GB2076145A

GB2076145A - Enclosures with a Gas Extraction System

Info

Publication number: GB2076145A
Application number: GB8109220A
Authority: GB
Original assignee: A C PLASTIC IND
Current assignee: A C PLASTIC IND
Priority date: 1980-03-24
Filing date: 1981-03-24
Publication date: 1981-11-25
Also published as: GB2076145B

Abstract

In a row of fume cupboards each fume cupboard 2 is closed by a door 4 and connected by a respective duct 5 and a common manifold 3 to a common air extraction fan 24. Within each duct 5 there is a rotatable damper 6 connected to an electric motor which incorporates a potentiometer. Incorporated in a sensor member adjacent the door is a plurality of magnetically actuated switches each associated with a respective resistance of a resistance ladder 10 and with a magnet which normally holds the switch closed. As the door is opened a plate secured to it progressively comes between the switches and their associated magnets thus permitting the switches to open and increasing the resistance of the resistance ladder 10. This resistance, which is indicative of the position of the associated door 4, is connected to an amplifier which moves the electric motor and thus the damper 6. The outputs of the potentiometers are summed by a summing amplifier 26 whose output varies the speed of the fan 24. <IMAGE>

Description

SPECIFICATION Enclosures with a Gas Extraction System The present invention relates to enclosures having a gas extraction system and in particular to fume cupboards in which air is continuously drawn through the enclosure and is subsequently expelled to the atmosphere. Such enclosures are used for carrying out reactions or for storage of chemicals which produce or potentially produce dangerous or noxious gases. These gases are entrained with the air, drawn through the enclosure by the gas extraction system and are then either expelled to the atmosphere with the air, or are removed from the air and disposed of by some other means.

Chemistry or research laboratories may have a number of fume cupboards, sometimes many hundreds, each of which is provided with its own air extraction fan or which is connected via a common manifold to a single high capacity extraction fan. The volume of air extracted through each fume cupboard is set in accordance with operational requirements such as the size of the cupboard and the toxicity of the gases which are likely to be present, but is generally in the region of 0.2 to 0.5 cubic metres per second.

Fume cupboards are generally provided with a vertically movable sash door counterbalanced by weights, and the air extraction rate is set so that when the sash is fully open the air velocity through the opening attains a predetermined value sufficient to entrain any gases that may be present. However, as the sash is closed the suction of the air extraction fan is applied across a decreasing cross-sectional area and the velocity through the aperture increases. In fact the volume of air removed from a laboratory by a conventional fume cupboard remains substantially constant regardless of whether the sash is open or closed, and thus when the sash is nearly closed the air velocity through the remaining cross-sectional area may be very considerable.

Air that is removed from a laboratory by a fume cupboard is replaced by unheated air from outside and although this may be acceptable in a laboratory having a single fume cupboard it has been found that laboratories having a large number of such fume cupboards are extremely expensive to heat by virtue of the rapid replacement of air in the laboratory that occurs. It has been calculated that the additional power required to heat the air that is sucked into the laboratory due to the presence of fume cupboards is of the order of 12 KW per day per fume cupboard.

According to the present invention an enclosure includes a door which may be opened to gain access to the interior of the enclosure, air extraction means connected to the interior of the enclosure by a duct, a movable ddmper situated in the duct, means to generate a signal indicative of the position of the door, means to adjust the position of the damper in accordance with the magnitude of the said signal and means to vary the extraction rate in accordance with the magnitude of the said signal.

The provision of the damper which opens and closes with the door ensures that the maximum extraction of air from the enclosure only occurs when the door or sash is fully open, and as the door is progressively closed so the air flow is progressively reduced. The damper is preferably so arranged that the air velocity through the open space is substantially constant regardless of the position of the door. Since in practice fume cupboard doors are open only for a minor proportion of the time, this leads to a considerable reduction in the total air throughput and thus a reduction in the energy used both to operate the air extraction means and to heat the room in which the enclosure is situated.Preferably the means to adjust the position of the damper comprises an electric motor and preferably there is an amplifier which produces a signal varying with the value of the said signal and drives the motor. The speed of the air extraction means may be varied directiy in dependence on the said signal or in dependence with a further signal which varies with the said signal. In one embodiment this further signal is produced by a potentiometer whose setting varies with the position of the electric motor.

The means to generate the position signal may take various forms but conveniently comprises a resistance chain in which resistances may be connected into or out of the chain by the interruption of a magnetic field acting on a magnetically actuated switch by a member secured to and moving with the door.

The invention is of particular value when the enclosure is one of many such enclosures connected by means of a manifold to a common air extraction means. Thus in a further embodiment of the invention there is provided a plurality of such enclosures each with an individual damper individually operable in dependence on the magnitude of the respective door position signal, summing means to sum each of the said signals or further signals which vary with the said signals to produce a summed signal and means to vary the speed of the air extraction means in dependence on the magnitude of the said summed signal.

Further features and details of the invention will be apparent from the following description of two specific embodiments of the invention which is given by way of example with reference to the accompanying drawings in which: Figure 1 is a scrap diagrammatic perspective view of the front of a single fume cupboard.

Figure 2 is a simplified block diagram of the control circuitry for a single fume cupboard; and Figure 3 is a schematic plan view of an array of adjacent fume cupboards showing both the electrical and air connections.

In its simpler form the invention is embodied in a single fume cupboard of generally conventional construction connecting at its upper end by means of a duct to an air extraction fan. The cupboard, designated 2 in Figure 1 has a vertically movable front sash 4 counter-balanced by two weights. Situated within the duct is a damper 6 (seen in Figure 2) connected directly or by means of an intermediate linkage to an electric actuating motor 8 which can rotate the damper from a position in which it substantially blocks the duct and permits only a relatively low rate of gas extraction to a position in which it offers substantially no resistance to air flow. The motor 8 rotates the damper to a position predetermined by the input from a sash position sensor circuit generally designated 10.This circuit comprises a plurality of resistors 12 in series, in this case 5, each of which is associated with a reed switch 14 by means of which the resistance can be short circuited. These switches 14 are spaced apart along, and secured to, or encapsulated in, one limb of a channel section sensor member 1 6 seen in Figure 1 situated above the sash when it is in its closed position. Normally each of the switches is held in the position in which it short circuits its associated resistor 12 by a respective permanent magnet 1 8 which is similarly secured to or encapsulated in the other limb of the sensor member. Thus when the sash is closed, each of the switches 14 is also held closed by the magnets and the total resistance of the circuit 10 is approaching zero.Projecting from the side of the sash 4 is a metallic plate 20 positioned to pass up between the two limbs of the sensor member. Thus as the sash is raised the plate 20 progressively comes between the switches and magnets causing the switches to open one by one under their natural spring bias and thus the resistance of the circuit 10 to rise incrementally, the size of each increment being equal to the value of a resistor 12.

The circuit 10 is connected to the input of an amplifier 22 which produces an output dependent on the magnitude of the resistance, and thus indicative of the extent to which the sash is open.

The output from the amplifier 22 is fed to the motor 8 which incorporates a potentiometer which is rotated with the motor shaft and has a voltag. across it which is indicative of the angular position of the shaft and thus of the damper. The output from the amplifier 22 is compared with the voltage across the motor potentiometer and the motor is rotated until these two values are in equality. The angular position of the damper is thus dependent on the extent to which the sash 4 is open. The output of the motor potentiometer is also connected to the control circuitry of the frame cupboard air extractor fan 22 so that the fan speed is also reduced as the sash is opened.

Thus the air throughput of the fan 24 and the available cross sectional area of the duct are increased as the sash is opened to maintain a substantially constant air flow velocity into the fume cupboard. By suitable adjustment of the motor potentiometer the angular rotation of the damper may be varied. The damper motor may also include stops to limit its travel.

Figure 3 is a schematic plan representation of a series of adjacent fume cupboards 2, of which only four are shown, each of which has its own sash 4 and sensor 10 as described above. Each fume cupboard is connected to a manifold 3 by a respective duct 5 in which is situated a damper 6.

Each damper 6 is actuated by a motor 8 powered by an amplifier 22 in dependence on the value of the resistance of the sensor circuit 10 as described above. Thus each damper 6 is positioned independently in dependence on the position of its associated sash 4.

The position of any one of the sashes 4 is not critical as regards the total air demand of all the fume cupboards and thus each of the potentiometers associated with the motors 8 is connected to the input of a summing amplifier which produces a single output which represents the total air demand, which output is fed to the control circuit of the extractor fan 24 and regulates its speed accordingly.

Thus the air supply to each fume cupboard is regulated individually by its damper 6 and the overall air throughput of the fan 24 is matched to the total requirement thus ensuring the maximum economy of energy.

In a modified embodiment which is not illustrated each of the sashes opens into the same large fume cupboard, and in this case the outputs from the position sensing circuits 10 will be fed to a summing amplifier whose output is used to vary both the position of a single damper and the speed of the air extraction fan.

In a further embodiment which is also not illustrated either of the two systems described above may be part of a larger system supplied by a single air extractor fan. In this case the air extractor fan 24 may be replaced by a manifold damper actuated by a motor in dependence on the output of the summing amplifier 26. The output of the summing amplifier 26 together with the output of further such summing amplifiers will be fed to a master summing amplifier whose output will be indicative of the total demand of the system and is used to regulate the speed of the fan accordingly.

As referred to above the total air extraction rate from the fume cupboards is varied in dependence on the sum of the outputs of the motor potentiometers 24, or alternatively of the individual resistive networks 10. These resistive networks may simply be connected in series, but it will be appreciated that in this case all the fume cupboards will cease to operate if a single fume cupboard should break down or be withdrawn from service since this will break the input control circuit. For this reason it is preferred that these networks be connected in parallel as shown in Figure 3. In this event any one cupboard may be taken out of service without affecting the operation of the others.

In research laboratories it is common to designate certain fume cupboards for hazardous or highly toxic chemicals and others for less toxic chemicals. It will be appreciated that for high toxic cupboards it is highly desirable that the air extraction rate from that cupboard should increase immediately the sash is opened whereas with low toxic cupboards this is not necessary and a slower response rate is acceptable and indeed desirable for energy conservation. For this reason the output from each fume cupboard which is used to determine the overall extraction rate, i.e.

From the motor potentiometer 24 or the resistive rietwork 10, preferably includes a response 3adjustment means which preferably comprises a potential divider connected in series with it and the input of the summing amplifier. The two limbs of the potential divider are preferably adjustable but present in dependence on the level of the toxicity of chemicals to be kept in that cupboard.

This enables the rate of response of the air extraction fan to the movement of any one fume cupboard sash to be varied in accordance with requirements. Each fume cupboard preferably has associated with it a daughter circuit board carrying this potential divider, in the form of two potentiometers, removably connected to a mother board whose output is connected to the summing amplifier. Thus any fume cupboard may simply be taken out of action by disconnecting its respective daughter board, and this in no way affects the operation or response of the remaining fume cupboards.

Similarly it will be appreciated that such a response adjustment means may alternatively or in addition be incorporated in the line leading from the sash position sensor to the damper motor so that the rate of response of the position of the damper to changes in position of the sash may be varied. If the sash position sensor is used to control both the damper position and to provide an input to the summing amplifier one response adjustment means for each fume cupboard will adjust the response of both the damper and the air extraction fan, but this arrangement will preclude independent adjustment of the operation of these two.

An array of fume cupboards'in accordance with the invention is thus extremely flexible and by adjusting the individual dampers, the response rate of the air extraction fan and/or that of the dampers to changes of position of the individual sashes the fume cupboards can be set so that individually and collectively they have exactly the required characteristics as regards optimising air .flow and with regard to the designated toxicity level of the different fume cupboards. These characteristics may then be altered as desired if the toxicity designation of any or all of the fume cupboards is to be changed. In addition any of the fume cupboards may be taken out of service and the associated daughter board removed entirely without in any way affecting the performance of the remaining fume cupboards.

It will be appreciated that many further modifications and alterations may be made to the embodiments described above. For instance, two or more extractor fans may be used operating in parallel and an alarm circuit may be provided to indicate that the air extraction fan is unable to meet the demand required of it. In addition the position sensing means may take a wide variety of forms such as a sliding resistance, a capacitatively or inductively tuned circuit or photo electric sensors.

Claims

1. An enclosure including a door which may be opened to gain access to the interior of the enclosure, air extraction means connected to the interior of the enclosure by a duct, a movable damper situated in the duct, means to generate a signal indicative of the position of the door, means to adjust the position of the damper in accordance with the magnitude of the said signal and means to vary the air extraction rate in accordance with the magnitude of the said signal.

2. An enclosure as claimed in Claim 1 in which the damper is so arranged that the air velocity is substantially constant regardless of the position of the door.

3. An enclosure as claimed in Claim 1 or Claim 2 in which the means to adjust the position of the damper comprises an electric motor.

4. An enclosure as claimed in Claim 3 including an amplifier whose output varies with the magnitude of the said signal and drives the electric motor.

5. An enclosure as claimed in Claim 3 or Claim 4 including a potentiometer whose setting varies with the position of the electric motor and in which the air extraction rate is varied in dependence on the potentiometer setting.

6. An enclosure as claimed in any one of the preceding claims in which means to generate a signal indicative of the position of the door includes a resistance chain and a plurality of magnetically actuated switches by which resistances may be switched into or out of the chain, movement of the door causing actuation of successive switches thereby altering the total resistance presented by the resistance chain.

7. An installation including a plurality of enclosures as claimed in any one of the preceding claims connected to a single common air extraction means.

8. An installation as claimed in Claim 7 including summing means to sum each of the door position signals or further signals which vary with the said signals to produce a summed signal and means to vary the speed of the air extraction means in dependence on the magnitude of the summed signal.

9. An installation as claimed in Claim 8 including a response adjustment means associated with each enclosure and arranged to adjust the speed of response of the air extraction means and/or the movable damper to changes in position of the associated door.

10. An installation as claimed in Claim 9 in which each response adjustment means is mounted on a separate removable daughter circuit board, the installation being so arranged that any daughter board may be removed without affecting the operation of the other enclosures.

11. A fume cupboard substantially as specifically herein described with reference to Figures 1 and 2 of the accompanying drawings.

12. A row of fume cupboards substantially as specifically herein described with reference to Figure 3 of the accompanying drawings.