Technical Field

The present invention relates to a multitube tubular heat exchanger and a method for changing the temperature of a liquid product flowing through such heat exchanger. More particularly, the present invention relates to an inner tube of such multitube tubular heat exchanger.

Background

Heat exchangers are well known for heating a flow of liquid product being transported through the heat exchanger. For example, such heat exchangers are widely used within liquid food processing, i.e. dairies, wherein heat treatment of the liquid food product is crucial for providing pasteurisation and/or sterilization of the product.

The main principle of heat exchangers in general is to transfer heat to the product, whereby the size and configuration of the heat exchanger depends on various parameters such as flow rate, physical properties of the liquid, desired pressure drop, temperature range etc. Typical examples of heat exchangers utilized in food processing systems include plate heat exchangers, tubular heat exchangers, and scraped-surface heat exchangers.

The choice of heat exchanger type is normally dependent on the type of liquid product to be heated. Plate heat exchangers are normally chosen for liquid products having a very low viscosity, whereby scraped-surface heat exchangers are used for high viscous liquids including large-sized particles. Tubular heat exchangers have been found to be suitable for medium viscous liquids including small-sized particles such as fibres etc.

One particular type of tubular heat exchangers is the multitube tubular heat exchanger in which the liquid product is transported through a group of parallel inner tubes, while the heat transfer media flows between the inner tubes thus surrounding the inner tubes.

In order to improve the heat transfer of multitube tubular heat exchangers it is known to provide the inner tubes with corrugations for increasing the turbulence of the liquid product flow inside the inner tubes. Examples of such corrugations are found in EP0052522 and in EP2149770.

Although these documents suggest specific deformations of the inner tubes for increasing the heat transfer a number of considerations need to be made in order to provide an efficient multitube tubular heat exchanger. Firstly, the inner tubes should provide turbulence of the liquid product flow. Further, it would be advantageous if the inner tubes also contribute to turbulence of the heat transfer media on the outside. However, the corrugations of the inner tubes should at the same time prevent particles of the liquid product, or heat transfer media if the heat exchanger is used in a regenerative mode, from being trapped. Finally, the inner tubes should also be configured for facilitating and improving cleaning of the heat exchanger.

In view of all these requirements there is a long felt need for an improved muititube tubular heat exchanger and in particular an improved inner tube of such heat exchanger.

Summary

Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a system according to the appended claims.

An idea of the present invention is to provide helical corrugations on the inner tubes.

A further idea is to increase the width of the helical corrugation pattern, such that the width-pitch ratio is increased.

According to a first aspect of the invention, it is provided a tube for a heat exchanger, said tube being provided with a helical corrugation, wherein the helical corrugation has pitch and a width, and the width being at least 20% of the pitch.

More specifically, the width may be 30 to 80% of the pitch. Even more specifically, the width may be 30 to 70% of the pitch.

The outer diameter of the tube may be between 1 and 4 cm. More specifically, the outer diameter may be between 1 and 2 cm.

The pitch of the corrugation may be in the interval of 1 to 4 cm. More specifically, the pitch of the corrugation may be in the interval of 2 to 3 cm.

The depth of the corrugation may be in the interval of 0.5 to 2 mm.

According to one embodiment the corrugation is symmetrical from a side view or longitudinal cross-sectional view of the tube.

According to another embodiment the corrugation is asymmetrical from a side view or longitudinal cross-sectional view of the tube.

According to a second aspect it is provided a tube set comprising a number of tubes according to the first aspect. The tube set may be comprised in an outer tube.

The number of tubes may be between 1 and 40. According to a third aspect, it is provided a heat exchanger for heat treating liquid food products comprising a tube set according to the second aspect.

In said heat exchanger, a first tube set may be connected to a second tube set by a connection tube, such that the first tube set is arranged in parallel with the second tube set.

According to a fourth aspect, it is provided a processing system for reducing a number of microorganisms in a product, said processing system comprising a product inlet and a product outlet, and a heat exchanger according to the third aspect, arranged downstream the inlet and upstream the outlet.

The processing system may be regenerative such that outcoming product acts as heating medium for incoming product.

The product may be milk.

Brief Description of Drawings

These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of

embodiments of the present invention, reference being made to the accompanying drawings, in which

Fig. 1 is a process scheme of a dairy system including a multitube tubular heat exchanger according to an embodiment;

Fig. 2 is a process scheme of a dairy system including a multitube tubular heat exchanger according to an embodiment

Fig. 3 is an isometric view of a multitube tubular heat exchanger according to an embodiment;

Fig. 4 is an isometric view of a part of the multitube tubular heat exchanger shown in Fig. 3;

Fig. 5 is a side view of an embodiment of an inner tube of the multitube heat exchanger, and

Fig. 6 is a side view of a further embodiment of an inner.

Detailed Description

Starting with Fig. 1 an example of a dairy system 10 is shown. The shown dairy system 10 is configured to perform various treatments of milk for providing long- life milk by means of ultra high temperature exposure. The dairy system 10 includes a product inlet 12 and a product outlet 14 arranged downstream of the product inlet 12. For this particular purpose, the dairy system 10 also comprises a number of equipment arranged between the inlet 12 and the outlet 14 through which the milk is flowing. The milk, entering the dairy system 10 at the inlet 12, is fed to a balance tank 16 from which it flows to a first preheater 20 by means of a feed pump 18. The preheater 20 is a multitube tubular heat exchanger configured to increase the temperature of the milk from approximately 4°C up to approximately 80°C. The milk is thereafter fed to a steam injection head 22 which serves to rapidly increase the temperature of the milk to 140- 150°C. After keeping the elevated temperature for some time the milk is quickly cooled down when entering vacuum vessel 24. The milk thereafter passes a centrifugal pump 26 and an aseptic homogenizer 38 before it enters a cooler 30 in the form of a multitube tubular heat exchanger 30 in which the temperature of the milk is lowered to approximately 25°C. The outlet 14 is capable of diverting the milk to either a storage tank, indicated by the letter "A", or a liquid product filling machine, indicated by the letter "B".

From hereon the multitube tubular heat exchanger, such as heat exchangers 20, 30 is denoted by the reference numeral 100.

The tubular heat exchanger 100 may also be used for cooling down a product by using e.g. cold water instead of hot water as thermal transfer medium, such as the case for with the multitube tubular heat exchanger 30 of Fig. 1 .

In plants where the product should be heated up at one stage of the process, e.g. in order to reduce the number of microorganisms, and cooled down at another stage, e.g. in order to cool down the product before it is stored and later put in packages, the tubular heat exchanger may be used as a so-called regenerative system, i.e. that the product itself is used as a thermal transfer medium. A regenerative system may be used in a dairy where incoming milk is heated up at the same time as outgoing milk is cooled down. An advantage of a regenerative system is that the energy consumption of the equipment may be significantly decreased. A regenerative system 10' according to one embodiment is disclosed in Fig. 2, said regenerative system 10' comprising the equipment disclosed in Fig. 1 together with the heat exchanger 100 in regenerative form.

Further, different products need to be treated in different ways, e.g. due to fibers or other food components, and different producers want to treat the products in different ways. In order to be able to offer a tubular heat exchanger 100 adapted to the specific needs and wishes of a producer a modular design is used. For example, in order to produce a wide range of models adapted to specific needs for each customer only a few frames are used. More particularly, even though tube sets with different diameters are used the same frame can be used, which of course provides for that a more economic production can be achieved and that a wider range of alternatives may be offered.

Fig. 3 illustrates one example of the multitube tubular heat exchanger 100, more particularly a Tetra Spiraflo™, suitable for heat treating milk, juice, nectar and other liquid food. In short, a food product, or any other product to be processed, flows in tubes bundled together in tube sets. The tubes are surrounded by a thermal transfer medium, such as hot water, heating up the product. It is common practice to connect the tube sets to each other by using connection tubes, such that long tubes are formed while keeping the tubular heat exchanger 100 compact.

As illustrated in Fig 4, a first tube set 102, in this particular example

comprising seven inner tubes 200 bundled together, are positioned within an outer tubular tube 103. The first tube set 102 may be connected to a second tube set 104, also comprising seven inner tubes bundled together, by a connection tube 106. For inspection inside the inner tubes 200 of the first tube set 102, the connection tube 106 may be released, thereby making it possible to look inside the inner tubes 200. Further, in order to inspect the outside of the tubes the tube sets 102, 104 may be pulled out from the housing holding the tube sets 102, 104.

Different number of tubes may be comprised in the tube sets 102, 104, such as between 1 and 40, such as 7 to 16, depending e.g. on the capacity of the heat exchanger. Of course the size of the outer tube 103 is adapted in accordance with the number of tubes in the tube sets 102, 104.

Now turning to Fig. 5 a side view of an inner tube 200 is shown. The inner tube 200 is preferably arranged adjacent to further inner tubes 200 of the same construction for forming a tube set 102.

The inner tube 200 has an outer diameter D1 . Along the longitudinal extension

L of the inner tube 200, the inner tube 200 is provided with a helical corrugation. The helical corrugation has an angle a in relation to a transversal plane of the inner tube 200. The helical corrugation has a width W1. Along a side view of the inner tube 200 the helical corrugation is separated by a separation distance S1 , as a result of the width W1 and the angle a. Thus, the separation distance S1 is equal to the pitch P1 of the corrugation minus the width W1 of the corrugation, from a side view or longitudinal cross-sectional view, of the inner tube 200. The corrugation has a depth H1 , which is the distance from the periphery of diameter D1 to the bottom of the corrugation from a side view or longitudinal cross-sectional view, of the inner tube 200. The corrugation is symmetrical, which means that the deepest part of the corrugation is positioned centrally of the borders of the corrugation, from said perspective. In another embodiment, as disclosed in Figs. 6a and 6b, the corrugation is asymmetrical, which means that the deepest part of the corrugation is positioned closer to one border of the corrugation than the other, from a side view or longitudinal cross-sectional view, of the inner tube 200. The width W2 may then be divided into two subwidths W21 , W22, defining the distance from the deepest part of the corrugation to the two borders, respectively, of the corrugation, from a side view or longitudinal cross- sectional view, of the inner tube 200. Along a side view of the inner tube 200 the helical corrugation is separated by a separation distance S2, as a result of the width W2 and the angle a. Thus, the separation distance S2 is equal to the pitch P2 of the

corrugation minus the width W2 of the corrugation, from a side view or longitudinal cross-sectional view, of the inner tube 200. The corrugation has a depth H2, which is the distance from the periphery of diameter D1 to the bottom of the corrugation from a side view or longitudinal cross-sectional view, of the inner tube 200. In accordance with above, this depth H2 is located closer to one border of the corrugation than the other. When using asymmetrical corrugations it has surprisingly been found that the pressure drop over the length L may be decreased with maintained or even better heat transfer coefficients for the food products, whereby heat exchanging may be improved.

The outer diameter D1 of the inner tube 200 is preferably between 1 and 4 cm, such as 1 to 3 cm, and preferably between 1 and 2 cm to obtain a good heat exchange for the products in question, to facilitate the arrangement of tube sets in a housing for heat-exchanging the food product in an effective way with a low pressure drop. The pitch P1 , P2 of the corrugation is preferably selected to be in the interval of 1 to 4 cm, such as between 2 and 3 cm, to in the same way obtain good heat- exchanging with low pressure drop and high heat transfer coefficient. The depth is preferably selected to be in the interval of 0.5 to 2 mm. If the depth is 1 mm to 2 mm, a greater effect of the asymmetric arrangement of the corrugation may be achieved, since the inclination may be increased. The width W1 , W2 should be at least 20%, such as at least 30 to 80%, such as 30 to 70% of the pitch P1 , P2. When the width W1 , W2 is at least 20% of the pitch P1 , P2 it is has been surprisingly found that the pressure drop along the inner tube may be decreased by as much as 30 % in comparison with pipes bearing a corrugation having a width of less than 20 % of the pitch thereof. By providing the corrugation as an asymmetric corrugation, in

accordance with above, the pressure drop may be additionally improved in comparison with a symmetric one having the same relationship between the width and the pitch.

Although specific embodiments have been described it should be appreciated that various modifications may be made to the printing systems without departing from the scope as defined in the accompanying claims.