GB1569499A

GB1569499A - Heat exchanger

Info

Publication number: GB1569499A
Application number: GB876377A
Authority: GB
Original assignee: Denso Marston Ltd
Current assignee: Denso Marston Ltd
Priority date: 1978-03-02
Filing date: 1978-03-02
Publication date: 1980-06-18

Description

(54) HEAT EXCHANGER (71) We IMI MARSTON LIMITED, formerly known as MARSTON ExCELSIOR LIMITED, a British Company, of Wobaston Road, Fordhouses, Wolverhampton WV10 6QJ, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to heat exchangers and has particular reference to plate fin type heat exchangers.

Plate fin heat exchangers are in extensive use for a number of applications where a high heat exchange capacity and low weight or low bulk is required. The heat exchangers are manufactured in a sandwich type construction in which a number of plates is arranged parallel to and above one another being separated from one another by corrugated fins. These fins together with suitable edge strips define ducts for the passage of fluids in heat exchange relationship with one another. The construction of these heat exchangers is well known to those in the art.

The heat exchangers are frequently made from aluminium or aluminium alloys and the assemblies are usually brazed together to form an integral block by dip brazing, flux dip brazing, vacuum brazing or other suitable means. Because it is frequently a requirement that the weight of the heat exchanger be as low as possible the thickness of the fins and the plates are kept to a minimum. Clearly reducing the thickness of the plates will increase the heat exchange efficiency of the heat exchanger and reducing the thickness of the fins enables the fin density to be increased without too rapidly increasing the weight of the heat exchanger.

Unfortunately, however, it has been found that when the fins become very thin, typically 0.012 in (0.305 mm) or less, they are susceptible to edge damage. Solid particles in the fluids passing through the ducts or even high speed liquid particles in a gaseous fluid can impinge on the thin edges of the corrugations which bend them over. The bent-over edges tend to obstruct the ducts and reduce the efficiency of the heat exchanger. Also during assembly of the heat exchangers the edges are liable to be damaged by accidental impacts.

In order that the edges may be protected, it has been the practice heretofor to use a thicker corrugated guard or reinforcing strip along the edges to protect the delicate thin corrugations. There are, however, difficulties associated in doing this in that it is awkward to locate the thickened strip during jigging and also it is not practicable to obtain efficient alignment of the thicker strip with the thinner corrugations. Also the thickened edge strip reduces the efficiency of the heat exchanger locally, increases its weight and increases its cost. At the plane of contact of the guard and main corrugation there is a potential dirt trap and corrosion zone which has proved troublesome in some applications.These disadvantages have, however, been accepted prior to the present invention in that they reduce the susceptibility of the heat exchanger to damage and this more than offsets the disadvantages referred to above.

By the present invention there is provided a plate fin heat exchanger including at least two opposed plates defining, at least in part, a duct for the passage of a fluid, a plurality of strip fins within the duct interconnecting the two opposed plates, the edge of some at least of the fins adjacent the inlet to the duct and between the regions of connection between the fins and the plates being recessed for at least a portion of its length so as to lie within the duct. The recess is to give enhanced resistance to deformation compared to a straight edged fin. The plurality of fins may be integral in the form of a corrugation, the edge of the corrugation being recessed. The invention is particularly applicable to fins of less than 0.012 in (0.305 mm) thickness when the fin material is aluminium.The fin material may have a thickness in the range 0.002 in (0.05 mm) to 0.008 in (0.2 mm) or in the range 0.006 in to 0.010 in. The fin material may be an alloy of aluminium.

The recess may be in the central region of the fins and the distal regions of the fins being unrecessed.

The recess may extend substantially from the edges of the fin adjacent the plates abutting both sides of the fin. The recess may be a part of a circle or may be sinusoidal in shape or may be formed with straight sides or curves of any suitable shape. The groove may be symmetrical or asymmetrical.

The fins and plates may be formed of aluminium, preferably the assembly of fins and plates is brazed together by, for example, flux dip brazing or vacuum brazing.

There may be a plurality of ducts defined by three or more plates, some or all of the ducts having concave edges at their inlets.

By way of example embodiments of the present invention will now be described with reference to the drawing accompanying the provisional specification of which: Figure 1 is a schematic perspective view of a 3-plate heat exchanger, Figure 2 is a partial end view of the heat exchanger of Figure 1 taken along the line of the arrow II, Figure 3 is a schematic end view of two plates and a corrugated fin, Figure 4 is a schematic end view of the assembly shown in Figure 3 once damaged, Figure 5 is a view similar to that of Figure 2 showing a reinforcing plate, Figure 6 is a view similar to Figure 2 showing a reinforced edge according to the invention Figure 7 is a scrap perspective view showing an arrangement according to the invention, and Figures 8-12 show alternative shapes for the groove in the edge of the fins.

Figures 1-5 relate to prior art arrangements. It is well-known to manufacture plate fin type heat exchangers formed basically of alternating plates and corrugated fins similar to a small module shown in Figure 1. Referring to Figure 1 three sheets of aluminium alloy 1, 2 and 3 are stacked one on top of the other being separated from each other by corrugated fins 4 and 5. These fins serve two main functions. Firstly they enable the heat exchanger stack to be manufactured by assembling a sandwich-like arrangement of alternating plates and corrugated fins and secondly when brazed to their adjacent plates they enhance the heat transfer of fluids flowing through the ducts. This arrangement is extremely well-known in the ar Because of the high thermal capacity of these heat exchangers in relation to their weight they are frequently used in for example aircraft.In these arrangements the weight of the heat exchanger should be kept to a minimum and this means that the thickness of both plates and corrugations is kept to a minimum. Referring to Figure 2 it can be seen that the corrugations conventionally come very close to the edge of the plates.

Because the corrugations are very thin the edges and in particular the centre such as 6 of the edges of the corrugations are quite susceptible to impact damage. This can occur during use by the impact of hail, rain or solid objects, in the event of a heat exchanger utilising atmospheric air or simply by damage during assembly. Referring to Figure 3 this shows a thin corrugated series of fins 7 in the undamaged condition and it can be seen that the amount of crosssectional area of the duct occupied by the fins is relatively low. If, however, the edges become distorted and flattened as is shown at 8, 9 in Figure 4 then this considerably reduces the cross-sectional area of the heat exchanger through which fluids can flow and this restriction reduces the efficiency of the heat exchanger.Conventionally when these problems have arisen in the past the solution adopted has been to use a thicker guard strip of corrugation such as 10 (Figure 5) which is fitted at the outer edge of the corrugated fins 11 to withstand impact damage caused by particles in the fluid flowing through the heat exchanger. These thicker strips 10 are, however, an added problem both regarding the manufacture of the heat exchanger, since they are an extra item to be inserted during jigging which can become displaced and also because the thicker material increases the weight of the heat exchanger. Both of these factors increase the cost of the prior art protected heat exchanger.

With the arrangement of the invention however, not only is there a significant increase in the resistance of the edges of the fins to deformation but there is also a very, very silght reduction in the overall weight of the heat exchanger, as compared to an unprotected heat exchanger, without significant loss of thermal efficiency. It is believed, however, that the reduction in weight is of far less significance than the increased resistance to damage.

It is expected that the concave shape will have better fluid pressure entry loss characteristics than either guarded or unprotected heat exchangers.

Referring to Figure 6 it can be seen that the edge of the fin 12 is concave as at 13.

This very significantly increases the resistance of the edge to damage. It is believed that the concave edge is more resistant to damage because it is more effectively supported than the free straight edge of the prior art. Also the most vulnerable centre part of the fin is recessed more out of harms way.

Referring to Figure 7 it can be seen that the concave edge may be formed by a simple groove along the length of the corrugation.

the groove being defined by part circular segments 14, 15, 16 etc cut out from the edges of the corrugation. These grooves can.

as shown in Figures 7 and 8, extend part way into the edge leaving a small portions 17, 18 on either side of the groove. Alternatively the grooves may extend fully to the edge of the corrugations as shown at 19 in Figure 9. The grooves can be in any shape desired such as a sinusoidal shape 20, Figure 10, a straight cut shape 21, Figure 11, or an asymmetric shape 22, Figure 12. All of these shapes of groove define a concave or hollowed section to the edge which has an increased resistance to deformation. The cut outs in the corrugation may be made in any suitable manner such as by stamping during the formation of the corrugation or by machining after the corrugations have been formed. For small quantities spark erosion machining is a suitable technique to use for machining preformed corrugations.

As an alternative method to spark eroding, the recess or grooves may be machined in the preformed corrugation. It may be desirable, to help support the thin corrugated material, to support the fins with a wax which is poured into the heat exchanger and solidifies around the fins. After the grooves or recesses have been machined away the wax can be melted out.

The heat exchangers according to the invention are manufactured in exactly the same manner as conventional heat exchangers, ie by jig assembly and flux dip brazing, vacuum brazing etc and the method of assembling the heat exchangers forms no part of the present invention. It will be appreciated that although the description of the invention relates to aluminium heat exchangers other suitable materials such as copper could be used as desired.

WHAT WE CLAIM IS: 1. A plate fin heat exchanger including at least two opposed plates defining, at least in part, a duct for the passage of a fluid, a plurality of strip fins within the duct interconnecting the two opposed plates, the edge of some at least of the fins adjacent the inlet to the duct and between the regions of connection between the fins and the plates being recessed for at least a portion of its length so as to lie within the duct.

2. A heat exchanger as claimed in Claim 1 in which the plurality of fins are integral in the form of a corrugation, the edge of the corrugation being recessed.

3. A heat exchanger as claimed in Claim 1 or Claim 2 in which the fins are formed of a material having a thickness less than 0.012 inch.

4. A heat exchanger as claimed in Claim 3 in which the thickness is in the range 0.002 to 0.008 inch, preferably 0.006 to 0.010 inch.

5. A heat exchanger as claimed in any one of claims 1 to 4 in which the fin material is chosen from the group aluminium, aluminium alloys, copper and copper alloys.

6. A heat exchanger as claimed in Claim 5 in which the fins and plates are made from aluminium or an aluminium alloy and are brazed together.

7. A heat exchanger as claimed in any one of Claims 1 to 6 in which the recess is in the central regions of the fin, and the distal regions of the fin being unrecessed.

8. A heat exchanger as claimed in any one of Claims 1 to 6 in which the recess extends substantially from the edges of the fin adjacent the plates abutting both sides of the fin.

9. A heat exchanger as claimed in any one of Claims 1 to 8 in which recess is partcircular or part-sinusoidal in shape or is formed with straight or curved sides.

10. A heat exchanger as claimed in Claim 8 or Claim 9 in which the recess is symmetrical or asymmetrical.

11. A heat exchanger as claimed in any one of Claims 7 to 10 in which the recess is formed from the preformed fins.

12. A heat exchanger as claimed in Claim 11 in which the recess is spark eroded.

13. A heat exchanger as claimed in Claim 11 in which the recess is machined from the formed fins.

14. A heat exchanger as claimed in Claim 13 in which the fins are supported during the machining operation.

15. A heat exchanger as claimed in Claim 14 in which the support is wax, the wax being melted out after the recess has been machined.

16. A heat exchanger as claimed in any one of Claims 7 to 10 in which the recess

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. damage because it is more effectively supported than the free straight edge of the prior art. Also the most vulnerable centre part of the fin is recessed more out of harms way. Referring to Figure 7 it can be seen that the concave edge may be formed by a simple groove along the length of the corrugation. the groove being defined by part circular segments 14, 15, 16 etc cut out from the edges of the corrugation. These grooves can. as shown in Figures 7 and 8, extend part way into the edge leaving a small portions 17, 18 on either side of the groove. Alternatively the grooves may extend fully to the edge of the corrugations as shown at 19 in Figure 9. The grooves can be in any shape desired such as a sinusoidal shape 20, Figure 10, a straight cut shape 21, Figure 11, or an asymmetric shape 22, Figure 12. All of these shapes of groove define a concave or hollowed section to the edge which has an increased resistance to deformation. The cut outs in the corrugation may be made in any suitable manner such as by stamping during the formation of the corrugation or by machining after the corrugations have been formed. For small quantities spark erosion machining is a suitable technique to use for machining preformed corrugations. As an alternative method to spark eroding, the recess or grooves may be machined in the preformed corrugation. It may be desirable, to help support the thin corrugated material, to support the fins with a wax which is poured into the heat exchanger and solidifies around the fins. After the grooves or recesses have been machined away the wax can be melted out. The heat exchangers according to the invention are manufactured in exactly the same manner as conventional heat exchangers, ie by jig assembly and flux dip brazing, vacuum brazing etc and the method of assembling the heat exchangers forms no part of the present invention. It will be appreciated that although the description of the invention relates to aluminium heat exchangers other suitable materials such as copper could be used as desired. WHAT WE CLAIM IS:

1. A plate fin heat exchanger including at least two opposed plates defining, at least in part, a duct for the passage of a fluid, a plurality of strip fins within the duct interconnecting the two opposed plates, the edge of some at least of the fins adjacent the inlet to the duct and between the regions of connection between the fins and the plates being recessed for at least a portion of its length so as to lie within the duct.

is formed prior to assembly of the fins and plates.

17. A heat exchanger as claimed in Claim 16 in which the recesses are formed by cutting or stamping the recesses out of the material from which the fins are formed.

18. A heat exchanger as claimed in any one of Claims 1 to 17 including a plurality of ducts formed by three or more plates some or all of the ducts having a plurality of recessed edge fins.

19. A plate fin heat exchanger substantially as herein described with reference to and as illustrated by Figures 6 to 12 inclusive of the drawings accompanying the Provisional Specification.