Apparatus for producing an aerosol jet

Apparatus for producing an aerosol jet comprising an atomizing means and a flow of carrier gas for entraining the particles formed. Downstream of the atomizing means, the conduit for carrying the aerosol constitutes the central passage (20) of a double nozzle (21) whose annular passage (22) receives another dry gaseous flow. This thus forms a central flow of aerosol, which is sheathed externally by a flow of dry gas. The invention is used for the analysis of solutions by spectroscopy.

BACKGROUND OF THE INVENTION 
The present invention concerns an apparatus for producing an aerosol jet, 
which is more particularly intended, by way of example, for an apparatus 
for analysis by spectroscopy. In an apparatus for analysing a solution by 
means of spectroscopy, the solution to be studied is firstly finely 
divided in the form of an aerosol, and then injected into a plasma flame 
such as of ionised argon, at a temperature of the order of 5000.degree. C. 
The aerosol is excited in the plasma and in turn emits radiation which is 
characteristic of the elements contained in the solution. This radiation 
is analysed in a conventional spectrometer. 
Various means are known for putting the solution into a finely divided 
form, and in particular the use of ultrasonics to cause the liquid mass to 
be broken down into very fine droplets; the droplets can then be entrained 
in a gas flow towards the position of use thereof and in particular 
towards the plasma torch. In the conventional apparatuses in their present 
state, certain droplets which are being transported can come into contact 
with the walls of the duct; in the vicinity of the plasma torch, the walls 
are hot, which, in that region, causes the droplets to evaporate, and the 
salts contained therein are deposited. This gives rise to the danger of 
restricting the flow section of the duct, and even blocking the duct. 
However, this also has the even more serious consequence of leaving in the 
apparatus a `memory`, which gives rise to the danger of falsifying the 
following analysis result, for example by a part of the deposits formed in 
the course of previous analysis operations being dissolved in the new mist 
of a subsequent analysis operation. In order to avoid such interference 
phenomena, the conduits for carrying the aerosol into the flame would have 
to be rinsed or changed for each analysis operation, which is incompatible 
with the increasingly felt need for the capability of carrying out 
sequential analysis operations at high rates. 
SUMMARY OF THE INVENTION 
The present invention provides a solution for overcoming such 
disadvantages, and makes it possible to improve the energy efficiency of 
the apparatus. 
The invention concerns an apparatus for producing an aerosol jet, 
comprising a means for atomisation of the substance to be transported, 
with the creation of a flow of carrier gas for entraining the particles 
formed. According to the invention, downstream of the actual atomising 
means, the conduit for carrying the aerosol formed by the carrier gas and 
the particles in a state of suspension constitutes the central passage of 
a double nozzle whose annular passage receives another dry gas flow 
thereby to form, in the conduit for carrying the whole towards the 
position of use, a central flow of aerosol which is externally sheathed by 
a flow of dry gas, the flow rates and the relative positioning of the 
annular and central passages of the nozzle being so determined that the 
flow patterns of the two flows remain laminar and at adjacent speeds. 
In accordance with a preferred embodiment, the axis of the nozzle and the 
conduit between the outlet of the nozzle and the position of use is 
vertical or only very slightly inclined.

DESCRIPTION OF AN EMBODIMENT 
The apparatus comprises an ultrasonic generator which is formed by a 
piezoelectric crystal 1 which is excited electrically from a generator 
(not shown) to which it is connected by connections 2. The ultrasonic 
waves, which are in a frequency range of from 200 KHz to 10 MHz, are 
transmitted by a liquid column 5 which maintains a constant load on the 
crystal. The liquid column is maintained in a body 3 which is placed on 
the crystal, with the interposition of sealing joints 4, continuous 
circulation of water being maintained between an inlet 6 and an outlet 7 
in order also to provide for cooling. 
The solution to be atomised is introduced by the pipe 9 into the cell 10 by 
means of a peristaltic pump 11 which is actuated by a stepping motor in 
order to maintain a constant amount of liquid in the cell, in the course 
of operation of the apparatus. The cell 10 which is closed in its lower 
part by a thin membrane 12, for example comprising polyester which is from 
10 to 100 microns in thickness, is fitted on to the body 3 with sealing 
joints 13 being interposed. 
The cavity in the body 3 forms an impedance matching means which is 
intended to increase the amplitude of the acoustic wave and the production 
of aerosol, while also making it possible for the power to be supplied to 
the crystal to be reduced. The amplification effect is achieved by a 
reduction in section from the inlet towards the outlet; as the density of 
acoustic energy required for the production of cavitation in the solution 
is constant, the energy to be applied to the crystal will be inversely 
proportional to the sectional area. 
The matching means is formed by a solid body (metal, glass . . . ) which 
has an acoustic impedance such that the coefficient of reflection of the 
wave at the interface between the body and the column of liquid is total. 
The matching means must be of a regular shape and, taking into account the 
fact that the dimensions are here markedly greater than the wavelength, a 
conical shape has been selected as being easier to produce. 
The internal surface condition of the matching means must be better than a 
fiftieth of the wavelength in order to achieve a satisfactory degree of 
efficiency. In order to avoid degradation of the surface condition, which 
would result in a deterioration in the degree of efficiency, the 
transmission liquid selected is a liquid which is chemically inert with 
respect to the material forming the guide. Particularly when the liquid 
used is water, it must be demineralised and degassed, as any deposit on 
the internal wall surfaces of the guide gives rise to the danger of 
interferring with proper functioning thereof. Its temperature must be 
stable and must be for example lower than 17.degree. C. when the liquid 
used is water. 
For any question in regard to diffraction of the sound waves which may be 
involved in calculation of the impedance matching means, reference may be 
made for example to a work such as `Fundamental of Ultrasonics`, J. BLITZ, 
Butterworths, London. 
The droplets formed by the cavitation phenomena produced by the ultrasonics 
in the vicinity of the surface of the solution in the cell 10 are 
entrained by a flow or argon which is introduced at 15. The aerosol which 
is thus formed is entrained into the extraction stack or funnel 16 whose 
base is of a bevel configuration in order to ensure that the 
large-diameter drops which are thrown up by the ultrasonics geyser are 
returned, without causing disturbances. The drops of excessively large 
diameter or which are re-deposited on the walls of the conduit are then 
recovered in the elbow-bent drain 17 provided with a discharge sylphon 18. 
The aerosol is then taken by way of the central tube 20 into the double 
nozzle 21 whose peripheral annular conduit 22 is supplied with a dry 
gaseous flow which is introduced by way of the passage 23. The flow rates 
and the speeds on the one hand of the central flow of wet aerosol, which 
carries the droplets of solution, and on the other hand the annular flow 
of dry gas, are so determined as to maintain laminar flow patterns at 
similar speeds, in order to avoid the formation of eddies at their 
interface. This thus produces, in the outlet pipe 25, a central jet of gas 
carrying the wet particles of the solution, and a real continuous outer 
sheath of dry gas which prevents the particles being transported from 
coming into contact with the walls. 
It is the conduit 25 which serves as a means for injecting the aerosol into 
the plasma torch 26 which is formed in the usual manner by an input of a 
flow of plasmagenic gas such as argon which is carried by the conduit 27 
between the turns of an inductor 28 which is supplied at very high 
frequency by a generator 29. 
It will be seen that, in the conduit 25 which is heated by the proximity of 
the torch 26, the droplets of the solution to be analysed, being in 
suspension in the aerosol, cannot be caused to evaporate by contact with 
the hot walls and deposit thereon the salts contained in the droplets. The 
conduit 25 therefore always remains clean and of constant section, and, at 
the end of an analysis operation, it is without any trace of the solution 
which has just been the subject of study. The apparatus can then be 
switched over to another analysis operation without any danger and 
instantly, and without requiring any dismantling or rinsing. 
In order to maintain the quality of the gaseous sheathing, care will be 
taken to ensure that the flow formed at the outlet from the nozzle does 
not have any transverse components due to gravitational pull. For that 
purpose, the system comprising the nozzle and the outlet conduit leading 
to the position of use will be disposed vertically, at least when the 
aerosol contains particles of relatively large sizes (1 micron in regard 
to water) or of high densities. For light particles or particles of small 
sizes (less than 0.1 micron for example for water), it would be possible 
to tolerate short sections in which the system extends at an inclined 
angle or even horizontally if the flow speed makes it possible 
sufficiently to limit the period of time for which the particles are 
flowing in those portions in which the gravitational pull could cause a 
flow across the gaseous protective sheathing. 
It will be appreciated that the invention is not limited to the single 
embodiment or the single use described above by way of example, but it 
also embraces constructions which would differ therefrom only in regard to 
details, design variations or by the use of equivalent means. 
Thus, the invention can be applied to the production of aerosol jets 
whenever the danger of hot walls suffering from the deposit of particles 
which are being transported, is likely to be prejudicial to good 
preservation or good functioning of the installation. The particles may 
then be solid or liquid, for example for feeding burners or boilers, and 
atomisation of the substance to be transported by the aerosol could be 
effected by known means other than ultrasonics.