DE10256232A1 - Method and 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

The invention relates to a method for extension 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 and one Device for performing the method according to the preamble of subsidiary claim 4.

Under a UHT process (UHT: Ultra High Temperature) with indirect product heating through 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, 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 for UHT systems of the generic type represent.

A UHT heating system with indirect Product heating initially includes 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, with up to 90% of the energy used be recovered can. 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-U-94 03 913 ; 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, flows in counterflow in the annular space of the jacket tube, which surrounds the inner tubes connected in parallel , 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. UHT heating systems with shell-and-tube heat exchangers take advantage of this last-mentioned effect particularly in comparison 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, which influences the flow boundary layer, from which one shorter dwell time of the milk ingredients follows. The Reynolds number Re is a flow state (degree of laminarity or turbulence) characteristic dimensionless parameter; it is calculated from the mean flow velocity ν in the inner tube, its hydraulic (equivalent) inner diameter d _hydr and the kinematic _viscosity ν of the product according to the relationship Re = ν d _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 ν in the inner tube of ν = 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 _ν in the inner pipes, which is inversely proportional to the fifth power of the inner pipe diameter D; is, 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 there is also a pressure loss Δp _ν 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.

UHT heating system operating times of up to 20 hours, which is also characterized by the term service life are 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.

Regardless of the respective cleaning process provides the cleaning of the deposits and the re-provision aseptic conditions is a business interruption that a considerable cost factor on the one hand with a view to an interruption production and on the other hand with a view to the cleaning process represents 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.

For this purpose, priority is given to one preferably full Denaturation of potentially forming protein by appropriate approach constructive measures 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.

On the other hand, the cleaning and sterilization time about 10 to 15% of the service life, so in this regard too there is a desire and a need to have these times absolutely and not only to be reduced relatively, based on an extended service life. When cleaning, especially one using chemical agents, however, it should be noted that the germs in topographic Shoals (roughness depths) especially for rolled, annealed, chemical 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.

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

SUMMARY OF THE INVENTION

This task is accomplished through a process solved with the features of claim 1. Advantageous configurations the procedure are the subject of the subclaims. An apparatus for performing the method according to the invention is given by the features in the independent claim 4. advantageous embodiments the proposed device are the subject of the dependent claims.

The procedural 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 by relatively large, on the surface of the inner tube wall of the inner tube effective macro-roughness structures, by which the aforementioned turbulence is generated. 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. 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 tubes for use in heat exchangers"; special print from PROCESS, 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 variable that cannot convey a real idea of the effective surface shape and other properties.

The prevention of the formation 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.

Simply 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, there is now 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 illustrate the effectiveness of proposed action comparative quantitative information is given below. This Information results 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.

Through electrochemical polishing becomes the surface one of these tubes is covered with a complete passive layer, this consists of a relatively thick chromium oxide layer (> 2 nm compared to <1 nm when using mechanical pre-grinding, without electrochemical polishing). Second is the surface due to the stress-free electrochemical polishing removal practical stress-free and shows a material-specific, specific energy level of about 1.3 N / m (compared approx. 2.2 N / m with mechanical pre-grinding). Passivation and reduction of the energy level primarily result in the reduced inclination for the Attachment of foreign substances, i.e. which significantly decreased Covering inclination.

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 Mechanical pre-sanding surface 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 following table shows the The above facts using the example of germ growth on stainless Stainless steel 1.4301 depending the processing method of the surface and the time.

Tabelle

table

The aim pursued by the invention, namely extend the service life of the UHT heating systems in question and Shortening the cleaning and sterilization time is significantly more important Way by combining the two described above activities 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, the service life of the UHT system can be further increased by an additional procedural measure, namely by significantly increasing the Reynolds number compared to previously UHT heating systems practiced relevant parameters. In this context, it is proposed that the Reynolds number Re of the tube flow in the inner tube is in the range 35,000 Re Re 45 45,000. 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 ν in the inner tube is in the range from ν = 2.9 to 3.75 m / s lies.

As stated above, is the critical area of a UHT heating system in terms of product approaches in the area the UHT heater and UHT hot holder 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, insofar as 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.

The device for performing the The process refers to the so-called tube bundle heat exchanger, which in the Usually consist of a large number of so-called tube bundles, each of which several inner tubes connected in parallel with a common entrance and a common exit for the product has. The respective group of inner tubes is enclosed by a tubular casing, each with the ends near a radially opening or ending 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 measure now consists in the device that the respective inner tube of a UHT heater and subordinate UHT hot holder has at least on the surface of its inner tube macro-roughness structures M _R , which are at an angle of attack 35 ≥ α ≥ 25 degrees to the longitudinal axis of the Are oriented, and that the surface of the inner wall of the tube 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.

Another proposal is the device according to the invention with a turbulent flow in the Operate inner tube whose Reynolds number Re is in the range 35,000 ≤ Re ≤ 45,000. This degree of turbulence ensures that impulses are exchanged and shear forces in the area of the flow boundary layer close to the wall, which further reduces the tendency to form a product batch and thereby the service life of the tube bundle heat exchanger further increased.

It is known from heat transfer technology the 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 turbulence increase is not in the thermal and hydraulic boundary layer, however without further ado quantitatively to the problem of the formation of a product approach transfer in the inner tube of UHT heating systems. 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).

Embodiments of the device to perform of the method 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 in view a section of a swirl tube in a third embodiment, the Dral cross-twisted at a twist angle of δ = 60 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 _ν

pressure drop

t

twist depth

ν

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

R _e

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

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

DETAILED DESCRIPTION

A tube bundle 1 ( 1 ; see also DE-U-94 03 913 ) 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 on the fixed pipe-side casing pipe flange 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 over a not Specified 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 bearing flange 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 wiring, the inner tubes 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 ν. The fixed flange exchanger flange 5 has a first connection opening 5a which corresponds on the one hand 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 ν 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 ν 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 the flow velocity ν 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 _ν 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 method according to the invention and the device for its implementation are reflected in the design 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 outside diameter D _a and an inside diameter D _i 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 the 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, which creates a micro-surface texture m _R , which 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 sections of be advantageous if the swirl angle δ is between 55 and 65 degrees.

Claims (10)

Process for extending the service life of tube bundle heat exchangers in indirectly heated UHT systems for food, in particular for milk or milk products, in which the product (P) to be heated is in tube bundles ( 1 ) each with several inner tubes connected in parallel ( 3 ) is guided and the heat transfer medium (W) in jacket pipes ( 2 ) which the respective groups of inner tubes ( 3 ) each enclose together, the outside of the inner tubes ( 3 ) acted upon in counterflow, characterized in that - an increase in turbulence in the thermal and hydraulic boundary layer of the pipe flow in the respective inner pipe ( 3 ) of a UHT heater and downstream UHT hot holder is provided by relatively large, on the surface of the inner tube wall ( 3a ) of the inner tube ( 3 ) effective macro-roughness structures (M _R ), - and that the surface of the pipe inner wall structured in this way ( 3a ) is provided with a micro-surface texture (m _R ) throughout, which is structurally and energetically characterized by a reduced tendency for the adhesion of foreign substances. A method 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. A method according to claim 1 or 2, characterized in that the inner tubes ( 3 ) the tube bundle heat exchanger of the UHT system upstream and downstream of the UHT heater and UHT hot holder, provided they are operated in a temperature range above 100 degrees Celsius, with the macro roughness structures (M _R ) and the micro surfaces -Condition (m _R ) are provided. Device for performing the method according to one of claims 1 to 3, 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 (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 ) for a heat transfer medium (W), characterized in that - the respective inner tube ( 3 ) of a UHT heater and subordinate UHT hot holder at least on the surface of its inner tube wall ( 3a ) Has macro-roughness structures (M _R ) which are at an angle of attack 35 ≥ α ≥ 25 degrees with respect to the longitudinal axis of the inner tube ( 3 ) are oriented, and - that the surface of the inner pipe wall structured in this way ( 3a ) is treated across the board by means of an electrochemical polishing process that produces a micro-surface texture (m _R ) that is structurally and energetically characterized by a reduced tendency for the adhesion of foreign substances. Apparatus according to claim 4, 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. Apparatus according to claim 4 or 5, 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. Apparatus according to claim 6, characterized in that the swirl ( 3a * ), can be selected for specific products, has a swirl depth (t) and a swirl width (b). Apparatus according to claim 6 or 7, characterized in that the swirl tube ( 3 * ) is catchy with a pitch (H _G ). Apparatus according to claim 6 or 7, 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 9, characterized in that the swirl width (b) results from the pitch (H _G ) divided by the number of gears (g) (b = H _G / g).