Heat exchange – Casing or tank enclosed conduit assembly – Manifold formed by casing section and tube sheet of assembly
Reexamination Certificate
1995-05-16
2001-06-26
Leo, Leonard (Department: 3743)
Heat exchange
Casing or tank enclosed conduit assembly
Manifold formed by casing section and tube sheet of assembly
C165S162000, C165S163000, C165S174000, C165S145000
Reexamination Certificate
active
06250379
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention involves a heat exchanger for the thermal conditioning of mixtures of substances or for the sterilization of liquids that are or may be contaminated with microorganisms, having an intake flange and an outlet flange and a tube bundle that connects these two flanges, which is sealed inside a shell having ports for the feeding-in and removal of the heating medium. The invention also involves a process for the thermal conditioning of mixtures of substances or for the sterilization of liquids.
Heat exchangers of this type are used, for instance, in the food processing industry, the pharmaceutics industry, and in biotechnology fields, as well as in other areas of process engineering in which liquid media must be heated to high temperatures in the shortest time possible. This heating results in the sterilization of these liquids by killing off undesirable microorganisms and microbes. One problem, however, is that heat labile components and useful materials, such as vitamins and proteins, also become denatured in the process of heat treatment, with the duration of the heat treatment being of primary importance in terms of this negative effect. This so-called denaturation is a particular problem in what is termed the discontinuous sterilization process, which generally involves long heating-up, residence, and cooling times. An additional disadvantage to the process of discontinuous sterilization is that the packaging must also be heated for sterilization. For this reason, continuous sterilization, in which short residence times are possible, is preferred. In the food processing industry, the ultra-high temperature processing of milk is a particularly well-known example of this.
For this known process of continuous sterilization, parallel-plate heat exchangers are generally used in industry. These are comprised of plates that are layered one on top of another and contain special, waved indentations that form the flow canals. These plates are generally pressed together in large numbers by means of tension rods between thick-walled holding plates, and support one another, according to the shape of the waves, at several points. The distance between the plates ranges from 2.5 to 12 mm, which creates correspondingly varied flow canal sizes. The product of value and the heat-exchanging medium flow alternatingly between every two plates. Depending upon the design, the flow paths for the product of value in the individual canals, from the intake opening to the outlet opening, are of varying lengths in all design types, regardless of whether the overflow of the plates is diagonal or curved. This necessarily creates a correspondingly broad residence time distribution for the products to be sterilized using these known parallel-plate heat exchangers, with the result that a certain portion of the heat-sensitive components, for which the period spent in the heat exchanger lies above the average residence time, are subjected to severe denaturation. An even wider range of residence times is created by the hydraulic boundary layers or dead areas that are created at the points of contact of adjacent plates, in which the rate of flow naturally drops to very low levels.
In general technology, tubular heat exchangers are known; these advantageously contain large flow areas and flow paths that are equal in length. Liquids that flow through these tubes have an equal distribution of residence times. The disadvantage of these heat exchangers, however, is that due to the correspondingly varied feeding-in of the medium from the central feed tube, radically dissimilar flow paths and varied residence times are created. In addition, these known tubular heat exchangers are unsuitable for the sterilization of liquids or for the conditioning of mixtures of substances because they do not permit short-time sterilization within the range of seconds.
SUMMARY OF THE INVENTION
It is thus the object of the invention to create a heat exchanger and a sterilization process that are also suited for short-time sterilization and that ensure a uniform residence time and sterilization temperature.
The object is attained in accordance with the invention in that the tube bundle contains several tubes that are equal in length and similar in cross-section, and that are connected to the central tube via distribution canals, positioned in the area of the intake and/or outlet flange, that are also equal in length and similar in cross-section.
This design for a heat exchanger, in which the medium flows through several tubes that are equal in length and similar in cross-section, but remains evenly distributed among the tubes, ensures a residence time distribution that remains within very narrow limits. The medium, which is fed through the tubes having the corresponding flow area, can, for example, be heated to 140° C. within tenths of a second, thus the required sterilization with the most extensive possible preservation of the vitamins and proteins can be guaranteed. It is thus possible to use a type of tube-bundle heat exchanger even in the food processing industry and in related industries, as equal residence times and sterilization temperatures can be guaranteed with these heat exchangers.
In relation to this, it is particularly advantageous for the tubes to have a very narrow flow area and thin walls, as this will permit an even and rapid increase in temperature over a short distance for the medium in the tubes. The capillary tubes that are used, which are equal in length and have very narrow flow areas, also make it possible for prime foodstuffs or similar goods to be sterilized gently but within the necessary time and at the required temperature, without danger of damage to the liquid itself.
In accordance with one advantageous embodiment of the invention, the tubes are either circular or oval in their cross-section. Although the tubes may theoretically possess any cross-sectional shape, the specified circular or oval tubes can be easily and accurately combined to form the corresponding tube bundles, even with their narrow or thin walls that are specified in the invention. They may also, in accordance with the invention, be flat or rectangular in cross-section, with the advantage that this shape provides, depending upon the volume of the liquid, a greater heat exchange surface in the capillary tubes.
In order to effect the heating of the medium to, for example, 140° C. within the required short period of time, correspondingly short tubes having an inside diameter, or transverse length, of between 0.5 and 5 mm, preferably 1.0 to 3.0 mm, are used. As a result of their relatively small cross-section, these tubes provide a defined length and a defined flow which enables a precise control of or compliance with the desired temperature values. This is also true for the further embodiment of the invention, in which the tubes have a wall thickness of 0.05 to 1 mm, preferably 0.1 to 0.3 mm. With these, and with a precisely defined cross-section, an even heating of the medium within the shortest time possible can be ensured.
Even with the shortest possible flow rates (laminar flow range), the desired heating is ensured under optimum conditions, since, in accordance with a further advantageous embodiment of the invention, the tubes are designed to be positioned evenly over the cross-section of the shell and to be curved or coiled into a helix or a meandering shape. Regardless of the cross-section of these tubes that have been curved or coiled into a helix or a meandering shape, so-called secondary flows are superimposed upon the primary flow, crosswise to the axial flow, which causes the pulse exchange as well as the heat exchange to be increased significantly due to the increased convection. Because such forced changes in the direction of flow occur over and over along the entire length of the tube, the result is the above-mentioned significant improvement in or acceleration of the heat exchanger, as well as a limitation of the residence time distribution. It is also an advantage that this arrangement a
Beykirch Helmut
Geissler Stefan
Langer Gert
Schweitzer Karl Heinz
Werner Udo
Creighton Wray James
HDE Metallwerk GmbH
Leo Leonard
Narasimhan Meera P.
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