FIELD OF INVENTION:
The field of invention pertains to mechanical engineering. More specifically it pertains to an improved heat exchanger.
BACKGROUND TO THE INVENTION (PRIOR ART): Definition of Important Terms:
1. Calandria: Equipment consisting of closely spaced metal tubes for heat exchange.
Vertical tube type falling film heat exchangers e.g. evaporators, condensers, evaporative condensers, find application in large number of industries where heat exchange is required. Vertical falling film tube heat exchangers typically include a heat exchanger chamber disposed between upper distribution chamber and a vapour separation chamber below. The heat exchanger chamber has plurality of heat exchange tubes positioned therein which vertically extend between a top tube sheet and a bottom tube sheet. The length of the tubes is 6-15 m, with 8-12 m being used most often. Evaporation occurs on the inner surfaces of tubes as the feed liquid falls as a film on their inner surfaces. The bank of tubes is also referred to as calandria. Feed material enters the upper distribution chamber through a feed inlet associated therewith and is directed downwardly through the vertical heat exchange tubes in the form of a falling film in a manner ensuring that the tubes receive approximately equal flows. Vapour-liquid separation usually takes place at the bottom, although some evaporators of this type are arranged for vapour to rise through the tube counter-currently to the liquid.
In falling film evaporators two factors are critical for evaporator efficiency viz.
a) uniform distribution of the feed liquid to every tube
b) even product wetting of the tubes over the entire length
The uniform distribution of the feed liquid is achieved by use of specially designed distribution devices whose proper functioning is decisive to the

operational safety of the heat exchanger. Numerous structures and devices are known for improving the distribution of the liquid, such devices generally being placed either on the top of tube bundle or inserted or screwed into the tubes. In the former type of distributors i.e. liquid feed distributor mounted on top of the tube bundle, generally the liquid falls on the tube sheet and then flows into the tubes (flush welded with the tube sheet) in the form of a film. In many cases where the tubes protrude above the top tube sheet forming a collector or reservoir with top tube sheet acting as a base for the collector or reservoir, the feed liquid overflows over the upper edge of the protrusion which may be serrated or toothed. The protrusion may also be slotted or provided with grooves or borings or depressions or the like.
To satisfy the requirement of minimum tube wetting, a flow rate of the feed liquid is maintained. In many cases, it is desirable to provide for recirculation of product through the heat exchanger. Longer the tube higher is the flow rate. This, however, is accompanied by a disadvantage that the upper portions of the tubes receive a thick film of liquid affecting the heat transfer coefficient. Further, while the film thins out towards the lower portions of the tube, the condensate film on the other side thickens in its downward descent thereby again affecting the heat transfer characteristics.
The fact that falling film heat exchangers operate at lower temperature differences makes it possible to use them in multiple effect configurations systems in modern plants with very low energy consumption. In certain apparatuses multiplicity of vertical tube bundles are stacked one above the other e.g. as in superimposed multi effect evaporators. Prior art is replete with apparatuses having vertical tube bundles in superimposed configurations. All, however, always require a separate liquid distribution means to be interposed between the vertically stacked tube bundles.

In constructing heat exchangers the aim is to achieve the maximum performance in apparatus of minimum size. One approach is to provide maximum heating surface in minimum possible area. Of the various means used to achieve this aim, use of finned or fluted tubes has been the most common. The design of the finned or fluted tubes is also adapted to increase the velocity of the descending liquid thereby increasing the heat transfer coefficient.
Another means to achieve the maximum heating surface in a given area is through the use of honeycomb calandria in which the calandria tubes are expanded to a hexagon at their ends and are welded directly to each other without the use of a tube plate in the form of a honey comb. The honey comb calandria provides the following distinct advantages:
1. Highest possible heating surface in a given area
2. Compact over-all dimensions.
3. Elimination of both the top & bottom tube-sheet
4. Greater structural integrity
5. Lower maintenance costs and long life
6. Lower welding requirements
The use of honey comb calandria has found wide application especially in calandria tube evaporators (Robert evaporators) and rising film tube evaporators (Kestner evaporators) especially in sugar industry since the feed enters at the bottom, there is no problem of uniform feed liquid distribution to all the tubes.
However, with the trend to install evaporators of larger size, Robert evaporators are generally restricted to about 3000 m2 each, as the diameter gets too large. Moreover, due to large liquid hold up, these evaporators cannot be used with heat sensitive materials. In case of Kestner evaporators, to establish a well-developed film, the rising film unit requires a driving film force, typically a temperature difference of at least 14 deg. C across the heating surface. In contrast, the falling film unit does not have a driving force limitation-permitting a greater number of

evaporator effects to be used within the same overall operating limits. In other words, the rising film evaporator requires a larger minimum delta T than a falling film unit. Further, in rising film evaporators, the hydrostatic head may create problem with heat sensitive products. The pressure drop is higher than for falling film types and there is a tendency to scale.
However, the application of honey comb calandria in vertical falling film tube heat exchangers is restricted mainly owing to:
a) difficulty in all important uniform and adequate liquid distribution due to
absence of intermediate space between the tubes, and
b) secondly, the tube length cannot be kept long due to difficulty in providing
support to the unsupported length of the tubes.
Further, while most of the existing distribution devices which are inserted or screwed in the tubes will require considerable design modification for being adapted to the honey comb arrangement, many of them e.g. those involving tangential entry or employing overflow, are rendered unsuitable for being used in honey comb calandria.
These features of the honey comb calandria henceforth considered as limitations for their application in vertical tube falling film heat exchangers are overcome through the use of a simple but novel tube extension resulting in an improved vertical tube falling film heat exchanger.
A search of Indian patent database reveals that no patent as for the present invention has been described in the prior art.
OBJECTS OF THE INVENTION:
It is therefore an object of the invention to provide an improved vertical tube falling film heat exchanger.

SUMMARY OF THE INVENTION:
The invention discloses a simple but novel tube extension for heat exchange tubes of a calandria having ends expanded in the form of a hexagon or any other angular form, which permits the use of honeycomb calandria (used widely in short tube calandria evaporators and rising film evaporators) in vertical falling film tube heat exchangers in a simple, efficient and inexpensive manner resulting in an improved vertical tube falling film heat exchanger. The novel tube extension results in complete elimination of separate liquid distributor means always required to be interposed between the vertical tube bundles in superimposed configurations. This has been achieved through the use of a substantially inverted funnel shaped tube extension of the end of the heat exchange tube which in use is its upper end. The said extension functions as an integral distribution device for vertical falling film tube heat exchanger. Alternatively, it can also function as a receptacle for distribution devices which are inserted or screwed into the tubes without any design modification.
STATEMENT OF INVENTION:
Accordingly, the invention provides an improved vertical tube falling film heat exchanger, comprising:
a single bank or multiple superimposed interspaced banks of vertical tubes within a housing,
the said vertical tubes sealed at the hexagonally or other angularly expanded ends or regions and forming said tube bank;
means for heating the said tube banks;
liquid distribution means;
means for removal and/or directional flow of vapours;
means for removal and/or directional flow of condensate and non-condensed gases;
means for liquid recirculation;
wherein the improvement comprises of a substantially inverted funnel shaped extension at the upper ends of the vertical tubes, whereby the said extension permits the use of honeycomb calandria in the vertical tube falling film heat exchangers by ensuring uniform liquid distribution in the vertical tubes and completely eliminating the need of a separate distribution means interposed between superimposed banks of vertical tubes.

BRIEF DESCRIPTION OF THE DRAWINGS:
Fig. 1 is the schematic isometric representation of a bank or bundle of vertical
tubes incorporating the novel tube extension of the invention.
Fig. 2 is the enlarged sectional view of the area shown in circle A in Fig. 1
Fig. 3 shows a schematic sectional elevation view of one embodiment of an
apparatus constructed according to this invention.
Fig. 4 is an enlarged schematic sectional elevation view of the top two stages of
the apparatus shown in Fig. 3.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE DRAWINGS:
It has now been found in accordance with the teaching of the instant invention that the features of the honey comb calandria henceforth considered as limitations for their application in vertical tube falling film heat exchangers are overcome through the use of a simple but novel tube extension resulting in an improved vertical tube falling film heat exchanger.
The invention is particularly useful where a multiplicity of banks of vertical tubes are arranged in superimposed configurations since the novel tube extension results in the following advantages:
(i) Complete elimination of separate liquid distribution means always required to be interposed between the vertical tube bundles in superimposed configurations.
(ii) Simple Construction: The novel tube extension makes the apparatus simple in construction. The cumbersome and complex fabrication and installation of a separate liquid distribution means is avoided, (iii) Compact: The tube extension of the invention has lent a degree of compactness to the apparatus due to complete elimination of intermediate liquid distribution means.

(iv) Operational Problems Overcome: Also the operational problems like misalignment or bending of the distribution means have been overcome through the use of the novel tube extension of the invention.
The invention provides a novel tube extension of a calandria tube having its ends expanded in the form of a hexagon or any other angular form, characterized by the fact that the end of the said calandria tube which in use is its upper end comprises of a substantially inverted funnel shaped extension with a predetermined length and diameter of the tubular portion of the said extension.
The said extension functions as an integral distribution device for each bank of vertical tubes in a falling film heat exchanger by permitting the liquid to accumulate around it, reach a certain level and then enter each tube individually and uniformly. The upper edges of the tubular portion of the said extension may be serrated or toothed. It may also be slotted or provided with grooves or borings or depressions or the like or of any other desired configuration. Alternatively, it can also function as a receptacle for distribution devices which are fitted into the tubes without any design modification.
Further, more advantageously, such modified tube design being simple is easy to manufacture and allows negligible wastage of raw material in its manufacture.
It is to be understood that while the drawings and accompanying description of the instant application will, for the most part, be confined to a tube extension, of a heat exchanger tube with ends expanded in the form of a hexagon or other angular form, as an integral part of such heat exchanger tube, the invention contemplated herein also includes formation of such a tube extension as a detachable or separate part thereof and either of the similar or different material of construction.
Corresponding and like parts are referred to in the following description, and indicated in all the views of the drawings, by the same reference characters.

Fig. 1 is the schematic isometric representation of a bank or bundle 1 of vertical tubes 2 incorporating the novel tube extension 3 of the invention. Fig. 2 is the enlarged sectional view of the area of the tube bundle shown in dotted circle A in Fig. 1 clearly showing the novel tube extension 3 in detail.
The said tube extension 3 of a calandria tube 2 having its ends expanded in the form of a hexagon or any other angular form is characterized by the fact that the end of the said heat exchanger tube 2 which in use is its upper end is comprised of a substantially inverted funnel shaped extension 3. The said extension 3 has a lower substantially conical portion 4 and an upper tubular portion 5 at the top end of each calandria tube 2. The said extension 3 functions as an integral distributing device for a vertical tube falling film heat exchanger. The ends of the tubular portion 5 of the said extension 3 may be serrated or toothed or of any other desired configuration. Alternatively, it can serve as a receptacle for fitting a distribution device for vertical tube falling film heat exchangers. The tubes 2 are sealed directly to each other without the use of tube plate at the hexagonally 6 or other angularly expanded zones.
The height and diameter of the tubular portion 5 of the substantially inverted funnel shaped extension 3 is determined according to the operational requirements. Preferably the diameter of the said tubular region 5 is the same as the diameter of the heat exchanger tube 2 in the section between the expanded hexagonal zones 6. The dimensions of the intermediate region 7 between the novel tube extensions 2 of the invention depend upon the dimensions of both the substantially conical section 4 and the tubular portion 5 of the said novel extension 3. This intermediate region 7 functions as a collector or reservoir for the liquid. In cases where the liquid is caused to overflow the periphery of the said novel extension 3, the length of the tubular region 5 is determined by the head of the liquid required to be maintained thereon to regulate liquid flow and distribute the liquid.

Fig. 3 shows a schematic sectional elevation view of an embodiment of an apparatus constructed according to this invention. Fig. 4 is an enlarged view of the top portion of the apparatus shown in Fig. 3 showing upper two stages. It highlights the advantages of the invention in superimposed configurations. A multiplicity of super-imposed banks 1 of vertical tubes 2 or stages is arranged in a cylindrical housing 8. The said stages may be operated in a single-effect as shown in Fig. 3 or in a multi-effect mode.
When operated as a single effect, the means of heating comprise a common steam conduit 9 interconnecting the steam-side 10 of the said multiplicity of superimposed banks 1 of vertical tubes 2 or stages in parallel. More of such single-effect units may also be applied in a multiple-effect arrangement. The tube bundles 1 may or may not have a casing depending upon the arrangement and design of the apparatus. Many such designs and arrangements are known in the prior art. A vapour space 11 vertically separates from one another said multiplicity of vertically stacked bundles 1 of vertical tubes 2 mounted within a single vessel 8 without any further distributor means interposed between them. If operated as a single effect the said vapour space 11 is connected to condenser means 12 for condensing the said vapour. A common entrainment separator (not shown) may also be provided in the passage of vapours from the vapour space to condensing means. On the other hand, if the apparatus is operated in a multi-effect mode, known means may be used for connecting the effects whereby in use vapour produced in one effect can flow into the next above or below it for condensation therein. Known means may also be used for removal or to facilitate directional flow of the condensate 13 and non-condensed gases 14.
In operation, the liquid to be treated may be once distributed through any conventional distributor means 15 over the topmost tube bundle 1. The liquid flows over the periphery of the said novel tube extension 3 forming a film on the inner surface of the tubes 2. The liquid film coming out from the lower

hexagonally or other angularly expanded ends of the heat exchange tubes 2 in the bundle above, falls in the intermediate region 7 (which serves as a collector or reservoir) between the novel tube extensions 3 of the lower tube bundle 1 and is distributed over the tube circumference of the lower tube bundles. This completely eliminates the liquid distributor means conventionally interposed between the vertically stacked tube bundles thereby making the arrangement simple and compact. The operational problems associated with conventionally interposed distributors are also thus totally avoided.
Means may also be provided for addition of feed liquid at any stage (when operated as a single effect) or in any effect (when operated in multi-effect mode). The advantage of addition of feed liquid at multiple stages or effects in comparison to single distribution once at the top is that a lower flow rate is required. This prevents the formation of a thick liquid film in the upper portions of the tubes ensuring high heat transfer coefficient.
Further, removal of condensate 13 in intermediate stages prevents formation of thick condensate layer as is formed in case of a conventional single long tube again benefiting the heat transfer characteristics. The removal of vapours also becomes easy and with a lesser energy expenditure. All these benefits have become economically viable owing to the use of novel tube extension 3 of the invention resulting in the elimination of separate liquid distributor means always required to be interposed between the vertically stacked tube bundles 1. Liquid recirculation means 16 may also be provided.
In order to ensure the uniform distribution of the liquid to all the tubes 2 in the bundle 1, liquid lanes are provided. These liquid lanes correspond to and overlie the steam lanes which are provided for even distribution of the steam or vapours especially in honeycomb calandria. While being advantageous for initial liquid distribution in the uppermost stage, the said liquid distribution lanes aid in uniform distribution of the feed liquid added in the lower stages or effects. The

collector or reservoir formed by the intermediate spaces 7 between the novel tube extensions 3 is also provided with a level controlling means.
While the invention has been illustrated and described as embodied in a simple and compact vertical falling film tube heat exchanger apparatus, it is not intended to be limited to the details shown, since it will be understood that various omissions, modifications, substitutions and changes in the forms and details of the apparatus illustrated and in its operation can be made by those skilled in the art without departing in any way from the spirit of the present invention which primarily comprises substantially inverted funnel shaped extension at the upper end of the vertical tubes which permits the use of honeycomb calandria in vertical tube falling film heat exchangers which was earlier not possible. The presence of of the said extension results in uniform liquid distribution in the vertical tubes and completely eliminates the need of a separate distribution means interposed between superimposed banks of vertical tubes.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

We CLAIM:
1. An improved vertical tube falling film heat exchanger, comprising:
a single bank or multiple superimposed interspaced banks of vertical tubes, within a housing,
the said vertical tubes sealed at the hexagonally or other angularly expanded ends or regions and forming said tube bank;
means for heating the said tube banks;
liquid distribution means;
means for removal and/or directional flow of vapours;
means for removal and/or directional flow of condensate and non-condensed gases;
with or without means for liquid recirculation;
wherein the improvement comprises of a substantially inverted funnel shaped extension at the upper ends of the vertical tubes.
2. An improved vertical tube falling film heat exchanger according to claim
1 wherein the said substantially inverted funnel shaped extension is an integral
part of the heat exchange tube.
3. An improved vertical tube falling film heat exchanger according to claim
1 wherein the said substantially inverted funnel shaped extension is a detachable
part of the heat exchange tube.
4. An improved vertical tube falling film heat exchanger according to claim
1 wherein liquid distribution means further comprise of liquid distribution lanes
corresponding to and overlying the steam lanes of the honey comb calandria.
5. An improved vertical tube falling film heat exchanger substantially as
herein described in the specification with reference to the accompanying
drawings.