Measuring and testing – Particle size
Reexamination Certificate
1999-09-14
2002-06-11
Noland, Thomas P. (Department: 2856)
Measuring and testing
Particle size
C324S071400, C324S464000
Reexamination Certificate
active
06401553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject of the invention is a method and an apparatus for measuring particle size distribution with an electrical impactor.
The subject of the invention is also an electrical impactor.
2. Background Information
As environmental standards are becoming stricter, the need for real-time measurement of particulate emissions increases. This need for measurement is especially important in the development of purification methods, in the research on different combustion processes as well as in the monitoring processes of the actual emissions. Traditionally, the so-called cascade impactors have been used in particle measurement. These impactors classify particles according to particle size.
Traditional cascade impactors do not allow for real-time measurement of particle size distribution and their changes. An electrical impactor, which has been developed from the traditional cascade impactor, has enabled this real-time measurement of particle size distribution.
FIG. 1
represents the operating principle of a known electrical impactor. The pump
14
produces underpressure, which then sucks the airflow
11
under observation first to the charger
12
, in which the particles are charged. Then, the airflow containing these charged particles is sucked through an impactor
10
, which consists of several stages. Charged particles, which are left behind on each of the stages, produce an electric current, which is then measured separately on each of the levels with a sensitive current meters located in current measurement unit
15
. The functions of the charger
12
, current measurement unit
15
and the pump
14
are controlled from the control unit
16
.
FIG. 2
presents a cross section of an electrical low pressure impactor, in which the chambers
29
a
and
29
b
can be seen, which are connected to the impactor's first two stages
20
a
and
20
b
. Airflow
21
is brought through the impactor's frame part
25
to the first chamber
29
a
through the inlet hole
28
. Each of the stages have a nozzle part
22
a
;
22
b
. The airflow
21
carrying the particles flows through the outlets of the nozzle parts. Behind the nozzle parts
22
a
;
22
b
lie the collector surfaces
23
a
;
23
b
. The collector surface has at least one outlet
30
, through which the flow
21
can flow to the next chamber or out of the impactor. Insulators
24
a
;
24
b
;
24
c
, which are situated between stages
20
a
;
20
b
, insulate different stages
20
a
,
20
b
from each other and from the cover section
26
of the impactor's first stage.
FIG. 3
presents a detail of the collector surface
23
. The direction of the airflow
21
flowing through the outlets of the nozzle part changes radically as it reaches the collector surface
23
. Particles
31
with a sufficiently low mechanical mobility are transported by the airflow
21
and cannot follow the radical and sudden change of the direction and impinge the collector surface
23
. The particles
31
, which have impinged the collector surface
23
, collect on the collector surface
23
and form a mass
32
.
As the charged particles impinge the collector surface
23
a
;
23
b
, as described in
FIG. 2
, they produce a change in the collector surface's charge level. Because the collector surface
23
a
;
23
b
is electrically connected to the impactor's stage in question
20
a
;
20
b
, which is, furthermore, connected to the current measurement unit
15
with an electric connection
27
a
;
27
b
, the change in the charge level of the collector surface
23
a
;
23
b
manifests itself as electric current, which can be perceived with the help of sensitive current meters, which are situated in the current measurement unit
15
.
The particles' mechanical mobility depends on their size in a known manner. This enables size selective classification of the particles. By choosing in a known manner the number and the size of holes in the nozzle parts
22
a
and
22
b
, the distance between the nozzle part
22
a
;
22
b
and the collector surface
23
a
;
23
b
and the velocity of the flow can each of the impactor's stages
20
a
,
20
b
be designed so, that on each stage the collector surface
23
a
;
23
b
draws only those particles with a mechanical mobility value lower than the desired value, or in other words, particles which are larger than a certain, set particle size.
FIG. 4
a
presents the collection efficiency
42
of a stage of impactor as function of the particle size (Dp).
FIG. 4
a
describes the collection efficiency of such an impactor stage, whose cut-off point is set at 1 &mgr;m. In an ideal situation, the collection efficiency of this stage would be step-like, but, due to non-ideal situations, in practise some particles which are larger than the cut-off point will pass the stage in question, and some particles which are smaller than the cut-off point will collect to the stage. This manifests itself as a deviation from the step-like shape of the efficiency curve
42
.
Certain collection efficiency, as demonstrated for example in
FIG. 4
b
, can be achieved by sequentially placing stages with a different cut-off point. An impactor with collection efficiency as demonstrated in
FIG. 4
b
has a first stage (collection efficiency curve
44
), which collects particles over 100 &mgr;m, a second stage (curve
43
), which collects particles between 10-100 &mgr;m and, correspondingly, a third and a fourth stage (curves
42
and
41
), which collect particles between 1-10 &mgr;m and 0.1-1 &mgr;m.
When the cut-off points of impactor's (
10
) different stages
20
a
,
20
b
and the average charge received by the particles at the charger
12
as a function of the particle size are known, size distribution of particles contained in the flow
11
can be determined real-time according to currents received by the current measurement unit
15
from each of the stages
20
a
,
20
b.
The problem of an electrical impactor construed according to the prior art as described above is the loss of smaller particles in the stages of impactor, which collect larger particles. Due to these losses impactor's collection efficiencies
41
,
42
,
43
,
44
can be significant to those particles which are much smaller than the cut-off point. Collection efficiency curves
41
,
42
,
43
,
44
presented in
FIGS. 4
a
and
4
b
demonstrate this problem: the curves do not zero in the sizes smaller than the cut-off point.
The inventors have noticed that a significant part of small particle losses in stages collecting larger particles are due to the accumulation of charges in the insulator between stages of impactor and to the Coulombic losses caused by these charges. These new discoveries by the inventors are represented in
FIGS. 5 and 6
, which illustrate how the effects on forces, which are caused by Coulombic interaction, produce the above-mentioned losses of small particles.
FIG. 5
presents impactor's first stage
20
. Negative charge has accumulated in the insulator
24
, either due to careless handling or due to external circumstances. Negative charges in the insulator
24
cause a Coulombic attraction force to the positively charged particles in the flow
21
, and pull the positively charged particles towards the insulator
24
.
FIG. 5
illustrates this attraction force with solid, unbroken arrows. The light the particle is, the easier it moves towards the insulator
24
, due to the force effect. As the particle impinges the insulator, the particles clings to it and thus leaves the flow under observation.
FIG. 6
illustrates how positively charged particles, which cling to the insulator
24
, produce around them a Coulombic force effect, which repels other positively charged particles. Due to this force effect small sized particles with a low mass do not follow the flow
21
through the holes in the nozzle parts
22
, but separate from the flow and impinge either the lower surface of the cover part
26
, nozzle part
22
or the walls of the stage
20
. Should this be the second or
Keskinen Jorma
Virtanen Annele
Burns Doane Swecker & Mathis L.L.P.
Dekati Oy
Noland Thomas P.
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