WO2009009341A2 - Improved efficiency falling film heat exchanger - Google Patents

Abstract

Provided is an improved falling-film heat exchanger that maximizes the amount of feed stream contacting the heat exchange surface of the heat exchange tubes. The falling-film heat exchanger includes two important features: a spray member and a tube insert for directing feed stream to the tube wall.

Description

IMPROVED EFFICIENCY FALLING FILM HEAT EXCHANGER

Field of the Invention

The invention relates to falling-film heat exchangers. More particularly, the invention relates to an improved falling film heat exchanger that includes a spray member and an insert in the heat exchange tubes for increased heat exchange. Background of the Invention

Falling-film heat exchangers are used for heating or cooling liquids such as in industrial processes. Generally, exchangers are made up of an array of tubes extending between and connected at their lower and upper ends to two spaced apart tube sheets. The arrangement is surrounded by a shell. The shell is provided with inlet and outlet port through which a suitable heat exchange liquid or gas can be circulated through the shell to cool or heat the feed stream flowing through the tubes. Because the retention time of the feed stream at the heat exchange surface is small and liquid inventory buildup is limited, falling-film heat exchangers are particularly well suited for feed stream materials that are sensitive to thermal history. For example, falling-film heat exchangers are particularly well suited for heating and vaporizing, or partially vaporizing, process fluids.

The effectiveness of falling film exchangers depends in part on how well the feed stream is distributed to the walls of the tubes where heat exchange primarily takes place. Greater distribution on the tube walls is generally more favorable. This invention provides falling film exchangers with increased fluid distribution at the heat exchange surface.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the invention provides a falling film heat exchanger. The heat exchanger comprises: a shell connected to vertically spaced apart horizontally arranged upper and lower tube sheets said shell and tube sheets defining a shell side of the heat exchanger, the shell defining entry and exit ports for feeding heat exchange fluid to the shell side of the heat exchanger; a plurality of vertically positioned parallel cylindrical tubes, with each tube extending through and connected to a hole in each tube sheet; a spray member disposed for distributing a feed stream into the upper ends of the tubes; and an insert positioned in each tube for directing the feed stream to the interior walls of the tubes. In some embodiments, there are two or more, preferably from two to a number equal to the plurality of vertically positioned parallel cylindrical tubes, spray members.

The invention also provides inserts for use in falling film heat exchangers.

The invention further provides methods for exchanging heat with a fluid stream using the heat exchangers described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross-sectional representation of a falling film heat exchanger according to one embodiment of the invention.

Fig. 2 depicts a spray member according to one embodiment of the invention being used to distribute fluid to the upper openings of heat exchange tubes. Fig. 3 depicts examples of various tube inserts.

Fig. 4 shows a preferred insert that is twisted and has an angled bottom edge. Fig. 5 is a schematic of a setup for testing falling film heat exchangers.

DETAILED DESCRIPTION OF THE INVENTION

The falling-film heat exchangers of the invention provide increased contact between the feed stream and the heat exchange surface of the exchanger, thereby increasing heat exchange efficiency with the cooling/heating fluid. To achieve this increased efficiency, the heat exchangers of the invention include two important features: at least one spray member

-?- for distributing the fluid to the exchanger tubes and an insert for directing fluid to the heat exchange surface of the tubes.

Fig. 1 illustrates a simplified cross-sectional representation of a falling film heat exchanger according to one embodiment of the invention. The heat exchanger 10 includes a vertical shell 20 which is joined to upper tube sheet 22 and lower tube sheet 24. Shell 20 with tube sheets 22 and 24 are preferably cylindrically shaped and made of metal, although other shapes and materials may be used.

A plurality of vertically positioned parallel heat exchange tubes 30, 32, and 34 extend, and are connected, to vertically aligned holes 40 and 42 in the upper tube sheet 22 and the lower tube sheet 24. Tubes 30, 32, 34 are preferably cylindrical and metal, but other shapes and materials may be used. For simplicity, Fig. 1 depicts three heat exchange tubes, although the invention is not limited to three tubes and encompasses any number of tubes.

Vertical shell 20 defines (i.e., includes) entry 50 and exit 52 ports for feeding and removing heat exchange fluid to the shell side 54 of the exchanger. Although shown in Fig. 1 at the top of the vertical shell 20, the entry 50 and exit 52 ports may be positioned at other locations at the vertical shell 20, such as both ports being on the bottom, or one port on the top and one port on the bottom. Various fluids are suitable for heat exchange, including for example gas and/or liquefied refrigerant.

A spray member 60 is disposed for distributing, and (as shown for illustration purposes) distributes, feed stream 62 into the upper ends of tubes 30, 32, and 34. Other dispositions of spray member 60 for distributing feed stream 62 to upper ends of tubes 30, 32, and 34 are possible, including dispositions wherein spray member 60 is centered over tube 30, 32, or 34 or is instead positioned over upper tube sheet 22. Feed stream 62, falling into tubes 30, 32, 34, is directed to the heat exchange surface (the inner walls) of the tubes by an insert 70, which is shown largely disposed within the upper end of tube 30. Heat is exchanged between the feed stream in the inside of tubes 30, 32, and 34 and the heat exchange fluid in the shell side of the exchanger.

An advantageous feature of the heat exchanger of the invention is that it is capable of providing sufficient liquid flow of feed stream to the heat exchange tubes so as to exceed the minimum liquid flow required to provide complete wetting at the tube exits. This advantage is achieved by using a spray member 60 to deliver the feed stream 62. Spray member 60 permits very low liquid residence times on the tube sheet. Fig. 2 is a schematic depiction of a spray member 60 being used to distribute fluid to the upper openings of tubes 30, 32, and 33 and to tube sheet 22. During use of spray member 60, some of the fluid may move downwardly and other of the fluid may move simultaneously downwardly and across into the upper openings of the tubes.

Commercially available spray nozzles can be used for the spray member 60 of the invention, including single nozzle and multiple nozzle systems. Suitable commercial solid cone spray pattern nozzles include, but are not limited to, the Bete SC , Bete MP from Bete Fog, Inc. (Greenfield, Massachusetts, USA) and Spraying Systems FullJet spray nozzles (Wheaton, Illinois, USA). The Bete MP nozzle has larger passages that are advantageous for passing particulate material in the process stream. The distance between the spray nozzle discharge and the tube sheet is preferably set such that the feed stream fully covers the area of the tube-sheet where the tube openings are located.

Although, as noted above, the spray member 60 is advantageous because it provides a low liquid residence time on the tube sheet, some of the sprayed liquid falls down the center portion of the tube bypassing the heat transfer surface. This undesirable effect is substantially mitigated or eliminated by insert 70. Insert 70 transfers feed stream that is free-falling through the tube to the tube's wall. The insert comprises a substantially an elongated body. In a preferred embodiment, the insert is rectangular, but other geometries may be used. The bottom edge of the insert is preferably angled. By "angled" it is meant that at least a portion of the bottom edge is not perpendicular to the tube in which the insert resides. Examples of angled bottom edges are shown in Fig. 3 for rectangular inserts. The angle of the edge (represented by "x" in Fig. 3) is preferably between about 5 degrees and 80 degrees, more preferably 30 and 60 degrees. Preferred bottom edge shapes include a wedge (Fig. 3(b)), a V-shape (Fig. 3(c)), and a half- moon or concave (Fig. 3(d)), with the wedge being more preferred. The angled bottom edge directs fluid captured by the insert to the heat exchange wall of the tube. While it is not required for the insert to contact the wall of the tube for effective direction of the captured fluid, it is preferred that the clearance between the wall and at least a portion of the bottom edge, preferably the lowermost portion of the bottom edge, not exceed about 3/16 inch, preferably not exceed about 1/ 8 inch, more preferably not exceed about 1/16 inch, and even more preferably not exceed about 1/32 inch. In a further embodiment, at least the lowermost point of the bottom edge contacts the tube wall.

The insert is positioned in the tube, preferably in the upper end of the tube. Various techniques can be used for maintaining the insert in its position, including providing the insert with sufficient width such it that it lodges in the tube. Other methods include configuring the top edge of the insert to suspend into the tube, for example, by providing hooks to the top edge of the insert. Preferably, the top edge of the insert is of a shape that permits ready suspension, such as a T-shape.

In a preferred embodiment, the insert is twisted, i.e., the top edge of the insert is rotated relative to the bottom edge. The twist permits the insert to capture more free- falling feed stream and thus increases its efficiency at directing feed stream to the heat transfer surface of the tube. Preferably, the twist in the insert is at least about 90 degrees (i.e., 1/4 turn) and no more than about 360 degrees. Further preferably, the twist is at least about 180 degrees (i.e., 1/2 turn), and even more preferably at least about 270 degrees. Fig. 4 depicts a preferred insert according to the invention having a twist of about 360 degrees.

The insert can be formed of various materials, but is advantageously of the same material from which the heat exchange tube, or inner portion thereof, is formed. The choice of material is generally dependent on the process fluid, and includes consideration of such factors as the wettability of the surface of the material by the process fluid. Examples of suitable materials include metals, plastics, and ceramics. In some embodiments, it is preferred that the insert be metal, and preferably stainless steel, further preferably stainless steel of about 16 gauge thickness.

The insert is generally of any width that allows it to fit within the heat exchange tube, provided that at least a portion of the bottom part of the insert is in close proximity to the inner surface of the tube as described above.

The insert is of sufficient length or shape such that free- falling fluid not contacting the wall on its way down the tube substantially contacts the insert. One convenient method of selecting an insert length is to base the length on the diameter of the tube. According to this measurement, it is preferred that the insert be at least about 3 tube diameters long (e.g., if the tube is 1.5 inches in diameter, the insert is at least about 4.5 inches long), more preferably at least about 6 tube diameters long, and even more preferably, at least about 8 tube diameters long. There is no particular upper limit on the length of the insert so long as the bottom edge of the insert resides substantially within the tube. In some embodiments, a shorter insert may be advantageous if it is desired to maximize the contact time between the feed stream and the heat exchange surface of the tube.

The following examples are illustrative of the invention but are not intended to limit its scope. EXAMPLES

In the examples, a single tube system is used to demonstrate the advantages of the invention by quantifying the quantity of liquid that passes through a tube. A schematic of a suitable setup is provided in Fig. 5. As shown, a 1 1/2" diameter tube 130 is fed using a commercial solid cone spray nozzle 160, resulting in approximately 2 to 3 kg per minute of liquid being fed to the tube (drop sizes at the bottom of the tube are about 2-3 mm in diameter). A weir 170 is used to remove excess liquid. A collection funnel 180 is used to collect separately the liquid that passes through the tube without contacting the walls. The flow rates are calculated from the material which contacts walls and the material that does not contact the walls. These flowrate values are used in the subsequent calculations. The inserts for this testing are fabricated from 16 gauge thickness 316 stainless steel, with each twisted unit having 360 degrees of twist as shown in Fig. 4.

The effect of the flow distribution insert is summarized in table 1. This table shows the influence of the insert on the percentage of liquid entering the tube as spray that exits the tube on the wall. The liquid flux column represents the mass flux at the perimeter. Table 1 also shows the percentage of material that is bypassing the tube wall. Because of the geometry, the data with no insert suggests that 70% of the drops and liquid entering the tube contact the wall in this tube. Table 1. Summary of results-

Insert Insert Liquid flux, Percent of flow Percent of description Description- Kg/m exiting the spray flow

Shape and clearance from tube bypassing entering tube length wall (each the tube wall not on wall side) none - open NA 318.6 5.16% 35.1 % tube flat ended 1 /16" 318.6 2.49% 16.9% v notched 1 /16" 318.6 0.75% 5.1 % v notched tight 1 /16" 318.6 0.17% 1 .1 % v notched tight 1 /16" 318.6 0.23% 1 .5% angled, - 1 /32 clearance 330.8 0.44% 3.0%

Generally, the Table shows that angled designs have significantly less bypass, and therefore improved efficiency, than the absence of insert or an insert with a flat, non-angled, bottom edge. More particularly, the v notched and wedged (angled, x in Fig. 3b = 45 degrees) designs exhibit the least amount of bypass.

While the invention has been described above according to its preferred embodiments, it can be modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using the general principles disclosed herein. Further, the application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A falling film heat exchanger comprising: a shell connected to vertically spaced apart horizontally arranged upper and lower tube sheets said shell and tube sheets defining a shell side of the heat exchanger, the shell defining entry and exit ports for feeding heat exchange fluid to the shell side of the heat exchanger; a plurality of vertically positioned parallel cylindrical tubes, with each tube extending through and connected to a hole in each tube sheet; a spray member disposed for distributing a feed stream into the upper ends of the tubes; and an insert positioned in each tube for directing the feed stream to the interior walls of the tubes.

2. The falling film heat exchanger of claim 1 wherein the insert comprises a substantially rectangular body having a bottom edge that is angled.

3. The falling film heat exchanger of claim 1 wherein the insert is configured at its top edge to suspend from the top of the tube.

4. The falling film heat exchanger of claim 1-3 wherein the top edge of the insert is T- shaped.

5. The falling film heat exchanger of claims 1-4 wherein the bottom edge of the insert is wedged.

6. The falling film heat exchanger of claims 1-4 wherein the bottom edge of the insert is V-shaped.

7. The falling film heat exchanger of claims 1-4 wherein the bottom edge of the insert is concave shaped.

8. The falling film heat exchanger of claims 1-7 wherein the insert is twisted.

9. The falling film heat exchanger of claims 1-8 wherein the insert is twisted by at least about 90 degrees.

10. The falling film heat exchanger of claims 1-9 wherein the insert is twisted by about 360 degrees.

11. An insert for increasing the efficiency of a falling-film heat exchanger, the insert comprising a substantially rectangular body having an angled bottom edge and configured at its top edge to suspend from the top of a tube of a falling film heat exchanger.

12. The insert of claim 11 wherein the top edge of the insert is T-shaped.

13. The insert of claims 11-12 wherein the bottom edge is wedge shaped.

14. The insert of claims 11-12 wherein the bottom edge is V-shaped.

15. The insert of claims 11-12 wherein the bottom edge concave shaped.

16. The insert of claims 11-15 wherein the insert is twisted.

17. The insert of claims 11-15 wherein the insert is twisted by at least about 90 degrees.

18. The insert of claims 11-15 wherein the insert is twisted by about 360 degrees.