WO1997014927A1

WO1997014927A1 - Heat exchanger

Info

Publication number: WO1997014927A1
Application number: PCT/EP1996/004490
Authority: WO
Inventors: Bjørn VESTERGAARD
Original assignee: Norsk Hydro Technology B.V.
Priority date: 1995-10-17
Filing date: 1996-10-16
Publication date: 1997-04-24
Also published as: AU7293796A; EP0769669A1

Abstract

A heat exchanger consists of a number of strips stacked adjacent one another in parallel relationship to provide air passages, and at least one flat tube for carrying a fluid. Each tube passes through holes in the strips and is fitted in the holes in heat exchange relationship with the strips, and is zigzag folded so as to have a number of rectilinear sections interconnected by bent portions.

Description

HEAT EXCHANGER.

The invention relates to a heat exchanger including a number of strips stacked adjacent one another in parallel relationship to provide air passages, and at least one flat tube for carrying a fluid, said tube or tubes passing through holes in the strips and being fitted in the holes in heat exchange relationship with the strips.

Such a heat exchanger is known from US - A - 4 428 419. In this known heat exchanger a number of flat tubes is passing through a number of holes provided in the strips. Each tube being fitted within one hole and being completely surrounded by the edges of that hole.

As such the tubes can be used for a fluid fiow passing in parallel through all the tubes at the same time. Otherwise it is possible to make the right connections between the ends of the individual tubes in order to provide one or more flows in series.

In this known heat exchanger type each individual tube must be connected to a manifold or header system after the fins and tubes have been assembled into a heat exchanger core. The connections to headers/Manifolds or other coupling systems are normally done by brazing or soldering, a process that may give reduced contact between the fins and the tubes in the heated zones. Furthermore, it is a costly procedure that may require the use of a brazing furnace, an operation that also may change the surface properties of the fins. Another type of heat exchanger is known from US - A - 5 372 188. In this unit, the fins are of a zig-zag type inserted as strips between the flat tubes. The assembly and joining of this type of heat exchanger is done by brazing of the complete heat exchanger in a special process. This involves a costly production line which is not suitable for small-scale production.

It is therefore an object of the invention to provide an heat exchanger of the above mentioned type, which is cheap to manufacture and to assemble, and with a high efficiency of heat exchange. It is a further object of the present invention to provide a heat exchanger which can be assembled without costly furnace brazing, and which do not require the operation of mechanical expansion of the tubes.

This object is achieved in that each tube is zigzag folded so as to have a number of rectilinear sections interconnected by bent portions or headers. These connection to the headers can be made by simple flame brazing or by adhesive bonding before the fins are inserted. Since the strips are not present during the assembly of tubes and headers, it is easy to gain access to the joints for brazing/soldering or gluing.

By providing such tubes composed of rectilinear and bent portions the assembling time may be heavily reduced while maintaining a low risk of leakage.

In one embodiment of the invention each strip is provided with holes with such dimension that each opening fits around two sections of the same tube.

In this way assembling is easy as the tube may be inserted in the hole by introducing the bent portion interconnecting the two sections into the hole. Otherwise the contact area between the strips and the tube is still high in order to have a good heat exchanger contact.

In another embodiment of the invention each strip is provided with elongated holes with such dimension that each hole fits around one section of the tube. In this case each elongated hole has a slot-like shape with an open end at one end of the strip. 97/14927 PC17EP96/04490

In this way each tube comprising rectilinear sections and bent portions can easily be fitted to the strips by inserting the same through the open ends of the slot-like holes.

Other advantages and characteristics of the invention will become clear from the following description, reference being made to the annexed drawings. In the drawings there is :

Fig. 1 a front view, partly in cross-section of a heat exchanger according to the invention, Fig. 2 a side view of the heat exchanger of Fig. 1 , Fig. 3 a cross-section, on enlarged scale, according to the line Ill-Ill in Fig.

2, Fig. 4 a perspective view of the heat exchanger of Fig. 1 before assembling, Fig. 5 a cross-section, as in Fig. 3 of a modified embodiment of the heat exchanger according to the invention.

Fig. 6a and 6b cross-section corresponding to the cross-section of fig. 3 of a conventional heat exchanger and a heat exchanger according to the invention. Fig. 7 a schematic representation of the heat flow in a heat exchanger having tubes and strips.

Fig. 8 a cross-section corresponding to the cross-section of Fig. 3 of a modified form of the invention.

As shown in Fig.'s 1 and 2 the heat exchanger 1 comprises a number of strips 2, which in the embodiment shown each consists of a rectangular plate made of a material with good heat conducting properties such as aluminium or aluminium alloys. The strips 2 are positioned in a parallel way with respect to each other thereby forming a cooling pack. The strips can be of a plain type or can have enhancements such as corrugations, louvers, etc. Each strip 2 is provided with a number of openings or holes 3 which in the

80 embodiment shown have an elongated form, and are open ended to one side so as to form cut-outs in the strip 2, as shown in Figs 1 and 3. A flat tube 4 is inserted in the openings 3 of the cooling pack, the flat tube 4 being bent in a serpentine way and as such is composed of a number of rectilinear sections 5 and bent portions 6. Otherwise the tube may be cut in a

85 number of straight sections that are connected with their ends to headers or manifolds locked on each side of the cooling pack. In the embodiment, several straight sections of flat tube are connected to headers or manifolds, thereby forming a fluid circuit. The tubes and headers/manifolds are connected by flame brazing or adhesive bonding before the fins are inserted. This eliminates the

90 problem of reduced tube/strip contact due to heating and thermal expansion. The flat tube is preferably of the type obtained by multiple port extrusion, i.e. a flat tube having a number of parallel internal flowing channels 10 (see Fig. 3); for _, a fluid (liquid or two phase mixture) separated by innerwalls 11 , the outer wails of such tube being formed by two parallel flat walls 13 and 14 interconnected by

95 two semi-circular sections 15 and 16 and the already mentioned inner walls 11. Such a tube may be produced by a single step extrusion process by using especially adapted extrusion dies. The main advantage of such flat tubes is that the heat exchange surface is much greater compared with traditional round tubes, so that heat exchangers based upon this type of tubes are more effective. 00 Preferably the slots 3 in the fin stock have a slightly tapered form, so that the finstock can more easily be positioned over the tubes 4. In order to improve the stability and to improve the thermal contact adhesives may be used between the fins and the tube.

However it is also possible to use flat tubes of the type described US-A-5 058 105 266. Flat tubes obtained by other type of processes are applicable as well. The height of the flat tube 4, i.e. the smallest dimension of the tube is somewhat bigger than the smallest dimension (i.e. the height) of the opening 3. The way in which the heat exchanger according to Fig. 1 and 2 is obtained is shown in Fig. 4. A piece of flat tube 4 is for instance folded in a serpentine 110 shape i.e. a construction comprising a number of rectilinear portions 5 interconnected by bent portion 6.

In the embodiment shown the rectilinear portions are parallel to each other and the distance between the facing outer walls of two adjacent rectilinear portions is equal to the smallest distance between two adjacent openings 3 in the strips 2. 115 Having formed the tube 4 in such a configuration the heat exchanger of Figs 1 and 2 can easily be obtained by laterally inserting the preshaped tube 4 into the openings 3 of a stack of strips 2. As a result of the chosen dimensions the tube will automatically fit in the openings 3 and there will be a good contact between the wall of the tube 4 and the individual strips 2.

120 Although the heat exchanger of the embodiment shown has parallel rectilinear sections of tubes it is possible, according to the invention, to use non-parallel . rectilinear sections. In this case the strips are to be provided with openings which are not located on the same place in each strip, but the position of the opening is gradually changing from one strip to another. It is even possible to

125 have a tube with not completely rectilinear sections, but with e.g. undulated sections between two bent portions 6. Basically it is even possible to have random configuration, provided the openings 3 in the strips are made accordingly.

In order to facilitate the assembly spacers are normally provided between the 130 strips of the fin stock. These spacers may be removed after assembly. Such spacers will make the fin stock more rigid, support the slotted portion of the finstock and facilitate assembly. They also will position the fins at this right location along the tube.

In fig. 5 there is shown a modified embodiment of the invention. Each strip 3 is 135 provided with a number of substantially rectangular openings 24 with four rounded corners 25. One dimension of the opening 24 corresponds to the width of the flat tube 4 to be used in connection with this strip 2. The other dimension is substantially bigger than the height of the tube 5. In fact that dimension is substantially equal to the distance between two non-facing walls of two adjacent

140 rectilinear portions of the tube as shown in Fig. 4. The heat exchanger is obtained in that the tube is shaped in accordance with Fig. 4. Thereupon the tube 4 is with the bent portion 6 first introduced in the openings 24 of the stack of strips 3. In this way a heat exchanger almost identical to the heat exchanger shown in the

145 figures 1 and 2 can be obtained.

In fig. 8 there is shown a modified shape of a tube which can be used in the heat exchanger according to the invention. A tube 41 , has a tapered corss-sectioπ and the cut-outs or opening 42 in the strips 2 have a corresponding shape, so that the tube 41 can be easily inserted into the cut-out and pushed further until a 150 good connection between tube 41 and strip is obtained.

. Such a tapered tube 41 can be produced either directly as extruded or extruded with parallel sides with subsequent wedge-rolling . Preferably the foliar of the cut-out 42 is designed in such a way that an elastic adjustment within the geometrical tolerances of tube and strip is possible.

155 In order to assess the improvement obtained by the invention a comparison between a traditional heat exchanger and a heat exchanger according to the invention has been made.

A traditional heat exchanger has a configuration as shown in fig. 6A, whereas the heat exchanger according to the invention has the configuration according to fig.

160 6B. The following dimensions are accepted:

Traditional heat exchanger: tubing outer diameter 8,0 mm wall thickness 0,7 mm flow area (internal) 34 mm²

165 wet surface (internal) 21 mm contact area with strip 12,6 mm Heat exchanger according to invention: tubing width (outer) 16 mm tubing height 3 mm

170 wall thickness 0,6 mm flow area (internal) 23 mm² wet surface (internal) 39 mm contact area with strip 30,7 mm strips thickness 0,17 mm

175 strip pitch 5 mm strip tube 375 mm²

Heat exchange characteristics:

Assumptions:

. Heat transfer coefficient (refr.-tube) α = 5000 W/(m² x K)

180 Heat transfer coefficient (tube/fin-air) α.,^ = 50 W/(m² x K)

Heat conductivity (aluminium) λ^ = 229 W/(m x K)

Temperature difference (refr.-air) Δt,._a = 30 K

Chain of Heat Transfer

This is shown in fig. 7. The heat flow is from Block 80, which represents the 185 refrigerant to block 81 representing the tubing. From block 81 (the tubing) the heat transfer flow goes either directly to block 82 representing the ambient art, or through block 83 representing the stripes. The different flows are indicated as 91 , 92, 93 and 94 respectively.

Comparison (per meter tubing^

190 Thermal Balance:

With reference to the chain of heat transfer:

Parallel : Heat transfer capabilities added together (+)

Serial : Heat transfer capabilities balanced out to reach equilibrium (=)

Calculated Performance:

195 Performance : 91 = { 92 + (93 = 94) }

It is obvious that by the invention there is obtained a heat exchanger with flat ^* tubes which can easily be assembled and constructed and wherein only very few connections have to be made. Other embodiments are possible especially with respect to the shape of the strips and the internal construction of the tubes and 200 the invention is only restricted by the scope of the claims.

Claims

1. A heat exchanger including a number of strips stacked adjacent one another in parallel relationship to provide air passages, and at least one flat tube for carrying a fluid, said tube or tubes passing through holes in the strips and being fitted in the holes in heat exchange relationship with the strips, characterised in that each flat tube is zigzag folded so as to have a number of rectilinear sections interconnected by bent portions or headers.

2. Heat exchanger according to claim 1 , characterised in that each strip is provided with holes with such dimensions that each hole fits around two sections of the same tube.

3. Heat exchanger according to claim 1 , characterised in that each strip is provided with elongated holes with such dimension that each hole fits around one section of a tube.

4. Heat exchanger according to claim 3, characterised in that each elongated hole has a slot-like shape having an open end at one edge of the strip.

Heat exchanger including a number of identical strips stacked adjacent one another in parallel relationship to provide air passages, each strip being provided with a number of elongated slots having an open end at one edge of the strip, and at least one flat tube for carrying a fluid, said tube being composed of a number of rectilinear sections interconnected by bent portions or headers, each tube being fitted in a set of openings provided in the strips.

6. Heat exchanger according to any one of the preceeding claims, characterised in that, the tube and the opening in the strip have correspondinly tapered corss-sections.

7. Heat exchanger according to any one of the preceeding claims, characterised in that the rectilinear sections are parallel to each other.

8. Heat exchanger according to any one of the preceeding claims, characterised in that the flat tube is extruded, drawn as welded tube.