, attaching the corners of the plate 102 to the part 1. Thus, the operation of motor 98 will rotate the disk 60 about the axis defined by the shaft 70 substantially fully or partially to occlude some openings of the second set whilst exposing other openings of the second set to either opening of the first set, thereby to change the connection state of the valve. In that connection, Figure 3 shows the disk 60 in an intermediate position, in which all of the openings of the second set are partially occluded.

The connection of the valve to a pressure box and a fan are shown in Figure 6. The pressure box 108 is connected to the port 34 whilst the fan 110 has an input (through which air is drawn into the fan) connected to port 8 and an output (from which the air is expelled by the fan) connected to port 6. The valve is referenced 112 in Figure 6. The particular type of fan shown in Figure 6 has a pair of annular rotor casings mounted on each end of an electric motor, with a corresponding pair of

rotors driven by a common shaft, and the ports in the rotor casings being connected by T-shaped manifolds to provide a single inlet and outlet.

In Figure 5, the pressure box is not shown, but it can be seen that the fan is connected to port 8 through a flexible pipe secured on the port by a band clamp 114. A similar flexible pipe and band clamp are used to connect the port 6 to the fan outlet (but are not visible in Figure 5). A similar arrangement of a flexible pipe (referenced 116) and band clamp connect the port 30 to an exhaust muffler 118 whilst flexible pipe 120 connects the port 36 to an inlet filter 122. The motor for operating the fan 110 is contained in a cylindrical housing 111 in-between the pair of annular rotor casings 113 and 115, and the electrical supply and control equipment is contained in a rectangular cabinet 124.

Figure 7 shows the arrangements of openings (in the first and second set) and their proportions in relation to the sectored disk for a modified version of the valve. The modification in this case is the omission of the lands and land plates in the part 2 of the housing. Thus there are no lands in the openings a, b and c which can therefore be considered to be simple sectors. In all other respects the modified version of the valve is identical to the first embodiment described above, and corresponding components are therefore denoted by the same reference numerals as those used in previous figures. The angular orientation of the openings in the two sets is illustrated in Figure 7, from which it can be seen that the partition 42 is aligned with the partition 12. Consequently, opening a is aligned only with the opening i, opening c only with the opening o whilst opening b straddles both openings i and o.

This relationship is further illustrated in Figure 8, in which the two sets of openings are shown superimposed on each other, with the openings of the second set being indicated by the broken lines. The first such diagram in Figure 8, referenced 126, shows the disk 60 in a position in which substantially the whole of the opening b is occluded by the sector 64 on the disk 60, whilst the openings in the disk connect the opening a to the opening i and the opening c to the opening o across the disk 60. This connection configuration is shown at 128, and in use equates to there being no fluid supplied to or drawn from the pressure box 108, and the fan being connected to atmosphere through the ports 30 and 36. Accordingly,

when the apparatus is in this neutral condition, the fan is not recirculating substantial amounts of air and is therefore less prone to overheating than known arrangements in which a substantial degree of recirculation occurs in the neutral position. The opening is not completely occluded (as discussed above), so the valve does allow a small amount of recirculation, when in the neutral position, as this is useful for obtaining a linear pressure versus disk angle characteristic when the valve is changing from the positive to the negative pressure conditions via the neutral position.

When disk 60 is in the position shown at 130 of Figure 8, the sector 62 substantially fully occludes the opening a, whilst the sector 64 only partially occludes openings b and c so that opening b is connected to opening i and opening c to opening o to give the connection configuration shown at 132, in which the port 34 is connected to the fan inlet and the port 30 to the fan outlet. In this condition, the fan draws air out of the pressure box 108, into the port 34, and into the fan across the valve and through the port 8. Air is expelled by the fan through the port 6, the valve and the port 30. Thus a negative pressure is set up in the pressure box 108.

The third position of the disk 60 is shown at 134 in Figure 8, from which it can be seen that the sector 62 substantially fully occludes the opening c whilst only partially occluding openings a and b in such a way that opening a communicates with opening i, and opening b with opening o, to give the connection condition shown at 136. In this condition, the fan outlet is connected, via the port 6 and valve to the port 34, whilst the fan inlet is connected to the port 36 through the valve and through port 8. Consequently, the fan blows air through the port 34 and into the pressure box 108, (thereby exerting a positive pressure in the box) whilst drawing air in through the port 36.

Each slot in the disk 60 subtends an angle which is approximately half that subtended by each opening a, b and c. As a result, the transition of the disk from one to the other of the positions shown in Figure 8 can leave one of the openings a or c in connection with the opening i or o while the other two openings in the second set are blocked or unblocked.

Thus, for example, Figure 9 shows the transition of the disk 60 from the position shown at 126 to the position shown at 130. As can be seen, this transition

blocks the opening a, whilst unblocking opening c, but that during the transition the opening c remains fully open to the opening o. As a result, the connection between the port 6 and the valve outlet is not impeded as the connection of the fan inlet is switched from port 36 to port 34. This means that the fan outlet is not strangled during this transition and this helps to prevent or reduce uncontrollable fluctuations in pressure supplied by the fan to the box 8.

With reference to Figure 10, the third embodiment of the valve is identical to the first embodiment in all particulars apart from the shape of the disk, constituting the valve member and the inlet and outlet openings o and i. Corresponding components are indicated by the reference numerals used in relation to the first embodiment, raised by 200. Thus, the first part of the housing 201 has an inlet i which is circular and is concentric with the rotational axis of the disk 260. The opening o is annular, surrounds and is concentric with the opening i. This type of inlet and outlet are considered to be particularly suitable for a connection to a centrifugal fan 310. The disk 260 has an inner opening 238 and an outer notch 240. The sides of the inner opening 238 lie on two radii of the disk 260, whilst the top and bottom (when the disk is viewed in the orientation shown in Figure 10) take the form of two arcs concentric with the disk 260. Similarly, the notch 240 has two radial sides separated by an inner arc. For any angular position of the disk 260, the hole 238 is in registry with the opening i whilst the notch 240 is in registry with the opening o. As shown in Figure 11 the disk can be rotated to place the valve in any selected condition corresponding to the conditions shown in Figure 8. Furthermore, the hole 238 can remain in registry with the opening a while the notch 240 transitions from opening c to opening b or as illustrated in Figure 12, the opening 240 can remain in registry with the opening c while the hole 238 is moved from opening a to opening b.

Figure 13 corresponds to Figure 7 but shows the lands 52 and 54 of the first embodiment of valve. This embodiment can achieve any of the connection states shown in Figure 8, but the transitional characteristics of the valve are modified by the lands 52 and 54 in a way which is illustrated in Figure 14. The first illustration of the sequence shown in Figure 14 has the disk 60 in a position in which the opening b is fully closed, whilst the opening a is fully opened to the opening i and the

opening c is fully opened to opening o. The disk 60 is then moved clockwise so that the sector 62 approaches the partition 40 and the sector 64 starts to extend over the partition 44. Initially, a given angle of movement of the disk in this way will occlude the area swept by the whole leading edge of the sector 62 (i.e. that moving towards the partition 40). However, once the leading edge reaches the land 52, the amount of the leading edge that is swept over the exposed part of the opening a reduces as the leading edge overlies the land 52. A similar thing happens, in reverse, in respect of the leading edge of the sector 54: initially, the land 54 reduces the amount of the opening c occluded per unit angle of rotation, but this increases as the leading edge of the sector 64 progresses along and beyond the land 54.

Consequently, for a given angular speed or of movement of the disk, the rate at which the opening a or c is uncovered by the disk progressively increases as the disk opens the opening. Conversely, the rate at which the inlet a or c is blocked by movement of the disk with a constant angular velocity will decrease progressively as the opening is blocked by the disk. The lands provide greater control for the pressure that is transmitted by the valve to the pressure box as the valve moves into and out of its neutral position, and help to ensure that the relationship between exerted pressure and the angle of the disk 60 is linear, even at the point at which the pressure changes from positive to negative. Another way of achieving this relationship is shown in the embodiment illustrated in Figures 15 and 16, in which the variation in the rate of blocking or unblocking of the opening a or c is achieved by means of having a disk 401 having curved portions 400 and 402. This embodiment is in all other respects identical to that shown in Figures 7-9. Figure 16 illustrates how the curved portion varied the opening/closing rate. However, it is also clear from the last frame of Figure 16 that the curved portions of the disk 401 protrude beyond the opening (a or c) which the disk is supposed to be blocking, and thus reduce the amount of area for fluid to pass through the openings which are intended to be connected to the openings i or o. Thus, this design is less efficient than that shown in Figures 13 and 14. In all of the described embodiments, the extent of allowable rotation of the disk is limited to 120 degrees, so that to pass from the condition shown in (for

example) Figure 8 at 130 to that shown at 134 it is necessary to pass back through the position shown at 126, i.e. the neutral position.

The circumferential surfaces and faces of the disk are spaced by a very small distance from the surfaces defining the openings in the two sets, and this substantially prevents leakage of air through those spaces, without the need to provide contacting seals which would be prone to wear and would reduce the responsiveness. In the present case, the upper face of the disk (as viewed in Figures 17 and 18 of the drawings) is axially spaced from the radial edges of the walls 10 and 12 by distance of a quarter of millimetre, whilst the opposite face of the disk is similarly spaced from the radial edges of the walls 40, 42 and 44 of the lower part 2. Similarly, the circumferential surfaces of the disk are both spaced by a quarter of a millimetre from the inner circumference of the valve casing. As indicated above, however, when a sector of the disk 60 is symmetrically placed over one of the three openings in the part 2, there is a one millimetre strip of the opening which is still exposed on either side of the sector. When the valve is in the neutral position (i.e. with the sector 64 over the opening c), a small proportion of air fed to the valve from the fan outlet can thus be recirculated to the fan inlet (via the opening b), which helps to provide the linear pressure versus disk angle characteristics discussed above in relation to Figure 8. As used herein, the terms "comprises", "comprising", "including" and

"includes" are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in this specification including claims, the terms "comprises", "comprising", "including" and "includes" and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.

The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.