Title:
Device for exchanging heat
Kind Code:
A1


Abstract:
A device for exchanging heat comprising at least one fluid inlet (2), at least one header pipe (4), a plurality of flat pipes (5) at least at one end being in flow connection with the header pipe (4), a fluid outlet (9) and at least one frame element (21, 23) being in connection with the header pipe (4) wherein said connection between the frame element (21, 23) and the header pipe (4) is a frictional connection due to auxiliary equipment (33).



Inventors:
Neumann, Olaf (Durban, ZA)
Birk, Peter (Centerville, OH, US)
Application Number:
11/473548
Publication Date:
12/27/2007
Filing Date:
06/23/2006
Assignee:
Behr America, Inc. (Troy, MI, US)
Primary Class:
International Classes:
F28F9/02
View Patent Images:
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Primary Examiner:
WALBERG, TERESA J
Attorney, Agent or Firm:
LEYDIG VOIT & MAYER, LTD (Alexandria, VA, US)
Claims:
1. A device for exchanging heat comprising at least one fluid inlet, at least one header pipe, a plurality of flat pipes being, at least at one end, in flow connection with the header pipe, a fluid outlet and at least one frame element being in connection with the header pipe wherein said connection between the frame element and the header pipe is a frictional connection by means of auxiliary equipment.

2. Device according to claim 1, characterized in that the header pipe comprises at least one projection which contacts the frame element and at least in one direction prevents movement of the frame element relative the header pipe.

3. Device according to claim 2, characterized in that the projection is a finger protruding from the header pipe.

4. Device according to claim 2, characterized in that the projection comprises at least one curved portion.

5. Device according to claim 1, characterized in that the frame element comprises at least one projection which contacts the header pipe and prevents movement at least in one direction of the frame element relative the header pipe.

6. Device according to claim 1, characterized in that the frame element comprises an end portion fitted to the geometrical shape of the header pipe.

7. Device according to claim 1, characterized in that the header pipe is not closed by the frame element.

8. Device according to claim 1, characterized in that the frame element comprises hook means connected to a portion of the header pipe.

9. Device according to claim 1, characterized in that the frame element comprises a curved section.

10. Device according to claim 1, characterized in that the frame element comprises an end portion running parallel to and laterally offset from the center region of the frame element.

11. Device according to claim 1, characterized in that the device comprises two header pipes positioned at the respective ends of the flat pipes and flow connected to them.

12. Device according to claim 11, characterized in that the frame element is connected with both header pipes wherein said connections are frictional due to auxiliary equipment.

13. Device according to claim 1, characterized in that the frame element comprises a plurality of projections along its length.

14. Device according to claim 1, characterized in that the frame element is positioned at an end portion of the header pipe.

15. Device according to claim 1, characterized in that two frame elements are provided.

16. Device according to claim 15, characterized in that the frame elements are positioned at end portions of the header pipe.

17. A device for exchanging heat comprising a fluid inlet, a header pipe, a plurality of flat pipes being at least at one end in flow connection with the header pipe, a header and a fluid outlet wherein the longitudinal direction of the header is substantially parallel to the longitudinal direction of the flat pipes.

18. Device according to claim 17, characterized in that the plurality of flat pipes generates a plane wherein the header is positioned parallel relative said plane.

19. Device according to claim 17, characterized in that the longitudinal direction of the header extends to an end region of the header pipe.

20. Device according to claim 17, characterized in that the device comprises a frame element connected to the header pipe.

21. Device according to claim 17, characterized in that the header is frictionally connected to the frame element due to auxiliary equipment.

22. Device according to claim 17, characterized, in that the header is positioned behind the plane of the flat tubes as seen in the direction of air flow through the device.

23. Device according to claim 17, characterized in that the header is positioned in front of the plane of the flat tubes as seen in the direction of air flow through the device.

24. Device according to claim 17, characterized in that at least the inlet or the outlet are positioned in an end region of the header pipe.

25. Method of manufacturing a device for exchanging heat in particular for motor vehicles comprising the following steps: arranging a plurality of flat pipes in flow-connection with at least one header pipe; arranging at least one frame element at the header pipe such that at least in one section said frame element contacts auxiliary equipment provided at the header pipe; braze welding the header pipe at least to the flat pipes and preferably also at least to the frame element.

Description:

FIELD OF THE INVENTION

The present invention relates to a device and method for exchanging heat, and more particularly, for exchanging heat in motor vehicles. Devices for exchanging heat are used in motor vehicles, for example, in air conditioning systems.

BACKGROUND OF THE INVENTION

Automotive heat exchangers typically include an inlet pipe and an outlet pipe for liquids and a header or distribution pipes for distributing the liquid to a plurality of flat pipes. With the liquid flowing through the flat pipes, heat is exchanged between the liquid and the ambient air. For this purpose, the flat pipes are connected to a collecting means such that the liquid may flow between the header pipes and the flat pipes. In addition, cooling fins are positioned between the individual flat pipes so as to enhance heat exchange with the ambient air.

The manufacturers of such heat exchangers connect the flat pipes to the header pipes by braze welding, which connections are intended to be fluid tight. For this purpose, the flat pipes and the header pipes are typically pre-stabilized in a soldering frame and subjected to the actual soldering process while in the frame.

Prior art heat exchangers may also include frames which serve to stabilize the device. Since such devices are designed to be used in motor vehicles, heat exchangers must possess adequate stability to withstand the expected vibrations of being placed in such an environment.

In certain prior art devices, a soldering frame is used to secure the frame element to the header pipe so as to allow the header pipe to be soldered to the frame element. The stability of the element during the manufacturing process is thus dependent on the quality of the soldering frame.

These prior art devices, however, suffer from instability. Insufficient stability between the individual flat pipes can cause the cooling fins to extend toward the header pipe and thus remove coating or solder from the joint of the header pipes to the flat pipes, thereby adversely affecting the sealing properties.

In addition, if the soldering frame does not support the frame element at its outermost end, visible gaps up to 1 mm can occur, which in turn, results in the cooling fins falling out of the device during manufacturing.

SUMMARY OF THE INVENTION

The present invention enhances the stability of automotive heat exchangers during the manufacturing process by, among other things, preventing the cooling fins from falling off or leaking.

In particular, the invention improves stability of the frame element relative the header pipe thereby permitting other components, such as a header which may receive dryer or filter units, to be attached to the frame element. Thus, the stability of the entire device increases further.

One embodiment of the invention includes at least one fluid inlet, at least one header pipe, a plurality of flat pipes in flow connection with the header pipe at least at one end, a fluid outlet and at least one frame element connected with the header pipe. According to an embodiment of the invention, the frame element engages a header pipe in a frictional relationship.

A frame element is understood to mean a device that serves to improve the stability of the heat exchanger. The frame element, or more than one frame element, may be arranged for example between individual flat pipes. A frame element further may be composed of several frame components. An assembled heat exchanger may include delimiting frame elements.

In one embodiment of the invention, the frame element is not joined to the header pipe by a soldered connection alone, but, alternatively or in addition, by auxiliary equipment. Such auxiliary equipment contributes to preventing the cooling fins from falling off during the manufacturing process. In addition, auxiliary equipment prevents the cooling fins from extending toward the header pipe during the manufacturing process and removing solder from the joints there.

In a preferred embodiment, the header pipe comprises at least one projection which contacts the frame element and prevents movement of the frame element relative the header pipe, at least in one direction. Preferably this direction is outward away from the heat exchanger i.e., toward one end of the header pipe. When such movement is prohibited to the frame element, the individual cooling fins remain secured between the flat pipes and thus cannot fall off the device during manufacturing.

A projection is understood to mean a device protruding out of the header pipe at least in one geometrical dimension.

A preferred embodiment has a finger-like projection protruding from the header pipe. In another preferred embodiment, the projection comprises at least one curved section. It is preferred that the finger or projection is curved such that its end portion contacts the frame element. It is preferred that the header pipe comprises several of these curved fingers against which the frame element rests.

In another preferred embodiment, the frame element also comprises a recess for supporting the header pipe. It is preferred that movement of the frame element is prevented on the one hand by said projection and on the other hand by the boundaries of said recess wherein said boundaries of the recess preferably prevent movement of the frame element in a direction perpendicular to the length of the header pipe.

In another preferred embodiment, the frame element comprises at least one projection which contacts the header pipe and prevents movement of the frame element relative the header pipe, at least in one direction. Both the frame element and the header pipe may comprise such projections which intermesh in a predetermined way to thus prevent movement of the header pipe relative the frame element, at least in one direction of movement.

In another preferred embodiment, the frame element comprises an end portion fitted to the geometrical structure of the header pipe. For example, the said end portion of the frame element may be a circle segment against which the outside face of the header pipe rests. In this way, the connection between the frame element and the header pipe is further stabilized. In another preferred embodiment, the frame element does not close the header pipe. In this case, the header pipe is preferably closed by a device positioned across or around the header pipe.

In another embodiment, the frame element covers the header pipe, and the header pipe comprises at least one projection to prevent outward movement of the frame element. Preferably said projection is positioned at the side or on top of the header pipe i.e., the projection contacts the frame element either at an end portion or along its length. More than one projection may also be provided as boundaries of the frame element.

In another preferred embodiment, the frame element itself is structured as a flat pipe, which means that the refrigerant may flow through the frame element.

In another preferred embodiment, the frame element comprises hook means connected to a portion of the header pipe. A hook means is understood to be a device which engages with another device so as to generate a substantially tension-resistant connection between the hook means and said other device.

In another preferred embodiment, the frame element comprises at least one curved section. Preferably, the frame element comprises an end portion running parallel to and laterally offset from the longitudinal direction of the frame element. Here, the frame element transits first into a curved portion which in turn transits into an end portion which runs parallel to and laterally offset from the main element of the frame element.

In a preferred embodiment, the device comprises two header pipes positioned at the respective ends of and being in flow connection with the flat pipes. In this embodiment, both ends of the header pipes comprise auxiliary equipment, particularly preferred projections which are a preferably designed as a frictional connection with the frame elements.

In another preferred embodiment, the frame element comprises a plurality of projections along its length. Said projections serve to stabilize the frame element and consequently to stabilize the entire heat exchanger.

In another preferred embodiment, the frame element is positioned at an end portion of the header pipe. Preferably, two frame elements are positioned at the two end portions of the one or two header pipes.

In this way, use of suitable auxiliary equipment on both sides allows the cooling fins to be pre-locked so as to prevent said cooling fins from falling off the flat pipe system during manufacturing.

In a preferred embodiment, at least one frame element is structured as a flat pipe, i.e., itself serves to convey fluids. It is preferred in this embodiment that the frame elements are also in flow connection with the header pipe.

It is preferred that the cross-sectional area of the frame elements is different from the cross-sectional area of the flat pipes. It is preferred that the flow cross-section of the frame elements is different from that of the flat pipes.

The present invention further relates to a device for exchanging heat comprising a fluid inlet, a header pipe, a plurality of flat pipes connected at least at one end to share the flow system of the header pipe, a header and a fluid outlet. Here the longitudinal direction of the header is substantially parallel to the longitudinal direction of the flat pipes.

The header may be a device which receives, for example, filter and dryer units for the refrigerant. Substantially parallel is understood to mean that an angle between the longitudinal direction of the header and the flat pipes is smaller than 20°, preferably smaller than 10°.

The prior art devices comprise headers positioned substantially parallel to the header pipe. The header arrangement of the invention has the advantage of a reduced size. And, the stability of the entire device for exchanging heat is improved because the header arrangement of the invention allows a mechanical connection both with the header pipe and with the preferred auxiliary equipment. Furthermore, the header arrangement of the invention also improves the filtering and/or drying properties.

In a preferred embodiment the plurality of flat pipes generates a plane where the header is positioned parallel relative said plane, meaning in front of or behind said plane. In another preferred embodiment, the longitudinal direction of the header extends to an end region of the header pipe. This means that the header is not positioned in a center area of the device for exchanging heat but near the boundary. In this way the flow conditions of the air passing through the device for exchanging heat can be further improved, i.e., there is little influence on the air flow.

In another preferred embodiment, the heat exchanger comprises a frame element connected with the header pipe. It is preferred that the header is frictionally connected to the frame element by means of auxiliary equipment.

Said auxiliary equipment is preferably, but not exclusively, a connection selected from a group of connections including screwed connections, rivet connections, clip connections and the like.

It is preferred to connect the header both with the frame element and the header pipe, thus further increasing the stability of the heat exchanger.

In another preferred embodiment, the header has a geometrical contour which benefits the flow conditions in the heat exchanger. This is preferably to be understood to mean a shape which improves the air flow through the heat exchanger for example by reducing turbulence.

In another preferred embodiment, the header is positioned behind the plane of the flat tubes as seen in the direction of the air flow through the heat exchanger.

Preferably, the header comprises a fluid inlet and a fluid outlet, both being positioned in an end region of the header. Inlet pipe and outlet pipe are understood to mean fluid lines, in particular, but not exclusively pipes feeding fluids to or removing fluids from the header.

It is preferred that at least the inlet pipe or the outlet pipe run in the longitudinal direction of the header in the connection area with the header.

This means that at least the inlet or the outlet ends in a front face of the header. This configuration allows particularly short connecting pipes, thus requiring minimum amounts of material.

Furthermore, the short connecting pipes offer high stability, thus themselves serving to enhance frictional connection from the header pipe to the header.

It is also preferred to position at least the inlet pipe or the outlet pipe at the circumference of the header.

In another preferred embodiment, the header is positioned in front of the plane of the flat tubes as seen in the direction of air flow through the device.

It is also preferred to position at least the inlet or the outlet in an end region of the header pipe.

The invention further relates to a method of manufacturing a heat exchanger. One process step provides for a plurality of flat pipes to be connected to a header pipe so as to allow flow. In a further process step, a frame element is arranged at the header pipe such that at least in one section it contacts auxiliary equipment provided at the header pipe. The header pipe is then braze welded to the flat pipes and the frame element.

It is preferred to provide another header pipe which is preferably pre-stabilized and braze welded to the flat pipes and the frame elements according to the invention. It is preferred to provide two frame elements as boundaries respectively for the flat pipes and the heat exchanger.

It is preferred to arrange in another process step cooling fins, in particular, but not exclusively, in a zigzag configuration between the individual flat pipes, preferably also between the one-piece flat pipes and the frame elements.

Other advantages of the embodiments can be taken from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the heat exchanger of the invention.

FIG. 2 is a perspective fragmentary view of the heat exchanger of the invention.

FIG. 3 is a perspective fragmentary view of the heat exchanger of the invention.

FIG. 4 is a side view of the heat exchanger of the invention.

FIG. 5 is a top view of the header pipe of the invention.

FIG. 6 is a side detail view of the header pipe of the invention.

FIG. 7 is a view of the header pipe in FIG. 5 along a line A-A of FIG. 5.

FIG. 8 is an enlarged top view of the end portion of the header pipe of FIG. 5.

FIG. 9 is another side view of the header pipe in FIG. 6.

FIG. 10 is a detailed view of FIG. 7.

FIG. 11 is a view of the header pipe in FIG. 5, along the line C-C of FIG. 5.

FIG. 12 is an enlarged top view of the area F in FIG. 5 on an opening for guiding a flat pipe through.

FIG. 13 is a detailed view of FIG. 9.

FIG. 14 is a top view of the frame element of the invention.

FIG. 15 is a side view of the frame element of the invention.

FIG. 16 is a detailed view of the end portion of the frame element of FIG. 15.

FIG. 17 is a detailed view of the projections in FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a top view of a device for exchanging heat according to one embodiment of the invention. The heat exchanger 1 includes an inlet 2 and an outlet 9 through which refrigerant is circulated. The inlet and the outlet are arranged at a common support 6.

The inlet 2 and a pipe 3, which includes a curved portion 3a to avoid an undesirably small curvature radius, feed the refrigerant into a header pipe 4. From this header pipe, refrigerant is distributed to at least some of the flat pipes 5 from which it is conveyed into a second header pipe 8.

One embodiment of the invention provides that the refrigerant is collected in the second header pipe 8 from where it is conveyed back up into header pipe 4 via further flat tubes. To this end, the header pipes 4 and 8 comprise partitions to prohibit at least in some sections a flow of the refrigerant in a horizontal direction.

From the first header pipe 4, the refrigerant is conveyed through a pipe into a header 26 and from there to the outlet 9. In this embodiment, the header 26 is not arranged parallel to the header pipes 4 or 8, but is instead oriented substantially parallel to flat pipes 5 seen in longitudinal direction.

Advantages of this embodiment include reduced size and higher stability. The latter advantage is made possible because the header 26 is directly connected with a frame element 23 of the heat exchanger. In the illustrated embodiment this connection is established by fasteners, such as screws 28. Clip connections, slip joints, rivet connections and the like may be used as alternatives.

Frame element 23 and frame element 21 each include end portions 24. These end portions are offset inwardly and to the side relative the longitudinal direction of the frame elements. End portions 24 rest against respective projections 33 of the header pipes and thus are prevented from moving outwardly away from the heat exchanger. Reference numerals 11, 13, 16 and 17 identify mounting elements for attaching the heat exchanger to a vehicle, for example to the car body.

FIG. 2 is a perspective detail illustration of a portion of the heat exchanger illustrated in FIG. 1. As illustrated, the leftward end of header pipe 4 is open, i.e., not closed by a portion of the frame element 23 or other component. Reference numeral 33 corresponds to projections arranged at the header pipe and bent downwardly. Reference numeral 31 corresponds to a curved portion of the frame element 23 which connects central portion 35 of frame element 23 with end portion 24. Here, end portion 24 and the central portion 35 are substantially parallel relative to one another. Reference numeral 34 corresponds to a projection positioned in the frame element 23. In this embodiment, the individual projections point inwardly, i.e., towards the flat pipes.

Reference numeral 37 identifies a recess positioned in the header pipe 4, and more precisely, in pipe element 4a. Pipe 4 further includes small projections 38. In this embodiment, projections 33 do not run parallel to the longitudinal direction of the frame element but at a specified angle.

FIG. 3 is another perspective illustration of the heat exchanger of the invention. It can be seen that the frame element 23 is delimited outwardly, i.e., leftward in FIG. 3, by header pipe projections 33. In this way, frame element 23 can be fixed in position as can the cooling fins (not shown) that are positioned between the individual flat pipes 5. Fixing the frame elements and cooling fins in a desired location and orientation during manufacturing prevents individual cooling fins from falling out of the spaces between the flat pipes. The illustrated embodiment of FIGS. 2 and 3 further provide for the end portion 24 and central portion 35 to protrude beyond the end portion of the header pipe.

As illustrated in FIGS. 2 and 3, header pipe 4 may be fabricated as a two-piece structure comprising a top pipe component 4a and a bottom pipe component 4b, wherein the top pipe component 4a engages the interior walls of the bottom pipe component 4b. This configuration may also be reversed, i.e., header pipe 4 includes a bottom pipe component 4b which engages a top pipe component 4a.

FIG. 4 is a side view of an embodiment of the invention. In this embodiment, both the top header pipe 4 and the bottom header pipe 8 include projections 33 against which the frame element 23 rests. In this way, a fractional connection is generated both at the top header pipe 4 and the bottom header pipe 8. Both the top header pipe 4 and the bottom header pipe 8 may consist of two pipe components each.

Reference numeral 7 illustrates a line or pipe through which the refrigerant is conveyed to the header 26. In this embodiment, the header is attached to the frame element 23 via connections 28, thereby establishing another fractional connection via pipe 7 such that external forces are also conveyed into the header pipe instead of only to the frame element. In this embodiment, header 26 or the dryer unit in the header are positioned behind a plane E where the flat pipes are arranged when viewed in the air flow direction, i.e., the air flows in the direction of the arrow P.

FIG. 5 is a bottom view of the header pipe 4 of the invention, i.e., seen in the direction in which the flat pipes are inserted into the header pipe. Header pipe 4 comprises a plurality of openings or slots 41 into which the flat pipes 5 are inserted. These slots are arranged substantially in parallel and spaced apart from one another by a predetermined distance. The distance of the individual slots 41 relative one another determines the width of the cooling fins positioned between said slots. The projections of the header pipe 4 are positioned at the respective end regions 43a and 43b as will be explained below in more detail. The capital letters E and F refer to the regions illustrated in detail in FIGS. 8 and 12.

FIG. 6 is a side view of the bottom part 4a in a location where the flat pipes are inserted (from above) into the slots 41. Projection 33 protrudes upwardly from the header pipe, i.e., in the direction of the flat pipes inserted into the header pipe.

FIG. 7 is a side view of header pipe 4 along the line A-A in FIG. 5, i.e., the portion where a flat pipe is inserted. Region 52 indicates the location of where a flat pipe may be inserted. Reference numeral 51 illustrates the top boundary region of the header pipe against which the side edge of the flat pipe (not shown) rests. FIG. 10, in turn, is an enlarged illustration of top boundary 51. Top boundary 51 comprises an inserting slope 52 and a region 53 substantially parallel to the longitudinal direction of the flat pipe to be inserted. As a flat pipe is inserted into a slot, regions 51 and 52 are bent, thereby applying a predetermined contact pressure to the flat pipe after inserting, which supports tight soldering.

FIG. 8 is an enlarged detail drawing of the end region of the header pipe of FIG. 5 designated with E. Reference numeral 5 identifies the location where a flat pipe may be inserted into slot 41. The frame element (not shown) is positioned to rest against the header pipe such that its outer surface contacts projection surfaces 33 pointing in the direction of the slot 41, i.e., outwardly in the plane of the drawing.

The end portion of the frame element adapts to the header pipe profile and in this way, upward or downward movement of the frame element is prohibited or restricted. Transverse movement is also substantially restricted or prohibited through the cooling fins inserted during manufacturing. Alternatively, or, in addition to the foregoing, inclines or projections inside the header pipe may be provided to restrict movement of the frame element.

FIG. 9 is a top view of the end portion of the header pipe. Flat pipe 5 is introduced from above to rest against the projections 33 from the rear. Persons of ordinary skill will recognize that more than two projections may be provided and that the profile of the individual projections may be different than that shown in FIG. 9. Furthermore, one or more of the projections may be designed as an annular protrusion around the header pipe.

FIG. 11 is a top view of the header pipe along the line C-C in FIG. 5, i.e., in a region where no slot is present. It can be seen that the header pipe comprises two portions 54 and 55 extending substantially horizontally, i.e., in the longitudinal direction of the flat pipes, and further includes a curved or bent portion, i.e., a portion extending substantially in a semicircle between portions 54 and 55.

FIG. 12 is an enlarged view of the area F in FIG. 5, i.e., the region in which a flat pipe may be inserted into the opening 5. It can be seen that the flat pipe is substantially rectangular in cross-section, and, in this embodiment, the short sides of the flat pipes 61 and 62 are slightly curved.

FIG. 13 is an inclined view of the end portion 43a in FIG. 9. It can be seen that in this embodiment the projection is not precisely perpendicular relative to the intermediate area 45 between the slot 41 and the projection 33, but is slightly tilted outwardly. Preferably the tilt angle relative the perpendicular is between 0 degrees and 10 degrees. Such tilting serves to facilitate insertion of the frame element (not shown) during manufacture.

FIG. 14 is a top view of the frame element of the invention which comprises a plurality of projections 34. Frame element 23 has an end portion structured as a circular ring against which the exterior profile of the header pipe rests. At the lower end thereof there is also provided an end portion 32 structured as a circular ring. Projections 34 are preferably positioned along the frame element such that the longitudinal direction of the projections is substantially parallel to the longitudinal direction of the frame element.

FIG. 15 is a side view of the frame element shown in FIG. 14. It can be seen that frame element 23 comprises a curved portion 31 in the region of an end portion 32. Furthermore, end portion 32 is arranged slightly below the plane of the actual frame element. It can further be seen that the projections 34 also protrude downwardly from the frame element 23 meaning, as specified above, that in assembled state they point toward the flat pipes.

FIG. 16 is an enlarged illustration of the end portion of the frame element, i.e., of the end portion 32 and the curved portion 31. It can be seen that the end portion 32 is positioned on a plane E2 which is provided opposite the plane E1 where the remaining area of the frame element 23, i.e., the region 35, is positioned. The area 31 between these two planes is preferably designed as a curve, not a step. In this way, excessive loads on the intermediate area can be prevented.

By way of providing different planes E1 and E2 on which the end region and the center region of the frame element are arranged, the entire frame element will ultimately be pushed outwardly, i.e., away from the flat pipes, relative the projection 33 which is positioned at a header pipe. The distance between the planes E1 and E2 determines how far said frame element can be pushed towards the flat pipes.

FIG. 17 is an enlarged illustration of the frame element in the region of the projection 34. It can be seen that the projection of this embodiment is not fixed to the frame element but it is an outward bend of the frame element itself and is approximately triangular in shape. Accordingly, the frame element also comprises a recess 37 in the region of projection 34. Providing such projections or notched regions will improve the overall stability of the frame element. Alternatively, projection 34 may be fabricated, for example, in a circular or ellipsoidal in shape or the like.