Patent Publication Number: US-6334484-B1

Title: Multi-pass heat exchanger

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a multi-pass heat exchanger having a tube body and a bonnet, which is connected to the tubesheet of the tube body and in which at least one web for directing the liquid conveyed into the bonnet is arranged, the web being connected to the bonnet. 
     2. Description of the Related Art 
     Multi-pass heat exchangers are used in the chemical industry. In a 2-pass heat exchanger a web separates the liquid conveyed into the bonnet from the liquid conveyed out of the bonnet. By fitting a plurality of webs into the bonnet of a 4 or 6-pass heat exchanger, the liquid flow is deflected repeatedly in the bonnet and forced to pass the heat exchanger repeatedly. As a result, high flow velocities are achieved in the tubes, and good heat transfer is obtained. 
     For use with liquids present in the ultra-pure form, in particular for use in ultra-pure-water systems (WFI=Water for Injection), these designs cannot be used, since seals are required to seal off the webs, and these seals involve the risk of bacterial deposit and contamination. For this reason, heat exchangers having only one passage and twin tubesheets have been used for ultra-pure-water systems. The flow velocities in the tubes of such heat exchangers are correspondingly low and poor coefficients of heat transfer are obtained. The consequence is that these heat exchangers, although satisfactory from the point of view of sterility, require a large overall length in order to achieve satisfactory cooling of the liquid. Heat exchangers for sterile liquids which measure several meters in overall length are not uncommon. 
     SUMMARY OF THE INVENTION 
     Against this background, the object of the present invention is to develop a multi-pass heat exchanger that can be used for cooling liquid present in the ultra-pure state, in particular ultra-pure water. 
     This object is achieved by modifying a multi-pass heat exchanger to eliminate the seal between the web and tubesheet and to form a gap between the web and the tubesheet. 
     According to the present invention, a multi-pass heat exchanger for cooling ultra-pure liquids is provided, the multi-pass heat exchanger including a tube body having a tubesheet, a bonnet connected to the tubesheet, and at least one web for directing the liquid conveyed into the bonnet, a portion of the web being secured to the bonnet and dividing the bonnet into at least two chambers, and wherein the web is not secured to the tubesheet. 
     According to another aspect of the present invention, a multi-pass heat exchanger is provided, the multi-pass heat exchanger including a tube body having a tubesheet, a bonnet connected to the tubesheet, and at least one web for directing the liquid conveyed into the bonnet, a portion of the web being secured to the bonnet and dividing the bonnet into at least two chambers, wherein there is a gap between the web and the tubesheet. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and other advantages of the invention will be realized and attained by the method and apparatus particularly pointed out in the written description and claims as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and together with the description serve to explain the principles of the invention. In the figures: 
     FIG. 1 is a longitudinal section through the region of the bonnet of a 2-pass heat exchanger of the present invention; 
     FIG. 2 is a cross-sectional view taken along line II—II of FIG. 1; 
     FIG. 3 is a detailed view of area III of FIG. 1; and 
     FIG. 4 is a cross-sectional view through the region of the bonnet of a 4-pass heat exchanger according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples of the present preferred embodiments of the invention are illustrated in the accompanying drawings. Wherever possible, similar reference numbers will be used throughout the drawings to refer to similar parts. 
     According to the invention, a 2-pass heat exchanger is provided for cooling sterile liquids. As embodied herein and shown in FIGS. 1 and 2, a 2-pass heat exchanger is generally designated by the reference numeral  1 . The 2-pass heat exchanger  1  includes a tube body  2  having a tubesheet  3  and sixteen tubes  4 , which are held in a sealed off manner in corresponding holes of the tubesheet  3 . A bonnet  5  connected to the tubesheet  3  covers the tube region of the tube body  2  and is provided with an inlet nozzle  6  and an outlet nozzle  7  for conveying the liquid, for example ultra-pure water, to be cooled in the heat exchanger  1 . The direction of flow of the liquid to be cooled is illustrated in FIG. 1 by means of the thick arrows. Apart from the two nozzles  6  and  7 , the bonnet is of rotationally symmetrical design. In the region of the plane of symmetry  8  of the bonnet  5 , a thin-walled web  11 , which ends at a distance from the tubesheet  3 , is provided. Web  11  is connected to the bonnet  5  at its top face  9  and its side face  10 . Thus, web  11  has a secured end  11   a  connected to the top face  9  of bonnet  5 , and an unsecured or free end  12  facing but not connected to the tubesheet  3 . 
     In a preferred embodiment of the present invention, free end  12  of web  11  is pointed. Due to this shape, which is made especially favorable from the fluidic point of view, deposits cannot occur in the region of the web end facing the tubesheet. 
     Web  11  does not subdivide the bonnet into completely separate chambers, but is produced in its length in such a way that a distance, gap  13 , remains between web and tubesheet. A leakage flow through gap  13  is thereby obtained, and this leakage flow serves to flush the gap clear to prevent deposits or contamination. 
     As can be seen in FIG. 3, a gap  13 , having a thickness of 0.2 mm for example, is formed between the pointed end  12  of the web and the tubesheet  3 . The liquid to be cooled therefore not only flows through the tubes  4  of the heat exchanger  1  but, as illustrated in FIG. 3 by the thick arrow, a leakage flow passes directly from the inlet chamber  14  of the bonnet  5  to its outlet chamber  15 . The width of the gap is to be optimized to the effect that a sufficient leakage flow flows through the gap during operation of the heat exchanger in order to flush the gap clear. As a rule, it is sufficient to select this gap to be less than 1 mm. However, it is possible to use a gap of larger dimensions. 
     Depending on the type of multi-pass heat exchanger to be designed, a plurality of webs may be provided. A 4-pass heat exchanger has, for example, two webs. As seen in FIG. 4, the webs are arranged in particular in a T-shape, so that the bonnet is subdivided into three chambers. If a 6-pass heat exchanger were to be provided, four chambers would result. The webs are preferably expediently arranged at an angle of 90° to one another. 
     A design of the bonnet  5  in the case of a 4-pass heat exchanger is shown in FIG.  4 . In the embodiment according to FIG. 4, two webs  11  or web regions are provided. The webs  11  are arranged like a T, one web  11  having a length which corresponds to the diameter of the bonnet  5 , whereas the length of the other web  11  corresponds to the radius of the bonnet  5 . With appropriate change to the arrangement of inlet nozzle  6  and outlet nozzle  7 , which are allocated to the chambers  14  and  15 , the liquid to be cooled enters the inlet chamber  14 , which is arranged in the region of the first quarter of the bonnet  5 . It then flows, apart from the leakage flow, through the tubes  4  allocated to this chamber  14  into the heat exchanger  1  and leaves the tube body  2  in the region of the second quarter of the bonnet; from there the liquid in the chamber  16  is deflected to the tubes  4  arranged in the region of the third quarter of the bonnet  5 . It enters the tubes  4  and leaves these tubes  4  in the region of the outlet chamber  15 , which is allocated to the fourth quarter of the bonnet  5 . The leakage flow described above is obtained in the region of the T-shaped webs  11 , which in accordance with the representation of FIG. 3, form a gap  13  relative to the tubesheet  3 . Thus, each web  11  has a gap between its free end  12  and tubesheet  3 . 
     In an alternative embodiment, it would be possible to provide a multi-pass heat exchanger having at least six passages. In doing so, it would be possible to use either four webs having a length corresponding to the radius of bonnet  5 , 2 webs corresponding to the diameter of bonnet  5  or a combination of the two which would result in at least 4 chambers in bonnet  5 . 
     The leakage flow in the present invention certainly leads to thermic losses, since it does not flow through the heat exchanger and thus is not subjected to any cooling. A mixed temperature occurs between the leakage flow and the cooled flow leaving the heat exchanger. Despite these losses, the advantage realized by using the present invention of a compact, short heat exchanger working in a sterile manner is considerable, since it conforms to GMP. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers all modifications and variations of this invention that come within the scope of the appended claims and their equivalents.