The present invention relates to fume cupboard extraction systems.

Fume cupboards as used in laboratories for the handling of materials which give off noxious fumes usually have an air/vapour extraction system associated with them. Many conventional extraction systems have a substantially constant volume flow rate.

As access to the interior of a fume cupboard is by way of a sliding sash, the velocity of air entering the fume cupboard will vary inversely with the size of the opening and will be greatest when the size of the opening is at its minimum. This variation in the velocity of the air entering the fume cupboard can cause operational difficulties as well as uncomfortable draughts for users of the fume cupboard. GB 2,142,425 discloses a fume cupboard the extraction system of which includes a vortex amplifier in an exhaust duct and means for supplying a control flow to the vortex amplifier, there being a control valve arranged to vary the control flow into the vortex amplifier in response to dimensional changes in the size of the opening of a sash door into the fume cupboard so as to regulate the flow into the vortex amplifier from the exhaust duct. The control flow is supplied by a fan which is connected to control ports of the vortex amplifier via a control valve, which is linked to the sash door so that the amount of air it passes is a function of the position of the sash door of the fume cupboard.

Another method of controlling the velocity of air flowing into the fume cupboard through the sash door opening is to maintain the interior of the fume cupboard

at a constant depression relative to the exterior of the fume cupboard. To a first approximation this will keep constant the velocity of the air entering the fume cupboard irrespective of the size of the sash door opening. However, the required depression is very small (of the order of 0.5 to 1 Pascal), and in practice it is difficult to maintain such a small pressure differential consistently.

According to the present invention there is provided an enclosure having an entrance aperture closed by a door adapted to provide a variable opening into the enclosure, and an exhaust duct leading from the enclosure, wherein there is included a two-stage vortex amplifier forming part of the exhaust duct, and means for so varying the total aerodynamic resistance of the two-stage vortex amplifier in response to variations in the size of the opening to the enclosure as to maintain substantially constant the velocity of gas flow into the enclosure through the opening to the enclosure irrespective of the size of the opening to the enclosure within a given operational range of movement of the door to the enclosure.

The means for varying the total aerodynamic resistance of the two-stage vortex amplifier may comprise a plurality of ducts leading to control ports of the two- stage vortex amplifier and means for sequentially closing off the ducts as the size of the opening to the enclosure is increased, the ducts leading to the control ports of the first stage of the two-stage vortex amplifier being closed off first and vice versa.

In many cases a suitable operating range of sizes for the opening to the enclosure is 4:1. However, this can be extended considerably by including a suitable

aerodynamic resistance, or choke, in the exhaust duct between the enclosure and the inlet to the first stage of the two stage vortex amplifier so as to reduce the inlet pressure to the first stage of the two stage vortex amplifier.

In use, the variation in the size of the opening to the enclosure acts as a variable aerodynamic resistance in the inlet to the first stage of the two-stage vortex amplifier. Initially the second stage of the two-stage vortex amplifier acts to provide a constant depression at the outlet from the first stage of the two-stage vortex amplifier, but as the control ports of the second stage of the two-stage vortex amplifier gradually are closed over the final part of the range of movement of the door, the depression at the outlet of the first stage of the two-stage vortex amplifier will gradually increase to a final value approximately equal to half that in the main extract duct of the outlet of the second stage of the two-stage vortex amplifier.

In a preferred embodiment ^" of the ^* invention, the enclosure is a fume cupboard having a sliding sa ^" sh ^v -door and the ducts leading to the control ports of the first stage of the two-stage vortex amplifier are opened or closed first as a result of movements of the door of the fume cupboard.

The invention will now be explained and described, by way of example with reference to the accompanying drawing, which is a diagrammatic representation of a fume cupboard system embodying the invention.

Referring to the drawing, a fume cupboard 1, shown schematically only, has a sliding sash door 2 which is raised and lowered in a conventional manner to provide

access to the interior of the fume cupboard 1. An exhaust duct 3 leads from the fume cupboard 1 to an air extraction system .

Two vortex amplifiers 5 and 6, respectively, connected in series to provide a two-stage vortex amplifier are included in the exhaust duct 3. The dimensions of the exhaust duct 3 in the region between the fume cupboard 1 and the inlet to the first stage vortex amplifier 5 are such that the aerodynamic resistance between the interior of the fume cupboard 1 and the inlet to the first stage vortex amplifier 5 is small so that, over a wide range of volume flows through the system, the depressions within the fume cupboard 1 and at the inlet to the first stage vortex amplifier 5 are the same.

Each of the vortex amplifiers 5 and 6 has four control ports 7. The control ports 7 communicate directly with the atmosphere via ducts 8 which terminate in a series of ports 9 formed in a plate 10. A cover plate 11 is attached to the sash door 2 of the fume cupboard 1 and is so arranged that when the door 2 of the fume cupboard 1 is closed, the ports 9 and hence the control ports 7 are all open so that the aerodynamic resistances of the vortex amplifiers 5 and 6 are at a maximum and decrease as the door 2 of the fume cupboard 1 is opened. The ports 9 associated with the control ports 7 of the first stage vortex amplifier 5 are arranged to be closed off first.

In the two-stage vortex amplifier system described above, the second stage vortex amplifier 6 acts initially to produce a constant depression at the outlet of the first stage vortex amplifier 5 over the range of flow rates caused by different degrees of opening of the door

2 of the fume cupboard 1, but as the control ports to 7 of the second stage vortex amplifier 6 gradually are closed over the final part of the range of opening of the door 2 of the fume cupboard 1, the depression at the outlet of the first stage vortex amplifier 5 gradually increases to approximately half that at the outlet of the second stage vortex amplifier 6. If each of the vortex amplifiers 5 and 6 has a ratio of inlet to outlet depressions over the operating range of 1 to n, then together the depression ratio is 1 to n ² . If, for example, the pressure in the exhaust duct 3 is about 100 pascals, then a ratio of inlet to outlet depressions for each of the amplifiers 5 and 6 of 1:10 is appropriate to enable the required constancy of depression and hence velocity of air flow into the fume cupboard 1 to be maintained, irrespective of the changes in the volume of air flowing into the fume cupboard as a result of the door 2 of the fume cupboard being opened to a greater or lesser extent.

Usually the operating range of door opening is about 4:1. However, if a fume cupboard requires a greater range of door openings, then the operating range of"the two-stage vortex amplifier volume flow rate control system can be extended by providing an aerodynamic resistance, or choke, in the duct 3 in the region between the fume cupboard 1 and the inlet to the first stage vortex amplifier 5 so as to lower the inlet pressure to the first stage vortex amplifier 5.

If a number of fume cupboards are to be connected into a common extraction system, then the aerodynamic conditions at each connection point could well differ and it may be necessary to match the aerodynamic characteristics of each fume cupboard to its appropriate part of the extraction system. This could be done by

varying the aerodynamic resistance, or choke, in the duct 3 in the region between the relevant fume cupboard and the inlet to the first stage of its associated two-stage vortex amplifier.