DE10256232B4 - Device for extending the service life of tube bundle heat exchangers in indirectly heated UHT systems for food - Google Patents

Abstract

The invention relates to a device for increasing the service life of a tube-bundle heat exchanger in indirectly-heated UHT units for foodstuffs, in particular for milk or milk products, according to the generic part of claim 1. According to the invention, the service life of UHT plants of the inventive type is significantly increased and, as a desired side-effect, the cleaning process optimised and the sterilisation frequency reduced. The above is achieved, whereby the structure surface of the tube inner wall (3a), achieved by means of macro-rough structures (MR) is surface treated by means of an electrochemical polishing method, which generates a micro-surface property (mR), structurally and energetically characterised in a reduced tendency for the adherence of foreign substances, that the incident angle (<) is in the range 35 = ? = 25 degrees and that said inner tubes (3; 3*) are provided in a UHT heater and a subsequent UHT heat retainer.

Description

TECHNICAL AREA

The The invention relates to a device for extending the service life of tube bundle heat exchangers in indirectly heated UHT systems for food, in particular for milk or milk products, according to the preamble of claim 1.

Under a UHT process (UHT: Ultra High Temperature) with indirect Product heating by heat exchange by means of a heat transfer medium on a wall one understands a thermal product treatment, which also called aseptic heating, in which almost everyone Microorganisms, but at least all that lead to corruption Microorganisms, killed be that during the storage phase of the product could grow at room temperature. Therefore have to all microorganisms with the exception of some, possibly surviving the heating process heat resistant spores killed become. However, these grow at normal room temperature during the Storage phase not over a critical value. Indirect product heating through a heat exchange on a wall can be done with so-called plate heat exchanger systems or also, as in the present case, with so-called tube bundle heat exchangers respectively. In the following the problem is consistently with milk or dairy products with approximately same kinematic viscosities ν, since these applications are an essential area of application of the device of the generic type represent.

A UHT heating system with indirect product heating included first a preheater for the heating up of the product. Then in this indirect UHT heater system the milk mostly over one so-called homogenizer leads. Another heat exchange follows a so-called preheating attitude for protein stabilization of the milk proteins for the subsequent milk heating process, then the actual UHT heating with keeping hot, then the cooling under heat exchange with the incoming milk and, if necessary, after-cooling. In dependence Depending on the technology used, the homogenization before or also take place after the UHT heating. Water acts as the heat transfer medium, which is circulated and in accordance with the temperature-time curve in the milk flow at higher temperature the milk is heated in countercurrent and the milk is also heated in the return flow cools in countercurrent. This heat exchange is regenerative, whereby up to 90% of the energy used is recovered can be. Here is the UHT heater from this regenerative heat exchange except and the necessary residual heating takes place here indirect heating with steam while redirecting the water cycle (Principle FINNAH, Ahaus; H. KESSLER, dairy technology, 3rd Edition, 1988).

An indirect UHT heating system that has been successfully used in practice realizes both the regenerative heat exchange and the non-regenerative heat exchange in the UHT heater using so-called tube bundle heat exchangers ( DE-94 03 913 U1 ), Principle Tuchenhagen Dairy Systems GmbH, Ahaus), whereby several parallel inner tubes are provided, through which milk flows, while the heat transfer medium, usually water or steam, in the annular space of the jacket tube, which surrounds the inner tubes connected in parallel, in Countercurrent flows. A one-pipe system without heat exchange is generally used for the UHT hot holder.

With the heating of food in general and in the present case with milk in particular, a problem occurs which is described with the term product batch formation. Milk proteins and mineral salts are mainly responsible for this build-up in milk. It is known that the rate of formation of the proteins increases with increasing product temperature, ie the reaction rate is greatly increased by the temperature increase. It is also known that with an upstream holding at 90 to 95 degrees Celsius less batch occurs in the downstream heater at a maximum of 142 degrees Celsius, which is due to the denaturation of whey proteins and the reduction of the supersaturation of Ca ₃ (PO ₄ ) ₂ by the formation of crystal nuclei in the hot holder. In addition to the composition, ie the basic contamination with germs, among other things, the pH value of the milk also influences the formation of the batch. If the pH value drops = 6.68 to 6.62, a strong increase in deposits can already be expected.

The flow rate of the product, the milk or the milk product has a decisive influence on the formation of the build-up, since here the deposition process due to build-up is overlaid by shear forces in the area of the flow boundary layer of the inner pipe wall. this the latter effect makes UHT heating systems with tube bundle heat exchangers particularly useful compared to plate heat exchangers, since here the flow velocity in the inner tubes is chosen to be relatively high due to the design and thus a relatively high Reynolds number, which influences the flow boundary layer, resulting in a lower number Residence time of the milk ingredients follows, is given. The Reynolds number Re is a dimensionless characteristic characterizing the flow state (degree of laminarity or turbulence); it is calculated from the mean flow velocity v in the inner tube, its hydraulic (equivalent) inner diameter d _hydr and the kinematic _viscosity ν of the product according to the relationship Re = vd _hydr / ν. Known tube bundle heat exchangers in UHT heating systems of the type in question are currently operated with Reynolds numbers in the range between Re = 15,000 and Re = 30,000 when applied to milk or milk products. For example, with a tube Φ14 × 1 mm and a kinematic viscosity ν = 1 · 10 ^–6 m ² / s for milk, this results in an average flow velocity v in the inner tube of v = 1.25 to 2.5 m / s.

Due to the aforementioned significant influence of the product temperature on the build-up speed, the UHT heater and the UHT hot holder are the critical system area of a UHT heating system, as far as the problem of build-up is concerned. It is obvious that on the one hand the formation of a product deteriorates the heat exchange on the inner pipes, ie the heat transfer coefficient k is significantly reduced, and on the other hand the formation of a pressure drop Δp _v in the inner pipes, which is inversely proportional to the fifth power of the inner pipe diameter D _i , increases significantly. The deterioration of the heat transfer ultimately leads to the fact that the heat flows necessary to ensure a sufficient temperature to kill the microorganisms can no longer be transmitted and that a pressure loss Δp _{v also} occurs, which in the limit case significantly exceeds the initial pressure loss. There is a malfunction in the form of a production stoppage; further operation of the UHT heating system is then no longer possible.

uptime the UHT heating system of up to 20 hours, which can also be used with the Term downtimes are characterized without interruption until cleaning under favorable Conditions possible. Then the production has to be interrupted, the system cleaned and if necessary, be re-sterilized. On the one hand, a so-called is used sterile intermediate cleaning, which, for example, takes 1.5 hours high temperature becomes. A second option on the other hand consists in performing a so-called main cleaning, which lasts about two hours, then the entire system over a period of time of about an hour to sterilize.

Independent of The cleaning of the deposits represents the respective cleaning process and the re-provision of aseptic conditions an outage representing a significant cost factor on the one hand an interruption of production and on the other hand with a view to represents the cleaning process itself. In the practice of ultra-high heating technology accordingly, extending the service life is a priority, to reduce production costs. An essential starting point to extend the service life consists in reducing the rate of formation.

To for this purpose, the most complete possible is denaturation potentially approaching protein by appropriate constructive activities desirable. If the possibilities the reduction in pre-heated milk is exhausted, must pay attention to further reduction to the area of tube bundle heat exchangers in the relevant high temperature ranges of the UHT heater system judge.

To the others make the cleaning and sterilization time about 10 to 15% of the service life, so that in this regard also the desire and the There is a need, these times absolutely and not only relatively, based on an extended Tool life, to reduce. When cleaning, especially one those using chemical agents, however, it should be noted that the germs in topographic shallows (surface roughness) specifically for rolled, annealed, chemically stained and then surfaces which have not been mechanically treated in accordance with extended exposure times need. After that, the hygienic end conditions now result the consequential problem that all organic and inorganic contaminants from the surface residue washed away Need to become.

From the publication DE 692 15 988 T3 a heat transfer tube is known which is used as an evaporation and condensation tube in devices such as heat exchangers and heat pipes and which has macro-roughness structures on the surface of its inner tube wall which extend at an angle to the longitudinal direction of the heat transfer tube. These macro-roughness structures are, on the one hand, a multiplicity of main grooves which are parallel to one another and which indicate under the Angle run, have a trapezoidal cross-section and their depth is in the range of 0.15 to 0.35 mm. On the other hand, a plurality of mutually parallel narrow grooves are provided, which extend at an angle to the longitudinal direction of the heat transfer tube, these narrow grooves only grasping the tube in regions and each of the narrow grooves has a base area and two side faces and within and to the main grooves is formed in parallel. The side surfaces of the narrow grooves mentioned are inclined closely towards the base surface, as a result of which each of the side surfaces and the base surface each form a sharp incision. With the known heat transfer tube, when used as a condensation tube, an improved liquefaction efficiency is to be achieved by increasing the turbulence of the vapors and an improved crystallization nucleus formation for the liquid phase, which is brought about by the effect of the surface unevenness. When using the heat transfer tube in evaporators, the edges of the grooves should act as nucleation sites for the bubbles. Such heat transfer tubes are not suitable for use in indirectly heated UHT systems for foodstuffs, since the narrow grooves in particular favor the formation of product batches and would thus reduce the service life of the UHT systems.

Furthermore, in the publication DE 197 51 405 A1 described a heat transfer tube in which the heat exchange surface on the side facing the flowing medium has zones of different surface roughness, these zones being strip-shaped and extending at a slight angle of inclination with respect to the main flow direction of the medium. The mode of operation of this known heat transfer tube obviously stands or falls with the alternating strip-shaped zones of different surface roughness, because this arrangement is intended to produce a distribution of the flow velocity tearing open the thermal boundary layer in the region of the transition between the flowing medium and the heat exchange surface. Said zones can hardly be referred to as relatively large macro-roughness structures of the tube, and with regard to the different surface roughnesses, there is no indication as to whether these surface roughnesses would favor or inhibit product build-up in UHT heaters and / or downstream UHT heat holders.

In the DE 35 36 525 A1 is a metallic heat transfer surface for boiling liquids with a rough surface layer, especially for liquids with a low boiling temperature, such as oxygen, nitrogen and refrigerants, in which the surface layer consists of metal-bonded, electrically practically non-conductive solid particles, which have a large number of each other and in their macrostructure has cavities connected to the boiling liquid and is notch-rich in the microstructure of the surface. The cavities in the macrostructure and the notch-rich microstructure on the surface create a large number of nucleation sites at which the formation of bubbles and thus the evaporation of the boiling liquid are promoted. This causes a high heat transfer to boiling liquid. A surface layer formed in this way in the area of the inner tube walls of the tube bundles of tube bundle heat exchangers, which are used in indirectly heated UHT systems for foodstuffs, would favor the formation of products in particular in their cavities and on their notch-rich surface and thus reduce the service life of the UHT systems ,

It is the object of the present invention, a device of the generic type to propose with which the service life of the UHT systems of the above Kind significantly extended and, as a side effect, the cleaning process is optimized and the sterilization frequency be reduced.

SUMMARY OF THE INVENTION

This Object is achieved by a device with the features of the claim 1 solved. Advantageous embodiments the proposed device are the subject of the dependent claims.

The proposed device according to the invention is part of the so-called tube bundle heat exchanger, which usually consists of a large number of so-called tube bundles, each of which has several inner tubes connected in parallel with one joint entry and exit for the product having. The respective group of inner tubes is made up of a jacket tube enclosed, which in each case near the ends with a radially opening out Connection piece for a heat transfer medium is provided that the jacket tube in counterflow to the tube flow in flows through the inner tubes.

The inventive solution makes use of two mechanisms, namely the mechanism of increasing turbulence in the thermal and hydraulic boundary layer of the pipe flow in the Inner tube. This is achieved by the fact that the respective inner tube of a UHT heater and subordinate UHT hot holder has macro-roughness structures M _R at least on the surface of its inner tube wall, which are oriented at an angle of incidence of 35 α α 25 25 degrees with respect to the longitudinal axis of the inner tube. This in turn results in a turbulent exchange of impulses transverse to the main flow direction in the area of the thermal and hydraulic boundary layer, which inhibits the formation of product batches. Such macro-roughness structures must be such that they protrude from the laminar lower layer of the boundary layer and thus generate or promote the desired pulse exchange. In addition, these macro-roughness structures must be oriented in relation to the main flow direction of the pipe flow in such a way that the accumulation of product batches is not favored.

The invention also makes use of a second mechanism which is decisively shaped by the microscopic nature of the surface of the inner tube wall of the inner tube. It is proposed here that the surface of the inner wall of the pipe structured in this way is treated over an entire area, also extending over the macro-roughness structures M _R , by means of an electrochemical polishing process which produces a micro-surface texture m _R which is structurally and energetically characterized by a reduced tendency for the attachment of foreign substances. However, this is not primarily about the structural feature of the surface that is determined by a so-called roughness depth value, such as the mean roughness depth R _z or the arithmetic mean roughness value R _a (definition according to DIN EN ISO 4287). It is a common fallacy (see also G. HENKEL, A-4830 Waidhofen / Thaya; article no. 35, 2001, "Modern surface treatment of high-quality stainless steel pipes for use in heat exchangers"; special print from PRO CESS, 8th year, March 2001, "Appearances are deceiving") to want to define the quality of a metal surface by specifying a surface roughness value, because this is only a quantitative comparison value that cannot convey a real idea of the effective surface shape and other properties.

The Prevention of deposits on stainless steel surfaces or at least their reduction or inhibition, which is the present Invention with a view to extension the service life or the reduction of the cleaning and sterilization time has set the primary goal, will be essentially through the physical relationships on the surface certainly. Here in particular are the microscopic binding mechanisms of contamination to the surface of interest. these are alongside Van der Waals forces especially bridge bindings and also mechanical anchors.

Simple standardized interior surface finishes the stainless steel pipes, such as by cold drawing or Rolling and subsequent glow and pickling, and in special cases these surfaces also can still be mechanically ground, are relative in the way active that there is between the surface and the flowing medium, the product, especially with turbulent flow, in the fluidic calmed boundary layer, the laminar lower layer, to so-called chemo-physical reactions of media particles with the contact surfaces. As a result, a coating layer builds up on the stainless steel surface, growing in a punctiform manner on the character of an intermediate or boundary layer represents, in which particles prefer to anchor and then others Particles of the medium accumulate, which ultimately makes them closed Layer formation occurs. In the present case it is with UHT heating systems the accumulation of substances that potentially form a deposit.

According to the invention a micro-surface texture suggested the more energetic than the structural nature the respective stainless steel surface changed. This is achieved through professional electrochemical polishing (see HENKEL pickling and electropolishing technology, A-3830 Waidhofen / Thaya, "Die surface saves the value of the component ").

to Showing the effectiveness of the proposed action comparative quantitative information is given below. This information result from tests on stainless steel pipes with a surface treatment of the aforementioned type, with the electrochemically polished, as well as those not treated in this way, mechanically in the initial state were ground. In tube bundle heat exchangers UHT systems of the generic type come for cost reasons usually longitudinally welded stainless steel pipes to use because of the continuous axial weld inside additionally calibrated, but not, as mentioned above, additionally mechanically ground become. The test results are therefore not easy quantitatively transferred to the inner tubes of tube bundle heat exchangers, however, these results at least make the qualitative change the physical nature of the surface of the inner tube wall electrochemical polishing clearly.

Due to the electrochemical polishing, the surface of these tubes is on the one hand with a gapless Passive layer coated, which consists of a relatively thick chromium oxide layer (> 2 nm compared to <1 nm in the execution with mechanical pre-grinding, without electrochemical polishing). On the other hand, the surface is practically stress-free due to the stress-free electrochemical polishing removal and shows a material energy level of around 1.3 N / m (compared to around 2.2 N / m with mechanical pre-grinding). Passivation and reduction of the energy level primarily result in the reduced tendency for the attachment of foreign substances, ie the significantly strongly reduced tendency to deposit.

The erosion caused by the electrochemical polishing process is about 10 to 15 μm, which has shown, for example, by an investigation (G. HENKEL), that 1 cm ² projected surface then has about 2.5 to 4 cm ² true surface ( compared to 12 to 14 cm ² with mechanical pre-grinding). This structural micro-surface texture also has an impact on the formation of the product batch, whereby this texture has hardly any effect on the average roughness value R _a , which in the investigation of the electrochemically polished surface at R _a = 0.2 to 0.25 μm and at Surface with mechanical pre-grinding was at R _a = 0.3 to 0.5 μm. It is also known that the tendency to form deposits is inhibited if the topographical shallows of a surface, the number of which is in relation to the aforementioned true surface, are at least the same, better smaller than the representative size of the particles accumulating in an undesirable manner. In milk heating, the product batches consist of proteins with a size of 1 to 2 μm, microorganisms> 1.5 μm and sugar and salts in the range of 0.7 to 0.8 μm.

Longitudinally welded inner tubes made of rolled steel with subsequent calibration, as they have been used up to now in the tube bundle heat exchangers of the type in question, generally have average roughness values R _a ≤ (0.7 to 0.8) μm and approximately in the area of the weld R _a ≤ 1.2 μm.

The The following table shows the above situation using the example of germ growth on stainless steel 1.4301 depending the processing method of the surface and the time.

Tabelle

table

The goal pursued by the invention, namely the service life of the in UHT heating systems in question and to extend the cleaning and Shorten sterilization time, is significant by combining the two above measures described to achieve the one you want Effectiveness once through the macro roughness structures outside the laminar underlayer and secondly due to the micro-surface properties essentially within the laminar underlayer of the flow boundary layer cover in the inner tube.

• – Ohne die erfinderischen Maßnahmen stellt sich am Milchaustritt des UHT-Erhitzers mit jeweils sieben parallel geschalteten Innenrohren Φ14 × 1 mm bei einer mittleren Strömungsgeschwindigkeit im Innenrohr von v = 2,4 m/s (Re ≈ 28.000) nach 20-stündiger Betriebszeit eine Temperaturdifferenz zwischen Wärmeträgermedium und Milch von Δϑ = ϑ_a – ϑ_E = 12 Grad Celsius ein.
• – Durch die erfinderischen Maßnahmen wird unter ansonsten gleichen Bedingungen an dieser Stelle eine Temperaturdifferenz Δϑ = 7 Grad Celsius erreicht, so dass im Vergleich zum Glatt-Rohr die Standzeit noch um einige Stunden verlängert werden kann, bis vergleichbare Temperaturverhältnisse vorliegen, die eine Unterbrechung des Betriebs der Anlage erfordern.

The effectiveness of the proposed inventive measures can be read quantitatively from the result determined in the practical test.

• - Without the inventive measures, there is a temperature difference at the milk outlet of the UHT heater, each with seven parallel inner tubes Φ14 × 1 mm with an average flow velocity in the inner tube of v = 2.4 m / s (Re ≈ 28,000) after 20 hours of operation between heat transfer medium and milk of Δϑ = ϑ _a - ϑ _E = 12 degrees Celsius.
• - Through the inventive measures, a temperature difference Δϑ = 7 degrees Celsius is achieved under otherwise identical conditions at this point, so that the service life can be extended by a few hours compared to the Glatt tube until comparable temperature conditions exist that interrupt the operation the system.

In addition to the two measures according to the invention described above, which shape the structural and physical properties of the surface of the inner tube wall, it is further proposed to operate the device according to the invention with a turbulent flow in the inner tube, the Reynolds number Re of which is in the range 35,000 ≤ Re ≤ 45,000. This degree of turbulence ensures a forced exchange of impulses and shear forces in the area of the flow boundary layer close to the wall, which is the tendency to the Pro Product build-up is further reduced, thereby further increasing the service life of the shell-and-tube heat exchangers. For example, for an inner tube with Φ14 × 1 mm and when applied to milk (ν = 1 · 10 ^–6 m ² / s) this means that the mean flow velocity v in the inner tube is in the range of v = 2.9 to 3.75 m / s lies.

Out of heat transfer technology is known for heat transfer in comparison to the so-called smooth tube by profiling the heat transfer To improve inner pipe and pipe outer surface. For this purpose, screw-shaped Indentations made by forming techniques in the pipe wall, whereby to produce this desired Macro roughness structures no additional material thickness, such as in the known finned tube, is required. This means that one outside the indentation on the inside represents a corresponding increase. The thin-walled so deformed Pipes are referred to as so-called swirl pipes (company publication from the company hde, swirl tube TURBO HELIX). It has been shown that the heat transfer coefficient k with a swirl tube Φ14 × 0.8 mm and a so-called swirl depth t = 0.75 mm, a heat exchange water for water, an average flow velocity of 1 m / s inside and outside and with a swirl angle δ = 45 degrees by about 60 to 65% the Glatt tube rises under otherwise comparable conditions.

This known effect of increasing turbulence in the thermal and hydraulic boundary layer, however, is not readily quantitative on the problem of product build-up in the inner tube of UHT heating systems transferred to. The thermal Optimal that at the above-mentioned twist angle δ = 45 degrees lies, the swirl angle the pitch angle of the helical profile corresponds, leads in an unforeseeable way to no reduction in the product approach compared to the corresponding smooth tube. Rather, it has pointed out that, contrary to the knowledge in the field heat transfer technology, a swirl angle in the range of δ = 60 degrees must be provided with that compared to the comparable Glatt tube a significant reduction in the rate of formation to achieve. A twist angle of δ = 60 degrees means that the Macro-roughness structures, in the present case the screw-shaped depressions, across from the longitudinal axis of the Inner tube are oriented by an angle of attack α = 30 degrees (α = 90 degrees - δ). useful Results also provide swirl angles in the range of δ = 60 ± 5 degrees, i.e. Angle of attack in the range of α = 30 ± 5 degrees.

It may be advantageous to design a swirl depth t and a swirl width b, which each characterize the swirl of a swirl tube, in a product-specific manner. For this purpose, the invention provides that the swirl, which can be preselected for specific products, has a swirl depth t and a swirl width b. In this context, it is further proposed to design the swirl tube in a single-start manner with a pitch H _G.

Since a swirl pipe designed with a swirl angle of approximately δ = 60 degrees according to the invention only covers part of the surface of the inner pipe with the desired macro-roughness structures, a further development of the device according to the invention provides that the swirl pipe has multiple gears with a number of gears and each is designed with a pitch H _G. In this way it is possible to cover the entire surface of the inner tube with the desired macro-roughness structures. According to a further advantageous embodiment, the complete covering of the surface of the inner tube is achieved when the swirl width b results from the pitch H _G divided by the number of gears g (b = H _G / g).

How The foregoing is the critical area of a UHT heating system in terms of product approaches in the area of UHT heaters and UHT hot holders to see. product approaches are also evident in other areas of a UHT heating system. To extend the service life here and the cleaning effort To reduce, it is proposed that the inner tubes of the the UHT heater and UHT hot holder upstream and downstream tube bundle heat exchanger the UHT system, provided that it is in a temperature range above 100 degrees Celsius can be operated with the macro-roughness structures and the micro-surface texture according to the invention are provided.

embodiments the device according to the invention are shown in the drawing and are described below. Show it

1 a middle section through a so-called tube bundle as a modular part of a tube bundle heat exchanger, on the inner tubes of which the measures according to the invention are applied;

2 in view a section of an inner tube designed as a five-course swirl tube, as in the tube bundle according to 1 is used;

3 in an enlarged view a middle section through the swirl tube according to 2 in the area of a detail marked there "X";

4 in view a section of an inner tube designed as a single twist tube, which otherwise has the same dimensions as that 2 owns and

5 a view of a section of a swirl tube in a third embodiment, the swirl being cross-twisted at a swirl angle of δ = 60 degrees.

DETAILED DESCRIPTION

A tube bundle 1 ( 1 ; see also DE 94 03 913 U1 ) consists of a casing tube in its middle part 2 with a jacket pipe flange arranged on the left-hand side in relation to the position of the illustration 2a and a jacket pipe flange arranged on the right-hand side on the floating bearing side 2 B , A first connection piece closes at the latter 4a and to the jacket pipe flange on the fixed bearing side 2a a second connection piece closes 4b on. Several inner tubes connected in parallel 3 . 3 * , preferably seven or even up to nineteen and possibly also more in number, each have a support plate on the fixed bearing end 7 or a support plate on the floating bearing side 8th welded, this overall arrangement via an unspecified opening of the second connection piece 4b into the casing tube 2 is introduced and via a fixed bearing-side exchanger flange 5 with the second connection piece 4b with the interposition of one flat gasket each 9 is tied together (fixed bearing 5 . 7 . 4b ). The two connecting pieces 4a . 4b are opposite the adjacent jacket pipe flange 2 B . 2a also with a flat gasket 9 sealed, the first connection piece arranged on the right 4a in connection with the casing pipe 2 via a floating flange on the floating bearing side 6 with the interposition of an O-ring 10 against the fixed bearing arranged on the left 5 . 7 . 4b is pressed. The support plate on the floating bearing side 8th engages through an unspecified hole in the floating bearing-side exchanger flange 6 through and finds its seal against the latter by means of the dynamically stressed O-ring 10 , which is also the first connection piece 4a static against the floating flange on the floating bearing side 6 caulk. The latter and the support plate on the floating bearing side 8th form a so-called floating bearing 6 . 8th , which shows the length changes of the carrier plate in the floating bearing side 8th welded inner tubes 3 . 3 * due to temperature change in both axial directions.

Depending on the arrangement of the respective tube bundle 1 In the tube bundle heat exchanger and its respective shading, the inner tubes can 3 . 3 * , based on the position of the illustration, a product P flows through it either from left to right or vice versa, the mean flow velocity in the inner tube 3 . 3 * is marked with v. The fixed flange exchanger flange 5 has a first connection opening 5a which, on the one hand, corresponds to the nominal diameter of the pipe (not shown) connected there and, on the other hand, is dimensioned such that there is a flow velocity v in the inner pipe 3 . 3 * corresponding flow velocity is present. In the same way there is also a second connection opening 8a in the support plate on the floating bearing side 8th dimensioned, the transition from the respective connection opening 5a respectively. 8a to the effective cross section in the area of the transition to the adjacent support plate 7 respectively. 8th through a conical extension 5b or conical taper 8b he follows.

The average flow velocity v in the inner tube has proven to be expedient 3 so far compared to the previous design recommendations (15,000 ≤ Re ≤ 30,000) that there is a turbulent pipe flow with a Reynolds number Re, which with the pipe _inside diameter D _i = d _hydr (see 2 ) is calculated in the range 35,000 ≤ Re ≤ 45,000.

Depending on the direction of flow velocity v in the inner tube 3 . 3 * the product P to be treated either flows through the first connection opening 5a or the second connection opening 8a the tube bundle 1 to. Since there is always a heat exchange between product P in the inner tubes 3 . 3 * and a heat transfer medium W in the jacket tube 2 has to take place in counterflow, this heat transfer medium W flows either to the first connection piece 4a or the second connection piece 4b with a flow rate c too. In the event that the product P the tube bundle 1 via the first connection opening 5a flows in, the inlet temperature of the product ϑ _E would be here. Accordingly, the heat transfer medium W would be the jacket tube 2 in counterflow via the second connection piece 4b leave with an outlet temperature of the heat transfer medium ϑ _a . The in the area of the second connection piece 4b The existing temperature difference at the product inlet Δϑ = ϑ _a - ϑ _E represents in practice, in addition to the above-mentioned pressure loss Δp _v in the inner tubes 3 . 3 * , a reliable indicator of the degree of product build-up in the inner tubes 3 . 3 * represents.

The proposed device according to the invention is reflected in the configuration of the surface of the inner tube wall of the respective inner tubes 3 . 3 * ( 1 ), with the inner tube in question 3 . 3 * , which has an outer tube diameter D _a , in the form of a so-called swirl tube 3 * is trained (see also 2 to 5 ).

A five-speed swirl tube 3 * (g = 5; 2 ) with the outer diameter Da and an inner diameter D; has a helix angle δ (pitch angle of the helical profile), which is in the range between 55 and 65 degrees. The so-called twist 3a * ( 3 ), which is defined by a swirl depth t and a swirl width b, forms a desired macro-roughness structure M _R , which extends over the laminar lower layer of the boundary layer within the pipe flow in the inner pipe 3 * raises and ensures the increased turbulence and the desired momentum exchange. Out 2 it can be seen that the macro-roughness structure M _{R has} an angle of attack α = 90 - δ with respect to the longitudinal axis of the swirl tube 3 * forms, this angle of attack α, complementary to the swirl angle δ, preferably in a range 35 ≥ α ≥ 25 degrees. An angle of attack α dimensioned in this way leads to an inhibition of the product batch, while, for example, a swirl angle δ = 45 degrees which is optimized with regard to heat transfer, accordingly an angle of attack α = 45 degrees, does not inhibit the formation of the product batch in comparison with a corresponding smooth tube, but rather to one leads to a stronger product approach. If the swirl width b results from a pitch H _G divided by the number of gears (b = H _G / g), as in the embodiment of the swirl tube 3 * according to 2 the case is the surface of the inner tube wall 3a of the swirl tube 3 * completely coated with the desired macro roughness structure M _R.

The surface of the inner pipe wall structured by the macro roughness structure M _R 3a of the swirl tube 3 * is also treated nationwide using an electrochemical polishing process that creates a micro-surface texture m _R that is structurally and energetically characterized by a reduced tendency for the adhesion of foreign substances. The professional electrochemical polishing process is generally applied to simply standardized inner surface designs of the inner pipe designed as a stainless steel pipe, the stainless steel preferably being austenitic chromium-nickel steel alloys. The stainless steel tube fed to the electrochemical polishing is generally longitudinally welded and, because of this longitudinal seam, calibrated and subsequently brightly pickled tubes. The starting sheet for pipe production was generally cold rolled, annealed and chemically pickled. The processing of the tubes in the tube bundle is advantageously carried out after the electrochemical polishing; there is no mechanical reworking of the circular welds. Before the electrochemical polishing of the mean roughness value is for the surface at R _a ≤ (0.7 to 0.8) microns and microns in the region of the longitudinal weld in R _a ≤ 1.2.

As explained above, the roughness is reduced by the electrochemical removal from the surface, but this aspect has only a relative effect on the desired micro-surface properties m _R , namely the reduction in the tendency for foreign substances to adhere to the surface minor influence. The influencing factors generated by electrochemical polishing are, compared to the untreated initial surface, essentially the reduction of the true surface compared to the projected surface, the reduction of the energy level of the surface (surface tension) and the complete, chromium oxide-rich passive layer (passivation).

With a catchy swirl tube 3 * ( 4 ), which has the same pitch H _G and swirl width b as that 2 has, compared to the latter is only about 1/5 of the surface of the inner tube wall 3a coated with the macro-roughness structure M _R , while the micro-surface texture m _R extends across the entire surface due to the treatment by means of the electrochemical polishing process. Which degree of surface coverage is to be aimed for or required by macro-roughness structures depends on the product-specific requirements. As long as an increase in the proportion of the surface covered by macro-roughness structures M _R results in an extension of the tool life, it will be called a swirl tube 3 * executed inner tubes 3 with the multi-turn twist required for this 3a * equip. If this is no longer the case, a further covering of the surface up to the complete covering with the condition H _G = b · g can be omitted. The electrochemical polishing always extends across the entire surface of the inner tube wall 3a , regardless of how high the degree of occupancy with macro roughness structures M _R is.

If at the same time as the inhibition of the product build-up on the inner pipe wall 3a of the inner tube 3 * Also, depending on the product, an intensification of the heat transfer is aimed for, a cross-twisted swirl tube can also be used 5 shows a section, be advantageous if the swirl angle δ is between 55 and 65 degrees.

1

tube bundle

2

casing pipe

2a

fixed bearing side Mantelrohrflansch

2 B

loslagerseitiger Mantelrohrflansch

3

inner tube

3a

Inner tube wall

3 *

twisted tube

3a *

Drallung

4a

first spigot

4b

second spigot

5

fixed bearing side exchanger flange

5a

first port opening

5b

conical extension

6

loslagerseitiger exchanger flange

7

fixed bearing side support plate

8th

movable bearing support plate

8a

second port opening

8b

conical rejuvenation

9

gasket

10

O-ring

b

swirl width

c

flow rate in the casing tube

d _hydr

hydraulic (equivalent) inner Φ of the inner tube

G

transfer coefficient

k

Heat transfer coefficient

m _R

Micro Surface Texture

Δp _v

pressure drop

t

twist depth

v

middle flow rate in the inner tube

D _a

External pipe diameter

D _i

Inside pipe diameter

H _G

pitch

M _R

Macro-roughness structures

P

product (temperature treated side)

R _a

Average roughness

re

Reynolds number of the flow in the inner tube (Re = d _hydr v / v)

W

Heat transfer medium

α

angle of attack (α = 90 - δ)

δ

angle of twist (δ = 90 - α)

ν

kinematic toughness of the product

ϑ _a

outlet temperature of the heat transfer medium

ϑ _E

inlet temperature of the product

Δϑ = ϑ _a

- ϑ _E temperature difference at product entry

Claims (8)

Device for extending the service life of tube bundle heat exchangers in indirectly heated UHT systems for food, in particular for milk or milk products, consisting of a tube bundle heat exchanger with tube bundles ( 1 ), each of which has several inner tubes connected in parallel ( 3 ) with a common inlet and outlet for the product to be heated (P), and the group of inner tubes ( 3 ) together from a casing tube ( 2 ) is enclosed, which in each case near the ends with a radially connecting or terminating connecting piece ( 4a . 4b ) is provided for a heat transfer medium (W) guided in countercurrent, the respective inner tube ( 3 ) at least on the surface of its inner tube wall ( 3a ) relatively large macro-roughness structures (M _R ) with a depth (t) that extends over the laminate lower layer of the boundary layer within the pipe flow in the inner pipe ( 3 ), which has an angle of attack (α) with respect to the longitudinal axis of the inner tube ( 3 ) are oriented, characterized by - that the surface of the inner pipe wall structured by macro-roughness structures (M _R ) ( 3a ) covering the entire surface, including the macro-roughness structures (M _R ), is treated by means of an electrochemical polishing process which produces a micro-surface texture (m _R ), which is structurally and energetically characterized by a reduced tendency for the adhesion of Characterized foreign substances, - that the angle of attack (a) is in the range 35 ≥ α ≥ 25 degrees and - that the respective inner tubes ( 3 ) are provided in a UHT heater and in a downstream UHT hot holder. Device according to claim 1, characterized in that the Reynolds number (Re) of the pipe flow in the inner pipe ( 3 ) is in the range 35,000 ≤ Re ≤ 45,000. Device according to claim 1 or 2, characterized in that the inner tube ( 3 ) as a tube with a helical profile on the inside and outside, as a so-called swirl tube ( 3 * ), whose swirl ( 3a * ) with a swirl angle (δ = 90 degrees - α) in the range 55 ≤ δ ≤ 65 degrees. Device according to claim 3, characterized in that the swirl ( 3a * ), can be selected for specific products, has a swirl depth (t) and a swirl width (b). Device according to claim 3 or 4, characterized in that the swirl tube ( 3 * ) is catchy with a pitch (H _G ). Device according to claim 3 or 4, characterized in that the swirl tube ( 3 * ) is multi-course with a number of gears (g) and each with a pitch (H _G ). Apparatus according to claim 6, characterized in that the swirl width (b) results from the pitch (H _G ) divided by the number of gears (g) (b = H _G / g). Device according to one of claims 1 to 7, characterized in that the inner tubes ( 3 ) the tube bundle heat exchanger of the UHT system upstream and downstream of the UHT heater and the UHT hot holder, provided that they are operated in a temperature range above 100 degrees Celsius, with the macro roughness structures (M _R ) and the micro Surface quality (m _R ) are provided.