Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F19/00—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/16—Polishing
  • C25F3/22—Polishing of heavy metals
  • C25F3/24—Polishing of heavy metals of iron or steel
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
  • F28D7/00—Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall
  • F28D7/16—Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall the conduits being arranged in parallel spaced relation
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F1/00—Tubular elements; Assemblies of tubular elements
  • F28F1/10—Tubular elements and assemblies thereof with means for increasing heat-transfer area, e.g. with fins, with projections, with recesses
  • F28F1/40—Tubular elements and assemblies thereof with means for increasing heat-transfer area, e.g. with fins, with projections, with recesses the means being only inside the tubular element
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F13/00—Arrangements for modifying heat-transfer, e.g. increasing, decreasing
  • F28F13/18—Arrangements for modifying heat-transfer, e.g. increasing, decreasing by applying coatings, e.g. radiation-absorbing, radiation-reflecting; by surface treatment, e.g. polishing
  • F28F13/185—Heat-exchange surfaces provided with microstructures or with porous coatings
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
  • F28D21/00—Heat-exchange apparatus not covered by any of the groups F28D1/00 - F28D20/00
  • F28D2021/0019—Other heat exchangers for particular applications; Heat exchange systems not otherwise provided for
  • F28D2021/0042—Other heat exchangers for particular applications; Heat exchange systems not otherwise provided for for foodstuffs

Definitions

• the invention relates to a device for extending the service life of a tube bundle heat exchanger in indirectly heated UHT systems for foods, in particular for milk or milk products, according to the preamble of claim 1.
• a UHT process with indirect product heating by heat exchange by means of a heat transfer medium on a wall means a thermal product treatment, which is also called aseptic heating, in which almost all microorganisms, but at least all, are used Corruption-causing microorganisms that could grow during the storage phase of the product at room temperature. Accordingly, all microorganisms with the exception of some heat-resistant spores that may survive the heating process must be killed. However, these do not grow above a critical value during normal storage at room temperature. Indirect product heating by means of heat exchange on a wall can take place both with so-called plate heat exchanger systems or, as in the present case, with so-called tube bundle heat exchangers. In the following, the problem is presented consistently on milk or milk products with approximately the same kinematic viscosities v, since these applications represent an essential area of application of the UHT systems of the generic type.
• a UHT heating system with indirect product heating initially contains a preheater for heating the product. Afterwards, the milk in this indirect UHT heating system is mostly passed through a so-called homogenizer. A further heat exchange follows, a so-called preheating for protein stabilization of the milk proteins for the subsequent milk heating process. zess, then the actual UHT heating with keeping hot, then the cooling with heat exchange with the incoming milk and, if necessary, post-cooling. Depending on the respective technology, the homogenization can take place before or after the UHT heating. Water acts as a heat transfer medium, which is circulated and heats the milk in countercurrent at a higher temperature in accordance with the temperature-time profile in the milk flow and also cools it down in counterflow when the milk returns.
• An indirect UHT heating system which has been successfully used in practice many times, realizes both the regenerative heat exchange and the non-regenerative heat exchange in the UHT heater by means of so-called tube bundle heat exchangers (DE-U-94 03 913; principle Tuchenhagen Dairy Systems GmbH, Ahaus), whereby several parallel inner tubes are provided, through which the milk flows, while the heat transfer medium, usually water or steam, flows in counter-current in the annular space (outer channel) of the outer tube (outer jacket), which surrounds the inner tubes connected in parallel.
• a one-pipe system without heat exchange is generally used for the UHT hot holder.
• 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.
• UHT heating systems with shell-and-tube heat exchangers make particular use of this latter effect compared to plate-type 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 that influences the flow boundary layer which is followed by a shorter dwell time of the milk ingredients.
• the heat transfer coefficient k is significantly reduced, and on the other hand the formation of a pressure drop ⁇ p v in the inner pipes, which is inversely proportional to the fifth power of the inner pipe diameter Di increases.
• the deterioration of the heat transfer ultimately means that the heat flows required to ensure a sufficient temperature to kill the microorganisms can no longer be transmitted and that a pressure loss ⁇ pv 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.
• the cleaning of the deposits and the renewed provision of aseptic conditions represent an interruption in operation, which represents a considerable cost factor on the one hand with regard to an interruption in production and on the other hand with regard to the cleaning process itself.
• An essential starting point for extending the tool life is to reduce the rate of build-up.
• the aim should be the complete denaturation of the protein that forms the starting point by appropriate constructive measures.
• the cleaning and sterilization time accounts for about 10 to 15% of the service life, so that there is also a desire and the need to reduce these times absolutely and not only relatively, based on an extended service life.
• cleaning especially using chemical agents
• the germs in topographical shallows require longer exposure times, especially on rolled, annealed, chemically pickled and then no further mechanically treated surfaces.
• the consequential problem now arises that all organic and inorganic contamination substances must be washed away from the surface without leaving any residue.
• a heat transfer tube 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 are at an angle to the longitudinal direction of the heat transfer tube extend.
• These macro-roughness structures are, on the one hand, a multiplicity of mutually parallel main grooves which run at the said angle, have a trapezoidal cross section and whose depth is in the range from 0.15 to 0.35 mm.
• a large number of narrow grooves parallel to one another are provided.
• each of the narrow grooves only grasp the tube in some areas and each of the narrow grooves has a base and two side surfaces and is formed within the main grooves and parallel to them.
• 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.
• the document DE 197 51 405 A1 describes 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 having an angle of inclination that is small 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 alternatingly arranged 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 area of the transition between the flowing medium and the heat exchange surface.
• Said zones can hardly be described as relatively large macro-roughness structures of the pipe, and with regard to the different surface roughness, there is no indication of whether these Surface roughness would favor or inhibit product build-up in UHT heaters and / or downstream UHT heat holders.
• the proposed device according to the invention is part of the so-called tube bundle heat exchanger, which generally consists of a plurality of tube bundles, each of which has a plurality of inner tubes connected in parallel with a common inlet and outlet for a product to be heated.
• the respective group of inner tubes is enclosed by an outer jacket, each of which is provided near the ends with a radially opening or ending connection piece for a heat transfer medium, which has an outer channel delimited by the outer jacket in relation to the inner tubes in counterflow to the pipe flow in one delimited by the inner tubes Flows through the inner channel.
• 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 pipe.
• This is achieved in that the respective inner tube of a UHT heater and subordinate UHT hot holder has at least on the surface of its inner tube wall macro-roughness structures M R , which are 35> ⁇ > 25 degrees at an angle of attack are oriented with respect to the longitudinal axis of the inner tube.
• 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.
• 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 pipe wall structured in this way is treated extensively 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 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 G.
• a micro-surface texture m R is now proposed which changes the energetic rather than the structural texture of the respective stainless steel surface. This is achieved through professional electrochemical polishing (see company brochure HENKEL Beiz- und Elektropoliertechnik, A-3830 Waidhofen / Thaya, "The surface secures the value of the component").
• Comparative quantitative information is given below to illustrate the effectiveness of the proposed measure. This information results from tests on stainless steel tubes with a surface treatment of the aforementioned type, the electrochemically polished as well as the ones not treated in this way being mechanically ground in the initial state.
• longitudinally welded stainless steel tubes are generally used for cost reasons, which are additionally calibrated on the inside due to the continuous axial weld seam, but not, as mentioned above, additionally mechanically ground.
• the test results can therefore not easily be transferred quantitatively to the inner tubes of tube bundle heat exchangers, but these results at least show the qualitative change in the physical properties of the surface of the inner tube wall by electrochemical polishing.
• Electrochemical polishing firstly covers the surface of these tubes with a complete passive layer, which consists of a relatively thick chromium oxide layer (> 2 nm compared to ⁇ 1 nm for the execution with mechanical pre-grinding, without electrochemical polishing).
• the surface is practically stress-free due to the stress-free electrochemical polishing removal and shows a material-specific, specific energy level of about 1.3 N / m (compared to about 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, i.e. the significantly reduced tendency to deposit.
• the erosion caused by the electrochemical polishing process is about 10 to 15 ⁇ m, 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).
• 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.
• the product batches consist of proteins with a size of 1 to 2 ⁇ m, microorganisms> 1, 5 ⁇ m and sugar and salts in the range of 0.7 to 0.8 ⁇ m.
• the aim pursued by the invention namely to extend the service life of the UHT heating systems in question and to shorten the cleaning and sterilization time, can be achieved in a significant manner by the combination of the two measures described above, which achieve their desired effectiveness once through the Macro roughness structures M R outside the laminar ter Mrs and on the other hand by the micro-surface properties m R essentially within the laminar lower layer of the flow boundary layer in the inner tube.
• the invention provides that the swirl, which can be preselected for specific products, has a swirl depth t and a swirl width b.
• the swirl tube in a single-start manner with a pitch H G.
• a further development of the device according to the invention provides that the swirl pipe has multiple threads Number of gears and each with a pitch H G is formed. In this way it is possible to cover the entire surface of the inner tube with the desired macro-roughness structures.
• the critical area of a UHT heating system can be seen in relation to product approaches in the area of UHT heaters and UHT hot holders.
• product approaches can also be seen in other areas of a UHT heating system.
• the inner tubes of the tube bundle heat exchanger upstream and downstream of the UHT heater and UHT hot holder should also be used, provided that they are in a temperature range above 100 degrees Celsius operated, are provided with the macro-roughness structures M R and the micro-surface texture m R according to the invention.
• Embodiments of the device according to the invention are shown in the drawing and are described below. Show it
• FIG. 1 shows a central 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;
• Figure 2. in view a section of an inner tube designed as a five-course swirl tube, as is the case in the tube bundle according to FIG.
• FIG. 4 shows a section of an inner tube designed as a single-start swirl tube, which otherwise has the same dimensional relationships as that according to FIG. 2
• a tube bundle 1 (FIG. 1; see also DE-U-94 03 913) consists in its middle part of an outer jacket 2 delimiting an outer channel 2 'with an outer jacket flange 2a arranged on the left-hand side relative to the illustration position and an outer jacket flange arranged on the right-hand side on the floating bearing side 2 B.
• the latter is followed by a first transverse duct 4a *, delimited by a first housing 4.1, with a first connecting piece 4a, and a second transverse duct 4b *, delimited by a second housing 4.2, with a second connecting piece 4b adjoins the outer casing flange 2a.
• the two housings 4.1, 4.2 are also sealed off from the respectively adjacent outer jacket flange 2b, 2a with a flat gasket 9, the first housing 4.1 arranged on the right-hand side in connection with the outer jacket 2 via an exchanger flange 6 on the floating bearing side, with the interposition of an O-ring 10 against the left-hand side arranged fixed bearing 5, 7, 4.2 is pressed.
• the floating bearing-side tube support plate 8 extends through a bore (not specified in more detail) in the floating bearing-side exchanger flange 6 and is sealed against the latter by means of the dynamically stressed O-ring 10, which also statically seals the first housing 4.1 against the floating-bearing exchanger flange 6.
• the latter and the loose side Pipe support plate 8 form a so-called floating bearing 6, 8, which permits changes in the length of the inner pipes 3, 3 * welded into the pipe support plate 8 on the floating bearing side as a result of temperature change in both axial directions.
• the inner tubes 3, 3 * can be flowed through either from left to right or vice versa by a product P to be heated, the mean flow velocity in Inner tube 3, 3 * and thus in the inner channel 2 'is marked with v.
• the cross-sectional design is usually carried out in such a way that this average flow velocity v is also present in a connecting bend 11, which is connected on the one hand to the fixed-side exchanger flange 5 and, on the other hand, indirectly to a floating-side connecting piece 8d which is firmly connected to the floating bearing-side tube support plate 8.
• the tube bundle 1 in question is connected in series with the adjacent tube bundle. Therefore, the fixed flange-side exchanger flange 5 forms an inlet E for the product P and the floating socket-side connection piece 8d houses an associated outlet A; In the neighboring tube bundle, these entry and exit conditions are reversed accordingly.
• the exchanger flange 5 on the fixed bearing side has a first connection opening 5a, which on the one hand corresponds to a nominal diameter DN and thus a nominal cross section Ao of the connecting bend 11 connected there, and which is dimensioned such that the average flow velocity v in the inner tube 3, 3 * or Flow velocity corresponding to inner channel 3 'is present.
• a second connection opening 8a is dimensioned in the connecting piece 8d on the floating bearing side, the respective connection opening 5a or 8a extending to a respectively enlarged passage cross section 5c or 8c in the area to the adjacent tube support plate 7 or 8 by a conical transition 5b or 8b expanded.
• It has proven to be expedient to increase the mean flow velocity v in the inner pipe 3, 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 that corresponds to the pipe inner diameter Dj d hydr (see also FIG. 2) is calculated, in the range 35,000 ⁇ Re ⁇ 45,000.
• the product P to be treated flows either through the first connection opening 5a or the second connection opening 8a to the tube bundle 1, so that either the fixed-side tube support plate 7 or the loose-side tube support plate 8 flows against becomes. Since in any case a heat exchange between product P in the inner tubes 3, 3 * and a heat transfer medium W in the outer jacket 2 must take place in countercurrent, this heat transfer medium W flows either to the first connection piece 4a or to the second connection piece 4b at a flow rate c to. In the event that the product P flows into the tube bundle 1 via the first connection opening 5a, an inlet temperature of the product ⁇ E would be present here.
• the heat transfer medium W would leave the outer jacket 2 in countercurrent via the second connecting piece 4b with an outlet temperature of the heat transfer medium ⁇ A.
• the temperature difference at the product inlet ⁇ ⁇ a - ⁇ E in the area of the second connection piece 4b, in addition to the aforementioned pressure loss ⁇ p v in the inner tubes 3, 3 *, provides a reliable indicator of the degree of product build-up in the inner tubes 3, 3 *.
• the proposed device according to the invention is reflected in the design of the surface of the inner tube wall 3a of the respective inner tubes 3, 3 *, the inner tube 3, 3 * in question in each case, which has the outer tube diameter D a , in the form of a so-called swirl tube 3 * is formed (see also Figures 2 to 5).
• a so-called swirl 3a * which is defined by a swirl depth t and a swirl width b (FIG.
• the desired macro-roughness structure M R which is formed via the laminar lower layer of the boundary layer within the pipe flow in the inner pipe 3, 3 * increases and ensures the increased turbulence and the desired impulse exchange.
• the surface of the inner tube wall 3a of the swirl tube 3 * which is structured by the macro-roughness structure M R is also treated extensively by means of an electrochemical polishing process which produces a micro-surface texture m R which is structurally and energetically characterized by a reduced inclination for distinguishes the attachment of foreign substances.
• the professional electrochemical polishing process is generally applied to simply standardized inner surface designs of the inner pipe 3, 3 *, which is designed as a stainless steel pipe, the stainless steel preferably being austenitic chromium-nickel steel alloys.
• the stainless steel tube supplied to the electrochemical polishing is generally a longitudinal Seam-welded and, because of this longitudinal seam, calibrated and then brightly pickled pipes.
• the starting sheet for pipe production was generally cold rolled, annealed and chemically pickled.
• the processing of the tubes in the tube bundle 1 is advantageously carried out after the electrochemical polishing; there is no mechanical reworking of the circular welds.
• the average roughness value for the surface is R a ⁇ (0.7 to 0.8) ⁇ m and in the area of the longitudinal weld seam is R a ⁇ 1.2 ⁇ m.
• 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, in comparison 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).
• a cross-twisted swirl tube 3 * can also be advantageous if the swirl angle ⁇ is between 55 and 65 degrees.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Crystallography & Structural Chemistry (AREA)
• Geometry (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat-Exchange Devices With Radiators And Conduit Assemblies (AREA)
• Food Preservation Except Freezing, Refrigeration, And Drying (AREA)
• Greenhouses (AREA)
• Confectionery (AREA)

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Cited By (7)

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Citations (8)

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• 2002
  • 2002-12-02 DE DE10256232A patent/DE10256232B4/de not_active Expired - Lifetime
• 2003
  • 2003-11-22 AT AT03780029T patent/ATE334371T1/de active
  • 2003-11-22 ES ES03780029T patent/ES2268454T5/es not_active Expired - Lifetime
  • 2003-11-22 DK DK03780029.9T patent/DK1567818T4/da active
  • 2003-11-22 EP EP03780029.9A patent/EP1567818B2/de not_active Expired - Lifetime
  • 2003-11-22 DE DE50304393T patent/DE50304393D1/de not_active Expired - Lifetime
  • 2003-11-22 WO PCT/EP2003/013131 patent/WO2004051174A1/de active IP Right Grant

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