Apparatus and process for producing acknowledged air flow and the use of such apparatus in measuring particle concentration in acknowledged air flow

Abstract

Apparatus (1) for generating acknowledged flow (Q), comprising a first passage (2) with ends (3,4) for acknowledged flow (Q) inlet and outlet, a discharge electrode (5) for generating airborne unipolar ions (8) positioned inside the first passage (2), a counter electrode (6) adapted to attract said airborne ions (8), thereby being adapted to cause a net flow (7) of airborne ions (8) and thereby generating an airflow (Q) in the direction of the net flow of airborne ions (8), sensing element (12, 13), the output of which is a function of the concentration of the airborne electric charge (8, 11), means (17) for switching or modulating a parameter which affects the output of the sensing element (12,13) and means for determining the volumetric flow (Q) on the basis of the time response which switching or modulation creates to the sensing element (12,13) output. 11. Use of apparatus (1) as described in the previous claims for determining ultrafine particle concentration. Process for generating acknowledged flow.

Claims

1. Process for generating acknowledged flow, comprising: a. generating airborne unipolar ions with a discharge electrode in a passage; b. using a counter electrode adapted to attract said airborne ions to cause a net flow of airborne ions and thereby generating an airflow in the direction of the net flow of airborne ions; c. determining concentration of airborne electric charge; d. switching or modulating a parameter which affects the concentration of airborne electric charge; and e. determining the volumetric flow on the basis of the time response which switching or modulation creates to the concentration of airborne electric charge.

2. Process of claim 1, comprising using a corona needle for generating airborne unipolar ions.

3. Process of claim 1, in which the passage comprises a first passageway and a second passageway, the process comprising dividing the acknowledged flow between the first passageway and the second passageway.

4. Process as in claim 3, comprising removing particles entering the second passageway, particle removal being carried out upstream of the discharge electrode.

5. Process as in claim 3, in which the passage comprises a third passageway, the process comprising combining the flows of the first passageway and the second passageway in the third passageway.

6. Process as in claim 1, in which: the step of generating airborne unipolar ions comprises electrically charging at least a fraction of particles entering an apparatus; the step of determining concentration of airborne electric charge comprises measuring the electrical current carried by charged particles; and the step of switching or modulating a parameter comprises switching or modulating the electrical discharge unit at least between first charging stage where the electrical discharge electrode provides a first charge amount to at least a fraction of particles and second charging stage where the electrical discharge electrode provides a second charge amount to at least a fraction of particles.

7. Process as in claim 1, comprising in which: the step of generating airborne unipolar ions comprises electrically charging at least a fraction of the particles entering an apparatus; the step of determining concentration of airborne electric charge comprises measuring the electrical current carried by the charged particles; the process comprises removing ions, charged ultrafine particles or charged fine particles from the aerosol passing through the apparatus; and the step of switching or modulating a parameter comprises switching or modulating the ion/particle trap at least between OFF-mode where the ion/particle trap essentially removes free ions and ON-mode where ion/particle trap essentially removes particles having a diameter smaller than d.sub.p.

8. Process as in claim 1, comprising determining the essential parameters of the transfer function of an apparatus.

9. Process as in claim 8, in which the step of determining the volumetric flow comprises: a. providing a computational reference signal; b. comparing the sensing element output to the reference signal; c. adjusting the reference signal for maximum correlation between the sensing element output and the reference signal; d. computing the transfer function of an apparatus from the reference signal with maximum correlation; and e. determining the volumetric flow through the apparatus using at least some parameters of the computed transfer function.

10. Process as in claim 9, in which the step of determining the volumetric flow comprises: a. providing a computational reference signal following at least a first-order low-pass filter; b. determining the delay time t.sub.d and time constant of the first-order low-pass filter; and c. determining the volumetric flow through an apparatus (1) using the inverse of t.sub.d, or the sum thereof, t.sub.d+.

11. Process as in claim 1, comprising adjusting the switching/modulation frequency of a parameter affecting the sensing element output, between 0.01 Hz and 10 Hz.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) In the following the invention will be described in greater detail, in connection with preferred embodiments, with reference to the attached drawings, in which

(2) FIG. 1 is a schematic view of one embodiment of an apparatus according to the present invention; and

(3) FIG. 2 is a schematic view of another embodiment of an apparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 shows the invented apparatus 1 for generating acknowledged flow Q. Apparatus 1 comprises a first passage 2 with ends 3, 4 for the inlet and outlet of the acknowledged flow Q. The first passage 2 is preferably designed so that it generates a pressure difference of less than 20 Pa with the documented flow Q, more preferably less than 10 Pa and most preferably less than 5 Pa, as such pressure levels can be typically achieved by using electric wind to generate flow. In most cases the ends 3, 4 of the first passage 2 are essentially free. Inside passage 2 is a discharge electrode 5 powered from a high voltage source 14, which is isolated from mains with an isolation transformer 15. Discharge electrode 5 is adapted for generating airborne unipolar ions 8 and a counter electrode 6 is adapted to attract said airborne ions 8, thereby being adapted to cause a net flow 7 of airborne ions 8 and thereby generating an airflow Q in the direction of the net flow of airborne ions 8. Apparatus 1 further comprises sensing element 12, 13, the output of which is a function of the concentration of the airborne electric charge 8, 11. The sensing element may be constructed as measuring the charge entering or passing a sensing element 12, or it may be constructed as an electrometer, which measures the electric current escaping from apparatus 1 as airborne electric charge. This so called escaping current technique for measuring particle concentration is described in detail in WO2009109688 A1, which is hereby incorporated by reference in its entirety. Apparatus 1 further comprises means 17 for switching or modulating a parameter which affects the output of the sensing element 12, 13 and means (not shown in FIG. 1) for determining the volumetric flow Q on the basis of the time response which switching or modulation creates to the sensing element 12,13 output. Preferably the volumetric flow Q is determined by providing a computational reference signal, comparing the sensing element 12, 13 output to the reference signal, adjusting the reference signal for maximum correlation between the sensing element 12, 13 output and the reference signal, computing the transfer function of apparatus 1 from the reference signal with maximum correlation and determining the volumetric flow Q through apparatus 1 using at least some parameters of the computed transfer function. In one embodiment of the present invention, the computational reference signal may follow at least a first-order low-pass filter, in which case determining the delay time t.sub.d and time constant Prof the first-order low-pass filter allows determining the volumetric flow Q through apparatus 1 using the inverse of t.sub.d, or the sum thereof, t.sub.d+.

(5) In one embodiment of the present invention, apparatus 1 comprises a corona needle adapted to work as the discharge electrode 5. To avoid corona needle soiling, apparatus 1 comprises in one embodiment of the present invention, shown in FIG. 2, a second passage 21 placed inside the first passage 2. The second passage comprises a particle removal unit 22 placed upstream of the discharge electrode 5. As the ion wind drags air into apparatus 1, a fraction of the air flow Q passes through filter 22 and particles are essentially removed from this flow fraction. Clean air then passes through second passage 21 and from the vicinity of corona needle 5, thus preventing the corona needle from soiling. As the fraction of the flow passing through passage 21 is small compared to the flow passing through passage 2, the use of such arrangement does not lead to harmfully erroneous results even when apparatus 1 is used to generate acknowledged flow for a particle measurement sensor. A surprising finding is that the flow of the ion wind does not follow the clean-air from the second passage 21. In that case the flow feeding force would direct mainly to the said clean-air flow and not to the flow from the first passage 2, thus feeding mainly clean-air flow from passage 21. The flow velocities in this kind of arrangements are too low to generate turbulent mixing. But, according to the finding the ion cloud from the passage 21 spreads effectively due to the electrostatic attraction to the whole cross section of the flow channel, where the flows of passage 2 and 21 have joined. For this reason the force caused by electrostatic field to the ions is directed to the whole cross section of flow channel, respectively. This feature enables also the particles passing from the passage 2 to be charged electrically by ions fed from the second passage 21. In order to enhance air withdrawal from passage 2, the counter electrode 6 is designed in such a way that the net flow 7 of airborne ions 8 is directed from the corona needle 5 towards the counter electrode 6, which essentially does not cover the straight flow direction from passage 21 towards end 4 of apparatus 1.

(6) In one embodiment of the present invention, apparatus 1 comprises a charging chamber 16 placed downstream the discharge electrode 5 for electrically charging at least a fraction of particles 10 entering apparatus 1 with the acknowledged flow Q, an ion/particle trap 9 for removing ions 8 which are not attached to particles 10, means 12, 13 for measuring the electrical current carried by charged particles 11 and means 17 for switching or modulating the electrical discharge unit 5 at least between first charging stage where the electrical discharge electrode 5 provides a first charge amount to at least a fraction of particles 10 and second charging stage where the electrical discharge electrode 5 provides a second charge amount to at least a fraction of particles 10. This embodiment provides the benefit that as the charged particles are more difficult to remove from the air stream Q than the ions (free charges) 8, the response to the modulation of the electrical discharge unit 5 is more accurate. The electrical discharge unit may be switched between ON and OFF stages only, in which case the volumetric flow is easily determined from the response of switching to ON stage only, by knowing the volume between the corona discharge unit 5 and the sensing unit 12, or when the escaping current technique with sensing element 13 is used, the distance between the corona discharge unit 5 and the output end 4 of apparatus 1. In another embodiment the electrical discharge unit 5 is modulated between at least two voltages (and/or between two discharging currents), each of which provides an air flow through apparatus 1.

(7) In another embodiment of the present invention apparatus 1 comprises a charging chamber 16 placed downstream said discharge electrode 5 for electrically charging at least a fraction of the particles 10 entering apparatus 1, means 12, 13 for measuring the electrical current carried by the charged particles 11, an ion/particle trap 9 for removing ions 8, and/or charged particles 11 having a diameter smaller than d.sub.p and means 17 for switching or modulating the ion/particle trap 9 power source 18 output at least between OFF-mode where the ion/particle trap 9 essentially removes free ions 8 and ON-mode where ion/particle trap 9 essentially removes charged particles 11 having a diameter smaller than d.sub.p. The advantage of such embodiment is that the flow Q can be kept essentially constant throughout the flow determination.

(8) In one embodiment of the present invention, apparatus 1 comprises means for determining the essential parameters of the transfer function of apparatus 1. These means may be constructed by analogue or digital means as obvious for a person skilled in the art and the means may be realized within one or several functional blocks.

(9) In one embodiment of the present invention, apparatus 1 comprises means for providing a computational reference signal and the signal is connected to the means for switching or modulating a parameter essentially affecting the sensing element output. Apparatus 1 further comprises means for comparing the sensing element output to the reference signal, means for adjusting the reference signal for maximum correlation between the sensing element output and the reference signal, means for computing the transfer function of apparatus 1 from the reference signal with maximum correlation and means for determining the volumetric flow Q through apparatus 1 using at least some parameters of the computed transfer function. In the preferred embodiment apparatus 1 comprises means for providing a computational reference signal following at least a first-order low-pass filter, means for determining the delay time t.sub.d and time constant of the first-order low-pass filter and means for determining the volumetric flow through apparatus 1 using the inverse of t.sub.d, or the sum thereof, t.sub.d+. It is obvious for a person skilled in the art that other dynamic models than the sum of delay and mixed reactor can be used, depending on flow behaviour inside the device.

(10) In one embodiment of the present invention, apparatus 1 comprises means for adjusting the switching/modulation frequency of the means 17 for switching or modulating a parameter which affects the output of the sensing element 12,13, between 0.01 Hz and 10 Hz. Such embodiment offers a fast flow determination.

(11) The present invention also includes use of apparatus 1 as described in the previous embodiments for determining ultrafine particle concentration. Such use of apparatus 1 comprises determining cumulative flow Q.sub.t for the period of time t on the basis of the time response which switching or modulation creates to the sensing element output, determining the cumulative particle mass M.sub.t or cumulative number of particles N.sub.t for the period of time t and determining particle mass or number concentration, M or N, by dividing cumulative particle mass M.sub.t or cumulative number of particles N.sub.t by the cumulative flow Q.sub.t, i.e. M=M.sub.t/Q.sub.t and N=N.sub.t/Q.sub.t. A significant advantage of the arrangement described above is that the most expensive components; sensing elements and discharge units, are common for both functions; controlled flow generation and particle concentration sensing.

(12) It is apparent to a person skilled in the art that as technology advanced, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.