METHOD AND APPARATUS FOR PROVIDING SUBSTANTIALLY CONSTANT AIR FLOW THROUGH THE DOOR OPENING OF A FUME CUPBOARD

Abstract: In a method for providing substantially constant air flow velocity through the door opening of a fume cupboard for laboratory applications and the like, independent of the size of the cupboard door opening, this cupboard door opening is sensed first, whereon the exhaust air flow from the cupboard is continuously controlled in response to the sensed door opening. In an apparatus for providing such substantially constant air flow velocity a measuring device (18, 20, 26) is disposed for delivering a signal varying continuously with the opening of the door (12), said signal controlling a regulator (34, 36) in the exhaust air duct (14) from the fume cupboard (10).

Description: Method and apparatus for providing substantially constant air flow through the door opening of a fume cupboard The present invention relates to a method and apparatus for providing substantially constant air flow through the door opening of a fume cupboard for laboratory applications and the like, independant of the size of the cupboard door opening. Handling substances hazardous to health, such as liquids and gases, in different laboratory environments, for research and educational purposes and the like often takes place in fume cup boards, correct ventilation of the cupboard then being of the greatest importance. Accordingly, there are special standards for the air flow through such cupboards. According to the art known up to now, the exhaust air flow from a fume cupboard is regulated in two steps, i.e. maximum exhaust air flow when the cupboard door is open, and minimum flow with the door practically closed. The cupboard is often made so that the door cannot be closed completely, thus ensuring basic venti lation of the cupboard. This prior art has two important disadvantages. In an installa¬ tion where there is a plurality of fume cupboards, the exhaust and supply air systems must be dimensioned for the maximum air flow from all the cupboards. This generally means heavy over- dimensioning of the systems. In practice it has been found that in installations with a large number of cupboards the total combined air flow from the cupboards attains at most about 50%- 60% of the theoretical maximum flow, corresponding to the sum o all maximum flows from all cupboards. Another disadvantage with the apparatus of the known art is caused by the exhaust air flo from the cupboard being regulated to its maximum value as soon as the door to the cupboard is opened slightly. This gives rise to an airflow velocity through the cupboard which is too great when its door has only been opened slightly. This can cause the formation of turbulence, which sucks out gases and other sub¬ stances from the cupboard, which can be dangeroeus, e.g. person in front of the cupboard can inhale such gases. This is illust¬ rated in Figure 1, schematically illustrating a fume cupboard 2 with an outlet 4 for exhaust air. In the Figure, the air flow movin towards the door o enin and throu h the cu board is illustrated with arrows, and if the air flow velocity is too great turbulence of the type illustrated at 6 will occur. This turbulence 6 can suck out substances hazardous to health from the fume cupboard, and can be inhaled by the person 8 standing in front of the cupboard door. The object of the present invention is to mitigate these defiencies in the previously known art and to provide a method of achieving an apparatus for generating a desired, substantial¬ ly constant air flow velocity through the door to a fume cup¬ board, irrespective of the size of the aperture the door has been opened to, while the need for over-dimensioning discussed above of the supply and exhaust air systems is eliminated, with accompanying saving in cost. This object is achieved by a method and apparatus of the kind mentioned in the introduction and having the characterising features disclosed in claims 1 and 3. In accordance with a preferred development of the invention, the total exhaust air quantity from one or more fume cupboards in a room is measured and the amount of supply air to the room is controlled in response to this amount. In this way an excess pressure or a sub-pressure can easily be set in the room, which is often desirable. In high-risk laboratories, e.g. laboratories for AIDS, it is desirable to have a sub-pressure to lessen the risk of injurious substances or organism leaving the laboratory room. On the other hand, in clean room laboratories an excess pressure is desirable to avoid contaminants migrating into such a laboratory. A further advantage with this embodiment of the invention is that the apparatus can readily be extended with more fume cupboards up to the limit of the capacities of the supply and exhaust air systems without any special setting of the apparatus being required, since there is local control of each room unit. Another advantage of the invention is that it can be readily applied to any existing fume cupboard or cupboards. According to still another advantageous embodiment of the appa¬ ratus in accordance with the invention the measuring device for the door opening includes an IR transmitter and receiver, as well as a reflector, the transmitter and receiver being fixedly disposed relative the cupboard, with the reflector attached to the door thereof, or vice versa. The reflector is then adapted for reflecting light sent from the transmitter to the receiver, the intensity of the reflected light decreasing with increasing distance between transmitter and receiver, on the one hand, and the reflector, on the other hand, there also being a photocell arranged to generate an electrical output signal varying in response to the intensity of the received light. In this way there is obtained simply and reliably a control signed varying continuously with the size of the door opening. Since the reflective power of the reflector is sensitive to dirtying of its reflectign surface, i.e. the intensity of the reflected light decreases as the reflector gets dirty, a still further embodiment of the invention provides for the reflector to move inside a protective tube when the door moves. Regulation of the exhaust air amount from minimum to maximum when the door is raised should take place within 3 - 5 sees, and in order to obtain a sufficiently rapid resetting of the regula¬ tors when the door opening is altered, yet another advantageous embodiment of the apparatus in accordance with the invention provides for the regulator to be pneumatically controlled,, and an electropneu atic converter is arranged to convert the output signal from the measuring device for the door opening to a pneumatic signal for controlling the regulator. According to an even further advantageous embodiment of the apparatus in accordance with the invention, suitable alarms can be arranged to be triggerd, e.g. the size of the air supply to a fume cupboard is incorrect for some reason, or the exhaust air fan unit capacity is exceeded as a result of too many fume cupboards being put into operation simultaneously, or if a cupboard door is left open after work has finished for the day. An embodiment of the apparatus in accordance with the invention, selected as an example, will now be described in more detail and with reference to Figures 2 - 5. Figure 1 illustrates the turbu¬ lence formation that can occur in fume cupboards when the air flow velocity through the cupboard is too high, Figure 2 illustrates a fume cupboard provided with a measuring device for sensing the door opening such as to generate a corresponding control signal, Figure 3 illustrates more closely the implement¬ ation of the measuring device, Figure 4 illustrates an embodi¬ ment with the reflector arranged in a protective tube, and Figure 5 is a general depiction of an apparatus in accordance with the invention installed in a unit of four rooms. In Figure 2 there is illustrated a fume cupboard 10 with a raisable and ^ lowerable front window or door 12, and from the upper part of the cupboard 10 there is an exhaust air duct 14 provided with a damper. Adjacent the working surface 16 of the cupboard 10 there is a photocell 18 with and IR transmitter and receiver fixedly moun¬ ted. A reflector 20 is disposed at the lower edge of the door 12, the photocell 18 and the reflector 20 being adapted such that light from the transmitter 22 is reflected towards the receiver 24 of the photocell 18, see Figure 3. The intensity of the received IR signal decreases as the distance between the photocell 18 and the reflector 20 increas¬ es, the photocell sending an output signal which is proportional to the intensity of the received IR light. In this way there is obtained an electrical output signal from the photocell 18 which is proportional to the distance between the photocell and the reflector, i.e. proportional to the opening of the door 12. The measuring device provided thus can suitable be implemented such that the photocell 18 sens an electrical output signal varying continuously from 0 to 10 V when the opening of the door 12 varies from 400 - 0 mm. In practice the cupboard door is not often closed completely, and there is a minimum opening, e.g. 5 cm, which ensures basic ventilation. In such an embodiment, the door opening may vary from 50 - 450 mm, and constant air flow velocity through the door opening is ensured by the apparatus in accordance with the invention. The reflector 20 is subjected to dirtying, thus causing its reflective power and consequently the reflected signal to de¬ crease. For the apparatus to function correctly, it must there¬ fore be ensured that the reflector 20 is kept clean all the time. For this reason, the reflector is suitably arranged in a protective tube 26, parallell to the side edge of the door 12, a longitudinal slot 28 being made in its side facing towards the door 12, a holder arm 30 for the reflector 20 being disposed in the slot for movement along it, see Figure 4. The ouput signal from the photocell 18 is converted in an electropneumatic con¬ verter 32 to a pneumatic signal for controlling an exhaust air regulator 34 on the exhaust air duct 14 of the cupboard 10, see Figure 2. Pneumatic control of the regulator 34 is necessary to obtain sufficiently rapid setting of the exhaust air flow when the door 12 is pulled up or down. The setting time from maximum to minimum air flow, or vice versa, should be about 3 - 5 sees. Flow variators can be used as regulators, and they are sold by the Applicant with the denotation EM(JP) (BC) . The damper blade 36 is suitably made from an acid-resistant material. In Figure 5 there is shown an apparatus in accordance with the invention installed in a unit of four rooms, each of which contains four fume cupboards 10. An exhaust air regulator 34 is arranged in the exhaust air duct 14 from each of the cupboards 10. The exhaust air is extracted from each room via a common plenum duct 38, in which a volume meter 40 is disposed. This meter suitably comprises a so-called volume measuring flange. The plenum ducts 38 merge into a common main plenum duct 42, provided with an exhaust air fan unit 44. For the supply of air there is an air supply fan unit 46, disposed for feeding an air supply plenum duct 48, common to the whole unit, from which supply air ducts 50 lead to each of the rooms. In each of these ducts 50 there is a volume meter 52 and a regulator 54. The volume meters 52 are suitably volume measur¬ ing flanges of the same kind as in the exhaust plenum ducts and the regulators 54 are the same kind as the regulators in the ex¬ haust air ducts 14 of the fume cupboards 10. Supply of air to each of the rooms suitably takes place using a supply air means 56 disposed in the ceiling of the rrspective room, and adapted such that a draught-free supply of air with a variable flow is obtained within the desired working volume. It is advantageous that the air supply takes place obliquely from above, since the bodily extension of persons working at the cupboards is less from above then seen from one side, thus reducing the risk of turbulence in the air supplied. As illustrated by dashed lines in Figure 5, the volume meter 52 and regulator 54 in the duct 50 to each room are controlled from the volume meter 40 in the appropriate plenum duct 38. By controlling the air supply such as to respond to the exhaust air flow in this way, a desired air flow velocity through the cup¬ boards 10 is maintained the whole time. Air velocities of the order of magnitude of 0,4 - 0,7 m/s are suitable in the door opening. By this regulation of the supply air amount in response to the exhaust air flow an excess pressure or sub-pressure is readily provided in the individual rooms, which is desirable in many applications as discussed above. In the exhaust air main plenum duct 42 there is a pressure transducer 58 for controlling the associated fan 44, and in the supply air plenum duct 48 there is a pressure transducer 60 for controlling the fan 46 so that the pressures in the ducts 42 and 48 are kept constant at a desired value. Since there is an individual regulation of each room, further fume cupboards can be added without any further rearrangement of the apparatus being required. The apparatus in accordance with the invention is furthermore suitably implemented such that the exhaust air regulator opens to a maximum if the control signal fails. The described embodiment example is implemented with respect to regulation so that increasing exhaust air flow is provided for a decreasing control pressure signal. For the purpose of providing emergency ventilation when needed, the photocell can also be disconnected by a switch simultaneously as the exhaust air flow increases to its maximum value. The fans 44 on the exhaust air side are suitably surface treated to withstand corrosive gases. The fan blades can therefore be epoxi anodised. The apparatus in accordance with the invention can with advantage be" provided with different alarms, such as an alarm triggered on detection of a too small amount of air in the fume cupboard, or when the fan unit capacity is exceeded, i.e. too many cupboards open at the same time, or when cupboard doors have been left open after work is finsished for the day. The alarm can be both visual and acoustic. When an alarm acknow¬ ledgement has been made, the acoustic alarm can be disconnected, although the visual alarm indication continues to be illuminated until the fault is remedied, this indication then being exting¬ uished by a further alarm acknowledgement.

Claims: Claims 1. Method for providing substantially constant airflow velocity through the door opening of a fume cupboard for laboratory applications and the like, independent of the size of the cupboard door opening, characterized in that the size of said cupboard door opening is sensed, and the exhaust air flow from the cupboard is continuously controlled in response to the sensed door opening. 2. Method as claimed in claim 1, characterized in that the total exhaust air amount from one or more fume cupboards in a room is measured and the amount of air supplied to the room is controlled in response to the measured exhaust air amount. 3. Apparatus for providing substantially constant air flow velocity through the door opening of a fume cupboard for laboratory applications and the like, independant of the size of the cupboard door opening, characterized in that a measuring device is disposed for delivering a signal varying continuously with the size of the door opening, said signal controlling a regulator in the exhaust air duct from the fume cupboard. 4. Apparatus as claimed in claim 3, characterized in that a volume measuring means is arranged for measuring the total exhaust air amount from one or more fume cupboards in a room, said measuring means controlling a corresponding volume measuring means and a regulator in the supply air duct for regulating the flow of supply air to the room in response of the total exhaust air flow. 5. Apparatus as claimed in claim 4, characterized in that the volume measuring means for the exhaust air is adapted to control the volume measuring means and regulator in the supply air duct, so that a sub-pressure or excess pressure is generated in the room. 6. Apparatus as claimed in any of claims 3 - 5, characterized in that the measuring means for the door opening includes an IR transmitter, IR receiver and a reflector, the transmitter and receiver being fixedly mounted to the cupboard, and the reflector fixedly mounted on the door, or vice versa, said reflector being adapted to reflect light sent from the trans¬ mitter to the reciever, the intensity of the reflected light decreasing with increasing distance between transmitter and receiver, on the one hand, and the reflector, on the other hand, and in that a photocell is arranged for generating an electric output signal varying in response to the instensity of the received light. 7. Apparatus as claimed in claim 6, the reflector being fixed to the door such as to accompany it in its movement, characterized in that the reflector is adapted for being moved inside a protective tube when the door moves. 8. Apparatus as claimed in any of claims 3 - 7, characterized in that the regulators are pneumatically controlled and in that an electropneumatic converter is adapted to convert the output signal from the measuring means to a pneumatic signal for controlling the associated exhaust regulator. 9. Apparatus as claimed in any one of claims 3 - 8, characterized in that main plenum duct for exhaust air and plenum duct for supply air are provided with pressure trans¬ ducers adapted to control their respective associated fan for maintaining the correct constant pressure in the ducts. 10. Apparatus as claimed in any one of claims 3 - 9, characterized in that an alarm is arranged to be started in the case of incorrect magnitude of the air supply to a fume cup¬ board, should the capacity of the exhaust air fan unit be exceeded as a result of putting too many fume cupboards into operation simultaneously, or if a door is left open after termination of work for the day.

Inventors: BOERRESEN ANDERS, HOMMERSTAD TERJE, BOERRESEN, ANDERS, HOMMERSTAD, TERJE
Applicants: FLAEKT AB, ABB FLAEKT AB
Application year: 1991
Main classification: B08B15
IPC codes: B08B15