GB2408793A

GB2408793A - Gas vent with linked control members contra-rotating at a predetermined pressure differential

Info

Publication number: GB2408793A
Application number: GB0328097A
Authority: GB
Inventors: Richard Gatley
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-12-04
Filing date: 2003-12-04
Publication date: 2005-06-08
Also published as: GB0328097D0

Abstract

A vent 1 for gas flow control through an opening 2 has first and second control members 7,8, and a link 9 constraining the control members to pivot together in opposite rotational directions. The control members are mounted on pivots 10,13 which are offset relative to the centres of pressure and gravity of the control members so that differential pressure across the opening will exert a moment on the control members. The link 9 balances the moment of the first control member 7 with the moment of the second control member 8. A counter-balance weight 27 allows the control members 7,8 to pivot from the closed position to the fully open position at a predetermined pressure differential. The link may comprise radius arms 24,25 and a coupling arm 26, or meshed gear wheels (not illustrated). Stops 20,21 limit the open and closed positions of the control members 7,8. The control members 7,8 have edges 22,23 which overlap or engage when the control members are in their closed positions, to substantially close the opening 2. Embodiments with three or four control members are described. The vent may maintain pressure in a room to suppress the spread of fire, or arranged to release excess pressure in a room during release of gas to combat a fire.

Description

Title: Improvements relating to Vents for Gases The invention relates to

improvements relating to vents for gases, and to providing a vent which has improved venting capacity, and which is weather resistant and provides a fire stop.

It is common to use water sprinkler systems, to combat fires. However, although such systems may reduce the damage caused by a fire, a significant amount of damage may be caused by the water itself. There is therefore a move to using other fire combatants rather than water. For example, gases which dilute the oxygen content of air in a room, but which do not react with fire, are being used. On detection of a fire, such a gas or gases are released into the room, suppressing the progress of the fire by diluting the oxygen in the room.

Gases such as argon or nitrogen may used in this way. These gases may be released into a room from, for example, cylinders in the room, which are generally arranged to vent gas quickly into the room, to quickly dilute the oxygen content therein. This in itself may lead to problems, in that there may be a sudden large increase in the pressure in-the room. It is therefore desirable to provide the room with at least one gas vent, which is capable of rapidly venting gas from the room. Rapid venting capability could be achieved by using a vent which maximises the area through which the gas can flow, without necessarily increasing the overall size of the vent. For example, a vent could be used which comprises two or more blades, and eliminating the mid-feathers between the blades. However, such a vent should also provide a fire stop.

In another application, vents for gases may be used to control the pressure in a pressurised environment. For example, the environment could be a pressurised stairwell, in which it is desired to maintain the pressure at a predetermined level above atmospheric pressure, to suppress the spread of fire into the stairwell. In such a pressurised environment, one or more gas vents may be used to closely control the pressure, to keep this as close as possible to the predetermined level above atmospheric pressure.

It is desirable to provide a vent for gases, which will be capable of use in both of the above applications, i.e. will be capable of rapid venting and close pressure control, whilst providing a fire stop.

According to the invention there is provided a gas vent adapted to be mounted in a opening to control gas flow through the opening, comprising at least a first and a second flow control member, at least one link connecting the flow control members, and at least one counter-balance wherein the first flow control member is mounted across the opening on a first pivot mechanism which provides a first pivot axis positioned with respect to the first flow control member to allow the first flow control member to pivot between a closed position and a fully open position, and to provide a moment of the first flow control member, the second flow control member is mounted across the opening, adjacent to the first flow control member, on a second pivot mechanism which provides a second pivot axis positioned with respect to the second flow control member to allow the second flow control member to pivot between a closed position and a fully open position, and to provide a moment of the second flow control member, opposite to and at least substantially equal to the moment of the first flow control member, the flow control members each comprise an edge extending across the opening, which edges overlap when the flow control members are in their closed positions, so that together the flow control members at least substantially close the opening, the link constrains the flow control members to pivot together and balances the moment of the first flow control member with the opposite and at least substantially equal moment of the second flow control member, and the counter-balance is operable to cause the flow control members to pivot from the closed position to the fully open position as a result of a pre- determined pressure differential across the opening.

The first pivot axis may be offset with respect to a cenke line of the first flow control member. The second pivot axis may be offset with respect to a centre line of the second flow control member. The offset of the first pivot axis with respect to the centre line of the first flow control member may be opposite to the offset of the second pivot axis with respect to the centre line of the second flow control member. For example, the first pivot axis may be offset above the centre line of the first flow control member, and the second pivot axis may be offset below the centre line of the second flow control member, or vice versa. Opposite offsets of the first pivot axis and the second pivot axis may allow the first flow control member to pivot in a first direction, between the closed position and the fully open position, and may allow the second flow control member to pivot in a second direction, between the closed position and the fully open position, which is opposite to the first direction of the first flow control member.

For each flow control member, offsetting the pivot axis from the centre line results in offsetting the pivot axis from the centre of pressure of the flow control member. When there is a pressure differential across the opening, resulting in a force on one face of the flow control member, as the pivot axis and centre of pressure do not coincide, the flow control member will be caused to pivot about the pivot axis.

Each flow control member may be constrained to pivot through a maximum of approximately 90 , from the closed position at 0 to the fully open position at approximately 90 . With such a fully open position, the vent is particularly useful for rapid venting, e.g. of a room in which a firesuppressant gas has been released. Each flow control member may be constrained to pivot through less than 90 , to provide differing degrees of pressure and flow control.

For example, each flow control member may be constrained to pivot through a maximum of approximately 45 , from the closed position at 0 to the fully open position at approximately 45 . At a maximum pivotal movement of approximately 45 , it is possible to provide very close pressure control, a feature which is particularly useful in environments such as pressurised stairwells. The vent may be provided with one or more stops, which may be used to constrain the pivotal movement of each flow control member.

The overlap of the edges of the flow control members may be achieved by providing the edge of the first flow control member with a channel, such that when the flow control members are in their closed positions, the edge of the second flow control member locates in the channel of the edge of the first flow control member. The channel may be of substantially Vshaped cross section, or U-shaped cross section. The channel may extend along the whole length of the edge of the first flow control member. The entire length of the edge of the second flow control member may be received in the channel.

The gas vent does not comprise a mid-feather between the two flow control members. Such a mid-feather is not wanted or useful. In vents of conventional design, the flow control members rotate the same direction, and a mid-feather is necessary for the aerodynamic separation of adjacent flow control members. In the vent of this invention, adjacent flow control members rotate in opposite directions, and a mid-feather between them is therefore not necessary. The absence of a mid-feather enables the edges of the first and second flow control members to overlap, e.g. by location of the edge of the second flow control member in a channel of the edge of the first control member. When the flow control members are in their closed positions, such overlapping provides weather resistance, e.g. resistance to the ingress of wind or rain through the vent, and a fire stop, e.g. resistance to the egress of smoke or flames through the vent. The vent may therefore be mounted in, for example, a wall or a ceiling of a room to vent gas to the outside of the room.

As no mid-feather is used, the area of the vent through which gas may flowis increased with regard to similarly-sized, conventional vents having one or more mid-feathers. Thus the speed at which the vent can vent gas is increased over such conventional vents. The area of the vent through which gas may flow may be in the range 80 to 85% of the total area of the opening.

When the first flow control member pivots in an opposite direction to the second flow control member, this allows the edge of the second flow control member to disengage from the channel of the edge of the first flow control member, on movement of the flow control members from their closed positions to their fully open positions. This also allows the edge of the second flow control member to locate in the channel of the first flow control member, on movement of the flow control channels from their fully open positions to their closed positions. This would not happen if the flow control members pivoted in the same direction.

The link may comprise a first radius arm, a second radius arm, and a coupling arm. The first radius arm may be connected between the first pivot axis of the first flow control member and a first end of the coupling arm, and the second radius arm may be connected between a second end of the coupling arm and the second pivot axis of the second flow control member. The first radius arm may be fixedly connected to the first pivot axis, and rotatably connected to the first end of the coupling arm. The second radius arm may be fixedly connected to the second pivot axis, and rotatably connected to the second end of the coupling arm. The positions of the first and second radius arms with regard to the first and second pivot axes respectively and the length of the radius arms and the coupling arm may be chosen such that the moment of the first flow control member is at least substantially equal to the moment of the second flow control member, when the flow control members are each in their fully open position. The link may comprise a gear mechanism, comprising a first gear wheel connected to the first flow control member, meshed with a second gear wheel connected to the second flow control member. The link may be driven, for example by a motor, to pivot the flow control members between their closed positions and their fully open positions. The link may thus link together the first and second flow control members. If either of the flow control members is constrained to pivot, e.g. by a pressure differential across the opening, the other flow control member will also be constrained to pivot, i.e. the flow control members are constrained to pivot together.

Linking the two flow control members so that they pivot together, allows the edge of the second flow control member to disengage from and locate in the lO channel in the edge of the first flow control member, on movement of the flow control members between their closed positions and their fully open positions.

The first pivot axis may be offset with respect to the centre of gravity of the first flow control member, resulting in the moment of the first flow control member. The second pivot axis may be offset with respect to the centre of gravity of the second flow control member, resulting in the moment of the second flow control member. For example, for each flow control member, offsetting the pivot axis with respect to the centre of gravity may be achieved by offsetting the pivot axis above or below the centre line of the flow control member. The offset of the first pivot axis with respect to the centre of gravity of the first flow control member may be equal and opposite to the offset of the second pivot axis with respect to the centre of gravity of the second flow control member, resulting in opposite and at least substantially equal moments of the first flow control member and the second flow control member. For each flow control member, the moment causes the flow control member to try to take up a neutral position, with regard to the action of gravity on the member.

Linking the two flow control members balances the moment of one flow control member against the moment of the other flow control member, and the tendency to return to a neutral position is negated. With the use of the counter- balance, the flow control members can therefore take up and maintain a position between their closed positions and their fully open positions, according to the flow of gas through the vent, without the tendency to return to their neutral positions.

The counter-balance may be provided by one or more weights. These may be attached, for example, to a flow control member, or to a radius arm or a coupling arm of the link, or a radius arm provided on the vent. The counter balance may be provided by a torsion spring attached between a flow control member and its pivot mechanism. The courter-balance may be provided by a radius arm attached between a flow control member and a tension or compression spring fixed to the opening. The counter-balance may be provided by electromechanical means, or hydraulic means or pneumatic means. The counter-balance allows the flow control members to move from their closed positions to their fully open positions when the pressure differential across the opening reaches the pre-determined pressure differential, and allows the flow control members to return to their closed positions when the pressure differential drops below a predetermined pressure differential.

The flow control members may each comprise a generally flat plate or a blade. The flow control members may be shaped, for examplFto increase the aerodynamics or the fire resistance of the members. Each flow control member may be manufactured from fire-resistant material. This will increase the capacity of the vent to act as a fire stop. As the flow control members are linked together, should any of the members distort out of shape, this will cause the other members to distort, and the fire stop function of the vent will be maintained.

The vent may comprise an even number of flow control members. For example, the vent may comprise four flow control members, and a first two flow control members may be linked and their moments balanced, a second two flow control members may be linked and their moments balanced, and the first two flow control members may be linked to the second two flow conkrol members, to allow all of the flow control members to pivot together. The vent may comprise an uneven number of flow control members. For example, the vent may comprise three flow control members, and a first two flow control members may be linked and their moments balanced, and the third flow control member may be linked to the first two flow control members, but not balanced, to allow all of the flow control members to pivot together.

The vent may be mounted in the opening such that it is at any orientation with respect to the vertical. For example, the vent may be mounted in the opening such that it is substantially vertical, e.g. the opening may be provided in a wall of a room, or the vent may be mounted in the opening such that it is substantially horizontal, e.g. the opening may be provided in a ceiling of a room. The pivot axis of each flow control member may therefore be placed in various orientations, unlike the pivot axes of conventional vents which are generally only placed in a horizontal orientation.

The vent may comprise a frame surrounding the flow control members.

The frame may be fixed to one or more portions of the opening to mount the vent in the opening.

- The vent may be provided with, or may be used in conjunction with, means to automatically move the flow control members to their closed positions, e.g. in the event of the vent being directly exposed to flames. For example, the means may comprise a plate provided with a peg, placed in proximity to one of the flow conko1 members. The plate may be soldered in place, in a first position in which the peg allows the flow control member to move between the closed position and the fully open position. When the plate is exposed to heat from the flames, the solder melts, and a spring attached to the plate may move the plate to a second position, causing the peg to engage a radius arm provided on the flow control member which forces the flow control member (and therefore all other flow control members) to move to their closed positions.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a side view of a gas vent according to the invention, and Figure 2 is a schematic representation of a front view of the vent of Figure 1.

Figures 1 and 2 show a gas vent 1 mounted for use in an opening 2 provided between walls 3 and 4 of a room 5. The vent 1 controls the flow of gas through the opening 2, from the room 5 to another space 6 e.g. the outdoors.

The vent 1 comprises a first flow control member 7, a second flow control member 8, and a link 9 connecting the flow control members. Each flow control member 7, 8 comprises a generally flat plate. The first flow control member 7 is mounted across the opening 2 on a first pivot mechanism 10. The first pivot mechanism 10 provides a first pivot axis 11 (Figure 2) , about which the first flow control member 7 pivots. Similarly, the second flow control member 8 is mounted across the opening 2 on a second pivot mechanism 13. The second pivot mechanism 13 provides a second pivot axis 14 (Figure 2), about which the second flow control member 8 pivots.

The first pivot axis 11 is offset above a centre line 12 of the first flow control member 7 (see Figure 2), i.e. is offset above the centre of pressure of the flow control member 7. When the gas pressure in the room 5 increases above the pressure of the space 6, a pressure differential is set up across the opening 2, and this results in a force being applied to a face of the first flow control member 7 adjacent the room 5. As the pivot axis 11 and the centre of pressure do not coincide, the force, if large enough, will cause the first flow control member 7 to pivot about the first pivot axis 11, in an anticlockwise direction. The first flow control member 7 can pivot between a closed position at 0 (as shown in the figures) to the vertical, and a fully open position at 90 to the vertical. The vent is provided with a stop 20 on the wall 3 of the opening 2, and a stop on the first pivot mechanism (not shown), which prevent the first flow control member 7 from pivoting past the closed and open positions.

The second pivot axis 14 is offset below a centre line 15 of the second flow control member 8 (see Figure 2), i.e. is offset below the centre of pressure of the flow control member 8. When the gas pressure in the room 5 increases above the pressure of the space 6, a pressure differential is set up across the opening 2, and this results in a force being applied to a face of the second flow control member 8 adjacent the room 5. As the pivot axis 14 and the centre of pressure do not coincide, the force, if large enough, will cause the second flow control member 8 to pivot about the second pivot axis 14, in a clockwise direction. The second flow control member 8 can pivot between a closed position at 0 (as shown in the figures) to the vertical, and a fully open position at 90 to the vertical. The vent is provided with a stop 21 on the wall 4 of the opening 2, and a stop on the second pivot mechanism (not shown), which prevent the second flow control member 8 from pivoting past the closed and open positions. Thus by offsetting of the pivot axes from the centre lines and the centres of pressure of the flow control members, each flow control member can pivot when subjected to a pressure differential across the opening. If the pivot axis and the centre lines/centres of pressure of each flow control member coincided, this would not be the case.

The first and second pivot mechanisms 10, 13 are positioned in the opening 2 such that the first and second pivot axes 11, 14 are parallel and aligned in a vertical plane. Thus when each flow control member 7, 8 is in its closed position, the flow control members 7, 8 are aligned in a vertical plane, and together at least substantially close the opening 2, reducing the flow of gas therethrough.

The first flow control member 7 is provided at a bottom edge thereof, with a channel 22, which runs along the length of this edge and has a substantially V-shaped cross section. The channel 22 is of sufficient dimensions, such that when the first and second flow control members 7, 8 are each in their closed position, a top edge 23 of the second flow control member 8 is located within the channel 22 of the bottom edge of the first flow control member 7. This provides an overlap between the bottom edge of the first flow control member 7 and the top edge of the second flow control member 8, as shown in Figure 1. This arrangement allows the vent to provide a fire stop, e.g. which resists the egress of smoke or flames from the room 5 to the space 6. As a mid-feather between the flow control members 7 and 8 is not used, the area of the vent 1 through which gas may flow is increased over a similarly-sized vent which uses a mid-feather. Therefore a vent with improved venting capacity is provided by the vent 1.

The first pivot axis 1 1 is offset above the centre line 12 of the first flow control member 7. The first flow control member 7 therefore pivots in an anticlockwise direction from the closed to the fully open position. The second pivot axis 14 is offset below the centre line 15 of the second flow control member 8. The second flow control member 8 therefore pivots in a clockwise direction from the closed to the fully open position. Opposite offsets of the first and second pivot axes 11, 14 are chosen so that the first and second flow control members 7, 8 will pivot in opposite directions from their closed to their fully open positions. When the first and second flow control members 7, 8 each start in the closed position and pivot towards their fully open positions, pivoting in the opposite direction allows the top edge 23 of the second flow control member 8 to disengage from the channel 22 of the first flow control member 7.

Similarly, as the first and second flow control members 7, 8 resume their closed positions, pivoting in the opposite direction allows the top edge 23 of the second flow control member 8 to locate in the channel 22 of the first flow control member 7. If the first and second flow control members 7, 8 were to pivot in the same direction, such disengaging and location would not be possible.

The vent 1 comprises a link 9, which connects the first flow control member 7 to the second flow control member 8. The link 9 comprises a first radius arm 24, a second radius arm 25, and a coupling arm 26. The first radius arm 24 is connected between the first pivot mechanism 10 of the first flow control member 7 and a first end of the coupling arm 26. The second radius arm 25 is connected between a second end of the coupling arm 26 and the second pivot mechanism 13 of the second flow control member 8. The link 9 thus links together the first and second flow control members 7, 8, via the first and second pivot mechanisms 10, 13. If either the first or the second flow control member is constrained to pivot, e.g. by a pressure differential across the opening 2, the second or the first flow control member respectively will also be constrained to pivot. The first and second flow control members 7, 8 will therefore pivot together. This allows the top edge 23 of the second flow control member 8 to disengage from the channel 22 of the first flow control member 7, when the first and second flow control members 7, 8 move from their closed positions towards their fully open positions,nd, as the first and second flow control members 7, 8 resume their closed positions, allows the top edge 23 of the second flow control member 8 to locate in the channel 22 of the first flow control member 7.

Offsetting the first pivot axis 11 from the centre line 12 of the first flow control member 7, and offsetting the second pivot axis 14 from the centre line of the second flow control member 8, allows the flow control members 7, 8 to pivot in response to a pressure differential across the opening 2. For each flow control member 7, 8, offsetting the pivot axis from the centre line also results in offsetting the pivot axis of the flow control member from the centre of gravity of the flow control member. This, in turn, results in a moment of each flow control member 7, 8, which causes the flow control member to try to take up a neutral position with regard to the action of gravity on the member.

For the first flow conkol member 7, the neukal position is the closed position of the member, as shown in Figures 1 and 2. When the gas pressure in the room 5 increases above the pressure of the space 6, a pressure differential is set up across the opening 2, causing the first flow control member 7 to pivot in an anticlockwise direction. When the gas pressure in the room 5 decreases again, the influence of gravity on the first flow conkol member 7 will cause it to pivot in a clockwise direction back to the closed position, i.e. its neukal position. For the second flow control member 8, the neukal position is the closed position of the member pivoted through 180 . When the gas pressure in the room 5 increases above the pressure of the space 6, a pressure differential is set up across the opening 2, causing the second flow conkol member 8 to pivot in a clockwise direction. If the pressure differential is great enough, the second flow conkol member 8 will pivot sufficiently far that the influence of gravity on the member causes it to continue to pivot in a clockwise direction to the closed position plus 180 , i.e. its neukal position. It is undesirable for the flow control members 7, 8 to be allowed to talce up their neukal positions. This is prevented by balancing the moment of the first flow conkol member 7 using the moment of the second flow conkol member 8. The offset of the first pivot axis 11 from the centre line 12 of the first flow conkol member 7 is equal and opposite to the offset of the second pivot axis 14 from the centre line 15 of the second flow conkol member 8. The moment of the first flow conkol member 7 will therefore be opposite to and at least substantially equal to the moment of the second flow conkol member 8. Connecting the two flow conkol members 7, 8 using the link 9, will therefore balance the moment of the first flow conkol member 7 using the moment of the second flow control member 8, and vice versa. The flow conko1 members 7, 8 are therefore able to pivot, under the influence of any pressure differential across the opening 2, rather than under the influence of gravity. With the use of the counter- balance 27, the flow conkol members 7, 8 can therefore take up and maintain a position between their closed positions and their fully open positions, according to the gas venting required from the room 5.

The vent further comprises a counter-balance 27, in the form of a weight, attached to the second flow control member 8, as shown. Although it is stated that the flow control members 7, 8 can take up and maintain a position between their closed position and their fully open position, in practice it is desirable that each flow control member 7, 8 remains in its closed position until the pressure in the room 5 exceeds an allowed maximum, i.e. the pressure differential across the opening 2 reaches a pre-determined level, when each flow control member 7, 8 moves to its fully open position, and when the excess pressure is vented, i.e. the pressure differential across the opening 2 falls below a pre-determined level, each flow control member 7, 8 moves from its fully open position to its closed position. The pre-determined level of the pressure differential across the opening at which each flow control member opens may be different to the pre-determined level of the pressure differential acrossthe opening at which each flow control member closes. The difference between the two pre-determired levels of the pressure differential may depend on the maximum pivotal movement of the flow controls members. For example, the difference between the two pre- determined levels of the pressure differential when each flow control member is constrained to pivot through a maximum of approximately 90 may be much greater than when each flow control member is constrained to pivot through a maximum of approximately 45 . Controlling of the operation of the flow control members 7, 8 in this way is achieved by the provision of the counter-balance weight 27. The location and size of the weight is chosen such that the flow control members 7, 8 will open at the predetermined pressure differential across the opening 2. Only one counter- balance is needed for both of the flow control members 7, 8, as these are connected by the link 9, and pivot together. It will be appreciated, however, that more than one counter-balance weight could be used.

In the embodiment described, the vent 1 comprises only one link 9. It will be appreciated that a second link could be provided, located on an opposite side of the first and second pivot mechanisms 10, 13 to the link 9. Further, in this embodiment, the link is described as comprising two radius arms connected together by a coupling arm. It will be appreciated that different methods of implementing the link could be used, for example the link could comprise a gear mechanism, in the form of a first gear wheel connected to the first flow control member 7, meshed with a second gear wheel connected to the second flow control member 8.

In the embodiment described, the vent l comprises two flow control members 7, 8. It will be appreciated that a vent according to the invention could be provided which comprises three, four or more flow control members.

When the vent comprises three flow control members, two of the flow control members will be linked and their moments balanced as above. The third flow control member will be linked to one of the first two flow control members, so that all three members can pivot together. The moment of the third flow control will not be balanced by either of the first two flow control members, and this may be balanced in a conventional manner. When the vent comprises four flow control members, a first two flow control members will be linked and their moments balanced as above, and a second two flow conko1 members will be linked and their moments balanced as above. The first two flow control members will be linked to the second two flow control members, so that all four members can pivot together.

In the embodiment described, the vent 1 is mounted vertically in the opening provided between walls 3, 4 of the room 5. It will be appreciated that the vent could be positioned horizontally in an opening, for example in a ceiling of a room, or could be positioned such that it is at any orientation with regard to the vertical.

In the present specification "comprises" means "includes or consists of" and "comprising" means "including or consisting of".

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A gas vent adapted to be mounted in a opening to control gas flow through the opening, comprising at least a first and a second flow control member, at least one link connecting the flow control members, and at least one counter-balance, wherein the first flow control member is mounted across the opening on a first pivot mechanism which provides a first pivot axis positioned with respect to the first flow control member to allow the first flow control member to pivot between a closed position and a fully open position, and to provide a moment of the first flow control member, the second flow control member is mounted across the opening, adjacent to the first flow control member, on a second pivot mechanism which provides a second pivot axis positioned with respect to the second flow control member to allow the second flow control member to pivot between a closed position and a fully open position, and to provide a moment of the second flow control member, opposite to and at least substantially equal to the moment of the first flow control member, the flow control members each comprise an edge extending across the opening, which edges overlap when the flow control members are in their closed positions, so that together the flow control members at least substantially close the opening, the link constrains the flow control members to pivot together and balances the moment of the first flow control member with the opposite and at least substantially equal moment of the second flow control member, and the counter-balance is operable to cause the flow control members to pivot from the closed position to the fully open position as a result of a pre deterrnined pressure differential across the opening.

2. A vent according to claim 1 in which the first pivot axis is offset with respect to a centre line of the first flow control member.

3. A vent according to claim 1 or claim 2 in which the second pivot axis is offset with respect to a centre line of the second flow control member.

4. A vent according to claim 3, as dependent from claim 2, in which the offset of the first pivot axis with respect to the centre line of the first flow control member is opposite to the offset of the second pivot axis with respect to the centre line of the second flow control member.

5. A vent according to claim 4 in which the first pivot axis is offset above the centre line of the first flow control member, and the second pivot axis is offset below the centre line of the second flow control member, or vice versa.

6. A vent according to claim 4 or claim 5 in which the opposite offsets of the first pivot axis and the second pivot axis allow the first flow control member to pivot in a first direction, between the closed position and the fully open position, and allow the second flow control member to pivot in a second direction, between the closed position and the fully open position, which is opposite to the first direction of the first flow control member.

7. A vent according to any preceding claim in which each flow control member is constrained to pivot through a maximum of approximately 90 , from the closed position at 0 to the fully open position at approximately 90 .

8. A vent according to any preceding claim in which the vent is provided with one or more stops, which are used to constrain the pivotal movement of each flow control member.

9. A vent according to any preceding claim in which the overlap of the edges of the flow control members is achieved by providing the edge of the first flow control member with a channel, such that when the flow control members are in their closed positions, the edge of the second flow control member locates in the channel of the edge of the first flow control member.

10. A vent according to any preceding claim in which the link comprises a first radius arm, a second radius arm, and a coupling arm.

11. A vent according to claim 10 in which the first radius arm is connected between the first pivot axis of the first flow control member and a first end of the coupling arm, and the second radius arm is connected between a second end of the coupling arm and the second pivot axis of the second flow control member.

12. A vent according to any preceding claim in which the link comprises a gear mechanism, comprising a first gear wheel connected to the first flow -control member, meshed with a second gear wheel connected to the second flow control member.

13. A vent according to any preceding claim in which the first pivot axis is offset with respect to the centre of gravity of the first flow control member, resulting in the moment of the first flow control member.

14. A vent according to any preceding claim in which the second pivot axis is offset with respect to the centre of gravity of the second flow control member, resulting in the moment of the second flow control member.

15. A vent according to claim 13 or claim 14 in which for a flow control member, offsetting the pivot axis with respect to the centre of gravity may be achieved by offsetting the pivot axis above or below the centre line of the flow control member.

16. A vent according to any of claims 13 to 15 in which the offset of the first pivot axis with respect to the centre of gravity of the first flow control member is equal and opposite to the offset of the second pivot axis with respect to the centre of gravity of the second flow control member, resulting in opposite and at least substantially equal moments of the first flow control member and the second flow control member.

17. A vent according to any preceding claim in which the counter-balance is provided by one or more weights, attached to a flow control member.

18. A vent according to any of claims 1 to 16 in which the counterbalance is provided by a torsion spring attached between a flow control member and its pivot mechanism.

19. A vent according to any of claims 1 to 16 in which the counterbalance is provided by a radius arm attached between a flow control member and a tension or compression spring fixed to the opening.

20. A vent according to any preceding claim which is mounted in the opening such that it is substantially vertical.

21. A vent according to any of claims 1 to 19 which is mounted in the opening such that it is substantially horizontal.

22. A vent substantially as described herein, with reference to Figures 1 and 2 of the accompanying drawings.

23. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.