Patent Application
20170113196
The present invention relates to a stirring member that comprises at least two blades and is able to be fastened to a rotation shaft.
The manufacture of numerous products requires a homogenizing, diluting, dissolving, reheating, etc. operation.
To this end, use is frequently made of mechanical stirrers having a rotary shaft, which are driven frequently by way of an electric motor, and are provided with a shaft and a stirring member or stirrer. The assembly is thus made up of a container, a product and a stirrer.
The present invention concerns the design of stirrers which are generally propellers or turbines that comprise a member known as a stirring member mounted on a rotation shaft.
A turbine is provided with straight blades at 90° to the vertical, but any member made up of straight blades, even ones positioned in an inclined manner, is customarily known as a turbine.
A turbine generates a radial flow that generates shear, dissipating energy.
A propeller is preferably formed by a steeply inclined portion of helical pitch of a curved or bent sheet.
A propeller produces an axial and methodical flow.
The rotation of the stirring member causes the liquid to be displaced, making it possible to carry out the desired operation, more or less effectively depending on the shape of the member, its size and the speed of rotation.
The rotation can also cause shear and dissipate energy in the liquid to be mixed.
Sometimes, these two phenomena are necessary, during a reaction, for the formation of an emulsion.
The invention deals more specifically with the case in which the aim is to minimize the losses of energy by shear in order to obtain a displacement of the liquid and the mixing thereof with small losses, this entailing increased efficiency.
In such a case, it is the use of a propeller that gives the best result. This is because these operations only require that the product is set in motion, i.e. a pumping flow.
The aim is to produce this flow with the least possible energy, and it is known that propellers consume less energy than turbines for an equivalent flow.
In previous centuries, use was made only of turbines, which did not require a particular design; then, around a century ago, marine propellers, which are more efficient and less energy-consuming, were developed.
Two large families of propellers, which are represented by the patents U.S. Pat. No. 4,147,437 and FR 1 578 991, can be distinguished.
These two families of propellers are still used today, given their performance in relation to marine propellers.
However, in some markets, it proves difficult to use turbines, on account of the high power required and consequently the cost, or high-efficiency propellers.
This is because such a use is frequently considered to be too expensive, since the benefits of the high efficiency are not fully appreciated, only the investment cost actually being taken into consideration. The high efficiency is considered to be advantageous only for large machines, or when the cost of the energy is high or at least taken into account.
It is a difficult and/or lengthy, and thus costly, process to manufacture high-efficiency propellers, and it can only be carried out by special machines. This is because there are numerous technical problems on account notably of the thickness of the sheet and the curves that are tricky to obtain. It is not possible to have these propellers manufactured at another workshop or on another continent, for example, which results in high transportation costs.
Bent propellers already exist on the market, but these have a very specific shape with a bend at the blade corner so as to limit radial leakage. The improvement in efficiency was not the technical problem that the designers thereof intended to deal with.
There is therefore a need for a propeller that is easy to construct, i.e. without special material or particular skill, provides a good flow, which is the essential determining factor of stirring, but without consuming too much power as would be the case for a simple blade having a flat and inclined shape, which would actually result in high power, a large shaft and a great thickness of the blade and ultimately in an uncompetitive manufacturing cost.
According to the invention, a stirring member that comprises at least two blades and is able to be fastened to a rotation shaft is characterized in that each blade has a leading edge facing the fluid to be stirred and a trailing edge facing away from the leading edge, characterized in that each blade is obtained by bending a flat sheet, each blade having two longitudinal bends, the length of each bend being greater than 60% of the maximum radius of the blade.
The length of each bend may be greater than 75% of the maximum radius of the blade.
Advantageously, the two bends are parallel.
At least one of the bends may be perpendicular to the outer edge of the propeller.
The angle between the leading edge and the radial axis of the blade passing through the center of rotation and perpendicular to the outer edge, referred to as the angle of incidence, is positive, the distal end of the outer edge, remote from the shaft, meeting the fluid before the proximal end when the member is in rotation.
The angle of incidence may be between 4 and 20°, preferably between 6 and 15°.
Advantageously, the stirring member comprises only two blades so as to make it easier to introduce it through the opening in the container of fluid to be stirred.
Each blade may have, on account of the presence of the two bends, a substantially U-shaped cross section in a plane parallel to the axis of rotation of the member and parallel to the outer edge of the blade.
The section of each blade may also be substantially Z-shaped in a plane parallel to the axis of rotation of the member and parallel to the outer edge of the blade.
The trailing edge may be at an angle of between 30 and 70° to the intersection with the section plane of a plane orthogonal to the axis of rotation of the member, this angle being referred to as the departure angle.
Preferably, if the width of the blade at its distal end is denoted 1 and the width of the blade at its base at the level of the axis is denoted L, then 1>0.5 L.
Preferably, for each blade, the angle of attack a between the face containing the leading edge and the central face is between 13 and 25°.
Further features and advantages of the invention will become apparent from the following description of a preferred embodiment with reference to the appended drawings but without any limiting nature. In these drawings:
Throughout the following description of different embodiments of propellers according to the invention, relative terms such as “upper”, “lower”, “front”, “rear”, “horizontal” and “vertical” should be interpreted as when the propeller according to the invention is installed in an operating situation.
Inasmuch as each blade of the propeller has two bends, each blade thus has three faces and, in a cross-sectional view, it is necessary to define three angles in order to define the profile of the blade. These angles are more particularly visible in
The angle of attack is the angle a between the face having the leading edge and the central face. The angle d is the positioning angle between the central face of the blade and the horizontal when the axis of rotation is vertical. The departure angle f is the angle between the face having the trailing edge and the central face.
This propeller has an angle of attack a and a departure angle f of 21°. The first bend A, that is to say the one which will meet the fluid first, is made along an axis passing through the axis of rotation of the propeller. The second bend is denoted B. It may be noted that the distal end of the trailing edge is situated forward of the proximal end of this same trailing edge and with respect to the direction of rotation of the propeller. The distal end will thus meet the fluid first.
The blades are bent so as to obtain a camber coefficient of less than 12%, so as to improve the energy efficiency. The angle of attack is between 13 and 22° so as to have a suitable Cx. Specifically, beyond 30°, the radial forces generated will be very high. This then approaches the situation of turbines.
The area of the blade is generous and virtually in the form of a quadrilateral, so as to obtain a high pumping flow since the volume displaced depends on the surface area of the blade.
If the width of the blade at its end is denoted 1 and the width of the blade at its base at the level of the axis is denoted L, the values of 1 and L are very close and 1>0.5 L and preferably 1>0.75 L.
This element has been preferred even if it runs counter to common practice. This is because the majority of propellers have a narrow end, in the form of a trapezoid, so as to limit the torque by narrowing the blade at its end.
Studies have shown that, given the combination of angles chosen, the bends in the blade and the shape of the latter, the performance compared with known propellers is quite acceptable.
If:
P=hydraulic pressure
ΔP=pressure difference between the inlet and the outlet of the member
Q=flow
D=diameter of the member
N=speed of rotation of the member
ρ=density
v=speed of the fluid
S=area of the member
k=constant
the flow of a propeller is given by the following simple equation:
Qp=Nq ND3
where Nq is the dimensionless number that characterizes the propeller (its shape, the number of blades, etc.).
The power consumed is calculated as follows:
P═Np ρ N3 D5
where Np is the dimensionless number that characterizes the propeller (its shape, the number of blades, etc.).
The efficiency is the ratio of the energy that produces the pumping flow and the energy necessary for turning the member.
The efficiency can be expressed simply by the general equation of fluid mechanics and the simplified Bernoulli equation:
General equation of fluid mechanics:
P1═ρΔPQ (1)
Pumping flow:
Qp=Nq ND3 (2)
Power needed to rotate the member:
P2═Np ρ N3 D5 (3)
Simplified Bernoulli equation:
Note that the calculation is identical when the power consumed in order to generate 1 m3/h is sought, for example.
The following is noted for example:
It can be seen that the efficiencies of the proposed propellers are particularly good compared with the prior art and conventional propellers and turbines such as the marine propeller or the turbine having blades inclined at 45°.
The number of blades on the propellers increases the amount of liquid displaced but also the power consumed.
Without being quite proportional, it is often noted that the power consumed increases proportionally with the number of blades by a factor of 0.8.
However, in the present case, given the surface area and the angles, the average speeds of fluid show that with two blades the power decreases by 31% compared with a propeller having three blades, while the flow decreases only by 13%.
There are thus multiple advantages in using a propeller with two blades.
From an economical point of view, manufacturing two blades instead of three allows a 33% saving of material, of labor for forming the blade and welding it on a hub.
It is easier to install the propeller. This is because, depending on the diameter of the shaft, it is sometimes not possible to install three blades around the latter.
In addition, some products are partially destroyed by the shear brought about by the blades. This is because, on each rotation, the blade “cuts” the product in order to break it (flocs, emulsion, polymers, etc.) and a member equipped with two blades will only shear twice per rotation and not three times.
Finally, the propeller can be made in one piece for different reasons, for example welded to the driveshaft to allow its possible coating for use in a corrosive or abrasive medium or when it is not possible to subsequently fasten it. The three-blade propeller is particularly difficult to introduce into a tube when the member exceeds 500 mm, but a two-blade propeller having the same diameter is easy to introduce, as illustrated in
The propeller is desired for its rather axial flow leaving the blade in order to be blown down to the bottom at the axis and to rise again at the wall so as to sweep any deposited particles from the bottom.
“Simple” propellers made up of blades that are inclined or formed with one bend do not make it possible to bring about a mostly axial flow on account of “radial and tangential leaks”; for the invention, a mostly axial flow is noted.
The manufacturing of the prior art propellers is complex.
In some cases, it requires a complex machine that can twist the blades for propellers having a diameter of 10 m, this being a single machine that is constantly in production.
Given their curvature, propellers of the saber type require a template for each diameter and shape, and hence a combination of more than one hundred templates.
It is relatively easy to manufacture the proposed propellers with the aid of a bending press; greater competitiveness of subcontractors is thus conceivable, i.e. a greater choice thereof.
The mechanical determination of a stirrer is dictated by its diameter and its speed of rotation for a given operation and consequently the power generated for the rotation of the member.
The saving in power for one and the same pumping flow, this being an essential calculation element for stirring to effect mixing, allows a saving in terms of the motor, the speed reducer transmitting the torque, in terms of the guiding system and in terms of the leaktightness, the shaft supporting the member and the thickness of the member. A saving of 20% in power between the proposed propeller and a marine propeller is noted, for example.
The economic saving that is brought about for the user from the point of view of investment is easily conceivable as the competitive advantage for the constructor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14305498.9 | Apr 2014 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/056951 | 3/31/2015 | WO | 00 |
