GB2222705A - Reduced pressure enclosure - Google Patents

Abstract

A reduced pressure enclosure (10) such as a fume cupboard comprising an outlet duct (11) and an access opening (12) closable by means such as a sliding sash (13), has a sensor (14) for sensing the velocity of air flow at the opening (12) and connected to circuitry (15) which controls flow control means (19) in the duct (11) which leads to an extractor fan (not shown). This allows the rate of extraction of air from the cabinet to be regulated in accordance with the size of the opening (12). The enclosure can be part of a system including a plurality of enclosures feeding a common exhaust system, each (reduced pressure enclosure (10) having a closed loop flow control system. For use with the above systems an anemometer is also described which includes a pair of matched electronic components such as diodes (30), (31) disposed side-by-side in an air stream (33), one of said components (30) having heating means (32) attached to it and the two components being connected to circuitry adapted to sense the difference in temperature of the components and thus the velocity of air therepast. Changes in air temperature are thereby compensated. <IMAGE>

Description

CLEAN ENCLOSURES This invention relates to clean enclosures, that is to say enclosures wherein dangerous/noxious/ unpleasant materials/experiments/animals/apparatus are dealt with and from which contaminated air should not escape except via ducting specifically for that purpose. Such enclosures are conventionally provided with a ducted suction system. This ensures that the enclosure itself is at a pressure generally below ambient pressure. Thus, through any openings into the enclosure ambient air enters the enclosure and contaminants from within the enclosure cannot travel, in air flow, from the interior to the exterior where they could be harmful to personnel or the surroundings.

The invention is particularly concerned with, and will be particularly described in relation to fume cupboards installed in laboratories and comparable installations. However, it will be appreciated that the invention is equally applicable to other enclosures wherein a reduced pressure has to be maintained within the enclosure. Examples are certain clean rooms and operating theatres.

In a laboratory which has several fume cupboards, each cupboard is often connected to a common manifold duct which leads to a fan at the outlet end of the duct usually at or above building roof level. The fume cupboards will have doors, usually in the form of horizontally or vertically sliding sashes which can vary the size of an inlet to each fume cupboard from virtually zero to a significant value. In order that there is always air flow into the fume cupboard the extractor fan has to have a certain power to ensure that when all the cabinets are open there is still a net inward flow into the cabinets. There is the further characteristic that the fan must expel the potentially contaminated air with a minimum efflux velocity in order to ensure that the expelled air clears the building envelope and does not return to earth significantly close to the building.Both these criteria mean that the fan has to be of significant power and cannot readily be altered in its power to meet varying conditions.

This has a signficant disadvantage. For example, considering only a single fume cupboard, when that fume cupboard is fully closed, there will be an influx of air via small gaps around the sashes. Because the fan is operating at a fixed power and because the sash is shut, there will be a very high rate of flow there into the fume cupboard through such narrow gaps. The velocity of the air will be far in excess of that needed to ensure cleanliness of the surroundings and is quite unnecessary. There is the additional problem that when a group of fume cupboards are connected in parallel to a common manifold extractor duct shutting down one fume cupboard causes an increase in flow through the other fume cupboards which are in an open condition. Again, such flow of air is usually in excess of what is required.

It has long been appreciated that drawing excess air through a fume cupboard, especially in laboratories which are in use for many hours each day, such as in schools or research or analytical laboratories can result in significant extraction of unnecessary air.

It must be borne in mind that the air which is being extracted and expelled to atmosphere has already been heated up to say 18 or 20"C in order to heat the building and is now being extracted at a very high rate by the fume cupboard system. In a laboratory having several fume cupboards the overall loss of hot air due to an inefficient fume cupboard system can place a significant load on the laboratory heating costs.

This loss has long been known, and it is not unusual for a fume cupboard to have a damper in its outlet duct connected mechanically to its sash(es).

Thus, when the entrance to the cupboard is open, opening movement of the sash opens the damper to ensure maximum air flow whilst the opening is at its maximum size. When the opening is fully closed, the damper is turned to a relatively closed position to reduce the amount of air which is drawn through the outlet duct.

However, such a system is an open loop system and cannot cater for variations in airflow which are caused by the presence of an apparatus inside the cupboard, which might easily reduce the flow of air around the inlet, nor can it deal with the problem of a number of cupboards in parallel. Even though on a particular cupboard the inlet might be half open and the damper set to an appropriate value for a half open inlet, if two more cupboards are shut down then the increased suction at the outlet duct simply causes a greater influx of air through the one duct which is unnecessary and is wasted.

It is an object of the present invention to provide an improved low pressure enclosure and associated equipment.

The invention provides a reduced pressure enclosure having an outlet duct and an access opening, a sensor for sensing the velocity of air inflow at the access opening and flow control means in the outlet duct, the sensor and the flow control means being interconnected so that variation in airflow from a desired value can be counteracted by modification of the flow control means in response to said sensor.

The sensing of the velocity of the input air can be effected directly, i.e. by measuring the velocity of the input air at the access opening itself, or the velocity can be measured at a remote point where the velocity of the airflow is directly related to the velocity of the airflow at the opneing. The sensor can be of any convenient form, but preferably is an electrical anemometer whse signals are processed, amplified and supplied to the flow control means which can be a damper or comparable air valve. The flow control means can be actuated by means of a pneumatic cylinder, an electrical/pneumatic transducer being provided between circuitry and the damper cylinder. As an alternative the damper could be operated via a stepper motor or by a linear motor.It is important, however, that whatever means is chosen to control the flow control means it must be able to respond very quickly to changes in access opening air velocity.

In the case of an enclosure which is closed by a door such as a sash, it is advantageous if a slot is formed between the sash and a part of the surround of the access opening, such slot remaining constant in its cross-section area during movement of said door (of whatever form) during its movement between open and closed positions. In such a case airflow through the slot is, we have found, directly proportional to the rate of airflow through the actual access opening itself and therefore measurement of the airflow can occur in that slot and not in the actual access opening where it could be disturbed by apparatus or operatives.

The invention also provides a system including a plurality of reduced pressure enclosures connected in parallel to a common extractor manifold which is in turn connected by a fan to an efflux outlet, each of the reduced pressure enclosures having a closed loop flow control system which includes a sensor, for sensing inlet flow via an access opening thereof, in operative connection with flow control means in an outlet duct from the enclosure to the manifold.

Preferably, the extractor manifold has a further inlet additional to the enclosure duct inlet, such inlet opening when pressure in the extractor manifold falls below a predetermined value. This prevents pressure reaching too low a value in the extractor manifold in the event that all or a significant number of the enclosures are fully closed (if this happens the eflux velocity can fall below a desired minimum).

Pressure sensing for the additional inlet can be effected by providing the inlet in the form of a flap valve which is exposed to atmospheric pressure on one side and to the pressure of the duct on the other side, being biassed, as by a spring or weight towards its closed position. Thus, when the difference between ambient and manifold pressure reaches a significant value the flap swings and allows ambient air to enter.

Alternatively, there can be an actual pressure sensor within the duct (or even a flow sensor) which sends a signal to open the inlet.

The air flow sensor is preferably as described hereafter.

As a further feature the invention provides an anemometer comprising a pair of matched semi-conductor components arranged fixedly side-by-side and electrically in parallel, controlled heating means being disposed closely adjacent one of the components.

the two components having matched thermal coefficients so that a change in the temperature of one results in a change in the voltage across it, the voltages across the two components being connected to circuitry which can compare those voltages and produce an output signal dependent upon the difference in temperature between the two components.

The components are preferably diodes.

The circuitry can comprise a first operational amplifier which produces an outlet signal which is proportional to the difference between the two input voltages i.e. is proportional to the difference in temperature between the two diodes. This signal and a calibration signal can be fed to a second operational amplifier whose output can be said to be an error signal indicative of how far from a norm the output of the first amplifier has varied. This is an indication of how far airflow velocity past the two components has varied from a desired airflow velocity.

The output from the first amplifier can also be compared with a second set "low" value and the output of a comparison between the two can be connected to a low air alarm. Such an alarm can give visual and/or audible warning to personnel working with the equipment.

The invention will be described further, by way of example, with reference to the accompanying drawings wherein: - Fig. 1 is a sketch illustrating, in general vertical cross-sectional view a preferred enclosure of the invention; Fig. 2 is a sketch illustrating a preferred system of the invention; Fig. 3 is a enlarged cross-sectional view illustrating that portion of Fig.l which is indicated III in Fig. 1.

Fig. 4 is an enlarged cross-sectional view of that portion of Fig. 3 which is indicated IV in Fig.3; and Fig. 5 is a circuit diagram of an airflow sensor according to the invention.

Referring firstly to Figs. 1 and 2 a first preferred reduced pressure enclosure (RPE) is in the form of a fume cupboard 10 having an outlet duct 11 and access opening 12 closable by a vertically sliding sash 13. Fume cupboard 10 is shown resting on a bench 14 as is conventional. However, it will be appreciated that the invention applys equally to RPE's of much greater size for example fume enclosures into which operatives can walk if necessary.

A sensor 14 senses the velocity of air entering opening 12 and feeds a signal to circuitry 15 which is indicative of the velocity of air entering opening 12.

The circuitry 15 processes such signals and produces an output at 16 which is in the form of an "error" signal in that it is indicative of whether and how far the airflow velocity. value at opening 12 has departed from a desired norm. A norm may be between 0 one 2 -1 -1 ms , such as 0.5 ms . Output 16 is fed to an electrical/pneumatic transducer 17 whose output is fed to a pneumatic cylinder 18 which controls a damper 19 in the duct 11.

In use, the sensor, such as sensor 14, generates a signal which is representative of the velocity of air entering via access opening 12 and feeds that signal to the circuitry 15. As will be described later, the circuitry 15 generates an output which is in the form of an error signal so that if the air velocity at opening 12 departs from a set norm output 16 operates the transducer 17 to operate the damper 19 to either increase or decrease effective cross-sectional area of duct 11. It will be appreciated that any alteration of the damper 19 will cause an alteration in the airflow which again will alter the velocity of air entering via access opening 12 which will effect the sensor. Thus, there is a closed loop control system which always returns the velocity value at 12 towards a desired value.As will be later described, the gain of circuitry 15 is controlled to prevent hunting about desired value.

The disposition of the sensor 14 is particularly important in relation to fume cupboards having sliding sashes. Unless the fume cupboard is designed in a very complicated way there is invariably a slot-shaped gap between the glass 21 of the sash 13 and the top member 22 of the cupboard 10. We have found that as the sash moves between its fully open and fully closed positions the velocity of air entering via the slot 20 is directly proportional to the velocity of air entering via the access opening 12. This enables the sensor to be placed in the slot 20 where it is protected against contact by users and is protected by damage, deflection or mis-use.

It would, of course, be perfectly satisfactory to measure the input velocity in the access opening 12, but any sensors in that region would be susceptible to damage and a single sensor might not be representative of airflow throughout the entire width of the access opening in view of the presence of operatives and/or apparatus within the enclosure. In such conditions, it would be prudent to provide a plurality of sensors around the opening 12 and to use some electronic method of averaging their outputs. Clearly, such a system would be significantly more expensive that the single sensor 14 used in the slot 20.

Fig. 2 illustrates part of a system embodying a plurality of fume cupboards such as fume cupboard 10 each having respective outlet ducts 11, lla, llb (there can be a very large number of such outlet ducts) which enter a common extractor manifold 23 which is connected to a fan 24 which has an efflux aperture or stack 25.

In the system which is partially shown in Fig. 2, when a particular one of the fume cupboards is adjusted such adjustment alters the pressure in manifold 23 which alters the flow through the remaining fume cupboards lla, llb etc. In previous, a direct mechanical connection between sash and damper could not have any effect on this alteration. In the present invention, however, each individual fume cupboard can sense any increased or decreased flow due to alteration in a particular fume cupboard and can adjust its own flow back to a minimum desired level.

Because of the close control which the present invention places on the amount of air which is extractable through each fume cupboard 10, it can often occur that the flow towards manifold 23 can become quite low. Because the fan 24 is essentially of a fixed power and rate (it has to be in order to ensure a minimum efflux velocity at maximum opening) the pressure within manifold 23 can often become low, possibly leading to distortion of the flow characteristics causing a too low efflux velocity. For this reason it is desirable that an additional inlet is in the form of a valve member such as a flap 26 connected to a sensor 27. Sensor 27 can sense when pressure drops below predetermined value, Opening of flap 26 allows ambient air from an inlet duct 28 to enter manifold 23 to maintain the flow volume and maintain the efflux velocity from stach 25.

In this way the amount of heated internal air which has to be pumped out to atmosphere can be minimised thus making significant economic savings in relation to the heating of the building concerned.

It will be appreciated that the sensor used in the apparatus aforedescribed is, in itself, equally applicable to use in many other embodiments and it will, therefore, be described individually as a separate invention.

Referring firstly to Fig. 3 and Fig. 4 it will be seen that the sensor 14 of the invention is mounted within the slot 20 between glass 21 and upper part 22 of the fume cupboard 10.

Referring to Fig. 4 it will be seen that sensor 14 includes a body 29 from which project a pair of semi-conductor components in the form of diodes 30 and 31. Diodes 30 and 31 are essentially identical in all characteristics. Modern accurate manufacture does mean that such diodes are identical to a very great degree.

By a simple sorting process from a small batch, virtually perfectly matched pairs can be produced with very little difficulty. A characteristic of such diodes is that they are temperature dependent in that the voltage across them varies linearly with their temperature. Diode 30 has a closely calibrated heating resistor 32 attached closely to it, for example by adherence. Heating resistor 32 is fed from a stabilized supply within circuitry of the apparatus to deliver a constant supply of heat to diode 30. In use, diode 30 is heated to a temperature above ambient, for example from 25 to 40"C, preferably 30 to 350C above ambient. In still air the voltage across diode 30 is significantly different from that across diode 31. The diode 30,31 are arranged side-by-side in the downward airflow indicated by arrows 33.The use of a pair of identical diodes side-by-side eliminates any possible error effects due to changes in the temperature of the air which is flowing. For example, if only a single diode was used an increase in the temperature of the air flowing would result in reduced cooling of the diode 30 which would signal to the equipment a reduction in airflow. This would be a spurious signal.

However, by using two diodes side-by-side, the reduction in cooling on the one due to an increased air temperature is balanced by a comparable effect on the other. Similar effects occur when the air temperature falls.

Referring now to Fig. 5 it will be seen that the two diodes 30 and 31 are connected across a 12v highly stabilized source each in series with a limiting resistor 34. The voltage across each diode at ambient temperature is about 600 millivolts and it falls about 2 millivolts for each increase in temperature of the diode by 1"C. The respective voltages across the diodes are fed via lead 35 and 36 to an operational amplifier Al which has again control 37. The output of amplifier Al varies between 0 and 10 volts d.c. and is a direct measure of the temperature difference between the two diodes 30 and 31. The output of amplifier Al is fed to a second amplifier A2 to whose other input is fed a voltage which is indicative of a norm.The sensor is calibrated by setting up the fume cupboard so that an input air velocity of 0.5 metres per second prevails at the opening 12. This is a "normal" condition. The voltage at the output of amplifier Al can be measured on meter 38 (permanently installed or used only during calibration and testing) and this voltage can be the norm which is set to the second input 39. The voltage fed to input 39 from the standard 40 can be adjusted by means of control 41.

The output of amplifier A2 is fed to a linear DC-to-pneumatic converter which in turn feeds a pneumatic cylinder 18 which operates damper 19.

In use, the output of A2 effectively generates an "error" signal which is indicative of departure from the normal set airflow. In the event that the airflow is too great a signal is given to the damper 19 to reduce flow through the duct and in the event that airflow is too small the damper 19 opens duct 11. The gain of amplifier Al is adjusted to ensure that there is not to much damping of the circuit, but not so little damping as to allow oscillation of the arrangement.

As a modification, the output of Al can also be fed to a second operational amplifier A3 where it is compared with a "low" signal. This low signal could indicate a very low value of the input airflow and, if and when the output of Al fell below such low value the output of A3 could be connected to an alarm which could audibly and/or visually warn users of the fume cupboards or could automatically close all the cupboards simultaneously.

The invention is not limited to the precise details of the foregoing and variations can be made thereto. For example, the air velocity sensor is not particularly limited to the application of the first invention and can be used in any condition wherein a relatively low air speed needs to be sensed accurately.

An electrical anemometer which is accurate at low air velocities is very desirable.

Although the anemometer has been described as using a pair of matched diodes, it is possible that other pairs of matched semi-conductor components could be used such as transistors The first invention of course is not only limited to fume cupboards and is applicable to all RPE's, be them very small enclosures or large enclosures such as rooms etc.

Many other variations are possible within the scope of the invention.

Claims (31)

1. A reduced pressure enclosure having an outlet duct and an access opening, a sensor for sensing the velocity of air inflow at the access opening and flow control means in the outlet duct, the sensor and the flow control means being interconnected so that variation in airflow from a desired value can be counteracted by modification of the flow control means in response to said sensor.

2. An enclosure as claimed in claim 1 wherein sensing of the velocity of the input air is effected directly.

3. An enclosure as claimed in claim 1, wherein the velocity is measured at a remote point.

4. An enclosure as claimed in claim 3 wherein said remote point is a point where velocity of the airflow at that point is directly related to the velocity of the airflow at the opening.

5. An enclosure as claimed in claims 2, 3 or 4, wherein the sensor is or includes an anemometor.

6. An enclosure as claimed in claim 5, wherein the anemometer is an electrical anemometer whose signals are processed, amplified and supplied to the flow control means.

7. An enclosure as claimed in any of claims 1 to 6, wherein the flow control means is a damper or comparable air valve.

8. An enclosure as claimed in any preceding claim wherein the flow control means is actuable by means of a pneumatic cylinder, a transducer being provided between control circuitry of the enclosure and the cylinder.

9. An enclosure as claimed in any of claims 1 to 7 wherein the damper is operable via a stepper motor.

10. An enclosure as claimed in any of claims 1 to 7 wherein the valve/damper is operable by a linear motor.

11. An enclosure as claimed in any preceding claim and being an enclosure which is closable by a door and a slot is formed between the door and a part of the surround of the access opening, such slot remaining constant in its cross-section area during movement of said door during its movement between open and closed positions.

12. An enclosure as claimed in claim 11 wherein the door is a sash.

13. An enclosure as claimed in claim 11 or 12 wherein measurement of the airflow occurs in that slot.

14. A reduced pressure enclosure substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15. A system including a plurality of reduced pressure enclosures connected in parallel to a common extractor manifold connected by a fan to an efflux outlet, each of the reduced pressure enclosures having a closed loop flow control system which includes a sensor, for sensing inlet flow, in operative connection with flow control means in an outlet duct from the enclosure to the manifold.

16. A system as claimed in claim 15 wherein the extractor manifold has a further inlet additional to the enclosure duct inlet, such further inlet being adapted to open when pressure in the extractor manifold falls below a predetermined value.

17. A system as claimed in claim 15 or 16 wherein pressure sensing for the further inlet is effected by providing the inlet in the form of a flap valve which is exposed to atmospheric pressure on one side and to the pressure of the duct on the other side.

18. A system as claimed in claim 17 wherein the valve is biassed, as by a spring or weight, towards its closed position.

19. A system as claimed in claim 17 or 18 wherein when the difference between ambient and manifold pressure reaches a significant value the flap swings and allows ambient air to enter.

20. A system as claimed in any of claims 15 to 17, wherein a pressure sensor is provided within the duct which sends a signal to open the inlet.

21. A system as claimed in any of claims 15 to 17 wherein a flow velocity sensor is used to open the further inlet.

22. A system as claimed in any of claims 15 to 21, wherein each enclosure is as claimed in any of claims 1 to 14.

23. A reduced pressure enclosure system substantially as hereinbefore described with reference to the accompanying drawings.

24. An anemometer comprising a pair of matched semi-conductor components arranged fixedly side-by-side and electrically in parallel, controlled heating means being disposed closely adjacent one of the components, the two components having matched thermal coefficients so that a change in the temperature of each results in a change in the voltage across it, the voltages across the two components being connected to circuitry which can compare those voltages and produce an output signal dependent upon the difference in temperature between the two components.

25. An anemometer as claimed in claim 24, wherein the components are diodes.

26. An anemometer as claimed in claim 24 or 25 wherein the circuitry comprises a first operational amplifier which produces an outlet signal which is proportional to the difference between the two input voltages i.e. is proportional to the difference in temperature between the two diodes.

27. A anemometer as claimed in claim 26, wherein this signal and a calibration signal are fed to a second operational amplifier whose output is an "error signal" indicative of how far from a norm the output of the first amplifier has varied (an indication of how far airflow velocity past the two components has varied from a desired airflow velocity).

28. An anemometer as claimed in claim 26 or 27 wherein the output from the first amplifier is also compared with a second set "low" value and the output of a comparison between the two is connected to a low air alarm.

29. An anemometer as claimed in claim 28 wherein the alarm gives visual and/or audible warning.

30. An anemometer substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

31. A low-pressure enclosure including an anemometer as claimed in any of claims 24 to 30.