WO1999058919A1 - Aluminium pipe suitable for the production of heat exchangers and heat exchangers made with this pipe - Google Patents

Abstract

The invention realizes an aluminium pipe (1, 6, 7) suitable for making heat exchangers having a basically quadrilateral external form and internally containing one or more ducts suitable for the passage of a heat exchanging fluid, said pipe being provided with outside fins (2, 3; 91, 92) belonging to two opposite outside surfaces of the pipe and obtained from said surfaces by raising the material. Each of the ducts belonging to said pipe has at least one wire (4) of basically round section spirally wound along the whole length of said pipe, such that a turbulence is created in the fluid that passes through said pipe.

Description

ALUMINIUM PIPE SUITABLE FOR THE PRODUCTION OF HEAT EXCHANGERS AND HEAT EXCHANGERS MADE WITH THIS PIPE The invention refers to a pipe especially suitable for use in heat exchangers and to the heat exchangers produced with this pipe. It is well known that heat exchangers are tubular devices with pipes generally set parallel to one another and generally belonging to a closed circuit within which a fluid is made to flow that yields heat to the environment, or absorbs heat from the environment. This exchange may occur directly between the exchange coil and the atmosphere or by means of another fluid, for instance water, if the heat exchange coil is immersed in the water.

One of the most common examples of heat exchangers using coiled pipes is the wall mounted gas boiler for heating sanitary water and radiators in households.

One recurrent configuration of the coil used to exchange heat consists of a single pipe bent in a U shape various times with several straight lengths parallel to one another so that two sections of the coil are running side by side where the fluid running through one pipe flows in one direction and the fluid in the other pipe at its side flows in the opposite direction. This type of construction means that the two sections parallel to one another are connected by a U junction.

The same occurs for refrigerant coils, in other words when a refrigerant fluid is flowing through the exchanger unit.

Both types of heat exchanger described above, usually have outside fins that are mostly spirals of sheet metal welded to the outside of round pipes. According to common techniques some known pipes frequently used in heat exchangers, for example in gas boilers for household heating, are made of copper and have outside fins.

In order to create turbulence within the fluid's flow, so that the exchange of heat is increased, some known pipes have fins or sections of diaphragms welded within, which break the flow of the fluid that runs within and thereby creates a turbulence. Some types of constructions have included spiralled plates inside the pipe. If on the one hand these devices create a turbulence, on the other they significantly increase the pressure drop, thereby losing any benefit that could be obtained from the increase in turbulence since the delivery of the fluid flowing in the exchanger has to be increased in order to obtain the same benefits in heat exchange, which often means an increase in pipe section and therefore in size and weight.

There are also other, different pipe configurations. One particularly well-known example is the aluminium pipe produced according to the patent belonging to PEERLES, where the pipes are made of aluminium and have outside fins obtained from spurring the metal sheath on two opposite surfaces of the pipe itself, so that the fins belonging to the pipe are obtained by mechanical tooling and not by welding. This kind of pipe is quite effective, seen from the standpoint of potential heat exchange against fluid flow standpoint, however they are rather costly so their current use has been somewhat limited.

One of the scopes of this invention is to produce a pipe, suitable for use in producing heat exchangers, that significantly increases the heat exchange over currently known pipes. In fact, with this pipe the intention is to make heat exchangers that, with equal heat exchange, are much lighter, cheaper, more compact or that, with equal volume and weight, can give markedly higher performance.

Another scope set for the invention is to make a pipe that creates high turbulence in the fluid running through it, although without significantly increasing the pressure drop. Yet another scope of the invention is to select a material, for the production of the pipe invention, that can support well chemical attacks from metals dissolved in the heat exchanging fluid that runs through the heat exchanger coil, with special reference to acid substances. Furthermore, the intention is to avoid the need of applying surface heat treatments such as Pb-Sn alloys that, in known technology, is normally carried out on copper pipes in order to increase their resistance to corrosive agents found in the flame or environment.

The aforementioned scopes, and others that shall be better illustrated below, are accomplished by a pipe in aluminium suitable for making heat exchangers, having a basically quadrilateral external form and internally containing one or more ducts suitable for the passage of a heat exchanging fluid, said pipe being provided with outside fins belonging to two opposite outside surfaces and obtained from said surfaces by raising the material, where, in accordance with the contents of the first claim said pipe is characterised in that each of the ducts belonging to said pipe has at least a wire of basically circular section spirally wound along the whole pipe length so that a turbulence is created in the fluid flowing through said pipe.

One advantage according to the invention, in the choice of a spiralled wire with circular section, is that it creates the optimum turbulent conditions with practically no pressure drop. All the beneficial effects due to the increase in heat exchange by the turbulent effect are thereby accentuated without giving rise to a significant increase in pressure drop. The heat exchanger made with the pipe invention includes a series of said pipes and is characterised in that it has two headers opposite to one another, between which the pipes are inserted and set basically parallel to one another with their ends connected to said headers so that the liquid in all sections of pipe flows in the same direction. This proves to be an evident advantage since the single flow through the header is divided in several pipes and the overall flow of exchange fluid can therefore be easily increased in the heat exchanger coil to markedly higher levels than current known technology thanks to its higher heat exchange capacity.

An advantage according to the invention, is the use of aluminium, and in particular the aluminium pipe having fins formed out of the pipe itself, is that it allows surface treatments to be avoided as would otherwise occur, as mentioned above, with copper pipes that are subjected to the direct action of the flame and environment, and for this reason have to be lined with lead and tin alloys. Because of the use of aluminium in the pipe invention there is an increase in the pipe's resistance to thermal shocks, since there is no surface film and so there is no possibility that this should peel or melt. What's more, since aluminium has a lower specific weight than copper the whole exchanger unit is lighter with an equal thermal power.

Additional characteristics and details of the invention shall be better illustrated in the description of the preferred form of execution of the pipe and the exchanger given as a guideline, but not a limitation and illustrated in the attached diagrams where:

- fig. 1 shows a partial prospective view of the pipe invention together with the spiralled wire inserted within it

- fig. 2 is a cross section of fig. I along line ll-ll;

- fig. 3 shows a variant of the pipe according to the invention with several ducts; - fig. 4 shows another variant of the pipe according to the invention

- fig. 5 shows a blown up view of a heat exchanger before its assembly using the pipes of the invention

- fig. 6 is a section of fig. 5. With reference to the above figures it can be seen that an aluminium pipe like the one represented in fig. 1 , indicated by 1 , has been made according to known technology and in other words by extrusion of the aluminium pipe and subsequent construction of the fins by spurring or raising the material by means of suitable tools on two opposite, parallel sides 2 and 3. The pipe 1 thereby obtains a double set of fins, one being on the one side indicated by 2, and one on the opposite side indicated by 3 as shown in fig.'s I and 2. The pipe section in fig. 1 is basically quadrangular and inside this pipe a wire 4 is inserted, wound in a spiral with a spiral diameter basically uniform to the inside measurement of the duct. An execution where the duct 5 of pipe 1 measures 4,5 x 4,5 mm., uses a round sectioned wire having approx. I mm. diameter, spiral wound according to a diameter of approx. 4 mm. and with a spiral gauge of approx. 6 mm. The geometries of the spiral inside the duct found in the pipe is particularly important and essential to optimising turbulence without creating an excessive pressure drop. In effect, in this case the substantially negligible size of the wire diameter used to make the spiral makes it so that the pressure drops are reduced to an absolute minimum, while the effects of this kind of wire configuration are optimised in terms of turbulence. Other dimensions and other geometries of the spiral, for instance a different gauge, may naturally achieve comparable effects. The important and essential fact nevertheless remains that the wire has a round section and a limited size with respect to the duct in which it is inserted in its spiral form.

Experiments conducted on heat exchangers of the type shown in fig. 5, made with the pipe invention and more precisely with aluminium pipes basically rectangular having a line of four ducts with a 4,5 x 4,5 square section, having a wire within having 1 mm. diameter spiralled to a gauge 6 mm., have proved how, with equal thermal power, the dimensions of the exchanger can be reduced by approximately 30% in plane and approximately 50% in volume. With regards to weight, an equivalent exchanger made according to known technology and in copper will be approx. 4,8 Kg, the exchanger invention approx. 2,8 Kg. This being for powers of around 24 Kilocalories. - 5 -

The result of the aforementioned spiral dimensions has given a heat exchanger, which shall be described below, with such a considerable improvement in performance that by substituting a traditional exchanger, for example in a 20 Kilocalorie boiler, with an exchanger using the pipe invention, with an equal size, the boiler power rises to 28 Kilocalories.

Fig. 3 shows a prospective view of a variant of the pipe invention indicated by 6 which shows four ducts 61 , 62, 63 and 64 which contain the spiral 4. According to a sectional view of the example shown in fig. 4, the pipe invention, here indicated in general by 7, has four ducts 71 , 72, 73 and 74 of circular section inside which an additional pipe has been inserted, respectively indicated by 75, 76, 77 and 78 made of material suitable to resist the corrosion of chemical agents in the fluid that passes through the pipe itself. Pipes 75, 76, 77 and 78 may, for example, be made of stainless steel, in order to be more effective in resisting against scaling or attack from free metal ions. Naturally, inside pipes 75, 76, 77 and 78 the spiral 4 shall still be inserted to accentuate the turbulence of the fluid that flows through said ducts belonging to the pipe. Fig. 5 shows an example of how the pipe invention has been used to make a heat exchanger suitable for gas boilers. It can be seen that the exchanger, indicated in general by 10, is composed of four sections of pipe 9, where each pipe 9 has four ducts 93, 94, 95 and 96 with straight lengths and fins 91 and 92 on both sides of each pipe. A wire spiral is inserted inside the ducts of each pipe. In another execution the spiral may even be continuous and run through all the pipes 93, 94, 95 and 96 bent to and fro within the pipes themselves, which does not create any difference in function. The ends of the pipes 9 are slotted into plates 11 each having openings 110 where said ends are welded. Once the pipes 9 are bonded to the plates 11 , they are then welded or fixed by means of ducts and suitable gaskets and screws to headers 12 and 13 respectively. As seen in fig. 6, the flow of fluid that enters the header 12 is divided into four branches of the pipes 9 to then be funnelled back together in the header 13 and exit after having undergone the heat exchange. It is precisely this increased heat exchange capacity of each of the pipes making up the exchanger that allows said exchanger to be produced with headers and with parallel pipes. Even though said pipes are short, the low thermal inertia ensures the most effective transmission of heat without being obliged to extend - 6 -

the lengths of the pipes under the flame. An evident advantage is gained from the fact that the pressure drop obtained with the exchanger configuration shown in fig. 6, and in other words with piping 9 set in parallel, is markedly lower than that of a traditional layout where the sections of pipe 9 would be set in series with U junctions on each end piece to reverse the flow and to connect each section 9 to the next.

The construction of the heat exchanger as illustrated in fig.'s 5 and 6 also emphasises the simplicity of the exchanger's design, which is limited to just two headers and equal sections of pipes according to the invention set between the two headers. The simplicity of the header's construction as described above is further increased by the relative ease with which current techniques can weld aluminium to aluminium such as, for instance, on the heads of the pipes 9 to the plates 11. One known technique is TIG welding, however this does not exclude other possible forms of welding or union even using special adhesives. It must be noted that the nature of aluminium, being a material that is particularly resistant to chemical attack from substances contained in the gas burner's flame, avoids a need, which until now has been inevitable, to carry out outside surface treatments such as applying PB-SN lead-tin alloys on copper pipes to prevent this attack from the flame on the actual copper.

With regards to the attack of the exchange fluid on aluminium, this can be preventively reduced if the fluid flowing through the exchanger is water, for instance by adding substances to the water in the exchanger's closed circuit making it chemically inert to aluminium. In other cases, as seen in fig. 4, a cladding can be given by pipes made of a basically corrosion-proof material like stainless steel.

Other possibilities may also be provided by coating the inside of the ducts within the aluminium pipes or may be provided with the anodisation or cataphoreses process of the pipe or exchanger. The pipe invention, and especially the exchanger made from the pipe invention, can be used to advantage in flash boilers currently constructed exclusively by pipes in stainless steel or metal cast, precisely because of the high aggressivity of the gas emissions that this type of boiler produces. Another interesting application for the heat exchangers, in addition to the heating sector, could be the refrigeration sector, both for cooling and for their heat exchange, for instance in exchangers for cooling transformer oil that is cooled by a coil through which the fluid requiring cooling passes, in other words the oil, and that is cooled by a flow of outside air channelled by special fans. The pipe invention also with more than one wire spirally wound along the whole lenght of the pipe, may naturally be used for exchangers applied in any most widespread of technological sectors always with surprising advantage, given the efficacy of heat exchange that, as said, with equal exchange capacity cuts dimensions by about 30% in plane and about 50% in volume, while the exchanger's weight is reduced by about half.

Claims

- 8 -

1 ) Aluminium pipe (1 , 6, 7) suitable for making heat exchangers, having a basically quadrilateral external form and internally containing one or more ducts suitable for the passage of a heat exchanging fluid, said pipe being provided with outside fins (2, 3; 91 , 92) belonging to two opposite outside surfaces of the pipe and obtained from said surfaces by raising the material, characterised in that each of the ducts belonging to said pipe has at least one wire (4) of basically round section spirally wound along the whole length of said pipe, such that a turbulence is created in the fluid that passes through said pipe.

2) Aluminium pipe according to claim 1 ) characterised in that said wire, present in each duct of said pipe, has a diameter of 0.8 to 1.2 mm. and a spiral gauge of approx. 6 mm. and spiral diameter of approx. 4 mm., said spiral being inserted in a duct with a minimum size basically the same as the diameter of said spiral.

3) Pipe according to claim 1 ) or 2) characterised in that said one or more ducts have a round section

4) Pipe according to claim 1 ) or 2) characterised in that said ducts have a square section. 5) Heat exchanger including a series of pipes according to any one of claims 1 ) to 4) hydraulically connected together, characterised in that it has two headers (12, 13) facing each other between which are inserted said pipes set basically parallel to one another and with their end pieces fixed to said headers, the liquid flow running in the same direction for each of said pipes. 6) Heat exchanger according to claim 5) characterised in that in every duct of each pipe contains an additional pipe (75, 76, 77, 78) of material resistant to chemical agents having outside dimensions equal to the inside dimensions of said duct (71 , 72, 73, 74).

7) Heat exchanger according to claim 5) characterised in that every duct of each pipe has its surfaces protected by a coating made with substances resistant to the chemical agents found in the fluids.

8) Heat exchanger according to claim 5) characterised in that every duct of each pipe has its surfaces treated with the anodisation process.

9) Heat exchanger according to claim 5) characterised in that every duct of each pipe has its surfaces treated with the cataphoresis process.