WO2002018904A1

WO2002018904A1 - Device for sampling and concentrating small particles in air by impaction

Info

Publication number: WO2002018904A1
Application number: PCT/SE2001/001854
Authority: WO
Inventors: Tage Berglund
Original assignee: Tage Berglund
Priority date: 2000-08-31
Filing date: 2001-08-31
Publication date: 2002-03-07
Also published as: EP1322933A1; SE0003079D0; AU2001282819A1; SE517047C2; SE0003079L

Abstract

A device (1) for sampling and concentrating small particles, such as bacteria, bacterial spores and pollen in air by impaction. The device has a rotor (3) arranged on a rotor shaft and having one or more wings (4) with a leading edge and a trailing edge adapted to rotate in said air. Moreover, it has an inlet slit (5) at the leading edge of the wing (4) for letting in an airflow enriched with particles from the air that hits the leading edge of the wing during rotation of the rotor and a channel (6) for conducting said airflow from the slit (5) through the wing and the rotor shaft to possibly further enriching steps (7, 8) and a detecting step (9).

Description

DEVICE FOR SAMPLING AND CONCENTRATING SMALL PARΗCLES IN AIR BY IMPACTION

The present invention relates to a device for sampling and concentrating small particles, for instance bacteria, bacterial spores and pollen in air by impaction.

More specifically, the invention relates to an aerosol concentrator for indicating an increased particle or microorganism content of an aerosol, such as air pollutants, pollen, bacteria or biological warfare agents. The present invention has occurred as a result of the development of apparatus for indicating biological warfare agents, and therefore the description will deal with this example. It goes without saying that the device is applicable to other types of particles and microorganisms.

Particles can be separated from a flow of air by impaction. The technique is based on inertia. The higher the speed of the particles, the finer particles can be separated by impaction if the flow of direction is changed abruptly. The coarsest particles can thus be separated at a relatively low speed whereas finer particles require a higher speed.

A virtual impactor causes separation of particles by abruptly deflecting the flow of air and simultaneously retaining a small flow of air in the extension of the original direction of flow. The latter flow of air will contain the greater particles while the abruptly deflected flow mainly contains finer particles. Virtual impactors are disclosed in e.g. US 4,670,135, US 4,927,957 and US 4,689,052.

Background

In biological warfare, B-agents such as pathogenic microorganisms, bacteria, virus and fungi are intentionally spread within an area in order to incapacitate people or reduce their power of resistance. Since B-agents multiply, only a small amount of organisms is necessary to cause people falling ill. B-agents can be spread by way of air, foodstuff, water, insects or on contact. They give a great effect in relation to their weight and volume. In spreading by air, which is the case when the present invention can be applied, liquids are atomised to aerosols where each individual particle contains one or more microorganisms. Only air attacks will be discussed below.

An indication of a B attack can be a large number of individuals being taken ill or an unexpected clinical picture. Since the effect of B weapons is not immediate and only small amounts are involved, it can thus take time before suspicion about an attack arises. A warning system for quick indication and possible identification raises the level of protection significantly by making it possible to prevent infection from being passed on by taking the requisite measures of precaution. It is also very useful if samples for indication can be continuously taken in order to detect an attack and take measures.

It is very difficult to detect a B attack. Immediately after and at the periphery of a B attack, the concentration of microorganisms in air is relatively low. In a B cloud there are sometimes only a few organisms per cubic meter. In indication of air samples it is therefore necessary to concentrate the micoorganisms for the content to be measurable.

Prior-Art Technique The air samples can be collected using apparatus of different kinds. Such apparatus are also used in time of peace to collect bacteria and fungal spores. For efficient study of the physical and chemical properties of the particles in the aerosol, it is necessary to separate the particles from the gas of the aerosol and convey the separated particles to a place where they can either be collected to be studied later or be analysed in situ using suitable instruments.

Most sampling methods are based on generating, using some kind of air pump, a constant flow of air through a filter where the particles are deposited. In many cases, the greater particles are separated by impaction. The higher the speed of the air, the finer particles can be separated.

Virtual impactors having holes or slits are normally used. This technique is based on the condition that air is exhausted by suction through holes or a slit, the airflow is deflected and the particles fall down in a lower space (Fig. 1). The drawback of these impactors is that the holes/slits are easily clogged by pollen and dirt, and that there practical effect will not be very great. In a slit impactor, the practical inflow can be about 50 dm³/min and less than half thereof in a hole impactor.

Patent Specification US 4,689,052 discloses an impactor which moves through an aerosol to cause a certain air speed towards an inlet slit. This device can be mounted on, for instance, an aeroplane. This is, of course, disadvantageous since a flight must take place for the sampling to be carried out, and thus the sampling is not continuous. Moreover it may be difficult to determine exactly where a B agent has been found.

The Present Invention

The object of the present invention is to provide an efficient, stationary device for continuous collection of air samples, which can process at least 1 m³/min. A further object is to provide a high relative speed between the air and the virtual impactor without pumping the air past the impactor. These objects are achieved by a device according to the claims.

According to the invention, the device comprises a rotor arranged on a rotor shaft and having one or more wings with a leading edge and a trailing edge adapted to rotate in said air. Moreover the device comprises an inlet slit at the leading edge of the wing for letting in a flow of air enriched with particles from the air that hits the leading edge of the wing during rotation of the rotor and a channel for conducting said airflow from the slit through the wing and the rotor shaft to possibly further enriching steps and a detecting step.

The invention concerns a stationary device for continuous collection of air samples and uses a rotating wing whose rotary speed gives the desired air speed towards the impactor. As a result, no pump is necessary to generate a sufficiently great airflow towards the impactor.

According to an embodiment, the device has an essentially cylindrical apparatus casing which is open for air to pass and in which the rotor is arranged to rotate at a certain angle of attack to cause air to flow through the apparatus casing. A certain particle fraction is sucked in through the slit in the wing and is thus enriched. From there the particles are passed on by suction through a channel towards possibly further enriching steps such as a conventional impactor.

The airflow can then pass through another enriching step consisting of particle- focusing lenses (displacement washers) arranged in succession. The particles are then conveyed to a detecting step for analysis and specific identification. The rotor is arranged to rotate with a number of revolutions which gives a relative speed between the leading edge of the wing and the ambient air of 10-50 m/s.

The invention will now be described in more detail by way of example and with refe- rence to the accompanying drawings in which

Fig. 1 illustrates a virtual impactor according to prior-art technique,

Fig. 2 shows the airflow passing a wing with a slit,

Fig. 3 schematically shows an embodiment of the device, Fig. 4 is a top plan view of Fig. 3, and

Fig. 5 shows the device arranged on a vehicle.

Fig. 1 shows a device (1) according to the invention consisting of a rotating rotor (3) arranged on a rotor shaft and having one or more wings (4). At the leading edge of the wing a slit (5) is formed for letting in an airflow enriched with particles. A channel (6) conducts the airflow from the slit (5) through the wing and the rotor shaft via a swivel (11) onto possibly further enriching steps (7, 8) and a detecting step (9).

According to an embodiment, the rotor is arranged in an essentially cylindrical appa- ratus casing (2) that is open for air to pass.

A further enriching step (7) consists of a conventional impactor. Yet another enriching step (8) consists of particle-focusing lenses arranged in succession in a conveying tube 12 for the enriched airflow.

The detecting step (9) consists of a particle analyser of a conventional type, for instance UV spectrophotometer or pyrolysis IMS.

Moreover the device comprises a vacuum pump (10) for conveying air enriched with particles to the detecting step (9).

The Function operates as follows.

The wing (4) rotates at a certain angle of attack and thus generates its own through- flow. A large amount of the airflow deflects round the wing (4) while a partial airflow containing particles continues forwards through the slit (5) of the wing. Owing to the inertia of the particles, a large amount of the particles do not manage to come along as the main airflow is deflected, and therefore they pass into the slit of the wing along with a smaller airflow.

A certain particle fraction is sucked in through the slit (5) in the wing and is thus enriched. From there the particles are conveyed by suction through the channel (6) towards a further enriching step (7), such as a conventional virtual impactor.

In the conveying tube (12), the airflow continues towards particle-focusing lenses (8) arranged in succession. The lenses have concentric holes whose diameter decreases along the direction of flow. This means that for each lens as passed, the particles, while being more inert than the air molecules, will assume a path closer to the centre of the conveying tube (12). Such lenses are described in, for instance, Aerosol Science, Vol. 24, March 1993, No. 7. Rao et al, Aerodynamic focusing of particles in viscous jets, pp 879-892.

The airflow, which is thus further enriched with particles, is conveyed by suction towards the particle analyser (9) by a vacuum pump (10) where identification of the particles takes place.

The device can also be arranged on a vehicle to be driven to a suitable location where collection and detection take place.

A motor for rotation of the wings and bearings therefor are conventionally designed and are not shown in the Figures. The number of wings is adjusted according to requirement. Typically, the wing attacks the air at a speed of 10-50 m/s, depending on aerosol fraction. The conveying channel may contain several channels.

Claims

Claims:

1. A device (1) for sampling and concentrating small particles, for instance bacteria, bacterial spores and pollen in air by impaction, c h a r a c t e r i s e d by a rotor (3) arranged on a rotor shaft and having one or more wings (4) with a leading edge and a trailing edge adapted to rotate in said air; an inlet slit (5) at the leading edge of the wing (4) for letting in an airflow enriched with particles from the air that hits the leading edge of the wing during rotation of the rotor, and a channel (6) for conducting said airflow from the slit (5) through the wing and the rotor shaft to possibly further enriching steps (7, 8) and a detecting step (9).

2. A device as claimed in claim ^ c h a r a c t e r i s e d by an essentially cylindrical apparatus casing (2) which is open for air to pass, and in that the rotor is arranged to rotate in said apparatus casing (2) at a certain angle of attack to cause air to pass through the apparatus casing (2).

3. A device as claimed in claim 1 or 2, c h a r a c t e r i s e d in that a further enriching step (7) consists of a conventional impactor.

4. A device as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that a further enriching step (8) consists of particle-focusing lenses arranged in succession in a conveying tube connected to the rotor shaft and intended for the enriched airflow.

5. A device as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the detecting step (9) consists of a particle analyser of a conventional type.

6. A device as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that the device (1) is stationary.

7. A device as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that the device (1) is placed on a vehicle.

8. A device as claimed in any one of claims 1-7, c h a r a c t e r i s e d in that the rotor (3) is arranged to rotate with a number of revolutions that gives a relative speed between the leading edge of the wing (4) and the ambient air of 10-50 m/s.

9. A device as claimed in any one of claims 1-8, c h a r a c t e r i s e d in that the inlet slit (5) is arranged along a stagnation line at the leading edge of the wing (4).

10. A device as claimed in any one of claims 1-9, c h a r a c t e r i s e d in that it further comprises a vacuum pump (10) for conveying air enriched with particles to the detecting step (9).