Gas separation – Multiple separators – each with discrete and longitudinally... – Centrifugal
Reexamination Certificate
1999-08-23
2001-07-17
Simmons, David A. (Department: 1724)
Gas separation
Multiple separators, each with discrete and longitudinally...
Centrifugal
C055S449000, C055S438000, C055SDIG003
Reexamination Certificate
active
06261330
ABSTRACT:
The invention relates to apparatus for separating particles from a fluid flow, particularly but not exclusively to apparatus incorporating a cyclonic separator for separating dirt and dust particles from an airflow.
Cyclonic separators for separating particles from a fluid flow are well known. Within the last 15 years, many developments have been made in relation to the use of cyclonic separators within vacuum cleaners. Examples of such developments are EP 0018197, EP 0134654, EP 0489565 and EP 0636338. Inevitably there is always a desire to make cyclonic separators more efficient; that is to say, to produce a cyclonic separator which will separate a higher proportion of particles of a given diameter than comparable cyclonic separators. Constraints of size, cost and materials mean that the basic characteristics of cyclonic separators specifically intended for use in vacuum cleaners cannot be varied outside particular limits. Therefore, other ways of improving the cyclonic separator's efficiency have to be explored.
Basic cyclone theory states that there are two opposing forces acting radially on a particle travelling within a fluid flow. A centrifugal force F
z
acts radially outwards on the particles, pressing them towards the cyclone wall, given by
F
z
=
(
ρ
p
-
ρ
)
V
t
,
p
2
π
x
3
6
r
where
&rgr;
p
=particle density, kg.m
−3
&rgr;=fluid density, kg.m
−3
V
t,p
=tangential velocity of particle, m/s
x=particle diameter, m
r=radius of particle orbit, m
However an opposing drag force F
w
acts towards the centre of the cyclone given by
F
w
=
C
D
ρ
π
x
2
8
(
V
r
-
V
r
,
p
)
where
V
r
=radial velocity of fluid, m/s
V
r,p
=radial velocity of particle, m/s
C
D
=drag coefficient (drag force per unit projected particle area/dynamic pressure)
When F
z
exceeds F
w
, the particle is thrown against the cyclone wall and separated from the air flow. When F
w
exceeds F
z
, the particle remains entrained within the air flow and exits the cyclone separator with it. Clearly it is highly advantageous to give F
z
as large a value as possible. Since F
z
is proportional to the square of the particle's tangential velocity, increasing the tangential velocity of the fluid will increase the cyclone's efficiency.
A parameter which is important in cyclonic separator theory is the cut diameter or cut point. This is the diameter of particle of which, for specified fluid flow conditions, 50% are removed by the cyclone and 50% remain entrained. The cut point is a measure of the separation efficiency of the cyclone: the lower the cut point for given conditions, the lower the proportion of particles remaining in the fluid flow; therefore, the higher the separation efficiency. It is therefore desirable to achieve as low a cut point as possible for specified conditions.
Semi-empirical cyclone theory has been developed to give an expression for the cut size X
50
for which 50% of the particles are collected.
X
50
=
kQ
μ
ρ
p
V
t
2
where
Q=volumetric flow rate, m
3
/s
&mgr;=viscosity, kg/ms
k=geometric factor
V
t
=tangential velocity, m/s
The theory states that V
t
in the cyclone vortex is a linear function of Q. This suggests that the cut size can be decreased by increasing V
t
(i.e. X
50
∝(1/V
t
)) and hence increasing the separation efficiency.
The computation of the separation efficiency for a range of particle sizes (commonly known as the grade efficiency curve) also depends on the cyclone parameters and flow characteristics of the fluid. The separation efficiency can be expressed empirically, for any particle size, as:
ξ
=
[
1
+
(
k
V
t
2
ρ
p
x
2
μ
Q
)
-
3.2
]
-
1
This also suggests that, for any fixed cyclone design and particle size, the separation efficiency is governed by the tangential velocity and the volumetric flow rate, the latter of which is related to the former. Hence, again, the separation efficiency will increase as the tangential velocity increases.
All of the above theory indicates that the efficiency of a cyclone can be increased by increasing the tangential velocity. The simplest way to achieve such an increase is merely to provide a more powerful motor to create a higher rate of fluid flow. Unfortunately, there are limits to the type of motor which can be utilized in some applications such as, for example, domestic vacuum cleaners. Such motors are limited by factors such as physical size, weight and cost. For this type of application other ways of increasing the tangential velocity of the fluid flow at the inlet to the cyclone must be sought.
It has long been recognised that fluid leaving a fan which incorporates rotating impeller blades has a tangential velocity component. Until now, only limited effective use has been made of that tangential component in cyclonic separators. The fan has normally been located at a position remote from the cyclone inlet, normally downstream of the separator, so that all of the tangential velocity of the fluid entering the cyclone has to be forcibly applied to the fluid flow upstream of the cyclone. The forced application of a tangential velocity component inevitably causes slight decreases in the flow rate overall due to friction losses.
There have been some attempts to utilise the tangential component of the velocity of the fluid leaving a fan in some cyclonic separators used to separate solids (or semi-solids) from liquids. Examples of such separators are shown and described in GB 727137 and FR 1077243. In each of these separators, a fan is located in the upper end of the cyclonic separator and arranged so that the tips of the blades run very close to the cyclone wall. This allows the fan to be responsible for setting up swirling cyclonic motion in the fluid entering the cyclone and the need for a tangential inlet is avoided. However, when an outlet of the separator is to be provided at the same end of the cyclone as the inlet, the arrangement is that a vortex finder is located so that it passes through the fan itself (see, for example,
FIG. 1
of GB 727137). This means that the incoming fluid cannot be passed directly to the eye of the fan and this will lead to limitations on the maximum speed of the fan and a consequential loss in performance of the fan.
It is therefore an object of the invention to provide apparatus for separating particles from a fluid flow having a cyclonic separator which, for given conditions, has a higher inlet tangential velocity and therefore a higher separation efficiency than corresponding known separators and overcomes the problems of the prior art. It is a further object of the invention to provide apparatus for separating particles from a fluid flow having a cyclonic separator which makes advantageous use of the tangential velocity component of fluid leaving a fan. A further object of the invention is to provide apparatus for separating particles from a fluid flow having a cyclonic separator which is efficient, compact, lightweight and easy to service and maintain.
The invention provides apparatus as set out in claim
1
. The location of the fan in the inlet to the cyclonic separator means that relatively high inlet tangential velocities can be achieved within the cyclone due to lower losses, hence improving the efficiency of the cyclone. This maximises the utilisation of the tangential velocity imparted to the fluid flow by the fan. Also, the arrangement of interleaved inlet and outlet conduits avoids the need to pass the vortex finder through the fan which optimises the fan's performance. Further advantageous features are set out in the subsidiary claims. It will be noted that, contrary to the arrangement in many known cyclonic vacuum cleaners, the fluid flow is in this case pushed through the cyclone separator instead of being pulled or sucked.
REFERENCES:
patent: 460152 (1891-09-01), Morse
patent: 2787374 (1957-04-01), Kreb
Burlington Geoffrey Michael
Dyson James
Hopkins Robert A.
Morrison & Foerster / LLP
Notetry Limited
Simmons David A.
LandOfFree
Apparatus for separating particles from a fluid flow does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus for separating particles from a fluid flow, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus for separating particles from a fluid flow will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2504052