GB2177198A - Detection of impurities in a fluid - Google Patents

Abstract

A detector for detecting the presence of dust and microbial impurities in a fluid comprises, in a casing, a rotary fan 11 associated with a box having a fluid inlet (diameter D0 as shown) and a fluid outlet (diameter D1 as shown) profiled so as to reduce the pressure losses, the hub of the fan being extended by a fixed box with profiled surfaces, defining a passage 18, 19, 17 connecting together two points of the fluid flow induced by the fan which are at different pressures. the passage comprising chamber 19 containing means for example removable disc 20 for detecting the impurities in the by-pass stream flowing through the passage under the effect of the pressure difference. <IMAGE>

Description

SPECIFICATION Detection of impurities in a fluid FIELD OF THE INVENTION The invention relates to detection of impurities in suspension in a fluid and it is particularly suitable for use in detecting impurities such as dust or microbial elements in the air of industrial or living premises.

PRIOR ART The rapid detection of the appearance or of the presence of impurities in the atmosphere of premises is often necessary. For example, radioactive or toxic dust, as well as the development of certain microbial colonies harmful for man or capable of disturbing certain food production processes must be detected at an early stage of appearance. But very often these impurities only invade the volume of premises very gradually. Most of existing detectors are proximity apparatuses which analyze a small volume of air per unit time, typically a few liters per minute. When a detector of this kind is placed at a location in a premises and when it is traversed at such a flow rate of analysis, it can only give indications concerning the impurity content in a very small fraction of the premises.The suction speeds towards the detector induced in the atmosphere decrease in proportion to the square of the distance to the inlet orifice of the apparatus. Even at a few centimeters away, the air speed becomes extremely low.

Consequently, the samples of air taken are only representative of the situation in a very restricted zone, so that the alternative is either to accept late detection or to increase the number of detectors.

SUMMARY OF THE INVENTION It is an object of the invention to provide an improved detector; it is a more specific object to provide detection in a larger volume around a detector.

To this end, there is provided a detector of impurities in suspension in a fluid, comprising a fan mounted for rotation about an axis in a casing defining a streamlined fluid inlet and a streamlined fluid outlet for said fan, said fan having a hub and said casing comprising a stationary axial housing located forwardly of said hub and extending it, wherein passage means are formed in said housing for connecting two points of the fluid flow induced by the fan during operation thereof whereby a by-pass flow occurs along said passage, and includes a chamber containing means for detection of impurities in said by-pass flow.

The by-pass flow may be of the order of a thousandth of the total flow passing through the fan. Thus, sampling is obtained on a very large air flow, which involves a volume very much greater than that monitored by a conventional detector. The part of the casing situated downsteam of the fan and comprising the fluid outlet may be shaped so as to form a nozzle delivering a jet which, by induction, causes stirring of the air even at an apprecia ble distance from the detector. The front part of the casing and the box may be formed so as to cause, in an upstream flow region, a homogeneizing turbulence.

The invention will be better understood from the following description of a particular embodiment of the invention, given by way of example only, formed by a detector of impurities in suspension in the air. The description refers to the single accompanying Fig. which shows the detector, in cross-section through a plane passing through the axis at the upper part of the Fig. in side elevation at the lower part.

DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT Referring to the Fig. detector 10 comprises a ducted fan 11 whose wheel 12 and electric drive motor 13 are shown with broken lines in the Fig. The duct has two flanges for connection to an input shroud 14 and to an output nozzle 15 so as to form a casing.

Inside the shroud 14 is placed a removable housing 16 consisting of several assembled parts. The housing is connected to the inlet shroud and constitutes an extension of the hub of fan 10. A flow passage for a by-pass flow is formed in housing 16. This passage comprises an axial hole whose diameter is small compared with that of the shroud, extended inwardly by a tube 18. This tube opens into a chamber forming part of the passage and communicating with an annular space defined by the shroud 14 and box 16 through radial holes 17. In chamber 19 is placed a sensor sensitive to the presence of the impurities to be detected. In the embodiment illustrated, this sensor is limited to a removable disk 20 placed facing tube 18 and able to receive an adhesive retaining the dust and aerosols contained in the air jet which leaves tube 18 and spreads over disk 20.

So that the by-pass flow flows through the passage formed in box 16, a pressure difference must exist between the inlet of tube 18 and the outlet of hole 17. This result is reached by designing box 16 and the internal face of the inlet shroud 14 so that a flow speed variation occurs which is the source of a pressure difference. In addition, shroud 14 advantageously has a throat situated at the front, having a diameter Do such that the flow area offered to the air increases over a length of a few diameters until it reaches the diameter of wheel 12 of the fan, in the vicinity of the cross-section where holes 17 are placed. Thus, tube 18 takes samples in a region where divergence of the flow increases its turbulence and so its homogeneity.

The hub of fan 18 is extended rearwardly by an ogival shaped piece 21, streamlined and cooperating with nozzle 15 for guiding the flow. The diameter D1 of the outlet of nozzle 15 is provided so as to give a cross-section comparable to that of the annular space in which the blades of the fan move. The nozzle and piece 20 are moreover streamlined so as to avoid abrupt variations of section downstream of the blades and so as to contribute to the formation of a smooth and penetrating jet, of circular cross-section, which induces a considerable air stream far behind the detector and further contributes to increasing the volume concerned by the sampling, by stirring the whole of the air of the premises.

The characteristics of a detector capable of satisfying a large field of application may be given by way of example. The fan is designed so as to provide a flow rate of several tens of liters per second, at a speed of a few meters per second, causing a pressure difference corresponding to a few millimeters of water between the inlet of tube 18 and the outlet of holes 17. Tube 18 may have a diameter of a few millimeters and a length of a few centimeters. The flow which feeds sensor 20 is then of the order of a thousandth of that of the fan, which is itself about ten times smaller than the flow induced downstream at a distance equal to about thirty diameters.

Numerous modified embodiments are possible. In particular the inlet shroud 14 and nozzle 15 may be in one piece instead of being in several assembled pieces; the casing formed as shown in the Fig, by the nozzle, the fan duct and the inlet shroud, could be in a single piece. The method of fixing and assembling the box 16 could be different from that illustrated, particularly to facilitate access to the sensor retaining the impurities and/or delivering a signal responsive to the presence of impurities of a predetermined nature. The sensor may be of any known construction; for instance, it may be of a type which provides a measurement in the form of an electric signal, rather than simply collecting impurities.

The detector may be used in a liquid medium and the term "fan" must be construed as also covering impellers for liquid flow. A plurality of passages, possibly having sensors for detecting different impurities, may be formed in the housing.

Claims (9)

1. A detector of impurities in suspension in a fluid, comprising a fan mounted for rotation about an axis in a casing defining a streamlined fluid inlet and a streamlined fluid outlet for said fan, said fan having a hub and said casing comprising a stationary axial housing located forwardly of said hub and extending it, wherein passage means are formed in said housing for connecting two points of the fluid flow induced by the fan during operation thereof whereby a by-pass flow occurs along said passage, and includes a chamber containing means for detection of impurities in said by-pass flow.

2. A detector according to claim 1, wherein the passage is dimensioned for circulating a by-pass flow having an order of magnitude of one thousandth of the flow induced by the fan.

3. A detector according to claim 1, wherein the part of the casing situated downstream of the fan and comprising the fluid outlet is shaped so as to form a nozzle delivering a jet which, by induction, causes stirring of the air.

4. A detector according to claim 1, wherein said passage means comprises an axially located conduit opening forwardly into a front surface of said housing and delivering a jet which strikes said means for collection of said impurities and comprises radially directed outlet ports.

5. A detector according to claim 1, wherein said casing and housing define a fluid inlet having a divergent intake zone for increasing the turbulence and homogeneizing the fluid.

6. A detector according to claim 1, further comprising a stationary streamlined portion located downstream of the fan and in extension with said hub cooperating with the casing to define a nozzle which delivers a fluid jet having a substantially circular cross-section for inducing a fluid circulation with a flow rate higher than that across the fan behind the detector.

7. A detector according to claim 1 for detection of impurities in suspension in atmospheric air, wherein said fan is constructed and arranged for delivering a rate of flow of several tens of liters per second and said passage means has a circular cross-section of from one to some millimeters at its inlet.

8. A detector of impurities in suspension in a fluid, comprising a fan mounted for rotation about an axis in a casing defining a path for circulation of a fluid from an inlet to an outlet and a housing secured in said casing in an axial portion thereof and defining at least one passage connecting a point of said fluid flow which is upstream of said fan to an other point of the flow upstream of the fan and where the fluid pressure is lower, said passage being arranged and dimensioned for accomodating a by-pass flow much lower than the flow across the fan and including a chamber containing means for detecting impurities in said by-pass flow.

9. A detector of impurities constructed and arranged substantially as shown in and as described with reference to the accompanying drawing.