GB2397342A - Fan duct with abradable coating surrounding blade tips - Google Patents

Abstract

A method of producing a duct section of uniform diameter for a generally cylindrical duct 2 containing a fan 1 for moving air through the duct comprises covering at least the part of the duct section swept by blades 9 of the fan 1 with a coating 10 of paint or plastic material. The coating 10, which is soft relative to the fan material, or remains soft whilst setting, has a diameter generally smaller than the diameter swept by the fan blades 9. When the fan 1 is rotated, the blade tips cut away surplus coating to form a duct section of substantially uniform diameter determined by the diameter swept by the fan, with a minimal clearance between the fan 1 and the coating 10. The method may also apply to a high temperature installation in which the coating is swept by the fan 10 at its maximum temperature expanded diameter.

Description

1 2397342 A duct and a method of producing a duct The present invention

relates to a duct for air and a method of producing a duct section of uniform diameter for a generally cylindrical duct containing a fan for moving air through the duct.

In such ducts it is necessary, for practical purposes, to have a clearance between the rotating fan and the interior surface of the duct to allow for variations in the diameter of the duct and variations in the diameter swept by the fan, caused by the necessity of providing tolerances on the dimensions of the individual components. However, this clearance or fan gap should be kept as small as possible since the larger the gap the lower the efficiency of the fan. The efficiency of the fan can be reduced by as much as 25 percent if the gap is too large. The problem is exacerbated in large diameter ducts which are formed of relatively thin sheet material. With such material it is difficult to prevent inaccuracies occurring in manufacture of the ducting lengths whilst there is also an added problem of distortion occurring during installation.

The reduction in efficiency is caused predominantly by air turbulence generated by air passing through the gap from the high pressure to the low pressure side of the fan, the greater the gap the larger the volume of air which can pass through the gap and hence the greater the turbulence generated. It has been found that the length of the gap in the axial direction has a significant effect on the volume of air passing through the gap, the longer the axial length of the gap, the less the air turbulence.

One method of increasing the gap length is to provide an actively extending flange or web on the peripheral edges of the tips of the fan blades. Where the fan blades are made of a moulded plastics material it is relatively easy to modify the moulds to mould the flange integrally with the remainder of the fan blades. However, this solution has a practical disadvantage since it is customary to vary the diameter of a basic fan by cutting off the tips to provide fans of the appropriate diameter for the particular installation. In this way, one set of moulds can provide fans adapted for different diameters. The disadvantage is that once the fan is cropped, the moulded flange is also removed so that fans of a reduced diameter have a reduced efficiency. (

It has been known to provide a fan with axially extending flanges secured to the tips of the fan blades by adhesive or a fastening such as pop rivets, but this design has the disadvantage that production costs are increased, while if the adhesive or fastening should fail whilst the fan is rotating, catastrophic failure results.

The present invention seeks to provide a solution to these problems.

According to the present invention there is provided a method of producing a duct section of uniform diameter for a generally cylindrical duct containing a fan for moving air through the duct, the method comprising covering at least the part of the duct section swept by blades of the fan with a coating of paint or plastic material which is soft relative to the fan material, or remains soft whilst setting, and has a diameter generally smaller than the diameter swept by the fan, the fan being rotated so that the blade tips cut away surplus coating to form a duct section of substantially uniform diameter determined by the diameter swept by the fan, with a minimal clearance between the fan and the coating.

In a preferred embodiment of the invention the coating is applied to a duct of a system adapted for use in a high temperature installation in which the minimum internal radius of the duct is greater than the maximum temperature-expanded radius of the fan blades. In this embodiment, the coating forms a sacrificial coating of a predetermined thickness, the coating thickness being such that at normal working temperature, the coating is swept by the fan to form said duct section of uniform diameter, and at the maximum temperature-expanded diameter of the fan, the coating is cut away sacrificially.

And embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figures 1A and 1B show known arrangements for reducing turbulence through the fan gap, and Figures 2A and 2B show an embodiment of the present invention.

Referring now to the drawings, in Figures 1, there is shown a schematic view of part of a multi-bladed fan 1 for impelling air through a generally cylindrical duct 2 by rotating about the duct axis 2a. The fan 1 is formed of a moulded plastics material and includes, on the radially outermost peripheral tip of each blade 3, an axially extending web or flange 4 moulded integrally with the blade which serves to increase the axial length of the gap 5 between the fan blades 3 and the duct casing 2.

Referring now to Figure 1B, there is shown a similar arrangement. In this case, the fan 1 is formed of aluminium. An axially extending flange element 6 is secured to the peripheral tip of each fan blade 3 by means of an adhesive or a fastening such as pop rivets. It is also possible that the flanges could be welded to the blade tips. The flange element 6 extends in the axial direction of the duct to increase the axial length of the fan gap 5.

Referring now to Figure 2A, there is shown an end view of a preferred embodiment of the present invention including a duct 2 incorporating a fan 1 for impelling air through the duct, and Figure 2B shows a schematic cross-section of the duct 2 and fan 1. The fan 1 has a hub 7, which may incorporate an electric motor 8, and a plurality of radially extending blades 9. In accordance with the method of the present invention, at least the section of the duct swept by the fan blades is coated with a paint or plastic material 10, the minimum radius of which is smaller than the radius swept by the fan blades 9. The coating of the paint or plastics material is softer than the material of the fan 1, or remains softer than the material of the fan until the coating sets, so that when the fan 1 is rotated, the fan blades 9 sweep over the coating 10 to cut away surplus material to form a duct section of uniform diameter determined by the diameter swept by the fan blades 9, with a minimal clearance between the fan and the coating.

In certain installations, the temperature of the surroundings or of the air flowing through the duct may, on occasion, increase significantly and this causes problems in maintaining a minimum gap because of the difference in the expansion coefficients of the different materials used. Typically, the ducts themselves are formed of mild steel sheet, whereas the fans and fan blades are frequently formed of aluminium, which has a much greater coefficient of expansion than steel. In practice, this means that the gap must be kept much larger than desired to ensure that the fan blades do not contact the ducting, which could lead to catastrophic failure, when their temperature increases. In a preferred embodiment of the invention, which is particularly suitable for such applications where there is a risk of high temperatures, the minimum internal diameter of the coating, as applied, is arranged to be smaller than the diameter swept by the fan blades 9 whilst the minimum internal diameter of the duct 2 itself, which is swept by the fan blades, is greater than the maximum temperature-expanded diameter of the fan blades at the maximum working temperature of the fan. In this way, a minimal clearance can be provided between the fan and the coating in accordance with the invention for normal operating temperatures, but if the temperature increases so that the swept diameter of the fan blades increases, the fan blades simply remove, sacrificially, more of the coating but cannot contact the duct itself.

Although described as being a duct for air, it will be understood that the invention is readily applicable to applications using other fluids, such as gases and liquids.

Claims (4)

1. A method of producing a duct section of uniform diameter for a generally cylindrical duct containing a fan for moving air through the duct, the method comprising covering at least the part of the duct section swept by blades of the fan with a coating of paint or plastic material which is soft relative to the fan material, or remains soft whilst setting, and has a diameter generally smaller than the diameter swept by the fan, the fan being rotated so that the blade tips cut away surplus coating to form a duct section of substantially uniform diameter determined by the diameter swept by the fan, with a minimal clearance between the fan and the coating.

2. A method according to claim 1, wherein the coating is applied to a duct of a system adapted for use in a high temperature installation, in which the minimum internal radius of the duct is greater than the maximum temperature-expanded radius of the fan blades, the coating forming a sacrificial coating of a predetermined thickness, the coating thickness being such that at normal working temperature, the coating is swept by the fan to form said duct section of uniform diameter, and at the maximum temperature-expanded diameter of the fan, the coating is cut away sacrificially.

3. A method of producing a duct section of uniform diameter for a generally cylindrical duct containing a fan for moving air through the duct, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.

4. A duct produced according to the method of claim 1, 2 or 3.