Falling film heat exchanger with member to distribute liquid on external surfaces of tubes

A falling film heat exchanger having a plurality of tubes is provided with a liquid distribution member surrounding each tube which directs liquid against the tube external wall to control the falling film flow and to form uniform films on each tube. The liquid can be a feed liquid or a heat exchange liquid such as a refrigerant.

This invention relates to vertical falling film heat exchangers containing 
tubes. More particularly, this invention is concerned with an improved 
heat exchanger which has a liquid distribution member around a portion of 
the tubes to direct a liquid against the outer surfaces of the tubes to 
form a thin falling film, thereby increasing heat exchange. 
BACKGROUND OF THE INVENTION 
Falling film heat exchangers usually include an array of vertical tubes. 
The tubes can be exposed or surrounded by a shell. The process liquid can 
be either inside or outside the tube with the heat exchange fluid on the 
other side. 
Although falling film heat exchangers are often used to heat a liquid feed 
stream, they can be used for cooling such a stream. Falling film heat 
exchangers of the described types can be used as freeze exchangers for 
producing fresh water from brackish water and seawater, for concentrating 
fruit and vegetable juices, and industrial crystallization processes. See 
U.S. Pat. No. 4,286,436. As the liquid flows through each tube, it can be 
cooled enough to crystallize a solid from the liquid. Thus, by cooling 
seawater, ice is obtained which when separated, washed and melted provides 
potable water. When a fruit or vegetable juice is similarly chilled, ice 
forms and is removed to provide a concentrated juice. 
Freeze exchangers of the described type can use any cooling fluid to cool a 
liquid flowing downwardly on the tubes. Some suitable cooling fluids are 
refrigerant gases such as ammonia and Freon brand refrigerants. 
Whether the heat exchanger is used to heat or cool a process or feed 
liquid, it is desirable to be able to control the thickness and uniformity 
of the falling film on each tube. Generally, acceptable results are not 
obtained by simply supplying enough liquid to flow down each tube because 
the feed to each tube is most often nonuniform, with some tubes receiving 
much more, and others much less, liquid than desired for optimum heat 
exchange results. There is a need, accordingly, for apparatus which will 
facilitate supplying the refrigerant and/or process or feed liquid to the 
tubes to produce falling films uniformly thick and evenly distributed on 
the surface of each tube. 
SUMMARY OF THE INVENTION 
According to one embodiment of the invention there is provided a falling 
film heat exchanger comprising a shell connected to vertically spaced 
apart horizontally arranged circular upper and lower tube sheets; a 
plurality of vertically positioned parallel tubes, with each tube 
extending through and sealingly connected to a hole in each tube sheet; a 
circular distribution plate spaced below the upper tube sheet and 
sealingly connected to the shell defining a heat exchange liquid 
distribution space, with said parallel tubes extending through oversized 
holes in the distribution plate; a heat exchange liquid distribution 
member surrounding each tube in the distribution space joined at a lower 
end to the distribution plate around said oversized hole and means 
preventing liquid from flowing between the member upper end and the tube, 
said member having means which directs liquid, in the distribution space, 
against the tube external wall and permits flow of liquid to the tube 
external wall only from said member; means to feed a heat exchange liquid 
to the distribution space and means to withdraw the heat exchange liquid 
from the shell side of the heat exchanger above the lower tube sheet; and 
a liquid distribution box positioned above the upper tube sheet and means 
to deliver a feed liquid to the liquid distribution box. 
The member which directs liquid to the tube external wall can include 
plurality of spaced apart holes which are sized so that the downwardly 
flowing liquid spreads out and distributes as a continuous liquid layer on 
the tube external wall. The holes can be at an ang1e of about 0.degree. to 
80.degree. measured from a plane horizontal to the tube axis. However, the 
holes are desirably at an ang1e of 20.degree. to 80.degree., and 
preferably 40.degree. to 70.degree.. 
The cooling liquid distribution member can be a truncated conical shell 
having its upper end sealing1y joined to the upper tube sheet. 
According to a second embodiment of the invention there is provided a heat 
exchanger comprising a vessel having a top, bottom and side wall enclosing 
a liquid feed box in the bottom portion, and a heat exchange liquid space 
in the top portion separated by a tube sheet; a plurality of heat exchange 
tubes, supported by the tube sheet, extending downwardly in the feed box 
and out through oversized holes larger than the tubes in the vessel 
bottom, said tubes being closed at their lower ends and having their upper 
ends in fluid communication with the heat exchange liquid space; a liquid 
feed distribution member surrounding each tube in the liquid feed box 
joined at a lower end to the vessel bottom around said oversized hole and 
means preventing liquid from flowing between the member upper end and the 
tube, said member having means which directs feed liquid, in the 
distribution space, against the tube external wall and permits flow of 
liquid to the tube external wall only from said member; means to deliver a 
liquid feed stream into the feed box; and means to deliver a heat exchange 
liquid to the heat exchange liquid space and means to remove heat exchange 
fluid therefrom. 
According to this second embodiment the liquid distribution member which 
directs liquid to the tube external wall includes a plurality of spaced 
apart holes in the member which are sized so that the downwardly flowing 
liquid spreads out and distributes as a continuous liquid layer or film on 
the tube external wall. The holes can be at an angle of about 0.degree. to 
80.degree. measured from a plane horizontal to the tube axis. Desirably, 
the holes are at an angle of 20.degree. to 80.degree., and preferably 
40.degree. to 70.degree.. 
In this second embodiment, the feed liquid distribution member also can be 
a truncated conical shell having its upper end sealingly joined to the 
upper tube sheet.

DETAILED DESCRIPTION OF THE DRAWINGS 
To the extent it is reasonable and practical, the same or similar elements 
or parts which appear in the various views of the drawings will be 
identified by the same numbers. 
With reference to FIGS. 1 to 3, the falling film heat exchanger 10 has a 
shell 12 connected to vertically spaced apart horizontally arranged 
circular upper and lower tube sheets 14 and 16. A plurality of vertically 
positioned parallel heat exchange tubes 18 extend through and are sealed 
to holes in the tube sheets 14 and 16 so that fluid cannot flow by. Shell 
12 extends above upper tube sheet 14 and supports removable cover 20. That 
portion of shell 12 extending above upper tube sheet 14 and supporting 
cover 20 defines a feed liquid supply box 22. Inlet conduit 24 is provided 
for feeding liquid to the supply box 22. 
Spaced below upper tube sheet 14 is a circular distribution plate 26 which 
is sealingly joined to shell 12. Distribution plate 26 contains an 
oversized hole 28 for each tube 18 to extend through. 
A truncated conical shell liquid refrigerant distribution member 30 is 
positioned around each portion of each tube 18 in the refrigerant 
distribution space 32 between upper tube sheet 14 and distribution plate 
26. Each distribution member 30 has its base welded to distribution plate 
26 around a hole 28. Similarly, the top and smaller end of member 30 is 
welded to the lower surface of upper tube sheet 14. The top of member 30 
contains a hole only big enough for a tube 18 to slide through it. 
Each distribution member 30 contains a plurality of radially positioned, 
spaced apart holes 34 located at an angle of about 30.degree. to 
40.degree. from a plane horizontal to the axis of tube 18. The number and 
size of the tubes is predetermined so as to have the refrigerant flowing 
through them contact tube 18 and then flow downwardly while spreading out 
on the tube to produce a falling film completely surrounding the tube 
circumference. 
Inlet conduit 36 provides for a liquid refrigerant, such as ammonia, to be 
fed to distribution space 32. Similarly, outlet conduit 38 is provided to 
remove excess refrigerant liquid from the lower shell side of the heat 
exchanger. Outlet 85 is provided to remove excess refrigerant vapor. 
As feed liquid is supplied to supply box 22 it flows downwardly through 
tubes 18 as a falling film. Simultaneously, a liquid refrigerant falling 
film flows downwardly on the outside surface of each tube 18 thereby 
cooling the feed liquid. If the feed liquid is an aqueous solution or 
dispersion, part of the water is converted to ice. The mixture or slurry 
of ice and solution flows out of the lower ends of tubes 18 into a 
collecting tank, not shown. The ice can then be separated and the 
remaining solution returned to the supply box 22 for further treatment as 
described. When the tubes 18 are about two inches in diameter, about ten 
to sixteen holes 34 can be used in each member 30 and the holes can be 
about 0.05 to 0.15 inch in diameter. It should also be understood that the 
rate of flow through the holes can be regulated by the depth of feed 
liquid in space 32 as well as the pressure therein. While the holes are 
shown as circular, they can also be square, triangular, rectangular and 
oval. They can be arranged in rows and columns as appropriate. 
FIG. 4 illustrates a second embodiment of falling film freeze exchanger 
according to the invention. However, in this embodiment the feed liquid, 
instead of the liquid refrigerant, is distributed as a falling film on the 
outside of the tubes. The freeze exchanger 40 includes a vessel 42 having 
a top 44, liquid feed distribution bottom 46 and side wall 48. Lower tube 
sheet 50 divides the vessel 42 interior into a liquid feed box space 52 in 
the bottom portion and a cooling fluid space 54 in the top portion. 
An upper tube sheet 58 is positioned in vessel 42 and divides the cooling 
liquid box space 52 into an upper space 62 and a lower space 64. Conduit 
66 feeds a liquefied refrigerant gas to upper space 62 and conduit 68 
removes it therefrom. Tube 70 extends upwardly from second tube sheet 58 
to provide vapor communication between upper space 62 and lower space 64. 
A plurality of freeze tubes 70, joined near their top ends to lower tube 
sheet 50, extend downwardly through oversized holes 72 in the bottom 46. 
The clearance between holes 72 and tubes 70 provides space for a film of 
feed liquid to flow down the exterior surface of the tubes. The bottom end 
of each freeze tube 70 is closed. 
A truncated conical shell distribution member 30 is positioned around each 
tube 70 in the feed liquid distribution space 52. The wider and lower end 
of member 30 is welded to bottom 46 around hole 72 and the narrower and 
upper end of member 30 is welded to the lower side of tube sheet 50. The 
member 30 is provided with holes 34 as already described above. The 
operation of member 30 is as already described except that in this 
embodiment it is used to form a falling film of feed liquid, instead of a 
falling film of refrigerant, on the exterior of the tube. 
Tubes 74 are joined at their upper ends to upper tube sheet 58. Tubes 74 
extend downwardly inside, and end near the bottom of, freeze tubes 70. 
Spacers 76 maintain tubes 74 centrally located in tubes 74. 
Feed liquid is supplied to feed liquid box space 52 by conduit 78 and the 
excess liquid is removed therefrom by conduit 80 and recycled. 
When liquefied refrigerant is supplied by conduit 66 to upper space 62, the 
refrigerant flows down tubes 74 and into tubes 70 thereby filling them and 
at least part of the lower space 64. As feed liquid flows out holes 34 in 
distribution members 30 and down the exterior surface of freeze tubes 70, 
it is cooled by heat exchange. When an aqueous feed liquid is being 
treated, ice crystals form in the liquid before it falls off the end of 
the freeze tubes into a collecting tank. The ice slurry can be separated 
from the concentrated liquid and be either used or discarded depending on 
the product desired from the concentration process. The liquid can be 
recycled for further concentration. 
The foregoing detailed description has been given for clearness of 
understanding only, and no unnecessary limitations should be understood 
therefrom, as modifications will be obvious to those skilled in the art.