WO2000079187A1

WO2000079187A1 - Tube for use in fluid evaporation techniques, in particular food fluid

Info

Publication number: WO2000079187A1
Application number: PCT/FR2000/001702
Authority: WO
Inventors: Laurence Boulange; Françoise Haegeli; Joël CHOBERT
Original assignee: Societe Meusienne De Constructions Mecaniques
Priority date: 1999-06-22
Filing date: 2000-06-21
Publication date: 2000-12-28
Also published as: BR0012526A; EP1188026A1; FR2795337A1; FR2795337B1; AU6450000A

Abstract

The invention concerns a welded stainless steel tube for use in fluid evaporation techniques, particularly a food fluid, characterised in that it comprises, on its outer heat-exchanging surface, mostly smooth, localised means for accumulating and conducting condensations generated on the heat-exchanging smooth surface, means consisting of elevations in the form of shoulders extending over the length of the tube and capable of forming recesses distributed over said tube length.

Description

Tube that can be used in fluid evaporation techniques, in particular edible fluid. The present invention relates to a tube which can be used in fluid evaporation techniques, in particular of edible fluid. During the evaporation of a food fluid such as a sweet liquid or milk, the heat exchanges on either side of the evaporator tube are very important because of the high energy balances used in this field.

In addition, upon evaporation of the fluid, mineral and or organic deposits are formed which it is necessary to eliminate by repeated cleaning and disinfection operations, in particular in the dairy sector. The cost of cleaning operations is high due to the amount of water and cleaning products required. In the case of a sugar refinery, processing sugar cane or beet and despite the use of anti-scale chemicals, production must be stopped to carry out the operations of cleaning the exchanger tubes.

A method of manufacturing a so-called fluted tube is known, in which a basic metal strip is corrugated, in the width direction, between two pairs of rollers comprising flutes. The corrugated metal strip is then rolled, also in the width direction, to form a tube having a corrugated profile, in section. The purpose of the tube thus produced with the corrugated strip is to increase the heat exchange surface for the same nominal tube diameter.

In this example, the production of the corrugations requires the use of a relatively thin sheet metal strip, of the order of 0.5 mm, to ensure shaping of the corrugations. As a result, the tube produced from a sheet of thin thickness leads to a risk of perforation during pitting corrosion of the tube following successive pickling treatments. The corrugation also has the function of stiffener, which allows the use of a thin base plate. In another exemplary embodiment, one of the rollers has a straight generator parallel to its axis of rotation and the other roller has grooves intended to form said grooves on one face of the strip laminated steel The tube thus produced, from the laminated strip only has grooves on the outside, said grooves, the purpose of which, also in this case, is to increase the heat exchange surface of the tube for the same diameter nominal In this example, the grooves formed locally generate a significant reduction in the thickness of the tube in the hollow of said grooves and, in the technical field considered, entails a risk of perforation during pitting corrosion.

It is also known a grooving technique by notching on the surface of a tube. In this case, too, grooving results in a relatively significant reduction in the thickness of the sheet metal of the tube and the risks of perforation during pitting corrosion.

In the examples of tubes known from the prior art, the function of grooving or corrugation is to increase the heat exchange surface without taking account of the wettability of the fluid.

The object of the invention is to propose a welded stainless steel tube, usable in evaporators, in particular in the sugar and dairy industry, the tube having an improved surface quality and a structure of its exchange surface ensuring a quality also improved heat exchange

The subject of the invention is a welded stainless steel tube which can be used in techniques for the evaporation of a fluid, in particular a food fluid which is characterized in that it comprises, on at least one of its external surfaces. or internal heat exchange, mostly smooth, localized means of accumulation and flow of condensations created on the smooth heat exchange surface

The other features of the invention are

- the means of accumulation and flow of condensations are distributed longitudinally to said tube and uniformly, - the means of accumulation and flow of condensations consist of gradients in the form of shoulders extending over the length of said tube and the height of which is less than 0.5 mm and preferably less than 0.2 mm.

- The means of accumulation and flow of condensations consist of hollow imprints distributed over the length of said tube, - the imprints have a width of between 0.2 and 1 mm, occupying in a distributed manner, less than 50% from the periphery of said tube,

- the impressions have a rounded bowl shape,

- The means of accumulation and flow of condensations consist of steps, distributed around the periphery of the tube and occupying less than 50% of the periphery of said tube.

- At least the opposite surface of the surface comprising the localized means of accumulation and flow of the condensations of the tube has a roughness of less than 0.2 μm, of the bright annealing type 2R.

The invention also relates to a method for improving the heat exchanges in a heat exchanger for the evaporation of a fluid, in particular in the food industry, the exchanger comprising tubes intended for heat exchange, characterized in that:

- Localized means of accumulation and flow of condensations are produced on at least one of the smooth surfaces of the exchanger tubes, means intended to attract, by capillarity, the film of condensed liquid which is deposited uniformly on the predominantly smooth surface of the tube;

- Draining by the localized means of accumulation and flow of condensations said film of condensed liquid.

Another characteristic of the invention is: - on at least one of the smooth surfaces, the exchanger tubes are produced at least one drop in the form of a shoulder extending over the length of said tube and whose height is less than 0.5 mm, and preferably less than 0.2 mm, difference in height defining a longitudinal imprint the edge of which attracts by capillary action the film of condensed liquid which is deposited uniformly on the predominantly smooth surface of the tube.

The following description, including the appended figures, all given by way of non-limiting example will make the invention easier to understand. Figures 1 and 2 each show, in section, an example of a stainless steel tube according to the invention.

In the shaping of a stainless steel tube that can be used in techniques for evaporating a fluid, in particular a food fluid, the tube shaping line, from a strip of steel includes rolling and edge-to-edge welding means of the strip. According to the invention, the line comprises a mechanical assembly comprising a so-called upper flat roller and a so-called lower roller. The roller, for example said to be lower, has equidistant protrusions or grooves of small height providing, between them, smooth surfaces, smooth surfaces occupying mainly the surface of the strip. The two rollers are adapted to the width of the starting steel strip, starting strip whose width corresponds to the average perimeter of the tube to be produced.

In the exemplary embodiment according to the invention, the mechanical assembly is placed at the head of an induction, laser or TIG welder.

Localized means are produced with the mechanical assembly, on at least one of the surfaces of the stainless steel strip, for the accumulation and flow of condensations constituted for example by linear imprints sufficiently spaced apart to release an internal surface. or mostly smooth external of the tube.

FIG. 1 presents in section a part of tube 1 according to the invention on which, for example, the localized means of accumulation and flow of condensations are produced on the external surface of the tube.

The means of accumulation and flow of condensations consist, in this embodiment of unevennesses 2 and 3 in the form of shoulders extending over the length of said tube and whose height is less than 0.5 mm and preferably less than 0.2 mm. The elevations 2 and 3 in the figure define indentations 4 whose depth is less than 0.5 mm and preferably less than 0.2 mm. It is noted that the weakness of the depth of the imprint does not significantly modify the overall thickness of the sheet metal of the tube. We can imagine the realization of a deeper footprint, but this has only drawbacks, including the appearance, on the tube, of thin areas.

The imprints 4, preferably distributed longitudinally on the tube 1, uniformly around its periphery, preferably have a width of between 0.2 and 1 mm, occupying in a distributed manner, less than 50% of the periphery of the tube 1. In the example presented in FIG. 1, the indentations 4, in the form of a rounded bowl, preferably occupy approximately 20 to 25% of the surface of the tube.

The capillary effect, generated by the edge of the cavity 4 moves the film of condensation of the vapor which forms on the surface 5 of the tube, towards said cavity 4. The cavity 4 behaves like a gutter.

The imprint, preferably in the form of a bowl, has the advantage of effectively attracting the film of the fluid distributed over the surface of the tube so as to form droplets which will be channeled and easily eliminated. In other words, the object of the invention is to remove as quickly as possible the drop of water which forms on the surface comprising the imprints so that it is replaced as quickly as possible by steam which will pass from the vapor phase to the liquid phase.

If the wettability was high, the drop would tend to spread over the surface and therefore not to be eliminated - the object of the invention avoids this phenomenon of wettability - the surfaces of the stainless steel tube do not undergo any treatment thermal to accentuate the evacuation.

Other means of accumulation and flow of condensations can be envisaged such as for example the production of redents 6 distributed over the length of the tube 1 as presented in FIG. 2.

The redents consist of two elevations 2 and 3 having a height of less than 0.5 mm, preferably less than 0.2 mm, and a width occupying in a distributed manner, less than 50% of the periphery of said tube and preferably between 20 and 25% of the surface of the tube. In the example, the internal surface of the tube has a roughness of less than 0.2 μm, of the bright annealing type 2R, surface not disturbed by the making of the impressions. The tube is made from a strip of sheet metal with a thickness of between 1.3 and 1.5 mm made of ferritic or austenitic type stainless steel such as AISI 439, 304, 304L, 316L although some steels seem preferable. to others as will be demonstrated later. The making of the imprint does not cause, because of the dimensions thereof, no hardening zone of the base steel, no increase in the hardness of the steel. In addition, the impression practically does not modify the thickness of the wall of the tube, which avoids any risk of premature piercing of the tube if there is a risk of pitting corrosion. In this embodiment, due to the small disturbance generated by the production of the imprint, it is possible to modify the external appearance of a cylindrical tube, to increase the heat exchange capacity of said tube, without modifying excessive surface condition of the inner part of the tube. Under these conditions, the internal part of the tube offers a surface limiting scaling and facilitating cleaning operations, the external part of said tube offering a practically smooth, cylindrical surface, and which ensures, due to the existence of fingerprints, a increase in heat exchange by draining and eliminating the condensation which forms continuously on the tube in its function of "hot" heat exchanger from steam.

The tube, according to the invention, can comprise the means of accumulation and flow of the fluid on its two external and internal surfaces so as to be able to be used, as desired, in the two functions of "hot" heat exchanger or "cold". In the context of "cold" heat exchangers with fluid phase change, tubes will be used comprising on their internal surface, the localized means of accumulation and flow of condensations.

In the context of a study on the scaling of tubes in contact with sugar juice, we find that the deposit consists mainly of CaC0 ₃ . A mechanism comprising three phases has been highlighted:

- an initiation of scale deposits, which corresponds to an increase in the evaporative flow. This primer depends on the surface properties of the tube such as micro-roughness, surface chemistry Two types of primers can be distinguished, one mineral of the MgO-CaC0 ₃ -Sι 0 _{2 type} for surfaces of the 2B, 2D type, pickled or not, the other mixed of type CaC0 ₃ plus organic matter, for type 2R and electro-polished surfaces, - a balance between the kinetics of scaling and the separation of layers At this stage, the composition of the steel of the tube is an important parameter because the thermal and expansion coefficients will play on this balance - a phase where scaling prevails over the separation of the layers of tartar The tube according to the invention has the advantage of less scaling compared to the other tubes tested Reduction of the scaling is a factor of 2 to 3, which increases the evaporative flow by almost 25%

From a cleaning point of view, there is a difference in kinetics and therefore the water consumption necessary to remove large deposits of scale can be reduced by half.

The cleanability of the tubes depends on both the chemistry and the topography of the surface Indeed, according to the surface chemistry of the tube, the initiation of scale deposits differs and therefore, the ruptures caused by pressurized water , in the scale deposit, are more or less easy The roughness of the internal surface is an important factor for the elimination of the deposit The lower the average surface roughness, the more the surface will be cleanable Due to the low deformation caused in external surface, when making the impressions, the internal surface condition and little or not changed The water consumption measurements clearly confirm the mechanisms governing surface cleanability, namely the surfaces of tubes made of AISI 439 stainless steel enriched in Si oxide and having an average roughness Ra of 0.2 μm will be scaled by a deposit of CaC0 ₃ associated with organic matter This deposit is more brittle and therefore more easily eliminated than a completely mineral deposit which preferably forms on the surfaces of a tube made, for example of AISI 304 steel enriched with iron or chromium oxide and having a lower average roughness Ra at 0.4 μm. Consequently, the volumes of water required to remove deposits from AISI 439 2R steel tubes are two times lower than those measured to remove deposits from pickled AISI 304 steel tubes.

It is thus highlighted, superior characteristics of a steel tube of composition AISI 439 in comparison with a steel tube of composition AISI 304.

The tube according to the invention ensures:

- a gain of at least 25% in the energy balance of the evaporation chambers, - a limitation of the liquid discharges to be reprocessed,

- a reduction in cleaning frequencies, in particular for sugar cane sweets that do not use anti-tartar products.

- a reduction in scaling.

- easier and faster cleaning.

Claims

1. Welded stainless steel tube that can be used in techniques for evaporating a fluid, in particular a food fluid, characterized in that it comprises, on at least one of its external and internal heat exchange surfaces, predominantly smooth, localized means of accumulation and flow of the condensations created on the smooth heat exchange surface.

2. Tube according to claim 1, characterized in that the means of accumulation and flow of condensations are distributed longitudinally on said tube and uniformly.

3. Tube according to one of claims 1 or 2, characterized in that the means of accumulation and flow of condensations consist of gradients in the form of shoulders extending over the length of said tube and whose height is less 0.5 mm and preferably less than 0.2 mm.

4. Tube according to one of claims 1 to 3, characterized in that the means of accumulation and flow of condensations consist of hollow imprints distributed over the length of said tube.

5. Tube according to claim 4, characterized in that the imprints have a width between 0.2 and 1 mm, occupying in a distributed manner, less than 50% of the periphery of said tube.

6. Tube according to claim 4, characterized in that the imprints have a rounded cup shape.

7. Tube according to one of claims 1 to 3, characterized in that the means of accumulation and flow of condensations consist of redents, distributed around the periphery of the tube and occupying less than 50% of the periphery of said tube.

8. Tube according to claim 1, characterized in that at least the surface opposite to the surface, comprising the localized means of accumulation and flow of condensations, has a roughness of less than 0.2 μm, of the bright annealed type 2R.

9. Method for improving the heat exchanges in a heat exchanger for the evaporation of a fluid, in particular in the food industry, the exchanger comprising tubes intended for heat exchange and comprising a smooth and uniform external surface, characterized in that:

- Localized means of accumulation and flow of condensations are produced on at least one of the smooth surfaces of the exchanger tubes, means intended to attract, by capillarity, the film of condensed liquid which is deposited uniformly on the predominantly smooth surface of the tube,

10. The method as claimed in claim 9, characterized in that at least one of the smooth surfaces of the exchanger tubes produces at least one drop in the form of a shoulder extending over the length of said tube and whose height is less than 0, 5 mm and preferably less than 0.2 mm, difference in height defining a longitudinal imprint the edge of which attracts by capillary action the film of condensed liquid which is deposited uniformly on the predominantly smooth surface of the tube.