Heat exchanger

Abstract

 A heat exchanger of the type comprising a substantially tube-shaped casing (1) with an inlet (5B) and an outlet (5A) for a first fluid, and a plurality of tubes (2) arranged longitudinally in the interior of said casing (1); each tube is connected to an entry element (7A) and an exit element (7B) for a second fluid, the first fluid flowing through the casing (1) in contact with the outer surfaces of said tubes; the tubes (2) within the casing (1) are substantially directly in contact with each other and with the inner walls (11) of said casing and have a cross-section such as to form within the casing (1) a plurality of microchannels (10), parallel to the tubes (2), for the flow of the first fluid.

Description

[0001] This invention relates to a heat exchanger in accordance with the pre-characterising part of the main claim.

[0002] Heat exchangers of the aforesaid type are known and are generally used as evaporators in refrigeration systems, in which case a refrigerant fluid flows through the tube bundle whereas a fluid to be cooled flows through the tube-shaped casing to make contact with the outer surface of the tubes. The refrigerant fluid flowing through the tubes evaporates to absorb heat from the fluid to be cooled.

[0003] Generally, in known heat exchangers, fins or deflectors are provided within the casing housing the tubes, to cause the fluid to be cooled to flow along a predetermined path, generally of sinusoidal type. In this respect, up to the present time it has always been considered that laminar flow of the fluid to be cooled along the outer surface of the tubes must be avoided, because an insulating layer would be created in contact with the tubes, resulting in decreased heat transfer between the fluids.

[0004] However, the fins or deflectors have the drawback of making the velocity of the fluid to be cooled variable within the heat exchanger, so decreasing heat exchanger efficiency. The fins or deflectors also complicate the heat exchanger assembly and compel the tubes to be maintained at a relatively large distance from each other, so making the heat exchanger more bulky.

[0005] Again in known heat exchangers, the tubes are maintained separated and equidistant by brackets which have substantially the same drawbacks as the fins or deflectors. In this respect, these brackets at least partly hinder the flow of the fluid to be cooled, so reducing heat exchanger efficiency, complicating heat exchanger assembly and at least partly increasing its bulk.

[0006] The object of this invention is to provide a heat exchanger in which the fluids transfer heat in a countercurrent laminar manner while providing a level of heat transfer comparable to that of known heat exchangers.

[0007] A further object is to provide a heat exchanger in which assembly is simplified and which is of smaller overall size than known heat exchangers.

[0008] A further object is to provide a heat exchanger which allows the use of tubes of usual form and which are able to withstand relatively high pressure (for example exceeding 50 bar).

[0009] A further object is to provide a heat exchanger in which the tubes are shaped in such a manner as to prevent stagnation of the refrigerant fluid and such as to facilitate mixing of the refrigerant fluid stream within the tube, so improving heat transfer.

[0010] These and further objects which will be apparent to an expert of the art are attained by a heat exchanger in accordance with the characterising part of the claims.

[0011] The invention will be more apparent from the accompanying drawings, which are provided by way of non-limiting example and on which:

Figure 1 is a partly sectional schematic front view of a heat exchanger according to the invention;

Figure 2 is a view thereof as in Figure 1, but with the heat exchanger shown without a part of the tubular casing containing the tubes, such that these are completely visible;

Figure 3 is a schematic view of the heat exchanger from above;

Figure 4 is a schematic side view thereof;

Figures 5, 6, 7 are schematic sections therethrough taken on the lines 5/5, 6/6 and 7/7 respectively (Figures 6 and 7 being on an enlarged scale).

[0012] With reference to said figures, a heat exchanger according to the invention comprises a substantially tubular casing 1, which in the example is of S-shape, the interior of which houses a plurality of tubes 2 (fortynine are shown in the example). The casing 1 comprises three rectilinear portions 1A, 1B, 1C formed from a usual rectilinear tube of square cross-section, and two curvilinear portions 1D, 1E formed from two shells 3A, 3B (Figure 3) joined together by welding (as described in detail hereinafter). The rectilinear portions (1A-C) are connected together by spacer and support elements 4 (Figure 1). At the ends of the casing 1 there are provided an entry port 5B and an exit port 5A enabling a fluid, for example to be cooled, to pass through the casing.

[0013] To the ends of the casing 1 there are welded two perforated plates 6A, 6B, to which the ends of the tubes 2 are welded. On these plates there are mounted and welded two caps 7A, 7B comprising a hole 8A, 8B respectively, for the entry and exit of a fluid flowing through the tubes 2.

[0014] The tubes 2 comprise a taper 2A, 2B at their ends in correspondence with the ports 5B and 5A for fluid entry into and fluid exit from the casing 1 respectively. The purpose of these tapers is to facilitate the connection of the tubes 2 to the end plates 6A, 6B without the need to widen or modify the ends of the tubular casing 1. These tapered ends also enable the fluid passing into the casing 1 to enter this casing via the port 5B by flowing between the tubes 2 and to leave from the port 5A. In this respect, by virtue of the tapers 2B, a space is created between the walls of the casing 1 and the tubes 2 which is sufficient to allow fluid passage. By this means the use of spacer brackets is unnecessary, they being however essential in known heat exchangers.

[0015] The tubes 2 are inserted longitudinally into the casing 1 and are dimensioned such as to lie in contact with each other (Figure 7) and with the inner walls 11 of the casing 1, so as to create microchannels 10 for passage of the fluid passing through the casing 1. Along those parts housed in the rectilinear portions 1A-C of the casing 1 the tubes 2 also comprise squeezed portions of "binocular" shape 9 (Figure 6).

[0016] As shown in Figure 2, along their bends the tubes do not comprise squeezed portions, whereas on leaving the first bend 1D the squeezed portions have a length such that regions exist in which the unsqueezed tube portions are still in mutual contact. In a modification (not shown), the squeezed portions could also be provided along the bends. The squeezed portions have a multiplicity of functions. On the one hand they maintain the tubes 2 uniformly spaced apart within the casing 1, and on the other hand they "break" the "core" of the fluid stream flowing through the tubes 2. In this respect, by virtue of the squeezed portions 9, the central part of the fluid stream flowing through the tubes, ie that part which undergoes less heat transfer, is mixed, so increasing the heat transfer coefficients within the tubes 2. In addition, the squeezed portions prevent any stagnation of the fluid flowing through the tubes because on the one hand their internal volume is reduced, and on the other hand if the heat exchanger operates as an evaporator, as the inner cross-section of the tube is reduced, the vapour velocity is increased enabling any stagnant pockets of refrigerant fluid for evaporation to be better "blown away".

[0017] Binocular-shaped squeezed portions 9 are preferred, because they are not particularly complicated to achieve and because they can be formed in the usual commercially available tubes without substantially altering their pressure resistance. Advantageously, the tubes 2 are conventional austenitic titanium steel tubes of circular cross-section which, before being inserted into the casing 1, are squeezed" in the previously described manner. However other conventional tubes could also be used, such as steel of a type other than austenitic, iron of low carbon content, or copper if ammonia is not used as the refrigerant.

[0018] The scope of the invention also includes other types of squeezed portions which vary the tube cross-section and are able to break up and mix the flow.

[0019] To construct an S-shaped heat exchanger of the aforedescribed type, the squeezed portions 9 are firstly formed in predetermined parts of usual commercially available tubes, the tubes then being inserted in the rectilinear parts 1A, B, C of the casing 1, spaced apart in such a manner as to allow them to be bent to S-shape. Before the tubes are S-bent, one of their ends is fixed to the entry plate 6A which is itself welded to the part 1C. The bundle of tubes 2 is then S-bent with a pipe bender of substantially conventional type.

[0020] The shells 3A, 3B of the bent parts 1D, 1E of the casing 1 are then associated with the two bends of the tube bundle 2. These shells are then welded together and to the relative ends of the parts 1A, 1B, 1C to form the tubular casing 1. The still free ends of the tubes 2 are then welded to the exit plate 6B, this plate then being welded to the part 1A. The fluid entry and exit ports 5A, 5B and 7A, 7B are then welded to the casing 1 obtained in this manner.

[0021] It should be noted that by virtue of the particular arrangement of the tubes 2 within the casing 1 and their particular shape, heat transfer within the heat exchanger of the invention takes place with the heat-receiving fluid and heat-yielding fluid being in directions substantially parallel to each other but countercurrent. In particular, this is also due to the absence of support brackets for the tubes 2, which could deviate the flow through the casing 1. The heat exchanger configuration achieves heat transfer coefficients and heat transfer area/volume ratios more similar to those of plate heat exchangers than of tube bundle or tube-in-tube heat exchangers, plate heat exchangers being very efficient and compact. In contrast to plate heat exchangers which have maximum pressure limits due to the flat geometry of the plates, the heat exchanger of the invention is able to operate at a pressure exceeding 50 bar inside the tubes.

[0022] Finally, it should be noted that the aforedescribed embodiment is provided by way of example and numerous modifications are possible all falling within the same inventive concept, for example the heat exchanger could be of rectilinear shape or could comprise only one or more than two bent portions. It should however be noted that in the case of a multi-bend heat exchanger it is preferable to provide an even number of bends because in this manner tubes of equal length can be used and there is no tube wastage.

Claims

1. A heat exchanger of the type comprising a substantially tube-shaped casing (1) with an inlet (5B) and an outlet (5A) for a first fluid, and a plurality of tubes (2) arranged longitudinally in the interior of said casing (1); each tube being connected to an entry element (7A) and an exit element (7B) for a second fluid, the first fluid flowing through the casing (1) in contact with the outer surfaces of said tubes; characterised in that the tubes (2) within the casing (1) are substantially directly in contact with each other and with the inner walls (11) of said casing and have a cross-section such as to form within the casing (1) a plurality of microchannels (10), parallel to the tubes (2), for the flow of the first fluid.

2. A heat exchanger as claimed in claim 1, characterised in that between their entry portions (5B) and exit portions (5A) the tubes (2) comprise at least one portion (9) having a cross-section (Figure 6) of different shape from that of the adjacent tube portions, said shape being such as to at least partly break up and mix the fluid stream flowing through the tube.

3. A heat exchanger as claimed in claim 2, characterised in that that portion (9) of tube (2) having a cross-section of different shape is of "binocular" shape.

4. A heat exchanger as claimed in claim 1, characterised in that those tube portions (9) having a cross-section of different shape are in mutual contact, so maintaining the tubes mutually spaced apart.

5. A heat exchanger as claimed in claim 1, characterised by comprising at least one bent portion (1D, 1E).

6. A heat exchanger as claimed in claim 5, characterised by being of S-shape.

7. A heat exchanger as claimed in claim 5, characterised in that in correspondence with the bent portion the casing housing the tubes (2) comprises two semi-circular shells (3A, 3B).

8. A heat exchanger as claimed in claim 1, characterised in that the tubes (2) comprise tapered entry portions (2A) and exit portions (2B), the inlet (5B) and outlet (5A) for the first fluid being provided in the tube-housing casing (1) in correspondence with said portions.

9. A heat exchanger as claimed in claim 1, characterised by comprising a perforated entry plate (6A) and exit plate (6B) to which the ends of the tubes (2) are welded; said plates having substantially the same shape as the cross-section of the casing (1) which houses the tubes, the ends of the tubes (2) being tapered.

10. A method for forming a heat exchanger claimed in claim 5, characterised by comprising a first step in which a bundle of rectilinear tubes is inserted into at least a first rectilinear tubular casing part (1A, B, C) in such a manner that the tubes lie substantially in contact with each other and with the internal walls (11) of said casing, a second step in which the entry ends (2A) of said tubes are rigidly secured to a plate-like element (6A) which is then fixed to one end of said rectilinear casing part (1C), a third step in which the tube bundle is bent, and a further step in which over the bent tube bundle portion and over the remaining tube bundle portion further elements are mounted which, together with the first tubular element, form a tubular casing (1) containing the tubes (2).

11. A method as claimed in claim 10, characterised in that, before being bunched into a bundle, the tubes are deformed along portions lying in predetermined positions so as to change the shape of their cross-section relative to adjacent tube portions, such that the fluid stream flowing through the tubes breaks up and mixes when passing through said deformed portions.

12. A refrigeration system of the type comprising an evaporator, characterised in that the evaporator is a heat exchanger as claimed in one of claims 1 to 9.

Drawing