DE102010033468A1 - Heat exchanger has ribs in fluid pipes that are comprised with rib combs between rib edges, such that rib combs of adjacent pipes are arranged alternately and rib edges are provided at right angle to longitudinal axis of pipes - Google Patents

Abstract

The heat exchanger (1) has fluid pipes (2) that are arranged on tube sheet (4) in parallel to each other. The ribs (7) are provided in the pipes along longitudinal direction and comprised with rib combs (8) between rib edges (9) such that rib combs of adjacent pipes are arranged alternately. Recesses are opened in the rib combs and the rib edges are provided at an angle of 90[deg] with respect to longitudinal axis of the pipes. An independent claim is included for method for manufacturing heat exchanger.

Description

The invention relates to a heat exchanger having the features of patent claim 1 and to a method for producing a heat exchanger according to the features of patent claim 12.

Heat exchangers are state of the art in a wide variety of designs. The heat exchange takes place between a liquid, which flows through tubes, and a gas, which flows around the tubes on the outside. By ribs, the heat exchanger surface is greatly increased and the heat transfer intensified. The fluid-carrying tubes usually pass through tube sheets, so that the liquid medium, starting from so-called water chambers, can flow into several tubes at the same time. The tubesheet serves as a partition. It is also possible to connect two pipes via manifolds so that on one side of the tubesheet the manifolds and on the other side the pipes are located.

It is known to form finned tube packages in which a plurality of fluid-carrying tubes jointly pass through stacked fin plates. The fixation of the ribs on the tubes takes place, for example, by widening the tubes, so that they are pressed against the openings in the ribs.

Although this manufacturing method is established, but always represents a certain manufacturing effort, since it must be ensured that the individual ribs lie close to the outer sides of the tubes for a good heat transfer.

Of course, cohesive connections, in particular solder joints between the ribs and the pipes are possible. By the US 5 042 574 A For example, it is known to bond aluminum-clad steel flat tubes using aluminum-silicon solder to wavy-folded aluminum ridge webs in a tempering furnace. This type of connection has the disadvantage that a soldering can only be made via the detour of aluminum-plated flat tubes. In addition to the comparatively high cost by providing various materials also has the disadvantage that the at least one longitudinal weld seam circumferentially closed flat tubes in the welding zone in the pipe production must not be clad with aluminum, otherwise no proper welding can be guaranteed.

When using different materials arise because of different thermal expansion coefficients of, for example, aluminum and steel due to the necessary high soldering temperatures and due to the subsequent cooling to ambient temperature strong material stresses, so that the interconnected parts can distort, and even the solder joint can break.

A fundamental problem in the production of large heat exchangers with correspondingly large rib areas is that the heating for soldering must be as uniform as possible and also slow, so that the solder used for connecting the ribs to the pipes is not heated too quickly in the region of the heat source and overheats while, for example, it has not yet reached a necessary temperature in a region remote from the heat source. The production is therefore time consuming.

Proceeding from this, the present invention seeks to provide a heat exchanger with a simple-to-produce, mechanical pipe-fin connection, which works without material connection between the pipes and the ribs but still allows a very good heat transfer and at the same time allows the combination of different materials and Moreover, it is quick to produce.

This object is achieved with a heat exchanger having the features of patent claim 1. A method of manufacture is the subject of claim 12.

The subclaims relate to expedient developments of the inventive concept.

• a) Zwischen parallel zueinander verlaufenden, Fluid führenden Rohren sind Rippen angeordnet;
• b) Die Enden der Rohre sind an Rohrböden angeordnet;
• c) Die Rippen sind als Rippenbänder ausgestaltet, die Zick-Zack-förmig verlaufen und sich in Längsrichtung der Rohre erstrecken;
• d) Die Rippen besitzen Rippenkämme zwischen denen sich Rippenflanken erstrecken;
• e) Die Rippen stützen sich mit ihrem Rippenkämmen abwechselnd an den angrenzenden Rohren ab;
• f) Die Rohre greifen in randseitig der Rippenkämme offene Ausnehmungen in den Rippenkämmen;
• g) Die Rippenflanken stehen in einem von 90° abweichenden Winkel zu Längsachse der Rohre.

The heat exchanger according to the invention has the following features:

• a) ribs are arranged between mutually parallel, fluid-carrying tubes;
• b) The ends of the tubes are arranged on tubesheets;
• c) The ribs are designed as ribbed ribs, which run in a zig-zag and extend in the longitudinal direction of the tubes;
• d) The ribs have rib combs between which rib flanks extend;
• e) The ribs are supported alternately on the adjacent tubes with their rib combs;
• f) The tubes engage in recesses open in the rib combs in the edge of the rib combs;
• g) The rib flanks are at an angle deviating from 90 ° to the longitudinal axis of the tubes.

In the heat exchanger concept of the invention, a special rib design is used, which is the thermal expansion makes use of the ribs and pipes to allow improved heat transfer between the pipe and the rib. The heat exchanger design according to the invention is independent of the materials of the tube and the rib. It can also be used to combine very different materials. Although an integral connection is additionally conceivable, it is not necessary in principle.

In the heat exchanger design according to the invention zigzag-shaped ribbed bands are provided, wherein the individual sections are designed to be somewhat trapezoidal. This results in a meandering shape. In each case recesses are arranged in the tips or rib combs of the adjacent trapezoidal regions, in which the tubes engage. It is not intended that the pipes pass through the ribs, but only that the pipes are arranged adjacent to the ribs. The ribs are pressed only peripherally against the tubes.

An essential feature of the invention is that the rib flanks are at an angle deviating from 90 ° to the longitudinal axis of the tubes. Since the tubes and the ribs stretch during operation of the heat exchanger, the contact pressure between the ribs and the tubes should increase when the heat exchanger is in operation. The ribs are therefore mounted so that the ribs can stretch but can not move indefinitely in the longitudinal direction of the tubes. This ensures that the contact pressure generated by thermal expansion actually transfers from the ribs to the tubes.

Another significant advantage is that over the previous production eliminates the expansion of the tubes. The expansion of pipes does not necessarily lead to a reduction in wall thickness, but to a reduction in length. Due to the existing tolerances in the wall thickness, however, the expansion also means that the tubes have different lengths after expansion and that the ends of different lengths must be adapted to the position of the tubesheet. It is therefore worked with an appropriate addition of the tube length, which is not required in each case. Excess pipe lengths must be separated, for example by drilling. By drilling relatively many chips, which is undesirable in terms of production technology, but is the better alternative to sawing, because vibrations are produced by sawing, which can affect the connection between the tubesheet and the tube. It would therefore be best if one could completely dispense with the step of separating or chipping. This is possible with the solution according to the invention. All tubes are the same length and do not have to be shortened.

Another advantage is that the tube itself compared to stacked finned tube heat exchangers of certain designs on the heat exchange part. This is to be understood that in the prior art with stacked ribs, the distance of the ribs on the height of the collar-like exhibitions is set, which are formed in the region of the tubes. In the assembly position, the exhibitions encounter each other so that the entire surface of the heat exchanger tube is completely enveloped by the ribs. In the heat exchanger according to the invention, this behaves differently. There are free surface areas of the tubes, which thus come directly into contact with the heat exchange medium and provide for an immediate heat transfer.

The heat exchanger according to the invention not only allows a very cost-effective production, but also still a high degree of flexibility in the choice of materials, as can be dispensed with more expensive, cohesive bonding techniques. In all candidate materials, a thermally induced expansion sets in, which inevitably always leads to the fact that the individual ribs are pressed by stretching firmly against the tubes.

It is irrelevant whether the pipe or the rib stretches more than the other component.

In an advantageous embodiment it is provided that the ribs are mounted biased by a biasing force acting in the longitudinal direction of the ribs and parallel to the tubes. The bias can be applied by reducing the pitch of the tubesheets. This assumes that the ribbed belts are supported on the tubesheets. Of course, tube sheets of different thickness can be used or uniform tube sheets, which are provided on your side facing the ribs with spacer elements on which the ribs are supported. By a certain biasing force, the operating pressure is reliably achieved. In addition, an improved heat transfer is achieved by the bias already at the start of operation, that is, at temperatures below the desired operating point.

The heat transfer can also be improved in that the recesses have a collar over which the recesses and thus also the ribs are supported on the tubes. About such a collar, the individual ribs over a larger area with the adjacent tubes in contact. This improves the heat transfer. At the same time, the surface pressure between the tubes and the expanding ribs is somewhat reduced, which can be advantageous for thin ribs or thin-walled tubes. The collar can also compensate for certain manufacturing tolerances.

It is considered particularly advantageous if the rib flanks have stiffeners which increase the buckling stiffness between the rib combs. The concept of the heat exchanger according to the invention is based on the fact that a positive connection between tubes and ribs is created by mutual expansion of the tubes and ribs. The contact force should not be reduced by a buckling of the rib edges. The rib edges must therefore have a predetermined minimum bending stiffness. In the context of the invention, this minimum buckling rigidity is preferably not achieved by accumulation of material, that is, by thick-walled rib flanks or local reinforcements on the rib flanks, but rather by stiffeners, which are designed in particular as embossings. Such embossings serve to displace material from the median plane of the rib flanks to the outside and thereby increase the flexural rigidity. The shape of the stiffening plays an important role here, since the buckling stiffness between two rib combs must be increased. In a preferred embodiment it is therefore provided that at least one stiffening extends between a recess in a first rib comb in the direction of the adjacent recess in the opposite rib comb. The stiffeners therefore, so to speak, run diagonally on the ribbed bands. Viewed in the longitudinal direction of the tubes, that is viewed from the direction of the tube plate, the stiffeners thus extend radially from one tube to the adjacent tube. An additional stiffening in the region of a rib comb is not required, since the rib comb itself is simple, preferably bent twice and thus is sufficiently rigid.

The exact contouring and position of the stiffeners depends on the geometry of the heat exchanger tubes. It is thus also conceivable that the stiffeners are perpendicular to the rib combs, which may be particularly advantageous in elliptical pipes in cross-section.

In principle, it is considered advantageous if the stiffeners in the form of embossings extend into the individual collars which are formed on the recesses. As a result, the wall of the stiffening passes into a certain extent directly into the collar, provided that the stiffeners in the form of imprints and the collar are oriented in the same direction.

A compact design of the heat exchanger is given in particular when the tubes are arranged in a plurality of mutually offset rows. This results in that the recesses are arranged offset to one another in the longitudinal direction of adjacent rib combs. The offset of the recesses thus determines the exact position of the stiffeners, in the form of web-like embossments.

• a) Bereitstellen von Rohren und Rippen, wobei die Rippen zu Zick-Zack-förmigen Rippenbändern gebogen und mit randseitigen Ausnehmungen versehen sind;
• b) Anordnen der Rohre zwischen den Rippen;
• c) Aufbringen einer Vorspannkraft auf die Rippen, um die Rippenkämme gegen die Rohre zu pressen, indem der Abstand zwischen Rohrböden reduziert wird.

A method for producing a heat exchanger according to any one of the preceding claims is the subject of claim 12. The method comprises the following steps:

• a) providing pipes and ribs, wherein the ribs are bent into zigzag rib ribbons and provided with edge-side recesses;
• b) placing the tubes between the ribs;
• c) applying a biasing force to the ribs to press the rib combs against the tubes by reducing the distance between tubesheets.

The method according to the invention has considerable advantages over both the production of heat exchangers with expanded tubes and heat exchangers in which the tubes and ribs are materially bonded because of the very high rationalization potential. In practice, the ribs and tubes are initially stacked alternately one above the other. Subsequently, the package consisting of ribs and pipes must be aligned so that the pipes fit exactly in or on the tubesheet. The package is placed between side walls and compressed by displacing at least one tube sheet relative to the other tube sheet. In this position, in which a biasing force is applied to the ribs, the tubesheet is fixed. The tubes can be connected to the tube bottom cohesively by welding, soldering, gluing or by expansion. Depending on the application, the tube sheet can be very tight or easily attached to tolerable leaks to a limited extent. Decisive for the present invention is not the type of attachment and the exact position of the pipe ends, which may rest only on the front side of the tube sheet or this alternatively partially or completely enforce, but the function of the tube sheet as an abutment for the ribs. It is within the scope of the invention also possible to provide separate abutment, on which the ribs are supported, while the tube sheets primarily have a sealing function. Functionally such abutments belong to the tubesheet.

So that the outer ribs of a heat exchanger lie close to the tubes, it must be ensured that the housing is sufficiently stable or exerts sufficient force on the peripheral ribs, so that the entire system does not bulge outwards. Therefore additional tie rods may be provided to remove the package To limit pipes and ribs in the transverse direction against lateral expansion and to maintain the biasing force between the ribs and the tubes.

The invention will be explained in more detail with reference to embodiments shown in the drawings. It shows:

1 a perspective view of an embodiment of a heat exchanger,

2 a view from above of the heat exchanger of 1 ;

3 a view in the direction of the longitudinal tubes of the heat exchanger of 1 ;

4 a view of the tubes and fins of the heat exchanger of 1 without surrounded housing and without tubesheets;

5 a side view of the tubes and fins of the heat exchanger of 1 without side panels and tube sheets;

6 a cross-sectional view through the tubes and fins of the heat exchanger of 1 ;

7 a perspective view of the tubes and fins of the heat exchanger of 1 ;

8th to 10 in plan view, side view and end view of a single fin plate of the heat exchanger of 1 ;

11 the ribbed plate of the 8th to 10 in perspective view;

12 a perspective view of a second embodiment of pipes and fins of a heat exchanger;

13 a side view of the parket of pipes and ribs according to 12 ;

14 one opposite the 13 90 ° offset views of the finned tube package 12 (View from above)

15 the finned tube of the finned tube pacts 12 in the front view;

16 a single fin plate of the heat exchanger of 12 in perspective view and

17 to 19 a plan view, side view and end view of the fin plate of 16 ,

1 shows a heat exchanger 1 , The heat exchanger 1 is very compact in this embodiment. It is noted that in practice the illustrated tubes can be many times longer. Also, the heat exchanger 1 be built much larger overall. The illustrated embodiment is merely illustrative of the inventive concept and is by no means to scale. In addition, in the following description of the figures, terms such as top, bottom, left, right, front, back, etc. refer exclusively to the exemplary representation and position of the depicted heat exchanger selected in the respective figures 1 and its components. The terms are not intended to be limiting, that is, in different working positions or by mirror-symmetrical design of the like, these references may change. The illustrated heat exchanger 1 owns several fluid-carrying tubes 2 , All pipes 2 run parallel to each other. The pipes 2 are at the moment. They extend between opposite and mutually parallel tube sheets 3 . 4 , The pipes 2 enforce each of the tube sheets 3 . 4 , are outward but not on the tube sheets 3 . 4 before, but are in the tube sheets 3 . 4 determined, for example, by a cohesive connection (welding, soldering) or by a positive connection such as curling.

In addition to the tube sheets 3 . 4 are as additional housing components sidewalls 5 . 6 provided, extending between the tube sheets 3 . 4 and define a rectangular interior. It is understood that the shape of the interior is determined decisively by the arrangement of the tubes and the shape of the tube bottom (circle, rectangle).

In this interior, arranged by the mutually parallel and mutually offset rows of tubes 2 is interspersed, there are also ribs 7 , There are nine rows of ribs altogether 7 , which enclose eight mutually offset rows of tubes. Ribs 7 are configured as a single rib bands, which run zig-zag and each extending between two adjacent rows of tubes. This is especially good on the basis of 2 and 3 to recognize. In 3 are due to the selected representation eight rows or columns of pipes 2 to recognize. There are always columns with three and four tubes 2 from. Between these columns are the in 2 illustrated ribs 7 in the form of ribbed ribbons. It can be seen that the ribs 7 the pipes 2 only partially encompass. Ribs 7 have rib combs for this 8th , The between the rib combs 8th extending rib flanks 9 are at an angle W deviating from the 90 ° to the longitudinal axis L of the tubes 2 ( 6 ). In this Embodiment, the angle W is 108 °. The angle W can be reduced by applying a biasing force F to the ribs 2 is applied. In 2 is the direction of this biasing force F drawn. It is assumed that the side walls 5 . 6 measured so long that a defined biasing force F on the ribs 7 established. In the relaxed state, the angle W between the rib edges 9 therefore, be larger than shown in the drawings.

From the representation of 4 it becomes clear that the individual ribs 2 in this perspective, as it were, have a rectangular basic contour, which points to the visible rib edges 9 also true. The rib flanks 9 are only marginal, that is in the area of their rib combs 8th with recesses 10 fitted to the outer contour of the tubes 2 are adjusted. Due to the cross-sectionally circular tubes shown here 2 are the recesses 10 a circular arc. They extend approximately over an angular range of 135 °, so that in this embodiment 270 ° or ¾ of the circumference of a tube 2 of recesses 10 the ribs 7 are enclosed. Thus, remain between the successive ribs 7 narrower column 11 , where the width of the column 11 according to the representation of 4 about half the width B of the ribs 7 corresponds in the illustrated direction.

All ribs 7 are uniformly configured. Due to the staggered arrangement of the tubes 2 are also the recesses 10 in the rib combs 8th arranged offset from one another. In this embodiment, each rib has 2 therefore in her one rib comb 8th four recesses 10 and in the opposite rib comb 8th three recesses 10 , The recesses 10 are also in the marginal ribs 7 to recognize how in particular from the perspective of 5 you can see.

A special feature of the illustrated ribs are stiffeners 12 that in representation of 4 run diagonally and always from a pipe 2 to the next pipe 2 that the ridge comb 8th touched on the opposite side, point. This results in a diamond-like pattern.

The stiffeners 12 increase the kink resistance between two rib combs 8th , These are bar-shaped forms, which are parallel to the radially shortest connection between adjacent tubes 2 run. Specifically, two web-shaped embossments are provided in this embodiment. Each of the embossments opens into the diagonally opposite recess 10 in the other rib comb 8th , The stiffeners 12 have this orientation as this is the shortest connection between two adjacent tubes 2 is and here due to the thermal change in length with the highest contact force between the pipe 2 and the rib 7 works, is expected.

The presentation of the 5 is a look at the rib combs 8th , It can be seen that the rib combs 8th not pointed, but plateau-like. As a result, they have a rectangular contour. The rib combs 8th are several times narrower than the adjacent rib flanks 9 , To the rib combs 8th also become the recesses 10 in the rib combs 8th counted, with the recesses 10 with collar 13 are provided. The collars 13 are each turned inward, as based on the 6 can be seen. That means on a rib comb 8th show the collar 13 not to the outside, but to some extent inside, so that in the rib comb 8th through the recess 10 given opening is reduced. From the perspective of 5 It can be seen that the opening thereby has the shape of a flat oval.

All ribs 7 are arranged mirrored. That means a pipe 2 as it is for example in 7 is marked, if it is on one side of a recess 10 from in the right in the image plane ribs 7 is included, at the same height or in its same length section also on the opposite side of a recess 10 in the other rib 7 is surrounded. As a result, no bending forces in the pipes 2 initiated. The circumferential support is always in the same length of a pipe 2 , This also ensures that thermally induced changes in length lead directly to an increased contact pressure and not to a deformation of the ribs 7 or the pipes 2 ,

To the most rigid design of thin-walled ribs 2 Also contributes to that the stiffeners 12 in the form of web-like embossing not before, that is at a distance to the collar 13 but the collars 13 enforce or end in this. This is especially good in 11 to recognize. There you can see the web-like beads or shapes, which are in the same direction as the collar 13 extend into which they lead. The collar 13 possesses a sort of openings. In this embodiment, there are four openings, which due to the selected distance of the stiffeners 12 arranged quasi equidistantly. While the stiffeners 12 for a power transmission in the transverse direction or diagonally to the rib edges 9 worry, settles in 11 also recognize well that the rib combs 8th due to their fold against the rib edges 9 also have sufficient bending stiffness, so that forces in the longitudinal direction of the rib comb 8th can be transferred well.

The embodiment of the 12 to 19 is different from the one of 1 to 11 by two characteristics: First, the pipes 14 not circular in cross-section, but elliptical and secondly, the stiffeners run 15 perpendicular to the rib combs 8th , Accordingly, the recesses 16 ( 15 ) also to the elliptical shape of the tubes 14 customized. The remaining reference numerals for this form of pipes 14 and ribs 17 were referring to the 1 to 11 taken over because they designate identical components. Significant, however, is the difference between the stiffeners 12 of the 1 to 11 and those stiffeners 15 , the two 12 to 19 is shown. Due to the differently oriented stiffeners 15 also results in a different force curve within the ribs 17 , The individual pipes 14 are the same as in the first embodiment ( 1 to 11 ) arranged offset to each other. The stiffeners 15 are in turn web-like characteristics, which are in the same direction as the collar 13 point. All stiffeners 12 are arranged equidistant, with two each of the stiffeners 12 in the ends of a recess 16 lead. The intermediate bracing 12 I am in the deepest or vertex of the recess 16 and ends in the opposite rib comb 8th , Through the middle stiffener 12 should the thermal expansion in the rib comb 8th be initiated so that the rib 17 thereby being stretched in this area. This stretching causes the adjacent recess 16 in this rib comb 8th close to the associated pipe 14 snugly.

LIST OF REFERENCE NUMBERS

1

heat exchangers

2

pipe

3

tube sheet

4

tube sheet

5

Side wall

6

Side wall

7

rib

8th

fin comb

9

rib flank

10

recess

11

gap

12

stiffening

13

collar

14

pipe

15

stiffening

16

recess

17

rib

B

width

F

preload force

L

longitudinal direction

W

angle

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 5042574 A [0005]

Claims (12)

Heat exchangers having the following features: a) between mutually parallel, fluid-carrying tubes ( 2 . 14 ) are ribs ( 7 . 17 ) arranged; b) The ends of the tubes ( 2 . 14 ) are arranged on tube sheets; c) The ribs ( 7 . 17 ) are designed as ribbed ribs, which run in a zig-zag and extend in the longitudinal direction (L) of the tubes ( 2 . 14 ) extend; d) The ribs ( 7 . 17 ) have rib combs ( 8th ) between which rib flanks ( 9 ) extend; e) The ribs ( 7 . 17 ) support themselves with their rib combs ( 8th ) alternately on the adjacent tubes ( 2 . 14 ); f) The pipes ( 2 . 14 ) grip in the edge of the rib combs ( 8th ) open recesses ( 10 . 16 ) in the rib combs ( 8th ); g) The rib flanks ( 9 ) are in an angle deviating from 90 ° (W) to the longitudinal axis (L) of the tubes ( 2 . 14 ). Heat exchanger according to claim 1, characterized in that the ribs ( 7 . 17 ) in the longitudinal direction of a ribbed belt and parallel to the tubes ( 2 . 14 ) acting biasing force (F) are biased mounted. Heat exchanger according to claim 1 or 2, characterized in that the recesses ( 10 . 16 ) a collar ( 13 ) over which the recesses ( 10 . 16 ) on the pipes ( 2 . 14 ) are supported. Heat exchanger according to one of claims 1 to 3, characterized in that the rib flanks ( 9 ) the buckling stiffness between two rib combs ( 8th ) increasing stiffeners ( 12 . 15 ) exhibit. Heat exchanger according to claim 4, characterized in that the stiffening ( 12 ) is formed as an embossing. Heat exchanger according to claim 4 or 5, characterized in that the at least one stiffener ( 12 ) between a recess ( 10 ) in the one rib comb ( 8th ) towards the adjacent recess ( 10 ) in the opposite rib comb ( 8th ). Heat exchanger according to one of claims 1 to 6, characterized in that the stiffeners ( 15 ) perpendicular to the rib combs ( 8th ). Heat exchanger according to claims 5 to 7, characterized in that the stiffeners ( 12 . 15 ) in the form of imprints down to the collar ( 13 ). Heat exchanger according to one of claims 1 to 8, characterized in that the stiffeners ( 12 . 15 ) are formed by a web-shaped embossing or by a plurality of mutually parallel ridge-shaped embossments. Heat exchanger according to one of claims 1 to 9, characterized in that the tubes ( 14 ) are elliptical in cross-section. Heat exchanger according to one of claims 1 to 10, characterized in that the recesses ( 10 . 16 ) in the longitudinal direction (L) of adjacent rib combs ( 8th ) are arranged offset from each other. Process for producing a heat exchanger according to one of the preceding claims, comprising the following steps: a) providing pipes ( 2 . 14 ) and ribs ( 7 . 17 ), the ribs ( 7 . 17 ), bent into zig-zag ribbed bands and with recesses ( 10 . 16 ) are provided; b) arranging the tubes ( 2 . 14 ) between the ribs; c) applying a biasing force to the ribs ( 7 . 17 ) to the rib combs ( 8th ) against the pipes ( 2 . 14 ) by reducing the distance between the tubesheets to which the tubes ( 2 . 14 ) to be ordered;

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