|Title of Invention||
"A METHOD OF DESIGNING INTERNAL SPIRAL RIBS AND SPIRAL RIBS DUCT THEREOF"
|Abstract||The method of designing optimum number of spiral ribs in a duct to optimize the net axial force for installation of a cable by cable blowing comprising the steps of : selecting an apex angle of rib in the range of 80 to 120 degrees; determining the longitudinal spacing between the ribs from the cable thickness and mass; deriving a range of values for rib density and helix angle from the longitudinal spacing, determining the optimum rib density and helix angle from the range of values by determining net axial force for the range of values; determining the optimum rib height for the optimum rib density and helix angle, such that a reduction in friction between the cable and the duct without a significant or no reduction in axial force is achieved. Figure 1|
|Full Text||A present invention relates method of preparing internal spiral ribs and duct thereof.
Background Of The Invention
Plastic ducts have long been used for installation of telecom or electrical cables. These ducts come with smooth inside surface or have multiple longitudinal ribs on the inside that are sometimes straight and sometimes helical. The number of such ribs in conventional ducts varies from 20-40 with varying helix angle in case of spiral ribs. Different manufacturcrs of such ducts also employ different rib heights with typical rib height in the range of 0.2 to 0.4 mm. However, the design of such ribs is not scientific and the efficiencies of cable installation shows only marginal, if any, improvement in a ribbed duct over the smooth wall duct. This has resulted in the industry often opting for a smooth wall duct over a nbbed duct.
Conventionally, cables were pulled through ducts by a winch line in which every time a bend or undulation in the duct was passed the pulling force is multiplied by a factor that depends on the friction. Higher the local pulling force, higher the friction that the cable is experiencing while being pulled against the internal duct wall resulting in an exponential force build-up with pull distance, producing generally high pulling forces. Tension in the cable on account of the pulling forces cannot exceed a certain safety margin of the cable or it may break.
Because of the forces necessary to pull the cable through a duct, it is highly advantageous to lower the friction between the cable and its duct. When the coefficient of friction of the duct is lowered, the resulting lower forces to pull the cable through the duct allow longer lengths to be strung without a relay.
Ducts with ribs, longitudinal or helical, are believed to reduce friction between the cable and duct. Examples of ribbed duct are shown in U.S. Patent No. 4565351 and US 5087153 that are hereby incorporated by reference.
In the case of a smooth duct, that is a duct having a smooth inner surface, the cable rests on one surface of the duct and the pulling force used to pull the cable through the duct has to overcome the frictional forces between the cable and the duct. The use of ribbed ducts, as described in US Patent No. 5087153 attempts to overcome this drawback and experimental results disclosed therein indicate that for the same pulling force spiral ribs offer a reduction in frictional force. The document mentions ranges for rib parameters such as rib height, distance between ribs and helix angle. US 5087153 however does not disclose ranges for these parameters for different duct sizes, but rather a universal range encompassing numerous duct sizes. Moreover, the preferred values for these parameters, as disclosed by US 5087153, do not indicate a dependency on the duct size.
In the last decade the use of cable blowing or cable jetting has gained popularity for laying cables in duct. Cable blowing involves the cable being installed through the duct using compressed air. This compressed air flows through the duct and along the cable at high speed. The friction of the cable is compensated locally by the distributed airflow and the large forces that would generate high friction are avoided. Special lubricants both solid as well as liquid have been developed for cable blowing that assist in the installation of cable by reducing friction.
Cable blowing has the advantage that longer installation distances can be reached and is less dependent on bends and undulations in duct. Forces exerted on the cable are also significantly lower. Cable blowing is especially popular for laying of fiber optical cables that are lightweight and flexible and require long lengths of uninterrupted cable. It is a requirement in the field of fiber optic cable blowing that greater length of cable be installed per day with the existing equipment and without a significant increase in the pressure of the compressed air.
For the cable blowing method of installation a variety of factors assume importance. The factors that influence the design of a rib for a duct in which the cables are to be installed by pulling, as disclosed by US 5087153, are different from those that are relevant when the rib is used in a duct in which the cable is to be installed by blowing.
Experiments conducted by the applicant have revealed that conventionally available ducts, with ribs when used in the cable blowing process though reduce friction between the cable and the duct also result in a net reduction in the air blowing force or axial force acting on the cable, thereby resulting in no significant improvement over a smooth walled duct.
The axial force may be defined as the force acting on the cable by the flowing air, while the net force or net axial force on the cable is calculated by deducting the frictional and the other forces from the axial force.
Laboratory and field experiments carried out on conventional ducts with spiral ribs when used in the cable blowing process result in a swirling flow that is found to be detrimental. Swirl reduces the axial force acting on the cable, while simultaneously increasing the friction between the cable and the rib. Thus it is important from a rib design point of view that the rib does not cause swirl in the airflow.
Moreover, it is important that the ribs be designed so that a reduction in friction force is achieved without a reduction in the net axial force.
There is therefore a requirement for a duct with ribs for the cable blowing process and particularly ribs for such ducts so that a reduction in friction force is achieved without compromising the net axial force. There is also a requirement of understanding the parameters involved in the design of ribs and their co-relation so
that optimum ribs may be designed. There is therefore a requirement for a method of designing optimum ribs for ducts used in the cable blowing process.
To overcome the aforementioned drawbacks the invention provides for a method of designing spiral ribs for a duct used in cable blowing comprising the steps of:
a. selecting an apex angle of rib in the range of 80 to 120 degrees;
b. determining the longitudinal spacing between the ribs from the
cable thickness and mass;
c. deriving a range of values for rib density and the helix angle from
the longitudinal spacing,
d. determining the optimum rib density and helix angle from the range
of values by determining net axial force for the range of values;
e. determining the optimum rib height for the optimum rib density and
helix angle, such that a reduction in friction between the cable and
the duct without a significant or no reduction in axial force is
The invention also provides for a duct with a forty-millimeter outer diameter for cables comprising an outer surface and an inner surface, wherein the inner surface is provided with 4 to 10 spiral ribs, each rib having apex angle in the range of 80 to 120 degrees; a helix angle in the range of 5 to 15 degrees and rib height in the range of 1 to 1.5 mm
Brief description of the drawings:
Figure 1 illustrates the geometry of the rib in accordance with an embodiment of the invention.
Detailed Description Of The Invention
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.
Throughout the patent specification, a convention employed is that in the appended drawings, like numerals denote like components.
The invention provides for a duct with ribs and more specifically to a duct with spiral ribs in which the ribs have been designed to optimize the net axial force.
The invention aims at optimizing the rib design so that a reduction in friction is achieved without compromising on the net axial force. The invention also aims at providing a method of rib design for optimum ribs.
The design of the ribs in accordance with the invention is on the assumption that a lubricant will be used in the cable blowing process. Though various lubricants are available, a standard liquid lubricant is typically employed.
The friction force between the duct and the cable varies with contact area and the parameter controlling this is the rib density and geometry. The friction force is directly dependent on rib density. The main parameter that is relevant to the blowing process is the net axial force on the cable at constant cable speed, that is the difference between the force exerted by the compressed air and the friction force between the cable and the duct.
For the sake of illustration, the invention has been described with reference to a duct having an outer diameter of 40 mm, inner diameter of 34.2 mm and for a cable having a 15 mm diameter. The 40 mm outer diameter duct is the most commonly used duct in industry and the optimization of the rib parameters has therefore been done for this duct. It is, however, within the scope of the invention to apply the principles taught herein to other duct diameters without sacrificing the spirit of the invention.
With reference to Table 1, results of experiment conducted on a smooth wall duct with a 15mm diameter cable are tabulated. The cable was suspended with the gap below the cable varying from zero to a concentric configuration. As the cable is lifted from the bottom the average velocity in the duct is seen to reduce, however, the axial force on the cable increases since progressively larger regions of the cable are subjected to a large shear stress. In a ribbed duct the cable is lifted off the bottom and it is therefore possible that lifting of cable compensate a reduction in the mean velocity due to ribs.
The aim of the rib is to move the cable towards the center of the duct without touching the duct inner surface so that the air force applied is all around the cable and greater net axial force may be achieved. Experiments conducted by the applicant have shown that the conventional spiral-ribbed ducts result in a reduction in friction force while also resulting in a 15 to 20 per cent drop in the effective axial or pulling force.
The applicant's research in the field of ducts with internal spiral ribs for the installation of cable by the blowing process has indicated that the design of the rib on the inside surface of the duct is dependent on the following parameters for optimum cable installation, namely:
1. Rib geometry.
2. Rib height.
3. Helix angle.
4. Rib density.
Research has also indicated that the parameter that affects the effective or net axial force is the mean hydraulic diameter of the duct. The mean hydraulic diameter has been defined as four times the cross sectional area of the duct divided by the wetted parameter of the duct (4 * cross sectional area/wetted perimeter). The cross sectional area referred to is the inner cross sectional area of the duct with or without ribs and the wetted perimeter refers to the inner perimeter of the duct with or without ribs. The smaller the mean hydraulic diameter greater is the resistance to flow and thus resulting in lower axial force. Conventionally available ducts tend to use a smaller rib height and a greater rib density. However, an increase in the rib density results in the cross sectional area decreasing and the wetted perimeter increasing, thereby resulting in a decrease in the mean hydraulic diameter and a decrease in the net axial force, in spite of the reduction in frictional force.
It is, therefore, important to determine the height of the ribs, the rib density, the helix angle and the rib geometry such that the effective axial force on the cable is not compromised or reduced for a friction force reduction, for a given air pressure. It is also necessary to reach the optimum rib design in view of these parameters.
The invention therefore provides for a method of optimum rib design applying the principle of mean hydraulic diameter. The method provides for first selection of a apex angle and base angle of the rib irrespective of rib height. On the basis of cable stiffness the longitudinal spacing between ribs is decided. This longitudinal spacing may be achieved by using various combinations of rib density and helix angle. This step is followed by the step of determining a range of helix angles for a range of rib densities and determining the optimum range of helix angles. For the given optimum range of helix angles the range of rib density is determined. Alternatively, the rib density range could be determined first followed by the computation of the range for helix angle. Using the range of helix angle and rib density the optimum helix angle and rib density are selected for the desired longitudinal spacing. Lastly, for the given helix angle and rib density the optimum rib height is determined.
In order to increase the cross sectional area and decrease the wetted perimeter, the applicant's research has indicated that the apex angle of the rib should be at least 80 degrees and may be as large as 120 degrees. Furthermore, the larger of the two base angles of the rib results in the rib being inclined in a particular direction. The duct having such ribs will have to be laid taking into consideration the direction of the inclination of the rib such that the net axial force is higher. In order to overcome this limitation of ensuring that the duct be laid with the ribs in the correct orientation, it is preferred that the base angle of the ribs be the same. In other words, it is preferred that the rib be an isosceles triangle with the apex angle of at least 80 degrees. Such a rib configuration maximises the mean hydraulic diameter for a given rib density while at the same time, allowing the
duct to be laid in any direction. Keeping the apex angle between 80-120 degrees maximizes the mean hydraulic diameter in all cases/configurations. From a manufacturing perspective, it is preferred that the angle be 90 degrees.
The appropriate ranges for the four parameters listed above is determined from a 1D analysis and then detailed investigations are carried out using a Computational Fluid Dynamics (CFD) package.
The longitudinal spacing between ribs is selected taking into account the stiffness of the cable to be blown and its mass per unit length. A stiffer cable affords a larger distance between points of support and hence a lower rib density may be employed.
For the duct as defined above, based on the stiffness of the cable it was ascertained that the longitudinal spacing between the ribs should lie between 10cm and 15cm. This spacing can be achieved using various combinations of rib density and helix angle.
Computer simulations with several rib densities showed that a helix angle greater than 15 degrees yields poor results in terms of pulling force on the cable. An angle less than 5 degrees would imply a very high rib density. Thus the optimal range for the helix angle was established as 5-15 degrees. Results of the simulations have been tabulated in Table 2.
To prevent singularities where the cable touches the ribs and to keep the number of nodes within limits, the cable has been kept 1mm above the ribs.
As may be seen from the table, as the helix angle is increased from 5° to 15° the drop in the mean velocity and the axial force is quite dramatic, particularly from 10° to 15°.
For a helix angle of 5°, the axial force for 6 and 8 ribs, when the rib height is 1mm is equal to or greater than that for the smooth duct. This holds promise since the friction is lower in the ribbed ducts by about 30 to 40 % and hence the blowing distance could then be 30% to 40% larger with these ducts.
This range of helix angles then yields a rib density between 4 to10 to obtain the required longitudinal spacing. 6 and 8 rib configurations were selected for detailed study on the computer. (Table Removed)
Table 3: Results for the cable in the ribbed duct. D = 34.2mm; cable diameter = 15mm.
To explore parameter range that showed promise in greater detail more calculations were conducted with a finer mesh and these are presented in Table 3. As in the results above the duct diameter was maintained at 34.2mm and the cable diameter was set at 15mm. The cable is closer to the ribs here, the gap being kept at 0.5mm. The main objective of this set of calculations was to choose between the 6-rib configuration and the 8-rib configuration. It is clear from Table 3 for all rib heights the combination of 6 ribs and helix angle of 10° is comparable to the case of 8 ribs with a helix angle of 5°. To ensure that the cable is well supported and that it does not sag between the ribs and scrape against the duct, it is desirable that the pitch of the ribs be not much greater than 10cm. With the 6 rib, 10° combination the pitch is 10.1 cm while with the 8 rib, 5° combination the pitch is 15.4cm. Thus, the configuration with 6 ribs of helix angle 10° appears to be optimal. The optimal rib height cannot be obtained with these calculations since the gap between the rib and the cable is comparable with the rib height. Thus a fourth set of calculations was done, wherein the gap between the cable and the rib was reduced to 0.25mm.
The table below lists the findings of the computer simulations conducted by the applicant on a smooth duct, a conventional ribbed duct and the duct in accordance with the invention. (Table Removed)
Table 4: Results for the cable in the ribbed duct. D = 34.2mm; cable diameter = 12.7mm and 6 ribs with a helix angle of 10°.
With reference to the table, it is shown that the smooth duct for a pressure drop of 4660 Palm provides a net axial force or pulling force of 1.16 N. A duct having thirty conventional spiral ribs with a rib height of 0.3 mm results in a drop in the effective axial force or pulling force.
With reference now to the duct in accordance with the invention, for a given rib density 6 to 8 and for a rib having an isosceles triangle with the apex angle between 80 degrees and 120 degrees, and a helical angle of 5 to 15 degrees the rib height was varied from 1 to 1.5 mm. As can be seen, for a rib height of 1.25 mm the effective axial force acting on the cable is nearly same as that in the case of smooth wall duct (1.17v/s 1.16 ). Experiments conducted at NT Delhi have also shown that a 15 to 20% reduction in friction was achieved with the conventional spiral duct over the smooth duct. The duct in accordance with the invention has much lower friction in comparison with the conventional spiral duct and hence is expected to yield a 35 to 40 % reduction in friction for the same axial force or blowing force.
Thus one optimum rib design for a 40 mm duct may be summarized as:
1. Apex angle of Rib: 90 degrees
2. Base angles: Isosceles 45 degrees
3. Helix angle: 10 degrees
4. Rib density: 6
5. Rib Height: 1.25mm
The rib density in the duct in accordance with the invention is much lower than that of the conventional ribbed duct. Applying the method of rib design as taught by the invention ribs for different duct sizes may be determined. It is also to be noted that the optimum ribs as defined by the invention may be uni-direction.
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the scope of the invention are desired to be protected.
1. A method of preparing internal spiral ribs for a duct in cable blowing
comprising the steps of :
a. selecting an apex angle of rib in the range of 80 to 120 degrees;
b. determining the longitudinal spacing between the ribs from the cable
thickness and mass;
c. deriving a range of values for rib density and helix angle from the
d. determining the optimum rib density and helix angle from the range of
values by determining net axial force for the range of values;
e. determining the optimum rib height for the optimum rib density and helix
angle, such that a reduction in friction between the cable and the duct
without a significant or no reduction in axial force is achieved.
2. A method as claimed in claim 1, wherein the base angles of the rib are equal.
3. A method as claimed in claim 1, wherein the said optimum range of the helix angle is determined before the determination of optimum rib density range.
4. A method as claimed in claim 1, wherein a range of rib heights is considered while determining the range of optimum rib density and helix angle.
5. Spiral ribbed duct for use in cable blowing with a 40mm outer diameter for cables comprising an outer surface and an inner surface, wherein said inner surface is provided with 4 to 10 spiral ribs, each rib having apex angle in the range of 80 to 120 degrees; a helix angle in the range of 5 to 15 degrees and rib height in the range of 1 to 1.5mm.
6. The spiral ribbed duct as claimed in claim 5 wherein said rib is an isosceles triangle.
7. The spiral ribbed duct as claimed in claim 5, wherein said preferred apex angle is 90 degrees.
8. The spiral ribbed duct as claimed in claim 5, wherein said preferred helix angle is 10 degrees.
9. The spiral ribbed duct as claimed in any of the preceding claims, wherein said number of ribs is 6.
10. The spiral ribbed duct as claimed in any preceding claim, wherein said preferred height of the rib is 1.25mm.
11. The spiral ribbed duct as herein described with reference to and as illustrated by the accompanying drawings and foregoing description.
12. A method of preparing internal spiral ribs for a duct used in cable blowing as herein described with reference to and as illustrated by the accompanying drawings and foregoing description.
|Indian Patent Application Number||1409/DEL/2006|
|PG Journal Number||31/2013|
|Date of Filing||14-Jun-2006|
|Name of Patentee||DURA-LINE INDIA PVT. LTD.|
|Applicant Address||A-10, SANSKRIT BHAWAN, OPP. JNU EAST GATE, QUTAB INSTITUTIONAL AREA, NEW DELHI-110 067, INDIA.|
|PCT International Classification Number||H02G 15/00|
|PCT International Application Number||N/A|
|PCT International Filing date|