Patent Publication Number: US-2019191729-A1

Title: Method And Assembly For Aseptically Heating A Liquid Product In A Heat Exchanger Unit Of The Heater Zone Of A UHT System

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to German Patent Application No. 10201700298.1, filed Mar. 28, 2017 and German Patent Application No. 102016010099.0, filed Aug. 24, 2016, each of which is herein incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The invention relates to a method and an assembly for aseptically heating a liquid product in a heat exchanger unit of the heater zone of a UHT system in which an indirect heat exchange on a wall takes place in the heat exchanger unit between the liquid product and a heating medium by a heating medium flow in a heat-releasing heating medium chamber being guided countercurrent to a product flow passing through a heat-absorbing product chamber, with the product flow being heated from a product input temperature to a product output temperature and at least the product output temperature and the heating medium inlet temperature being monitored and regulated. The invention further relates to a heat exchanger unit for such an arrangement. 
     The liquid products subjected to the heat treatment under discussion can, for example, be not only milk products but also temperature-sensitive food products, in particular desserts or dessert-like products with the entire range of possible viscosities. The invention displays its intended effect in a particularly significant way in the pasteurization zone of a UHT system. There is generally a treatment zone upstream of the heat exchanger unit, such as a preheater zone, or downstream, such as a heat maintenance or cooling zone. 
     BACKGROUND 
     A UHT (ultra-high temperature) process carried out with the UHT system initially mentioned with indirect product heating by heat exchange on a wall using a heat transfer medium or heating medium is understood to be a thermal product treatment, also referred to as aseptic heating, in which virtually all microorganisms are killed, or at least all microorganisms which lead to spoilage, which can propagate at ambient temperature during storage. 
     Indirect product heating above 100° C. is carried out in a particularly advantageous manner by heat exchange on a wall with tubular heat exchangers, in particular a so-called shell-and-tube heat exchanger. In the latter, the heat energy is transmitted by the tube walls of a group of parallel interior tubes which are preferably oriented horizontally. Here the liquid product to be treated flows in the interior tubes while a heating medium, generally water heated by steam, flows countercurrent in the annular gap space of a jacket tube which surrounds the interior tubes connected in parallel. A shell-and-tube heat exchanger in this regard is known from DE 94 03 913 U1. Indirect product heating of the aforementioned type can also take place with other heat exchanger designs, such as plate heat exchangers. 
     A known UHT heating device with indirect product heating for producing a UHT milk (DE 10 2005 007 557 A1) contains a preheater in a so-called pre-warming zone for heating the standardized milk. Then the milk is passed through a so-called homogenizer to disperse fat and is then preheated further afterward. So-called maintenance of preheating follows to stabilize the milk proteins. After a further heat exchanger, which is generally run “regeneratively” and is provided for the subsequent milk heating process, the actual UHT heating then takes place in a so-called heater zone with the product kept hot, followed by cooling in a cooling zone with heat exchange using a “regenerative” heat transfer medium, usually water. A “regenerative” heat transfer medium with which a “regeneratively” conducted heat exchange is carried out is understood to be a heat transfer medium which is run in a circuit and, with reference to the direction of flow of the liquid product to be treated, absorbs heat energy from the product in areas of high temperature and “regeneratively” transfers it to the product in areas of low temperature. 
     Regenerative heat exchange of the aforementioned type is also to be included by the present invention, even if the description below is limited to a heating medium that is not liquid product. The aseptic heating of the liquid product under discussion is effected in a heat exchanger unit of the heater zone of a UHT system, which in particular can also include a pasteurization zone, by a heating medium, such as water heated by steam, which necessarily has a heating medium inlet temperature above that of the product output temperature from the heat exchanger unit characteristic of the aseptic heating process. The aseptic heating is product-specific and takes place in the following exemplary embodiment between a product input temperature T PE =125° C. and a product output temperature T PA =140° C., with the walls of the heat exchanger unit in contact with the product, through which the indirect exchange of heat takes place, needing to have a higher temperature in the assigned temperature curve to ensure the necessary driving forces for the transfer of heat between the walls and liquid product as well as the required efficiency for the heat exchange. Such high wall temperatures pose difficulties, as will be described below. 
     In the heating method under discussion, more or less heavy deposits occur, particularly in the heater section and in the downstream heat maintenance section, which is not heated externally, or in the heat retention unit. Heat sensitive or temperature sensitive liquid products—with this generic term to be understood particularly as a liquid food product below—can contain a relatively large number of proteins, a lot of dry mass and little water, and their viscosities can cover the entire possible range. Liquid products in this regard, preferably at temperatures above 100° C., tend to scorch, i.e., tend to form deposits on the walls of the heat exchanger unit under these conditions. This deposit formation is also referred to as product “fouling” and can lead to quality problems in the heated liquid product, an end product or an intermediate product and/or to serious cleaning problems. The latter require intensive cleaning cycles and thus reduced operation times for the heat exchanger unit. Thus product fouling reduces the service life and operation time respectively of the heat exchanger unit between two cleaning cycles and is undesirable. 
     In any case, an effort must be made to ensure that, on one hand, all simultaneously flowing portions of the liquid product to be treated and, on the other hand, all sequential portions are subject as much as possible to the same residence time, particularly in the pasteurization section and the heat maintenance unit, because different dwell times—and thus different treatment times—can work disadvantageously in the manner described above, particularly at high treatment temperatures. The formation of deposits on the hot walls during UHT heating can be significantly reduced by regulated heating of the liquid products in the pre-warming zone with a specific dwell time. Therefore it is important that the process unit situated upstream of the heat exchanger unit provides liquid product to be heated aseptically so that it has the required product input temperature T PE . 
     The present problem and the disadvantages of prior art in this regard are to be made clear below based on a known heat exchanger unit and a known method which can be carried out with it, which comprise the starting point of the present invention. It is shown in  FIGS. 1 and 2 . 
     A detail from an arrangement  10  according to the prior art, which is designed in its un-displayed entirety in  FIG. 1  as a UHT system for aseptic heating of a liquid product P, has in its heater or pasteurization zone at least one heat exchanger unit  22 , for which, seen in the direction of flow of the liquid product P, there is an upstream process unit  21 , for example a heat exchanger of a preheater zone, and a downstream process unit  23 , such as a heat maintenance section in the form of a heat retention unit. The schematic representation of the heat exchanger unit  22  can be for a tubular heat exchanger, preferably a so-called shell-and-tube heat exchanger, or for another design as well, with each of these embodiments being able to be subdivided into multiple sequentially connected sections. It is of decisive importance that a total heat exchanger path L be formed between a product input E P  and a product output A P  of a heat-absorbing product chamber  22 . 1 , through which a product flow F P  of the liquid product P passes from right to left with reference to the position in the drawing. The product flow F P  enters the product input E P  with a product input temperature T PE  and exits the product output A P  with a product output temperature T PA . 
     The product chamber  22 . 1  is in an indirect heat exchange with a heat-releasing heating medium chamber  22 . 2 , through which a heating medium flow F M  of a heating medium M passes countercurrent to the product flow F P  between a heating medium inlet E M  and a heating medium outlet A M . The heating medium flow F M  enters the heating medium inlet E M  with a heating medium inlet temperature T ME  and exits the heating medium outlet A M  with a heating medium outlet temperature T MA . A heat flow Q is exchanged between the heating medium chamber  22 . 2  and the product chamber  22 . 1 . The factors indicated above which include “flow” are to be understood as time-related physical parameters, specifically mass/time (kilograms/second, kg/s) or volume/time (liters/second, dm 3 /s) and heat quantity/time (joules/second, J/s=W). 
     A measuring apparatus for product flow  26  measures the product flow F P , a measuring apparatus for product input temperature  28 . 1  measures the product input temperature T PE  and a measuring apparatus for product output temperature  28 . 2  measures the product output temperature T PA , a measuring apparatus for heating medium flow  29  measures the heating medium flow F M  and a measuring apparatus for heating medium inlet temperature  30 . 1  measures the heating medium inlet temperature T ME . 
     The measurement variables F P , T PE , T PA , F M  and T ME  listed above are transmitted to a control and feedback unit  24 , which provides a control signal for a target medium inlet temperature T ME * on an output for target heating medium inlet temperature  31 . 1  and a control signal for a target heating medium flow F M * on an output for target heating medium flow  31 . 2 , with both control signals being in effect for the heating medium M at the heating medium inlet E M . 
     The temperature curves T P (I x ) and T M (I x ) shown in  FIG. 2  are observed in practice via the operation time of a heat exchanger unit  22  of the type under discussion, with the operation time generally corresponding to the so-called service time between two necessary cleaning cycles. Assigned product temperatures T P  and assigned heating medium temperatures T M  (both assigned to the Y-axis, for example in degrees Celsius (° C.)) are plotted versus a variable heat exchanger path I x  (X-axis). The variable heat exchanger path I x  has its origin (I x =0) at the product input E P , and it ends at the product output A P  after completing the entire heat exchanger path L (I x =L). 
     The product-specific temperature curve of the specified, heat-absorbing product flow F P  to be treated between the product input temperature T PE  (for example, 125° C.) provided by the upstream process unit  21  and the product output temperature T PA  (for example, 140° C.) necessary to ensure sufficient aseptic heating is designated by T P (I x ). T M (I x ) is the designation for two heating medium-specific temperature curves of the heat-releasing heating medium flow F M . The lower temperature curve, with reference to the position in the drawing, between a first heating medium inlet temperature T ME ( 1 ) (for example, 140.9° C.) and a first heating medium outlet temperature T MA ( 1 ) (for example, 130.6° C.) is at the beginning of the operation time if the heat exchanger unit  22  is still free of any deposits (product fouling) on the product side. 
     The temperature difference T ME ( 1 )−T MA ( 1 ) results from the following balance equation (1): 
         =F P   c   P ( T   PA   −T   PE ) =F   M   c   M ( T   ME (1)− T   MA (1)) =AkΔT   m ,   (1)
 
     where A is an entire heat exchange surface of the heat exchanger unit  22 , K is a heat transfer coefficient (see  FIG. 1 ), ΔT m  is the average logarithmic temperature difference (see  FIG. 2 ), c P  is a specific heat capacity of the liquid product P and c M  is a specific heat capacity of the heating medium M. 
     For the countercurrent at the start of the operation time (label ( 1 ) in  FIG. 2 ) and the average logarithmic temperature difference ΔT M  contained in equation (1), a first average logarithmic temperature difference ΔT M ( 1 ) applies according to equation (2.1) with the first heating medium inlet temperature T ME ( 1 ) and the first heating medium outlet temperature T MA ( 1 ), with the last term of equation (2.1) and the usual abbreviations ΔT large ( 1 ) and ΔT small ( 1 ) for the respective temperature differences on the end side resulting as follows: 
     
       
         
           
             
               
                 
                   
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     In the course of the operation time, the deposits increase continuously on the product side and the heat transfer coefficient k is likewise continuously reduced by this. Then the temperature differences between the liquid product P and the heating medium M provided at the beginning of the operation time no longer suffice to transfer the necessary heat flow Q for heating the product flow F P  to the necessary product output temperature T PA . At the end of the operation time, after 12 hours for example, the control and feedback unit  24  has increased the heating medium inlet temperature T ME  so much that a second heating medium inlet temperature T ME ( 2 ) (for example, 144.5° C.) is now necessary at the heating medium inlet E M , from which, according to equation (1), a second heating medium outlet temperature T MA ( 2 ) (for example, 134.2° C.) results. 
     For the end of the operation time (label ( 2 ) in  FIG. 2 ), analogous to equation (2.1), according to equation (2.2), a second average logarithmic temperature difference ΔT M ( 2 ) results with the second heating medium inlet temperature T ME ( 2 ) and the second heating medium outlet temperature T MA ( 2 ) and the abbreviations introduced above and adapted correspondingly (ΔT large ( 2 ), ΔT small ( 2 )): 
     
       
         
           
             
               
                 
                   
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     The second heating medium outlet temperature T MA ( 2 ) necessarily set at the heating medium outlet A M  is, as can be derived from equation (1) with corresponding values, substantially dependent on a second mass flow ratio f( 2 ), formed as a quotient of a second heating medium flow F M ( 2 ) divided by the product flow F P  on one hand (f( 2 )=F M ( 2 )/F P ; generally: f=F M /F P ), the respective specific heat capacities c M  of the heating medium M and c P  of the liquid product P as well as from the heat transfer conditions (characterized by the heat transfer coefficient k) also influenced by the growing deposits on the walls of the heat exchanger unit  22  on which the heat exchange takes place. In the present case, to ensure that the necessary product output temperature T PA  is achieved under all operating conditions and this is therefore also applicable to the lower heating medium-specific temperature curve set at the beginning of the operation time with a first heating medium flow F M ( 1 ), i.e., with f( 1 )=F M ( 1 )/F P  during the entire operation time the heat exchanger unit  22  is operated with a constant mass flow ratio f, in which the heating medium flow F M  exceeds the product flow F P  by almost 50% (f=f( 1 )=f( 2 )=1.43=constant; see  FIG. 2 ). 
     A further increase of the heating medium inlet temperature T ME  is no longer possible, because the heater power cannot be or is not permitted to be increased further via the heating medium M and/or because the pressure drop due to accumulated deposits on the product side exceeds a permitted extent. 
     The deposits accumulated during the operation time can also be recognized by the specialist from the average logarithmic temperature difference ΔT M , which, according to equation (1), is required in order to transfer the heat flow Q in the respective load condition of the heat exchanger unit  22  with these deposits. In the present case, at the beginning of the operation time, the first average logarithmic temperature difference ΔT M ( 1 ) is 2.6° C. and at the end of the operation time the second average logarithmic temperature difference ΔT M ( 2 ) is 6.6° C. 
     As  FIG. 1  shows, in the heat exchanger unit  22  in the arrangement  10  according to prior art necessary measurement is performed for the product input temperature T PE , the product output temperature T PA , the heating medium inlet temperature T ME  and the product flow F P  and heating medium flow F M  at the respective assigned product input E P  and/or product output A P  and/or heating medium inlet E M  and used for control and/or regulation. The temperatures T P  of the product flow F P  inside the heat-absorbing product chamber  22 . 1  and temperatures T M  of the heating medium flow F M  inside the heat-releasing heating medium chamber  22 . 2  and in its direction of extension are not recorded, so the actual temperature curves are not known in the course of the operation time, with the exception of the previously mentioned marginal temperatures T PA , T PE  and T ME . 
     A product-specific temperature limit curve of the product flow F P —designated in  FIG. 2  as T P (I x )′—is theoretical in nature with respect to its linear plot between the product input temperature T PE  and the product output temperature T PA , just like a linear temperature curve in the heating medium flow F M , which is not shown. These linear plots would only occur if the specific heat capacities c P  and c M  of the product P and heating medium M respectively and the physical parameters determining the heat throughput, indicated by the heat transfer coefficient k, were independent of temperature and a quantitatively and qualitatively uniform deposit formation were to occur over the entire heat exchange surface A, which is not the case in practice. Nevertheless, as part of the influencing parameters available, it is worth the effort to bring the actual product-specific temperature curve T P (I x ) and the heating medium-specific temperature curve T M (I x ) as close as possible to the respective linear temperature limit curve, because the quantitative heat exchange over the entire heat exchanger path L is more uniform by doing so. 
     Based on the case example of  FIG. 2  and the underlying design data for it, measurement of the product flow F P  shows that after a discrete heat exchanger path I x =I x1 , already about at the beginning of the last third of the entire heat exchanger path L, there is a discrete temperature of the liquid product T P (I x1 ) which nearly corresponds to the product output temperature T PA  first required at the product output A P . This circumstance is not easily anticipatable without discrete measurement during the dimensioning of the heat exchanger unit  22  and in the definition of the operating data, particularly as the heat exchanger unit  22  is operated with the previously mentioned mass flow ratio f=1.43 for safety reasons. Furthermore, the greatest variety of liquid products P with the most diverse formulations are heat treated in an arrangement  10  of the type under discussion, with the most diverse raw material requirements, viscosities, quality criteria and production conditions to be considered. It is to be assumed that the aforementioned circumstance, which in the final result means that the heat exchanger unit  22  is either over-dimensioned or is not operated in an optimal manner, is no isolated case under the boundary conditions mentioned which are to be met. 
     Even further disadvantages are apparent from the case example described, which concern an undefined residence time of the liquid product P at the level of the product output temperature T PA  and the degree and distribution of the deposit formation in each case in the heat exchanger unit  22 . 
     It is known that the tendency to form deposits and the rate of deposit buildup are significantly influenced not only by the level of temperature for the heat-releasing wall itself but also decisively by the difference between the wall temperature and the temperature of the liquid product P at this point. In the case example shown by  FIG. 2 , at the product output A P  and heating medium inlet E M  respectively the product temperature T P  and thus necessarily also the heating medium temperature T M  are at their highest in each case, while at the beginning of the operation time the necessary temperature difference between the first heating medium inlet temperature T ME ( 1 ) and the product input temperature T PE  is kept as small as possible (in the case example, T ME ( 1 )−T PA =ΔT small ( 1 )=0.9° C.). 
     At the product input E P  and heating medium outlet A M  respectively, the product input temperature T PE  and thus also the heating medium outlet temperature T MA  is naturally at the lowest, while at the beginning of the operation time according to equation (1) the change of the heating medium temperature T M  between the heating medium inlet E M  and the outlet A M  does not depend only on the necessary temperature difference between the product output temperature T PA  and the product input temperature T PE , but also on the mass flow ratio f=f( 1 )=1.43. In the case example, T MA ( 1 )−T PE =ΔT large ( 1 )=5.6° C. The temperature difference T M −T P  thus increases continuously in the clean heat exchanger unit  22  not fouled with product from the product output A P  with ΔT small ( 1 )=0.9° C. to the product input E P  with ΔT large ( 1 )=5.6° C. (by a factor of 6.2), which, in the course of the operation time, leads to further deposit growth due to the the deposit formation caused by the product-specific temperature curve T P (I x ), said growth being approximately proportional to the temperature difference T M −T P  and magnified by a factor of 6.2 at the start of the operation time. 
     At the end of the operation time in the case example, the result is T ME ( 2 )−T PA =ΔT small ( 2 )=4.5° C. and T MA ( 2 )−T PE =ΔT large ( 2 )=9.2° C. (about a factor of 2). Seen altogether, in the course of the operation time one finds that the deposit grows continuously everywhere on the entire heat exchanger path L, with the deposit thickness increasing from the product output A P  to the product input E P  because the temperature differences between the heating medium M and thus between the wall and liquid product P increase in this direction and at all times. The deposit has a significant influence on the heat transfer on the product side and thus on the heat transfer coefficient k. Since the heat exchanger unit  22  is operated during the entire operation time with a constant mass flow ratio f=f( 1 )=f( 2 )=1.43, the increase of the temperature difference from ΔT small ( 1 )=0.9° C. to ΔT small ( 2 )=4.5° C. at the product output A P  and from ΔT large ( 1 )=5.6° C. to ΔT large ( 2 )=9.2° C. at the product input E P  can only be explained by the change in the thickness of the deposit on one hand and by the increase of the deposit thickness toward the product input E P  on the other hand and thus essentially by the change of the heat transfer coefficient k as a function of the heat exchanger path I x  and of the operation time. 
     With the current manner of operation for the heat exchanger unit  22 , the disadvantages to be found are summarized as follows. 
     After starting the clean heat exchanger unit  22  and setting a stationary product input temperature T PE  and a stationary product output temperature T PA  it is not recognizable whether the latter is already reached prior to the product output A P  and thus whether the heat exchanger unit  22  is operated in an optimal manner. 
     For the case that the product output temperature T PA  or one which deviates only slightly from it is reached prior to the product output A P , for example at the position I x =I x1 &lt;L, the remaining section L−I x1  acts as a heat maintenance section in the heat exchanger unit  22 , and the liquid product P already experiences an undefined and undesired residence time from this, which can adversely affect its quality. 
     The high mass flow ratio f=(f( 1 )=f( 2 )=constant, which remains the same throughout the entire operation time, results in acceptance of uneconomical operation, at least in the first part of the operation time. 
     The significant increase of the temperature difference over the entire heat exchanger path L from ΔT small ( 1 ) to ΔT large ( 1 ) (a factor of 6.2) at the start of the operation time and then until the end of the operation time again from ΔT small ( 2 ) to ΔT large ( 2 ) (about a factor of 2), considered over the entire heat exchange surface A, results altogether in a load quantity from product fouling which is generally, and particularly in the area of the product input E P , larger than it would be if the preceding increase of the temperature differences, which is fundamentally and as a tendency to be tolerated throughout the operation time, were smaller. 
     The load mass which collects determines the service time of the heat exchanger unit  22  in the pasteurization zone of the arrangement  10 , i.e., the possible operation time as a time between two necessary cleaning cycles. The deposit formation observed with the manner of operating the heat exchanger unit  22  up to now and the resulting load size by mass and distribution lead to a reduction of the service time. 
     WO 2014/191062 A1 describes a method for determining the degree of heat treatment for a liquid product in a processing system for liquid products in which this known method preferably refers to the pasteurization of these liquid products in the temperature range from 10 to 100° C. and contains no indication of transfer to a heating or pasteurization in UHT processes. The determined degree of heat treatment is a so-called heat treatment index value comparable with so-called pasteurizing units, which the specialist can determine from a generally known mathematical relationship for the respective liquid product into which the temperatures imposed on the liquid product in particular time segments in the course of its heat treatment are input. 
     It is a task of the present invention to create a method of the generic type and an arrangement for carrying out the method and a heat exchanger unit for this arrangement, which, in the treatment of liquid products, particularly temperature sensitive food products of the type initially mentioned, can altogether reduce the product fouling in the areas adjacent to the product input of the heat exchanger unit and beyond and thus significantly extend the service time of the heat exchanger unit. 
     BRIEF SUMMARY 
     In terms of technical method, this disclosure starts from a method for aseptic heating of a liquid product, such as temperature sensitive food products, in particular milk products, desserts or dessert-like products, with the entire range of possible viscosities, in a heat exchanger unit of the heater or pasteurization zone of an arrangement in a UHT system. An indirect heat exchange on a wall takes place here in the heat exchanger unit between the liquid product and a heating medium by a heating medium flow running in a heat-releasing heating medium chamber countercurrent to a product flow running in a heat-absorbing product chamber. The product flow is heated from a product input temperature to a product output temperature, with at least the product output temperature and the heating medium inlet temperature being monitored and regulated in the process, with the product input temperature also being monitored as part of a particularly secure process control and possibly being regulated by a process unit upstream of the heat exchanger unit. 
     An object underlying the invention is solved according to a first method if, in the method of the generic type, the following method steps (A 1 ), (B 1 ), (C), (D 1 ), (E), and (F) are provided. An object underlying the invention is solved according to a second method if, in the method of the generic type, the following method steps (A 2 ), (B 2 ), (C), (D 2 ), (E), and (F) are provided. 
     The basic idea herein consists for both methods of the necessity of solving the object at hand by ensuring an optimal product-specific and an optimal heating medium-specific temperature curve throughout the entire operation time of the heat exchanger unit, and that this can only succeed if at least information is available about the temperature of the product flow at least in an area upstream of the product output, said information enabling suitable control and regulation of the heating medium flow. With this information and the proposed method steps, product fouling is reduced in the heat exchanger unit on the whole and particularly in the regions adjacent to the product output. 
     The first method includes: (A 1 ) setting an unknown product-specific temperature curve between the product input temperature and the product output temperature with the aid of a supply of the required heating medium flow with the required heating medium inlet temperature into the heating medium chamber at a heating medium inlet. This setting is accompanied by measuring discrete product temperatures at specified measurement points in the product flow, with at least the product output temperature and usually also the product input temperature being recorded via further specified measurement points. The product-specific temperature curve resulting from these measurements is provided for further processing according to method step (D 1 ). 
     The method step (A 1 ) is applied if no adequate empirical values are available for the liquid product and only the endpoints of the temperature curve, particularly the product input temperature and product output temperature, are necessarily specified. The method step of adding the heating medium flow with the heating medium inlet temperature is to be understood such that at first minimum values are chosen for both the heating medium inlet temperature and the heating medium flow, with which is precisely guaranteed to reach and maintain the product output temperature and product input temperature. Consequently, operation is not carried out as previously with a high mass flow ratio (=heating medium flow/product flow) throughout the entire operation time for safety reasons, with the ratio being sufficient in magnitude for the end of the operation time, but instead with a significantly smaller ratio. 
     The first method includes: (B 1 ) specifying the product input temperature at a product input into the product chamber and the product output temperature at a product output from it and providing the heating medium inlet temperature and heating medium flow. 
     The method steps of specifying and providing are to be understood such that these quantitative instructions are saved in a control and feedback unit in conjunction with the necessary control algorithms. 
     The first method includes: (C) measuring a product-specific temperature curve between the product output and the product input at the specified measurement points. 
     This method step is to be understood such that in the course of the operation time, if the formation of deposits increases, and in fact after setting the unknown product-specific temperature curve according to method step (A 1 ), the product-specific temperature curve is measured in each case and provided for further processing according to the subsequent method step (D 1 ). 
     The first method includes: (D 1 ) comparing the temperature curves for method steps (A 1 ) and (C) and calculating temperature deviations at the specified measurement points. 
     This method step provides, as a consequence of the growing deposit, possible changes to the product-specific temperature curve upward or downward, expressed by the respective temperature deviation determined, where a “drop” of the product output temperature by 3° C., for example, can mean that the liquid product is no longer aseptic when it leaves the heat exchanger unit. The temperature deviation determined can be positive or negative. 
     The first method includes: (E) specifying a permitted temperature deviation. 
     This specification is dependent on the liquid product and the respective formulation and is saved in the control and feedback unit for further processing. Due to the possible positive or negative temperature deviation, it is positive and negative and may differ in amount. 
     The first method includes: (F) changing of the heating medium inlet temperature to a target heating medium inlet temperature when the permitted temperature deviation is exceeded by the calculated temperature deviation. 
     The method step of changing is to be understood such that when the permitted temperature deviation is exceeded on the high or low side, an instruction or algorithm is stored in the control and feedback unit, according to which at first only the target heating medium inlet temperature is changed with which the product-specific temperature profile is brought back into the range of the permitted temperature deviation. The corresponding magnitudes of the deviations in question are compared with one another for this purpose. 
     In the second method, the explanation can be limited to the method steps (A 2 ) and (B 2 ), because the further method steps (C), (D 2 ), (E) and (F) are identical in content to the corresponding method steps (C), (D 1 ), (E) and (F). 
     The second method includes: (A 2 ) setting a known product-specific target temperature curve with the aid of measuring discrete product temperatures at specified measurement points in the product flow and with the aid of a supply of the required heating medium flow with the required heating medium inlet temperature into the heating medium chamber at a heating medium inlet. 
     This method step of setting is to be understood such that a known product-specific target temperature curve stored in the control and feedback unit is controlled and adjusted with the aid of measurements for discrete product temperatures at specified measurement points in the product flow, during which at least the product output temperature and generally also the product input temperature are recorded at other specified measurement points. This temperature curve, which is set and measured and corresponds as much as possible to the specified known product-specific temperature curve, is provided for further processing according to method step (D 2 ). 
     The method step is applied if sufficient empirical values are available from previous heating processes for the liquid product to be heated and thus an achievable, product-specific target temperature curve is available which includes the endpoints of the temperature curve which need to be specified, specifically the product input temperature and product output temperature. 
     The method step of supplying the heating medium flow with the heating medium inlet temperature is to be understood such that these operating data are known and kept ready to ensure that the known product-specific target temperature curve is achieved and maintained. Consequently, operation is not carried out as previously with a high mass flow ratio throughout the entire operation time for safety reasons; instead, these operating data are minimized or optimized at least for the beginning of the operation time. 
     The second method includes: (B 2 ) specifying the known product-specific target temperature curve; this includes the product input temperature at a product input into the product chamber and the product output temperature at a product output out of it, and providing a stored supply of the heating medium flow with the heating medium inlet temperature. 
     The method steps of specifying and providing are to be understood such that these quantitative instructions are saved in the control and feedback unit in conjunction with the necessary control algorithms. 
     Considered over the operation time, the heating medium inlet temperature must be changed on account of the deposit growth, i.e., it must be increased in order to compensate for the decreasing heat throughput. According to one proposal, this is achieved by changing the heating medium inlet temperature to the required target heating medium inlet temperature in each case either in temperature steps, which can preferably be very small, or by a continuous temperature change. In both cases, very sensitive temperature control can be achieved. 
     The increase of the heating medium inlet temperature is limited on one hand by the options available in the process installation for representing these temperatures and on the other hand by considerations of efficiency. A further limitation of the heating medium inlet temperature is imposed by the rate of temperature increase, i.e., by the change of temperature in a specified time span. This temperature gradient, for example in degrees Celsius per hour (° C./h), provides an indication of the rate of growth for the deposit and thus of the available service time for the heat exchanger unit. 
     A further embodiment of the method which applies equally to the first and second methods provides the following method steps:
         (G) Determining a temperature/time gradient from a change of the heating medium inlet temperature in a specified time span.   (H) Specifying a reference gradient for a permitted temperature increase of the heating medium inlet temperature in the time span.   (I) Comparing the results of method step (G) with the specification according to method step (H).   (J) Changing the heating medium flow to a target heating medium flow when the reference gradient is exceeded by the temperature/time gradient determined.       

     Because the first and second methods are started with an exactly necessary mass flow ratio at the beginning of the operation time, significant quantity-related increases in the heating medium flow up to the end of the operation time, which find their limit where the known method starts from at the beginning of the operation time due to safety considerations, remain as part of the resources of the process installation and the required efficiency. 
     In consideration of the change to the heating medium flow, one embodiment of the method provides that the change of the heating medium flow to the necessary target heating medium flow takes place in each case either by a stepwise or a continuous increase. In both cases, with corresponding design, this can support finely adjusted regulation of the medium&#39;s inlet temperature on one hand and on the other hand limit a temperature difference between the product and heating medium temperature in the direction of the product input or the heating medium outlet to the degree exactly necessary. This measure ensures that the tendency to increased deposit formation driven by the temperature difference is minimized. An indication of the increasing growth of the deposit is also given by another embodiment of the method in which a product inlet pressure is measured at the product input and a product outlet pressure is measured at the product output. 
     One arrangement for carrying out a method according to the invention starts from a known UHT system with a heat exchanger unit in the heater zone which, seen in the direction of flow of a liquid product to be heated indirectly, is situated between an upstream process unit and a downstream process unit. The heat exchanger unit has a flow-through heat-absorbing product chamber and a flow-through heat-releasing heating medium chamber. Furthermore, at least one measuring apparatus for product flow, one measuring apparatus for product input temperature, one measuring apparatus for product output temperature, one measuring apparatus for heating medium flow and one measuring apparatus for heating medium inlet temperature are provided. These measuring apparatuses are connected with a control and feedback unit which, dependent on these measuring apparatuses, controls an output for target heating medium inlet temperature and an output for target heating medium flow which are provided on the control and feedback unit. 
     According to the teachings herein, it is provided, starting from the previously specified known assembly, that at least one temperature measurement point be provided in the product chamber of the heat exchanger unit upstream of a product output and adjacent to it with a defined spacing, said measurement point being connected in each case to the control and feedback unit via an associated measuring apparatus for discrete product temperature for measuring discrete product temperatures. Information on the product-specific temperature curve inside the product chamber is obtained with this at least one temperature measurement point, and this is done in fact in an area adjacent to the product output. In each case, this area has a defined spacing from the product output; this spacing is preferably directly adjacent to the product output. 
     The product-specific temperature curve in the area under discussion is recorded all the more exactly according to one proposal if more than one temperature measurement point is provided. In this case, the temperature measurement points are situated in series with respect to one another and with defined spacing from one another contrary to the direction of flow of the liquid product. 
     It has been found to be sufficient if the at least one temperature measurement point or points is or are arranged at least in the last third of the flow-through product chamber. This area can be detected in this way, enabling it to be recognized whether the heat exchange surface of the heat exchanger unit is utilized optimally and thus efficiently and whether the quality of the liquid product is at risk from a maintenance of heat with undefined residence time already occurring in this zone. 
     One heat exchanger unit according to the teachings herein, which is suited in the sense of an object according to the invention for aseptic heating in a heater zone of an arrangement in a UHT system, is subdivided into multiple sections connected to one another in series. Here, adjacent sections on the product side are connected to one another in each case via a first connecting element through which liquid product flows and on the heating medium side via a second connecting element. The respective temperature measurement point is provided in the first connecting element. The sectional construction of the heat exchanger unit enables conceivably simple access to the area of the heat-absorbing product chamber under discussion upstream of the product output. One very simple arrangement of a temperature measurement point is given in each of these first connecting elements assigned to this area without having to engage in a complicated manner with the product chamber itself, where the heat exchange takes place. 
     The preceding measures are accomplished in a particularly simple and useful manner according to one further proposal if the heat exchanger unit is designed as a tubular heat exchanger and if the individual section of the tubular heat exchanger is formed in each case on the product side as a monotube through which liquid product flows or as a tube bundle with a number of parallel interior tubes through which liquid product flows. Here the first connecting element is preferably formed in each case as a connecting bend or as a connection fitting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed representation of the invention is found in the following description and the drawing figures provided as well as in the claims. While the invention is implemented in the most varied embodiments of a first and a second method of the generic type, with the most varied embodiments of an arrangement for carrying out the method and the most varied embodiments of a heat exchanger unit for such an arrangement, the two methods, a preferred embodiment of an arrangement according to the invention which accommodates an heat exchanger unit according to the invention, and two advantageous embodiments of the heat exchanger unit are described below based on the drawing. 
         FIG. 1  is a schematic representation of a section from a prior art UHT system for aseptic heating of a liquid product and a heat exchanger unit of the heater or pasteurization zone. 
         FIG. 2  a qualitative representation of the temperature curves of the liquid product to be heated and of the heat-releasing heating medium, which show the temperatures on the Y-axis and a variable heat exchanger path in a schematically shown prior art heat exchanger unit on the X-axis. 
         FIG. 3  is a flow diagram of a first and a second method for aseptic heating according to the teachings herein. 
         FIG. 4  is a schematic representation of an arrangement with a heat exchanger unit for carrying out the two methods according to  FIG. 3 . 
         FIG. 5  is a diagram for representing the temperature curves in the heat exchanger unit according to  FIG. 4 . 
         FIG. 6A  is a front view of a preferred embodiment of the heat exchanger unit according to  FIG. 4 . 
         FIG. 6B  is a schematic and enlarged representation of a first embodiment of the heat exchanger unit according to  FIG. 6A  based on a detail in reference to the formation of the heat-absorbing product chamber labeled there with “Z”. 
         FIG. 6C  is a schematic and enlarged representation of a second embodiment of the heat exchanger unit according to  FIG. 6A  based on a detail in reference to the formation of the heat-absorbing product chamber labeled there with “Z”. 
     
    
    
     DETAILED DESCRIPTION 
     An arrangement  20  according to  FIG. 4 , which represents a section from a UHT system, is largely identical in its basic construction with the previously described arrangement  10  according to  FIG. 1 . Therefore, a renewed description in this regard is omitted. The difference between the arrangement  10  and the arrangement  20  consists of the fact that in the product chamber  22 . 1  of the heat exchanger unit  22  at least one temperature measurement point  22 . 3  is provided upstream of the product output A P  and adjacent thereto. The at least one temperature measurement point  22 . 3  in the embodiment has a spacing from the product input E P , which is designated with a discrete heat exchanger path I x1 , and thus has a defined spacing L−I x1  from the product output A P  according to the measure of an entire heat exchanger path L. The temperature measurement points  22 . 3  are situated in series with respect to one another and spaced from one another with a defined measurement point interval Δl, contrary to the direction of flow of the liquid product P. Each of these measurement points  22 . 3  is connected to the control and feedback unit  24  via an associated measuring apparatus for discrete product temperature  25  in each case for measuring discrete product temperatures T P  or T P1  to T Pn . Furthermore, it is provided that a measuring apparatus for product inlet pressure  27 . 1  measures a product inlet pressure p E  and a measuring apparatus for product outlet pressure  27 . 2  measures a product outlet pressure p A . An optional measuring apparatus for heating medium outlet temperature  30 . 2  measures the heating medium outlet temperature T MA . 
     The features and reference values in  FIG. 2 , which were defined and explained above, are also found identically or in a modified form only with regard to designation to some extent in  FIG. 3  and predominantly in  FIGS. 5 and 6A-6C . In this regard as well, a renewed definition and explanation is omitted below. With reference to the subject matter of the invention, only the additional or different features and reference values will be introduced and explained. 
     The first and second methods according to the invention are illustrated in  FIG. 3 , in each case in connection with a further embodiment advantageous for both methods, in the form of a flow diagram during the time t increasing downward (on the vertical axis). 
     First Method 
     The first method starts from the known method for aseptic heating of a liquid product P in a heat exchanger unit  22  of the heater zone of an arrangement  20  in a UHT system in which an indirect heat exchange on a wall takes place in the heat exchanger unit  22  between the liquid product P and a heating medium M by a heating medium flow F M  in a heat-releasing heating medium chamber  22 . 2  being guided countercurrent to a product flow F P  passing through a heat-absorbing product chamber  22 . 1 , with the product flow F P  being heated from a product input temperature T PE  to a product output temperature T PA  and at least the product output temperature T PA  and the heating medium inlet temperature T ME  being monitored and regulated. 
     The first method is characterized by the following method steps (A 1 ), (B 1 ), (C), (D 1 ), (E), and (F), which are shown graphically in their conditional relationship and meaning in  FIG. 3 . 
     The method step (A 1 ) includes setting an unknown product-specific temperature curve [T P (I x )] PE-PA  between the product input temperature T PE  and the product output temperature T PA  with the aid of a supply of the required heating medium flow F M  with the required heating medium inlet temperature T ME  at a heating medium inlet E M  into the heating medium chamber  22 . 2  and measuring discrete product temperatures T P  or T P1  to T Pa  at specified measurement points  22 . 3  in the product flow F P . 
     The method step (B 1 ) includes specifying the product input temperature T PE  at a product input E P  into the product chamber  22 . 1  and the product output temperature T PA  at a product output A P  from it and providing the heating medium inlet temperature T ME  and the heating medium flow F M . 
     The method step (C) includes measuring a product-specific temperature curve T P (I x ) between the product output A P  and the product input E P  at the specified measurement points  22 . 3 ; 
     The method step (D 1 ) includes comparing the temperature curves for method steps (A 1 ) and (C) and calculating temperature deviations ΔT P  at the specified measurement points  22 . 3 . 
     The method step (E) includes specifying a permitted temperature deviation [ΔT P ] 0 . 
     The method step (F) includes changing of the heating medium inlet temperature T ME  to a target heating medium inlet temperature T ME * when the permitted temperature deviation [ΔT P ] 0  is exceeded by the calculated temperature deviation ΔT P . 
     Second Method 
     The second method also starts from the previously described known method and is characterized by the following method steps (A 2 ), (B 2 ), (C), (D 2 ), (E), and (F), with the method steps (C), (E), and (F) being identical to the method steps having the same labels in the first method. The method steps of the second method are also illustrated graphically in  FIG. 3  in their conditional relationship and their meaning. 
     The method step (A 2 ) includes setting a known product-specific target temperature curve [T P (I x )] 0  with the aid of measuring discrete product temperatures T P  and T P1  to T P , respectively at specified measurement points  22 . 3  in the product flow F P  and with the aid of a supply of the required heating medium flow F M  with the required heating medium inlet temperature T ME  at a heating medium inlet E M  into the heating medium chamber  22 . 2 . 
     The method step (B 2 ) includes specifying the product-specific target temperature curve [T P (I x )] 0 , which includes the product input temperature T PE  at a product input E P  into the product chamber  22 . 1  and the product output temperature T PA  at a product output A P  out of it, and providing a stored supply of the heating medium flow F M  with a heating medium inlet temperature T ME . 
     The method step (C) includes measuring a product-specific temperature curve T P (I x ) between the product output A P  and the product input E P  at the specified measurement points  22 . 3 . 
     The method step (D 2 ) includes comparing the temperature curves for method steps (A 2 ) and (C) and calculating temperature deviations ΔT P  at the specified measurement points  22 . 3 . 
     The method step (E) includes specifying a permitted temperature deviation [ΔT P ] 0 . 
     The method step (F) includes changing of the heating medium inlet temperature T ME  to a target heating medium inlet temperature T ME * when the permitted temperature deviation [ΔT P ] 0  is exceeded by the calculated temperature deviation ΔT P . 
     Both the first and the second method can be advantageously embodied in each case with additional method steps (G), (H), (I), and (J), which are also illustrated graphically in  FIG. 3  in their conditional relationship and their meaning. 
     The method step (G) includes determining a temperature/time gradient ΔT ME /Δt from a change of the heating medium inlet temperature T ME  in a specified time span Δt. 
     The method step (H) includes specifying a reference gradient [ΔT ME /Δt] 0  for a permitted temperature increase of the heating medium inlet temperature T ME  in the time span Δt. 
     The method step (I) includes comparing the results of the method step (G) with the specification according to the method step (H). 
     The method step (J) includes changing the heating medium flow F M  to a target heating medium flow F M * when the reference gradient [ΔT ME /Δt] 0  is exceeded by the temperature/time gradient ΔT ME /Δt determined. 
     Analogous to the representation in  FIG. 2 , the temperature curves T P (I x ) and T M (I x ) shown in  FIG. 5  are observed during the operation time of the heat exchanger unit  22 , entered over the variable heat exchanger path I x . The product-specific temperature curve in the specified, heat-absorbing product flow F P  to be treated between the product input temperature T PE  (for example, 125° C.) provided by the upstream process unit  21  and the product output temperature T PA  (for example, 140° C.) necessary to ensure sufficient aseptic heating is designated in turn by T P (I x ). T M (I x ) is the designation for two heating medium-specific temperature curves in the heat-releasing heating medium flow F M . The lower temperature curve, with reference to the position in the drawing, between a third heating medium inlet temperature T ME ( 3 ) (for example, 141.7° C.) and a third heating medium outlet temperature T MA ( 3 ) (for example, 128.8° C.) is at the beginning of the operation time if the heat exchanger unit  22  is still free of any deposits (product fouling) on the product side. 
     At the end of the operation time, after 12 hours for example, the control and feedback unit  24  has increased the heating medium inlet temperature T ME  so much that a fourth heating medium inlet temperature T ME ( 4 ) (for example, 144° C.) is now necessary at the heating medium inlet E M . The third heating medium outlet temperature T MA ( 3 ) necessarily establishing itself at the heating medium outlet A M  at the beginning of the operation time is essentially dependent on a third mass flow ratio f( 3 ), comprised as a ratio of a third heating medium flow F M ( 3 ) and the product flow F P  on one hand (f( 3 )=F M ( 3 )/F P =1.14) and the other influencing parameters cited above in conjunction with  FIG. 2 . 
     A fourth heating medium outlet temperature T MA ( 4 ) (for example, 134.6° C.) necessarily establishing itself at the heating medium outlet A M  at the end of the operation time is essentially dependent on a fourth mass flow ratio f( 4 ), comprised as a quotient of a fourth heating medium flow F M ( 4 ) and the product flow F P  on one hand (f( 4 )=F M ( 4 )/F P =1.57) and the other influencing parameters cited above in conjunction with  FIG. 2 . 
     At the beginning of the operation time, the heat exchanger unit  22  is operated with a minimum value for the third heating medium flow F M ( 3 ), with which, in conjunction with a minimum value for the third heating medium inlet temperature T ME ( 3 ), it is ensured to achieve and maintain the product output temperature T PA  and the product input temperature T PE . 
     In contrast to the known method, as part of the increase of the heating medium inlet temperature from T ME ( 3 ) to T ME ( 4 ), the heating medium flow F M  is increased from the minimum value F M ( 3 ) to the maximum value F M ( 4 ) in a stepwise or continuous manner. This results in a significantly smaller temperature difference between the product temperature T P  and the heating medium temperature T M  at the product input E P  and heating medium outlet A M  respectively compared to the known method. Advantages in this regard with respect to a lesser buildup for the product input E P  were already described above in conjunction with  FIG. 2 . 
     The specialist recognizes the accumulated product fouling deposit during the operation time from the average logarithmic temperature difference ΔT M  as already described. In the present case, according to this disclosure, at the beginning of the operation time, a third average logarithmic temperature difference ΔT M ( 3 ) is 2.6° C. and at the end of the operation time a fourth average logarithmic temperature difference ΔT M ( 4 ) is 6.4° C. In this respect, these values correspond approximately to those in the method of prior art. 
     In contrast to the known method, equations (2.1) and (2.2) with correspondingly modified parameters yield altogether a lower quantity of accumulation compared to the known method as a result of the significant reduction of the temperature difference throughout the entire heat exchanger path L from ΔT small ( 3 )=1.7° C. to ΔT large ( 3 )=3.8° C. (a factor of 2.2) from the beginning of the operation time up to the end of the operation time, then still from ΔT small ( 4 )=4° C. to ΔT large ( 4 )=9.6° C. (a factor of 2.4) at the end of the operation time from the product fouling, considered over the entire heat exchange surface A. This circumstance is particularly due at the beginning and in the first half of the operation time where the temperature difference, i.e., the ratio of ΔT large ( 3 )=3.8° C. to ΔT small ( 3 )=1.7° C., is only a factor of 2.2, whereas in the known method with ΔT large ( 1 )=5.6° C. to ΔT small ( 1 )=0.9° C. it acts on the product fouling with a factor of 6.2. 
     A comparison of the relevant data for heat exchange in the known method according to  FIG. 2  and the method according to  FIG. 5  is shown in the following table. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Designation 
                 FIG. 2 
                 FIG. 5 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Unit of measure 
                   
                 [° C.] 
                   
                 [° C.] 
               
               
                   
               
               
                 Product input temperature 
                 T PE   
                 125.0 
                 T PE   
                 125.0 
               
               
                 Product output temperature 
                 T PA   
                 140.0 
                 T PA   
                 140.0 
               
               
                 Heating medium inlet temperature 
                 T ME (1) 
                 140.9 
                 T ME (3) 
                 141.7 
               
               
                 Heating medium inlet temperature 
                 T ME (2) 
                 144.5 
                 T ME (4) 
                 144.0 
               
               
                 Heating medium outlet temperature 
                 T MA (1) 
                 130.6 
                 T MA (3) 
                 128.8 
               
               
                 Heating medium outlet temperature 
                 T MA (2) 
                 134.2 
                 T MA (4) 
                 134.6 
               
               
                 Small temperature difference 
                 ΔT small (1) 
                 0.9 
                 ΔT small (3) 
                 1.7 
               
               
                 Small temperature difference 
                 ΔT small (2) 
                 4.5 
                 ΔT small (4) 
                 4.0 
               
               
                 Large temperature difference 
                 ΔT large (1) 
                 5.6 
                 ΔT large (3) 
                 3.8 
               
               
                 Large temperature difference 
                 ΔT large (2) 
                 9.2 
                 ΔT large (4) 
                 9.6 
               
               
                 Average logarithmic temperature difference 
                 ΔT M (1) 
                 2.6 
                 ΔT M (3) 
                 2.6 
               
               
                 Average logarithmic temperature difference 
                 ΔT M (2) 
                 6.6 
                 ΔT M (4) 
                 6.4 
               
               
                   
               
               
                 Unit of measure 
                   
                 [1] 
                   
                 [1] 
               
               
                   
               
               
                 Mass flow ratio 
                 f(1) 
                 1.43 
                 f(3) 
                 1.14 
               
               
                 Mass flow ratio 
                 f(2) 
                 1.43 
                 f(4) 
                 1.57 
               
               
                   
               
            
           
         
       
     
     A product-specific temperature limit curve of the product flow F P  designated in turn in  FIG. 5  as T P (I x )′—is theoretical in nature with respect to its linear plot between the product input temperature T PE  and the product output temperature T PA , just like a linear temperature curve in the heating medium flow F M , which is not shown, as already indicated above in conjunction with the known method. As  FIG. 5  shows, in the method described herein, success was achieved in the context of the available influencing parameters in bringing the actual temperature curve in the product P and in the heating medium M closer together than in the known method. 
       FIG. 5  also graphically shows the method steps D 1  and D 2  respectively and E and F. A permitted downward temperature deviation is designated as −[ΔT P ] 0  and an upward one with +[ΔT P ] 0 . This results in a lower temperature limit curve [T P (I x )]* and an upper temperature limit curve [T P (I x )]**. The product-specific temperature curve T P (I x ) is measured via the discrete temperatures T P  in the region close to the product output A P  by the arrangement of the temperature measurement points  22 . 3 . Here the first product temperature T P1  is located at the discrete heat exchanger path I x1  (T P (I x1 )) and the second product temperature T P2  and third product temperature T P3  are measured in each case at measurement point intervals ΔI one after the other in the direction of flow of the liquid product P. If the product-specific temperature curve T P (I x ) diverges particularly from this region, then, in accordance with the invention as described above and illustrated in  FIG. 3 , this is counteracted by the influencing parameters for the target heating medium inlet temperature T M * and target heating medium flow F M *. 
     As shown in  FIG. 6A , the heat exchanger unit  22  is subdivided into multiple sections  22   a  connected in series to one another. Here, adjacent sections  22   a  are connected to one another in each case via a first connecting element  32  through which liquid product P flows on the product side and via a second connecting element  33  on the heating medium side, whereby, if required, the respective temperature measurement points  22 . 3  are provided in a necessary number of first connecting elements  32 . 
     The instrumental embodiment of the heat exchanger unit  22  is accomplished in a particularly easy manner if it is implemented as a tubular heat exchanger as shown in  FIG. 6A , in which the heat-absorbing product chamber  22 . 1  and the heat-releasing heating medium chamber  22 . 2  which surrounds the product chamber  22 . 2  externally preferably have in each case the form of a straight section of tubing. The subdivision of the length of tubing in sections of equal length or also of different lengths results in the sections  22   a.  Here there are two fundamentally differing embodiments, specifically a first in which the individual section  22   a  of the tubular heat exchanger  22  is formed on the product side in each case as a monotube  22 . 1 * through which liquid product P flows, said monotube being concentrically enclosed by the heating medium chamber  22 . 2  in the form of a tube-shaped external jacket as shown in  FIG. 6B . 
     In the second embodiment, a so-called shell-and-tube heat exchanger  22 , the individual section  22   a  is formed as a tube bundle  22 . 1 ** with a number of parallel interior tubes  22 . 1 *** through which liquid product P flows as shown in  FIG. 6C . Here these interior tubes  22 . 1 *** are not only arranged in the Meridian level of the heating medium chamber  22 . 2 , which surrounds the interior tubes  22 . 1 *** altogether as a tube-shaped external jacket as shown in a simplifying manner in  FIG. 6C , but are also distributed as evenly as possible over the entire cross-section of this external jacket. 
     As shown in  FIG. 6A , the first connecting element  32  is preferably formed in each case as a connecting bend, for example as a 180° pipe bend, or as a connection fitting with another geometric form which necessarily ensures an interior passage. The second connecting element  33  is designed, for example, in the form of a short pipe connection which connects adjacent external jackets of the heating medium chamber  22 . 2  to one another in their end region in each case. 
     The arrangement of the necessary temperature measurement points  22 . 3  is very easily possible by the embodiment of the heat exchanger unit  22  shown above in the form of a tubular heat exchanger or shell-and-tube heat exchanger  22  subdivided in sections  22   a,  because access to the product flow F P  is given directly at defined measurement point intervals ΔI in each case via the first connecting element  32  without needing to reach into the section  22   a  itself and through the heating medium chamber  22 . 2  in a complicated manner. The first, second and third product temperature—T P1 , T P2  and T P3  respectively are obtained at the temperature measurement points  22 . 3  in the embodiment example by the respective measuring apparatus for discrete product temperature  25 . The arrangement of the associated temperature measurement points  22 . 3  in the embodiment example is done based on  FIG. 4  in such a way that, viewed in the direction of flow of the liquid product P, upstream of the product output A P  and at a defined spacing from it, they are arranged necessarily in series with respect to one another and with defined spacing from one another, specifically with the spacing of the preferably equal length of the section  22   a  in each case. 
     The list of reference numbers used in the drawing figures is as follows.
       10  arrangement according to prior art     21  upstream process unit     22  heat exchanger unit     22 . 1  heat-absorbing product chamber     22 . 2  heat-releasing heating medium chamber     23  downstream process unit     24  control and feedback unit     26  measuring apparatus for product flow (F P )     28 . 1  measuring apparatus for product input temperature (T PE )     28 . 2  measuring apparatus for product output temperature (T PA )     29  measuring apparatus for heating medium flow (F M )     30 . 1  measuring apparatus for heating medium inlet temperature (T ME )     31 . 1  outlet for target medium inlet temperature (T ME *)     31 . 2  outlet for target heating medium flow (F M *)   A heat exchange surface (of the heat exchanger unit  22 )   A M  heating medium outlet   A P  product output   E M  heating medium inlet   E P  product input   F M  heating medium flow in kg/s, for example   F M * target heating medium flow   F M ( 1 ) first heating medium flow   F M ( 2 ) second heating medium flow   F P  product flow in kg/s, for example   L total heat exchanger path   M heating medium   P liquid product   Q heat flow, for example in W=J/s   T M  heating medium temperature   T M (I x ) heating medium-specific temperature curve, general   T MA  heating medium outlet temperature, general   T MA ( 1 ) first heating medium outlet temperature   T MA ( 2 ) second heating medium outlet temperature   T ME  heating medium inlet temperature, general   T ME * target heating medium inlet temperature, general   T ME ( 1 ) first heating medium inlet temperature   T ME ( 2 ) second heating medium inlet temperature   T P  product temperature   T(I x ) product-specific temperature curve, general   T P (I x )′ product-specific temperature limit curve   T P (I x1 ) discrete temperature of the liquid product   T PA  product output temperature   T PE  product input temperature   ΔT large ( 1 ) first large temperature difference   ΔT large ( 2 ) second large temperature difference   ΔT small ( 1 ) first small temperature difference   ΔT small ( 2 ) second small temperature difference   ΔT m  average logarithmic temperature difference, general   ΔT M ( 1 ) first average logarithmic temperature difference   ΔT M ( 2 ) second average logarithmic temperature difference   c M  specific heat capacity of the heating medium (M)—in J/(kgK) for example   c P  specific heat capacity of the liquid product (P)—in J/(kgK) for example   F mass flow ratio, general   f( 1 ) first mass flow ratio   f( 2 ) second mass flow ratio   K heat transfer coefficient, for example in W/(m 2 K)=J/(m 2  sK); K=Kelvin   I x  variable heat exchanger path   I x1  discrete heat exchanger path (at the point I x1 )   A 1  setting of an unknown product-specific temperature curve [T P (I x )] PE-PA      A 2  setting of a known product-specific target temperature curve [T P (I x )] 0      B 1  specifying the product input temperature T PE  and the product output temperature T PA  and providing the heating medium inlet temperature T ME  and heating medium flow F M      B 2  specifying the known product-specific target temperature curve [T P (I x )] 0  and providing the heating medium flow F M  with a heating medium inlet temperature T ME      C measurement of a product-specific temperature curve T P (I x )   D 1  comparing the temperature curves for the method steps (A 1 ) and (C) and calculating temperature deviations ΔT P      D 2  comparing the temperature curves for method steps (A 2 ) and (C) and calculating temperature deviations ΔT P      E specifying a permitted temperature deviation [ΔT P ] 0      F changing the heating medium inlet temperature T ME      G determining a temperature/time gradient ΔT ME /Δt   H specifying a reference gradient [ΔT ME /Δt] 0      I comparing the results of method step (G) with the specification according to method step (H);   J changing the heating medium flow F M        20  arrangement     22   a  Section     22 . 1 * Monotube     22 . 1 ** tube bundle     22 . 1 *** interior tube     22 . 3  temperature measurement point     25  measuring apparatus for discrete product temperature T P ; T P1  to T Pn        27 . 1  measuring apparatus for product inlet pressure (p E )     27 . 2  measuring apparatus for product outlet pressure (p A )     30 . 2  measuring device for heating medium outlet temperature (TMA)     32  first connecting element     33  second connecting element   F M ( 3 ) third heating medium flow   F M ( 4 ) fourth heating medium flow   T MA ( 3 ) third heating medium outlet temperature   T MA ( 4 ) fourth heating medium outlet temperature   T ME ( 3 ) third heating medium inlet temperature   T ME ( 4 ) fourth heating medium inlet temperature   T P  discrete product temperature, general   T P1  first product temperature   T P2  second product temperature   T P3  third product temperature   T Pi  i th  product temperature   T Pn  n th  product temperature   [T P (I x )] PE-PA  unknown, product-specific temperature curve between the product input temperature T PE  and the product output temperature T PA      [T P (I x )] 0  known product-specific target temperature curve   [T P (I x )]* lower temperature limit curve   [T P (I x )]** upper temperature limit curve   ΔT ME /Δt temperature/time gradient   [ΔT ME /Δt] 0  reference gradient   ΔT M ( 3 ) third average logarithmic temperature difference   ΔT M ( 4 ) fourth average logarithmic temperature difference   ΔT large ( 3 ) third large temperature difference   ΔT large ( 4 ) fourth large temperature difference   ΔT small ( 3 ) third small temperature difference   ΔT small ( 4 ) fourth small temperature difference   ±ΔT P  temperature deviation (+: upward; −: downward)   ±[ΔT P ] 0  permitted temperature deviation (+: upward; −: downward)   f( 3 ) third mass flow ratio   f( 4 ) fourth mass flow ratio   ΔI measurement point interval   p A  product outlet pressure   p E  product inlet pressure   t Time   Δt time span