WO1998041858A2

WO1998041858A2 - Fume extraction apparatus

Info

Publication number: WO1998041858A2
Application number: PCT/GB1998/000780
Authority: WO
Inventors: Neil Edward Flynn
Original assignee: Ok International Limited
Priority date: 1997-03-14
Filing date: 1998-03-16
Publication date: 1998-09-24
Also published as: WO1998041858A3

Abstract

Fume extraction apparatus comprises filters (4, 5, 6) to remove fumes from air, and a semi-conductor type detector (13) which is located downstream of the filters to detect fumes in the filtered air and which is adapted to issue a warning signal to indicate the presence of fumes. Control means (15) calibrates the apparatus under one or more known fume conditions so as to trigger said warning signal at a predetermined level of fumes. A sample of air is tested on a continuous basis by drawing air via conduits (10, 11) through a chamber (12) containing the detector (13). Filters (4, 5, 6) remove particles from the air, and a pressure detector (16) detects the drop in pressure across the filters and issues a warning signal for a predetermined pressure drop corresponding to a predetermined level of blockage of the filters.

Description

FUME EXTRACTION APPARATUS

This invention relates to fume extraction apparatus, especially gas filtering apparatus, fume/gas detection apparatus, and a gas pump .

Fume extraction apparatus is needed to remove harmful fumes from the work-place, such as soldering equipment. Generally, extractor ducting is provided adjacent the work-place, and a pump sucks air through the ducting, and a filter system removes the fumes, both particulates and gases from the air, before it is recirculated. The filter system includes a particle pre- filter, a HEPA (High Efficiency Particle Arrestor) filter, and a gas absorption filter, such as an active carbon filter; and safety regulations require that these filters be replaced regularly as they become blocked or exhausted.

According to one aspect, the invention consists in fume extraction apparatus comprising filtering apparatus to remove fumes from air passed through it, and a fume detector located downstream of the filter to detect fumes in output air from the filtering apparatus and adapted to issue a warning signal when fumes are detected in said output air.

The fume detector is preferably set up to produce said warning signal at a predetermined level of detected fumes in the output air, the detector output signal being calibrated in an initial setting-up procedure by reference to its response to air samples containing zero fumes and a known level of fumes, respectively. The warning signal may simply trigger an audible or visual warning, or may operate a trip, or trigger a shut-down sequence of the apparatus.

The fume detector is preferably a pellister, which is a semiconductor device that is heated to an operating condition at which it produces an output signalling current in response to fume molecules contacting it.

Preferably, the fume detector detects fumes in a sample of the output air which is taken from the output air, preferably on a continuous basis using the output air flow itself to produce a pressure differential between two sampling points which are connected to a chamber containing the detector device, preferably in the form of a pellister. The sampled air flow through said chamber will be a fraction of that of the output air and will be proportional to the output air flow so that measurements made by the detector device will be representative of the fume level in the output air flow. This arrangement with a pellister as the detector device has the particular advantage that the pellister is less exposed to the cooling effect of the greater output air flow. According to a second aspect, the invention consists in a fume detector as described in the previous paragraph comprising a pellister located in a sampling chamber connected between sampling points in an air flow line being tested so that air is drawn through the chamber between these points by the pressure differential created by the air flow itself across these sampling points .

According to a third aspect, the invention consists in fume extraction apparatus comprising filtering apparatus to remove particles from air passed through it, and a pressure detector to detect the drop in pressure across the filter and adapted to issue a warning signal when a predetermined pressure drop is detected corresponding to a predetermined level of blockage of the filter.

Preferably, the filtering apparatus comprises a particle pre- filter and a main particle filter, and the pre-filter is renewed when said predetermined level of blockage is detected, and this process is repeated a number of times as the main filter becomes progressively blocked, until the pressure detector indicates that the main filter is blocked to a predetermined maximum level and must be changed.

Preferably, the pressure drop detector is calibrated by reference to its output signal under zero pressure conditions when the filters are clean and new, and under 100% blocked conditions when the filters fully block all air flow through the filters.

According to a fourth aspect, the invention consists in fume extraction apparatus comprising filtering apparatus with filter elements that have a limited life before they need replacing or servicing, a pump that moves air through the filtering apparatus to be cleaned, and a controller including a timer that determines a maximum period of use of the apparatus, preferably through disabling the pump at the end of this period, possibly following one or more intermediate warning periods at each of which the apparatus can be reset to operate for a further limited period of time.

According to a fifth aspect, the invention consists in fume extraction apparatus comprising filtering apparatus to remove fume particles from air, a pump that sucks air to be cleaned through the filtering apparatus, and a temperature sensor in the output side of the pump to sense the temperature of the output air and to issue a warning signal that the temperature has reached a predetermined maximum level corresponding to a minimum air flow through the pump.

It will be appreciated that the invention also consists in a combination of any of the five aspects of the invention stated above . According to a sixth aspect, the invention consists in a centrifugal pump having an impeller comprising a series of radially extending blades located between a pair of coaxial discs to which they are connected by a series of projections or tabs, preferably integral with the blades, which engage in apertures in the discs .

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of fume extraction apparatus according to the invention;

Figure 2 is a diagram of the fume detector used in the apparatus of Figure 1;

Figure 3 is a front view of the fume extraction apparatus of Figure 1 as incorporated into a mobile housing unit;

Figure 4 is the rear view of the apparatus of Figure 3 ;

Figure 5 is an enlarged view of the control panel of the apparatus of Figures 3 and 4;

Figure 6 is a schematic diagram of a centrifugal gas pump according to the invention; and Figure 7 is a diagram of a blade of the pump of Figure 6.

The fume extraction apparatus illustrated in the drawings consists of a portable unit 1, as shown in Figures 3 and 4, for location in the work-place, for example, adjacent a soldering bench for connection to fume collector ducting via an inlet port 2. The unit 1 consists of a housing 3 with a removable lid for access to a set of three replaceable filters 4,5,6 and a pump 7 within the housing. The pump 7 sucks air through the filters from the inlet port 2 and vents this to atmosphere via an exhaust port 8. The filters, shown in Figure 1, comprise a pre-filter 4 to remove further particulates, a HEPA filter 5, and an active carbon filter 6 to remove toxic/obnoxious gases and chemicals .

A fume detector 9 is connected to the output duct 18 on the exhaust side of the pump 7 to sample the exhaust air for the presence of toxic/obnoxious gases such as acetic acid, ammonia, and carbon monoxide. Sampling takes place on a continuous basis via a pair of conduit connections 10,11 from the exhaust duct to a sampling chamber 12 containing a semi-conductor gas detection device in the form of a pellister 13. The different cross-sectional areas Al,A2 of these two conduits 10,11, respectively, with A2>A1, serve to create a pressure differential between them as the exhaust air flows across them both, and this in turn circulates a sample of air through the chamber 12 and across the pellister 13. The pellister 13 is mounted on a printed circuit board 14 within the chamber, as shown in Figures 1 and 2. The pellister 13 is connected in a bridge circuit and is driven with a constant voltage so as to stabilise the standing resistance necessary because the pellister has a negative temperature coefficient. Also, because pellisters vary in their resistance from one unit to the next, it is calibrated in an initial set-up procedure in which its output is registered by the controller 15 of the fume extraction apparatus under zero fume conditions and known fume conditions, the controller then applying an algorithm to match these registered values so that the output signal of the pellister 13 is appropriately scaled. In this way, the pellister can be set up to operate between zero and lpp or up to lOOOppm of toxic gas at full scale. The controller 15 can then be set to respond to any desired level of fume detection to issue a warning signal or close down operation of the apparatus by de-energising the pump 7.

The signal generated by the pellister 13 is a current signal, but this is converted to a corresponding frequency signal which is then digitised for inputting to a micro-processor of the controller 15 for calibration.

A differential air pressure detector 16 is connected across the filters 4,5,6 and serves to give an indication of the state of cleanliness or blockage of the filters, the detector producing a signal which is frequency converted and digitised and fed to the controller 15.

A temperature detector 17, for example, a thermocouple, is located in the pump output duct 18 to protect the pump 7, especially against air starvation due to filter blockage, the detector 17 producing a signal which is frequency converted and digitised and fed to the controller 15.

The controller 15 includes a timer 19 that counts days and serves to control the length of time for which the fume extraction apparatus is used before the filters need replacing. The controller serves to give a warning of the need to replace the filters at predetermined intervals, such as 365 days, 30 days, 15 days, and 5 days, before finally closing down operation of the system if the filters have not been replaced and the timer reset to zero .

The controller 15 includes a display panel 20, as shown in Figure 6 , in the form of a membrane keyboard through which the operator controls the fume extraction apparatus and which displays the state of the apparatus.

A remote control unit (not shown) is provided that works via a tonal link with the controller 15 to allow an operator to operate the controller remotely.

The four parameters that are measured and displayed on the display panel are: (i) "Pressure loss" across the three stage filter, displayed as 0 to 100% blocked (ii) "Air purity" passing into the workplace displayed as

0-100% impurity, (iii) "Temperature" of the pump, displayed as degrees

Celsius 0-100°C. (iv) "Time", between filter changes, displayed in days to run counting down.

These four parameters are displayed on a digital display 23, one by one, as selected by pressing a button 21 to step round the different parameters.

As each parameter is selected a flashing LED 22 indicates beside a corresponding one of four logos 24, which parameter is being selected. Each parameter has three levels of operation indicated by a Green LED, a Yellow LED, or a Red LED, as follows: (i) Normal level, shown by a Green LED; this is the area of normal, expected, conditions, (ii) Warning level, shown by a Yellow LED; the conditions of the parameter have moved outside normal conditions and actions must be taken to return to normal operation; a warning bleeper will be sounded, the bleeper may be muted. (iii) Emergency level, shown by a Red LED; the conditions of the parameter have now moved beyond the warning level and the system may be shut down until remedial action has been taken. The bleeper will sound and may be muted.

A mute button 30 serves to switch off the warning bleeper once it has been triggered.

The pump 7 is started and stopped manually from the front panel by buttons 25,26. Also, the pump may be stopped automatically, according to the relevant emergency condition at that time. The monitoring and display continues to operate regardless of automatic shutdown or manual stop/start.

A suitable pellister 13, for example, an NAP-llAS, is connected to the controller 15 by a four-way, polarised, connector 27. Two ways are used for the pellister heater and two ways are used for the pellister sensing element. The heater needs a constant voltage supply, 5 volts AC or DC, at 200 milliamps. Because the current requirement for the heater is a thousand times greater than the current required by the micro-processor of the controller, there are two separate 5 volt regulators. The 5 volt regulator, for the heater, at 1 amp capability is placed on a separate printed circuit board that includes the power supply for the pump 7, and the 5 volt regulator, for the micro-processor, at 100 milliamp capability, is placed on the printed circuit board for the micro-processor.

The pellister is connected with three other resistors in a bridge circuit, and connected across the 5 volt regulated supply. Generally, the pellister provides a signal variation, from "pure" to "impure", of approximately 500 millivolts, the differential signal being digitised to 3 decades, or 10 bits.

Upon installation of the pellister, a "Set-up" routine is followed in order to ensure that air, with a quality better than a given level, displays "zero" impurities, and air, with a quality, worse than a given level displays 100%. This level is the Red or Emergency level. A lower, warning level, the yellow level will be set at 25% lower. Thus, as the gas filter 6 becomes saturated, the controller issues an audible warning at the 25% level, and then an audible warning and system shutdown at the 100% level.

A suitable temperature sensor 17 is a 35DX, which is connected to the controller by a polarised 3-way connector 28. Two ways are used for 5 volt regulated supply and one way for the temperature signal. The sensitivity is 10 millivolts per degree Celsius, and the signal at 75 degrees C will be 750 millivolts. This signal is digitised to 3 decades, or 10 bits . Should the temperature rise above 50°C the yellow LED will illuminate and cause an audible alarm which can be muted. The pump will be allowed to continue. However, should the temperature rise to 60°C this will automatically shut down the pump. It will not be possible to re-start until the unit has cooled down. This function of re-start is not automatic and must be manually instigated by pressing the start button.

A suitable differential, air pressure sensor 16, is a silicon bridge, uncompensated type; with a full scale range of 1 psi or 32 inches of water, or 60mm of mercury, or 80 milliBars, or 8 kiloPascals. This is connected to the controller by a connection 29.

A typical signal output for this kind of air pressure sensor would be around 16 millivolts for full scale pressure. Under normal conditions, say "4 inches water gauge", the output signal would be 2 millivolts, which digitised to 2 decades, or 8 bits.

Upon installation of a new three stage filter 4,5,6, the display must show zero, i.e. a perfect filter, no pressure loss. Subsequently, with use, a degree of blockage will occur. At some point this degree of blockage will be classified as unacceptable, and the filter should be changed. The display must then show 100% blocked. To do this involves scaling the digitised input. Having scaled the digitised input as required, this "100% blocked" level will be referred to as the Red, or Emergency, level. A lower warning level, the yellow level, is set at 85%.

In use, the particulate filters will begin to block as they clean the air passing through them, causing a reduction in flow. The controller will alert the user at an 85% blocked condition, both visually and audibly. No shutdown will occur and the alarm may be muted. This is a warning that only 15% of operational life is available. If no action is taken, the unit will run on to 100% blocked and a shutdown will take place. It will not be possible to run the unit in this condition. The alarm will also sound, although this can be muted .

To restore operation, it is necessary to replace the pre-filter 4. This will then enable the reset/start to be actioned. The display will then indicate the condition of the secondary main particulate HEPA filter 5. The reading will show the percentage of blockage in this unchanged filter. This procedure of pre-filter changes may be carried out several times before the HEPA filter 5 has become 100% blocked. At this point, both particulate filters 4 and 5 must be replaced to restore normal running. A failure to read ZERO may by a result of contamination of particulate on the gas filter 6. It is recommended, therefore, that when the main HEPA filter 5 is ready for change, that routinely the gas filter 6 be changed at the same time.

Time is derived directly from the micro-processor of the controller 15, via software, and is displayed initially as 365 days. This number is then counted down, day by day until time has run out. This time is generated digitally and displayed digitally throughout.

The overall life of the main filters and general maintenance is designed to cycle around 365 days. In order to give a conscious effect over the passage of time, the display will normally read "days to run", sequentially running down one day at a time.

The other parameters are selected by pressing the step button 21 on the display. The LCD display will revert to time automatically two minutes after the last key pressed.

Starting with a new system, 365 days is installed. As the display runs down and reaches 30 days left to run, the green LED will extinguish and the yellow LED will illuminate, the bleeper will sound and may be muted. The pump continues to run, no shutdown having occurred. The LCD continues to display the reducing number of days of normal running, less than 30. On reaching zero days the pump shuts down and the LCD will flash its display. The yellow LED will extinguish, the red LED will illuminate, and the bleeper will sound. The bleeper may be muted by pressing the mute/reset button 30. On pressing the mute/reset button at this time, the bleeper will be muted and will reinstate 30 days on the display (flashing). The controller now permits restart of the pump using the ON-button 26. The red LED will extinguish and the yellow LED will illuminate. The flashing display will continue to count down in days from 30.

On reaching zero DAYS, the pump 7 will shut down, the yellow LED will extinguish, the red LED will illuminate and the bleeper will sound. The bleeper may be muted by pressing the mute/reset button 30. On pressing the mute/reset at this time, the bleeper will be muted and reinstate 15 days on the flashing display. The controller now permits to restart the pump using the "ON" button, the red LED will extinguish and the yellow LED will illuminate. The flashing display will continue to count down in days from 15.

On reaching zero days, the pump will shutdown, the yellow LED will extinguish, the red LED will illuminate and the bleeper will sound. The bleeper may be muted by pressing the mute/reset button. On pressing the mute/reset button at this time, the bleeper will be muted and will reinstate 5 days on the LCD (flashing) . It is now permitted to restart the pump using the "ON" button, the red LED will extinguish, and the yellow LED will illuminate. The flashing LED will continue to count down in days from 5. On reaching zero days, the pump will shut down, the yellow LED will extinguish, and the red LED will illuminate. The bleeper will sound and may be muted by pressing the mute/reset button. Zero time will reinstate and therefore no running time is left. The pump will not run. To reset and install, or reinstate a full cycle of 365 days from the start, it will be necessary to input a four digit code via a hand-held remote tonal device held by a service engineer or responsible person. The code also serves to trigger a re-calibration routine for the sensor .

The overall permissible operation time will therefore be locked into a sequence of 420 days. If a delay in pressing the reset takes place, the real time clock will keep running, thus preventing the instigation of reset time if the 420 days is exceeded.

Figures 6 and 7 illustrate a centrifugal air pump suitable for use in fume extraction apparatus according to the invention, but not limited to such use. The pump has an impeller 31 constructed from three basic components, a back plate 32, a front plate 33, and blades 34. The back and front plates are connected by the blades which have three tabs 35 along each side that engage holes in the back and front plates and are bent over both forwards and backwards. The blades 34 are spaced circumferentially about the impeller and are all offset at an angle to the radius of the impeller so that the radially inner edge of each leads its radially outer edge when rotated. Typically, the inclination of each blade to the radius is 14°- 17°. However, the blades are not all set at the same angle or circumferential spacing relative to one another so as to avoid peak vibration and noise characterisation included by resonances. Nevertheless, the blades are still arranged so that the nett effect is to balance the rotary forces exerted by the impeller.

The radially inner end of each blade on that side adjacent the front plate and a central air inlet of the pump 36, is chamfered, as shown in Figure 7, to increase performance and reduce noise.

The impeller is mounted inside a scroll cavity 38 of the pump housing 37 and rotated anti-clockwise, as seen in Figure 7, at a speed ranging from 2700rpm to 3600rpm. A motor (not shown) drives the impeller, the latter being connected directly to the drive shaft of the motor, and air delivered to the outlet 39.

Claims

1. Fume extraction apparatus comprising filtering apparatus to remove fumes from air passed through it, characterised by a fume detector (13) located downstream of the filtering apparatus (4,5,6) to detect fumes in output air from the filtering apparatus and adapted to issue a warning signal when a predetermined level of fumes are detected in said output air.

2 . Apparatus as claimed in claim 1, in which the fume detector (13) includes calibration means (15) by which an output signal of the detector is calibrated under one or more known fume conditions so as to trigger said warning signal at a predetermined level of detected fumes in the output air.

3. Apparatus as claimed in claim 2, in which the calibration means (15) is adapted to calibrate the detector (13) by reference to its operation under zero fume conditions.

4. Apparatus as claimed in claim 3, including warning means (15) which is operated by the warning signal.

5. Apparatus as claimed in claim 3 , which is adapted so that it is shut down by the warning signal.

6. Apparatus as claimed in any one of the preceding claims, in which the detector (13) comprises a semiconductor device responsive to fume molecules.

7. Apparatus as claimed in any one of the preceding claims, including air sampling means (10 to 13) comprising a chamber (12) that houses the detector (13) and two air flow connections (10,11) between the chamber (12) and respective sampling points in the output air flow (18) so that a pressure differential is generated between them by the output air flow, thereby circulating a sample of air through the chamber (12) .

8. A fume detector to detect fumes in an air flow, comprising a semiconductor device responsive to fume molecules, characterised by a chamber (12) in which the device (13) is located, and two air flow connections (10,11) between the chamber (12) and respective sampling points in the output air flow (18) so that a pressure differential is generated between them by the output air flow, thereby circulating a sample of air through the chamber (12) .

9. Fume extraction apparatus comprising filtering apparatus to remove particles from air passed through it, characterised by a pressure detector (16) to detect the drop in pressure across the filtering apparatus (4,5,6) and adapted to issue a warning signal when a predetermined pressure drop is detected corresponding to a predetermined level of blockage of the filtering apparatus .

10. Apparatus as claimed in claim 9, in which the filtering apparatus comprises a pre-filter (4) and a main filter (5) , and the detector (16) is set to detect the pressure drop across both filters .

11. Apparatus as claimed in claim 10, including calibration means (15) by which an output signal of the detector (16) is calibrated under one or more known pressure drop conditions so as to trigger the warning signal at a predetermined pressure drop across the filtering (4,5,6).

12. Apparatus as claimed in claim 11, in which the calibration means (15) is adapted to calibrate the detector (16) by reference to its operation when the filtering apparatus is new and when the filter is fully blocked.

13. Apparatus as claimed in any one of claims 10 to 12 including warning means (15) adapted to monitor the degree of blocking of the main filter (5) each time the pre-filter (4) is replaced, and which triggers a warning signal at a predetermined maximum level of blockage, at which the main filter (5) should be changed.

14. Fume extraction apparatus comprising filtering apparatus with filter elements that have a limited life before they need replacing or servicing, a pump that moves air through the filtering apparatus to be cleaned, characterised by a controller (15) including a timer (19) that determines a maximum period of use of the apparatus comprising a number of successively shorter periods of use, each determined by the timer (19) , which is re-settable by the operator at the end of each period of use, except the last.

15. A centrifugal pump having an impeller characterised by a series of radial extending blades (34) located between a pair of coaxial discs (32,33) to which they are connected by a series of projections or tabs (34) which engage in apertures in the discs .

16. Fume extraction apparatus substantially as herein described with reference to the drawings 1 to 5.

17. A centrifugal pump substantially as herein described with reference to Figures 6 and 7.