GB2161261A

GB2161261A - Fume cupboard

Info

Publication number: GB2161261A
Application number: GB08513389A
Authority: GB
Inventors: James Burns
Original assignee: MOGUL REFRIGERATION Ltd
Current assignee: MOGUL REFRIGERATION Ltd
Priority date: 1984-05-25
Filing date: 1985-05-28
Publication date: 1986-01-08
Also published as: GB8513389D0

Abstract

A fume cupboard intended principally for operation in a super-atmospheric working environment has a chamber having an opening access hatch, and a recirculation system including means for withdrawing atmosphere from the chamber, gas scrubbing means, means for returning scrubbed atmosphere to the chamber and including valve means downstream of the scrubbing means for discharging a portion of the scrubbed gas to atmosphere external the chamber.

Description

SPECIFICATION Fume cupboard This invention relates to fume cupboards for use in handling toxic, noxious, corrosive or flammable materials (hereinafter referred to, for convenience, as "toxic" materials).

Traditional fume cupboards are simple boxes with a sash window opening, the atmosphere within the box being continuously vented to atmosphere by an extractor fan or, sometimes, merely by convection. Thus the box is maintained at a slight sub-atmospheric pressure so that air tends to be drawn in rather than the internal contaminated atmosphere escape.

In certain applications it is necessary that the fume cupboard be capable of operating in an environment which is itself maintained, normally for reasons of safety, at a superatmospheric pressure.

For example, on off-shore oil rigs accommodation is maintained at superatmospheric pressure to retard the spread of fire should it occur. A fume cupboard in the geology laboratory of such a rig, for example, could not be allowed to be open to outside atmosphere as this would lead to loss of the pressurised atmosphere within the laboratory. A traditional fume cupboard cannot be used in such an environment.

More sophisticated fume cupboards are of the "total containment" type. In this arrangement the box is totally closed to ambient atmosphere and the atmosphere within the box is continuously recirculated in a closed circuit which includes a gas scrubber or other means for total destruction of contaminants. Since the box is totally enclosed, the handling of materials within can only be effected using remote handling equipment such as robot arms or by a "glove box" arrangement.

An object of this invention is to provide a fume cupboard combining ease of access with efficiency of containment of the undesirable materials therein.

Accordingly, the present invention provides a fume cupboard comprising a chamber having an opening access hatch, and a recirculation system including means for withdrawing atmosphere from the chamber, gas scrubbing means, means for returning scrubbed atmosphere to the chamber and including valve means downstream of the scrubbing means for discharging a portion of the scrubbed gas to atmosphere external the chamber.

Preferably, the recirculation system includes a gas analyser downstream of the scrubber for monitoring the concentration of toxic gases therein.

It is further preferred that the discharge valve means is operatively connected to the said analyser and discharge is activated in accordance with preselected safe concentration levels detected by the analyser.

Preferably also, the fume cupboard of this invention includes means for monitoring the pressure within the chamber said means being operatively connected to the discharge valve, the chamber being operated in use at sub-atmospheric pressure.

The fume cupboard of this invention has been designed particularly for use in a positive pressure working environment. For example, it is normal for confined spaces on offshore oil platforms to be maintained at a positive pressure: the fume cupboard of this invention operates satisfactorily in such spaces as well, of course, as normal atmospheric environments.

While the fume cupboard of the invention is in use, air is drawn into the chamber around the hatch opening (normally a sash opening) and thus there is no need for an air-tight seal around the hatch. The portion of scrubbed gas which is discharged from the system is replaced by an equivalent amount of air drawn in around the hatch opening.

The fume cupboard of this invention combines the advantages of traditional fume cupboards in respect of operator visibility and ease of access and of the glove box type of system in respect of the efficient scrubbing of the toxic gases.

The invention will now be described by way of example with reference to the accompanying drawings, of which Figure 1 is a schematic flow diagram of a fume cupboard of this invention, and Figure 2 is a cut-away view of the working chamber showing air-flow therethrough.

Referring to Figure 1 a fume cupboard 1 of this invention has a working chamber 2 from which air is withdrawn by a fan 3 through a gas scrubber 4 in which toxic materials are removed from the atmosphere. The scrubbed atmosphere is monitored continuously by an analyser 5 which measure the concentration of toxic materials in the air flow. A control valve 6 is operatively connected to the analyser 5, 7. In normal operation the valve 6 allows clean scrubbed gas to return to the working chamber 2. In order to maintain in the working chamber 2 a pressure which is lower than ambient pressure in the laboratory a small amount of the recirculating atmosphere is vented off view the valve 6 and vent 8.The operation of the valve 6 is also controlled by a pressure differential controller (not shown) which monitors the pressure difference between the working chamber 2 and the ambient atmosphere in the laboratory.

In the event of emergency conditions being detected by the toxic gas analyser 5 the valve 6 may be set to divert all the flow on to total recirculation or to total venting. Which manner of operation is used is a matter of choice for the user as is the selection of the pressure differential and the level of concentration of toxic gas which activates the valve 6. As a matter of course, when dangerous conditions are detected the analyser output operates alarm circuits and/or other safety systems.

Figure 2 of the drawing shows a typical working chamber used in this invention: it has a working chamber 10 with a vertical sash window 11. An air baffle plate 12 is located proximate the rear wall 12 of the chamber 10 with openings 14, 15 at the top and bottom. An air inlet port 17 and outlet port 18 are located in the roof of the chamber 10. Air is withdrawn via port 18 and sent through the gas scrubber and returned to the chamber 10 by port 17. Air to make up for what has been vented to atmosphere is drawn in to the chamber around the bottom of the sash window 11. The air flow in the chamber 10 is shown by arrows in Figure 2.

The design of the gas scrubbing unit depends, of course, on the nature of the toxic materials to be removed. It may utilise cyclonic1venturi action or wet filters all of them to maximise the removal. A demister may be incorporated to prevent recirculation of droplets.

Likewise, the design of the gas analyser depends on the nature of the materials to be monitored. For example, in one application where the most common toxic chemical is hydrofluoric acid, such as in a geology laboratory, this may be analysed using a technique where the hydrofluoric acid is detected electrometrically using the reaction of the acid with potassium iodide to liberate iodine. The actual analyser used is not of prime importance to this invention as it depends wholly on the material to be detected.

Claims

1. A fume cupboard comprising a chamber having an opening access hatch, and a recirculation system including means for withdrawing atmosphere from the chamber, gas scrubbing means, means for returning scrubbed atmosphere to the chamber and including valve means downstream of the scrubbing means for discharging a portion of the scrubbed gas to atmosphere external the chamber.

2. A fume cupboard as claimed in claim 1, in which the recirculation system includes a gas analyser downstream of the scrubber for monitoring the concentration of toxic gases therein.

3. A fume cupboard as claimed in claim 1 or claim 2, in which the discharge valve means is operatively connected to the said analyser and discharge is activated in accordance with preselected safe concentration levels detected by the analyser.

4. A fume cupboard as claimed in claim 1 or 2 or 3, including also means for monitoring the pressure within the chamber said means being operatively connected to the discharge valve, the chamber being operated in use at sub-atmospheric pressure.

5. A fume cupboard substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.