GB2155610A - Heat exchanger - Google Patents

Description

(12) UK Patent Application (ig)GB 1) 2 155 610 A (43) Application

published 25 Sep 1985 (21) Application No 8406530 (22) Date of filing 13 Mar 1984 (71) Applicants Petrocarbon Developments Limited (United Kingdom), Petrocarbon House, Sharston Road, Manchester M22 4TP. DUT Pty Limited (Australia-New South Wages), Killangolo, Burradoo Road, Burradoo. Nr Bowral, -New South Wages 2576. Australia (72) Inventors Terence Ronald Tomlinson Donald Ray Cummings (74) Agent and/or Address for Service Mathys & Squire, 10 Fleet Street, London EC4Y 1 AY (5 1) INT CL' F28B 1 /00 F28F 3/02 (52) Domestic classification F4S 4E1A 4E1 B 4E2X 4G 4U1 1 51J (56) Documents cited GB 1570728 GB 1375503 GB 1256964

(58) Field of search

F4S ERRATUM SPECIFICATION NO 2155610A

Page No 1 Line No 73 for outer read outlet Page No 1 Line No 87 for passaes read passages Page No 2 Line No 99 after sheet insert lies. The degree of extension of the corrugated sheet Page No 2 Line No 119 for information read formation Page No 3 Line NO 5 for 4 read 5 THE PATENT OFFICE 15 January 1986 A D?,2D / 7/O AW,Al Sj, e ACCESS FDR FEED LIQUID our The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

G) cc) l\i 01 cl m 1 0 1 GB 2 155 61 OA 1 SPECIFICATION

Heat exchanger This invention relates to refluxing heat ex- 70 changers sometimes known as dephlegmators, and in particular to such exchangers of the so called plate-fin type.

Such exchangers may be described as corn prising a block which in its simplest form includes at least one first hollow section hav ing an inlet and outlet for the passage of fluid heat exchange medium therethrough and at least one second hollow section which is de l 5 fined by a pair of spaced parallel plates subdi vided by a corrugated sheet into a plurality of generally parallel and vertically disposed pas sages for the upward flow of vapour there through from an inlet at the bottom to an outlet at the top. The said first and second hollow sections are separated by a common wall, which is usually one of the flat plates of the second section, for the transfer of heat between the fluid heat exchange medium and the vapour. Usually, the first section, like the second, is also divided into a plurality of parallel passageways eg. by a corrugated sheet.

In another form, the second hollow section may be divided by two or more corrugated sheets each separated from the next by a flat sheet, to form, in effect, two or more banks of passages for the upward flow of the vapour which is to be cooled. In operation, the va pour to be cooled enters the passages at the bottom and travels upwards while being cooled by indirect heat exhange with the fluid circulating through the first hollow section.

It is thus a feature of these exchangers that within some, at least, of the said passages, condensation of the vapour occurs so that there is a liquid phase which runs down the passage under gravity in counter-current to the rising vapour stream. A certain minimum flow area is required to ensure that a pheno menon termed 'flooding' does not occur.

Flooding occurs when moving vapour exerts an upward shear force on the liquid sufficient to thicken the liquid film and cause bridging of the passage with a consequential large increase in gas pressure drop and liquid en trainment in the vapour stream. The minimum cross-sectional (flow) area is thus primarily dictated by the need to avoid flooding, and not by heat transfer considerations.

At the bottom of each passage, the de scending liquid film must thicken and form drops of sufficient size to overcome surface tension forces before the liquid film can break away. These drops tend to reduce the flow area for ascending gas and thus the gas velocity and vapour shear will locally be higher than elsewhere and the greatest ten dency for flooding is at the bottom of the passage.

1 The present invention provides an improved design of refluxing plate-fin heat exchanger in which the tendency to flood at the bottom of the passage is reduced or avoided.

According to the present invention, there is provided a refluxing platefin heat exchanger comprising a block including at least one first hollow section with an inlet and outer for the passage of a fluid heat exchange medium therethrough and at least one second hollow section defined by a pair of generally parallel and flat plates the space between which is divided by a corrugated sheet, or two or more corrugated sheets each separated from the next by a flat sheet, into a plurality of separate and generally vertically disposed passages for the upward flow of vapour therethrough from an inlet at the bottom to an outlet at the top, said first and second sections having a common wall for the transfer of heat between them, and wherein each of at least some of said passaes are provided with means, such as a surface extending generally downwardly below the bottom of the passage, for lowering the droplet forming zone for any liquid flowing down the wall of the passage to a levol at which droplet formation does not constrict the opening into the passage.

The surface extending generally down- wardly below the bottom of the passage may be provided in many ways. In one embodiment the wall of the passage may be extended below t he passage opening for a part only of its periphery; for example, by extend- ing the corrugated sheet, a corrugation of which forms part of the passage wall, downwardly below the bottom of the flat sheet which forms the remainder of the passage wall, or vice versa. In this case, the corru- gated sheet or flat sheet which is extended downwards relative to the other may be perforated in its extension, if desired, to reduce interference with horizontal gas flow below the passage opening.

In another alternative, an extension which may be in the form of one or more wires or strips may be attached to the bottom of the passage e.g. by brazing or other suitable means. The extension may, for example, cornprise a bundle of wires in the form of a brush. Such extensions may, if desired, be attached to the walls of the passages above the bottoms thereof and extend below the bottoms whereby to conduct any liquid flowing down the passage wall to a level below the passage opening. It will also be understood that these extensions need not be attached to, or even touch, the passage wall provided they are in sufficiently close proximity as to permit flow of liquid down the passage wall to transfer to and continue to flow down the extension.

The invention will now be described in greater detail with reference to certain embodiments thereof and with the aid of the accompanying drawings in which:

GB 2155 610A 2 Figure 1 is a perspective view showing a conventional plate-fin refluxing heat exchanger of the kind having a plurality of alternating first (or coolant) and second (or refluxing) sections and one bank of passages, ie. one corrugated sheet, in each second or refluxing section; Figure 1A is a cross-section through one of the reflexing sections of the heat exchanger of Fig. 1; Figure 2 is a corresponding view of the heat exchanger of Fig. 1 modified in accordance with one embodiment of the present inven tion; Figure 3 is the corresponding view of the 80 heat exchanger of Fig. 1 modified in accor dance with another embodiment of the pre sent invention; Figures 4 and 5 are diagramatic vertical cross-sectional views showing an adaptation of the embodiment of the invention illustrated in Fig. 2 to refluxing heat exchangers having two and three banks, respectively, of passages in each second or refluxing section; and ---Figures6 and 7 are diagramatic vertical cross-sectional views showing the adaptation of the embodiment of the invention illustrated in Fig. 3 to refluxing heat exchangers having two and three banks, respectively, of passages in each second or refluxing section.

Fig. 1 illustrates one form of conventional plate-fin heat exchanger having a single bank of passages in each second or refluxing section. In effect, the exchanger comprises a plurality of spaced parallel flat plates 10, 20, 30 etc. which are generally known as parting sheets. The spaces between adjacent plates define alternate first (coolant) and second (refluxing) sections of the heat exchanger.

Thus, the spaces between eg. plates 10 and 20, and between plates 30 and 40, define first sections of the heat exchanger through which a fluid coolant is passed. Likewise, the spaces between eg. plates 20 and 30 and between plates 40 and 50 define second or refluxing sections of the heat exchanger through which the vapour feed to be cooled is passed in an upward direction as shown.

Each of the refluxing sections is divided into a plurality of vertical passageways by corrugated sheets 12, 22 etc. these corrugations having in cross-section the general configuration of a square wave with rounded corners as shown in Fig. 1 A. The first coolant sections are also divided likewise into passageways by corrugated sheets, not shown.

In operation, vapour feed enters the bottom of the refluxing sections and, as it travels upwardly through the sections, is cooled by heat exchange with a coolant medium that is circulated through the coolant sections of the heat exchanger. Condensate formed by the cooling of the vapour travels back down the passages in counter-current direct contact with the rising vapour stream and exits from the bottom of the passages.

At the bottom of each passage down which condensate is descending, the descending film of condensate must thicken into drops of sufficient size to overcome surface tension forces before the drops can break away from the bottom of the passages. The formation of these drops reduces the available area of the passage for the entry of the ascending vapour. Thus, the velocity of the ascending vapour and its shear will be increased in the vicinity of the bottom of the passage bringing a tendency to flooding.

In the arrangement illustrated, the walls of each of the passages in each of the refluxing sections of the heat exchanger are formed in part by a corrugation of the corrugated sheet 12, 22 etc. and in part by one of the two parting sheets between which the corrugated sheet lies. In the embodiment illustrated in Fig. 2, in which the parts corresponding to those of the structure illustrated in Fig. 1 are accorded the same reference numbers, a part of the periphery of the wall of each passageway is, in accordance with the invention, extended downwards a sufficient distance to lower the droplet-forming zone below the plane in which the openings of the passages lie. This is effected by arranging for the bottom end of each of corrugated sheets 12, 22, etc. in the refluxing sections to extend below the bottom of the pair of parting sheets 20, 30 or 40, 50 between which the corrugated sheet is not critical provided that the desired end of lowering the zone in which the droplets form to a level at which the forming droplets do not constrict the passage openings is achieved. In general an extension of about the same order as the corrugation height, which is typically about 0.25 inch (6mm), is adequate.

In the embodiment illustrated in Fig. 3, in which the parts corresponding to those of the structure illustrated in Fig. 1 are accorded to the same reference numerals, the solution according to the invention is provided by arranging for a parting sheet, eg. 30 or 50, which provides a part of the wall for passageways in a refluxing section, to be extended below the bottom of the adjacent corrugated sheet 12, 22. Preferably, as illustrated, the lower edge of the parting sheet is serrated or otherwise suitably profiled to encourage localised areas of concentration of the liquid film and assist droplet information. The zone at which the downflowing liquid film forms into droplets is thereby brought to a level below the passage openings thus reducing or avoid ing altogether a tendency of the openings to become blocked by the forming droplets.

Figs. 4, 5, 6 and 7 are vertical cross sections through refluxing heat exchangers of the general kind described above with refer ence to Figs. 1 to 3 but wherein each reflux ing section comprises, respectively, two or three layers of corrugated sheet each sepa- 3 GB 2 155 61 OA 3 rated from the next by a flat secondary parting sheet, whereby to provide, respectively, two or three banks of passages.

In the arrangements illustrated in Figs. 4 and 4, which illustrate the application of the embodiment of the invention illustrated in Fig. 2, ie. downward extension of a corrugated sheet, to a heat exchanger having respectively two or three banks of passages in each reflux- ing section, the features which correspond to those of Fig. 2 are given the same reference numerals. As illustrated, each refluxing section contains two or three layers, respectively, of corrugated sheet, identified as 1 2A, 1 2B, 22A, 22B etc. in Fig. 4 and 12A, 1213, 12C, 22A, 2213, 22C etc. in Fig. 5, each pair of adjacent corrugated sheets being separated by a flat secondary parting sheet 14, 24 etc. in Fig. 4 and 14A, 1413, 24A, 24B etc. in Fig.

5.

To achieve an improvement in performance in heat exchangers of this kind wherein a refluxing section has two banks of passages, as illustrated in Fig. 4, it is necessary for only one of each pair of the corrugated sheets in each refluxing section to be extended. In Fig. 4, this is shown as the left hand corrugated sheet 1 2A, 22A etc. although it may alternatively be the right hand sheet. Extending this one sheet benefits all the passages in one bank, and those in the other bank to a lesser extent. If both corrugated sheets are equally extended, then only half the passages in each of the banks will be benefited. However some benefit of the invention may be achieved for all the passages in the refluxing sections of the embodiment illustrated in Fig. 4 if, for example, both corrugated sheets are extended, with one being extended to a lower level than the other.

In heat exchangers of the kind having three banks of passages in a refluxing section, as illustrated in Fig. 5, a valuable improvement is obtained if just the central corrugated sheet 1213, 22B etc. is extended since this will benefit all the passages of the central banks and those of the passages of each of the other banks to a -lesser, extent.

All the passages may have the benefit of the invention if each of corrugated sheets 12A, 12B and 12C etc. is extended downwards but by differing amounts, eg. with middle sheet 1213, 2213, etc. extended below each of the other two sheets 12A, 1 2C; 22A, 22C etc.

Figs. 6 and7 illustrate the application of the embodiment of the invention illustrated in Fig. 3 to heat exchangers of the kind having two or three banks of passages, respectively. The same features as those of the embodiment of Figs. 4 and 5 have the same reference numerals.

In the embodiment illustrated in Fig. 6, the single secondary parting sheet 14, 24 etc. in each refluxing section is extended relative to the level of the adjacent corrugated sheets 12A, 12B, 22A, 22B etc., thus benefiting half the passages in each bank. All the passages may be benefited if each of the primary parting sheets 20, 30, 40, 50 etc. is also extended but ideally to a different level than that of the secondary parting sheet.

In the embodiment illustrated in Fig. 7, one of each pair of secondary parting sheets 14A, 14B, 24A, 24B, etc. in each refluxing section is extended below the level of the adjacent corrugated sheets.

If desired, however, both secondary parting sheets may be extended to the same level or, preferably, to different levels. Similarly one or more of the primary parting sheets may be extended, the criterion for optimum improvement being that adjacent parting sheets (primary or secondary) are not extended to the same level.

The benefit of the invention may also be obtained for heat exchangers of the kind illustrated in Figs. 4 and 6 by arranging for the bottom ends of the corrugated sheets 12A, 12B, 22A, 22B etc. to be at a level above the bottom ends of the primary and secondary parting sheets 20, 30, 40, 50 etc and 14, 24, 34 etc.

Of course, it is also possible in heat ex- changers of the kind having two or more banks of passages in a refluxing section to extend a combination of parting and corrugated sheets.

While the invention has been described with reference in particular to heat exchangers where the 'fins' of the plate-fin assembly are corrugated sheets with a square wave crosssection, it will be understood that it is not limited thereto and may be applied also to those heat exchangers where the fins have different cross-sections, eg. sinusoidal or Vshaped.

Likewise, the invention is also applicable to heat exchangers having more than three banks of passages in a refluxing section.

Moreover, as indicated above, the lowering of the droplet forming zone to a level at which droplet formation does not constrict the mouth of the passage may be achieved in other ways, e.g. by attaching strip- or wire-like extensions to the walls of the passages. Wire bundles may be used to form brush-like extensions. The extension may be fixed to the bottom of the wall of the passage or at a location on the passage wall above the bottom. Furthermore, the extension need not be attached to the wall or even touch it provided that it is in sufficiently close proximity to the wall to permit liquid flowing down the wall to transfer to and continue to flow down the extension.

Claims (5)

CLAIMS.

1. A refluxing plate-fin heat exchanger comprising a block including at least one first 4 GB 2 155 61 OA 4 hollow section with an inlet and outlet for the passage of a fluid heat exchange medium therethrough and at least one second hollow section defined by a pair of generally parallel and flat plates the space between which is divided by a corrugated sheet, or two or more corrugated sheets each separated from the next by a flat sheet, into a plurality of separate and generally vertically disposed passages for the upward flow of vapour therethrough from an inlet at the bottom to an outlet at the top, said first and second sections having a common wall for the transfer of heat between them, and wherein each of at least some of said passages are provided with means, such as a surface extending generally downwardly below the bottom of the passage, for lowering the droplet forming zone for any liquid flowing down the wall of the passage to a level at which droplet formation does not constrict the opening into the passage.

2. A heat exchanger as claimed in claim 1 in which the surface extending generally downwardly below the bottom of the passage is provided by extending the corrugated sheet, a corrugation of which forms part of the passage wall, downwardly below the bottom of the flat sheet which forms the remainder of the passage wall, or vice versa.

3. A heat exchanger as claimed in claim 1 in which the surface extending generally downwardly below the bottom of the passage is provided by one or more wires or strips.

4. A refluxing exchanger as claimed in claim 1 substantially as hereinbefore described.

5. A refluxing exchanger as claimed in claim 1 substantially as shown in any one of Figs. 2 to 7 of the drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office. Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY. from which copies may be obtained.