Heat exchanger comprising a finned pipe

A heat exchanger comprises a vertically arranged circular-cylindrical holder closed at its upper end and at its lower end, and a pipe coaxially arranged within the holder. A fin is secured to the inner pipe and consists of a plurality of interconnected lamellae extending radially from the pipe, the fin being helically wound around the pipe so that the adjacent lamellae of successive turns of the resulting helix overlap each other in part, viewed in the circumferential direction of the pipe. A capillary passage is located in each lamella above the surface of an underlying lamella in a position shifted with respect to the capillary passage of the underlying lamella.

This invention relates to a heat exchanger comprising a finned metal pipe 
arranged in a holder, in which a metal fin secured to the pipe consists of 
a number of interconnected lamellae, the metal fin being helically wound 
around the pipe so that the adjacent lamellae of successive turns of the 
resulting helix overlap each other in part, viewed in the circumferential 
direction of the pipe. 
In a known heat exchanger of this type (see U.S. Pat. No. 3,723,693), heat 
exchange takes place between a first liquid flowing in the pipe and a 
second liquid flowing between the holder and the pipe along the lamellae 
of the fin. Such a heat exchanger is not intended for the mass transfer 
(enrichment or impoverishment) between a gas and a liquid both situated 
between the holder and the pipe, while at the same time heat transfer 
takes place by means of a cooling liquid or a heating liquid flowing in 
the pipe. In this case, the heat exchanger would act as a so-called 
heat-mass-exchanger. The mass transfer would then be comparatively small, 
however, because no intimate contact is present between the gas and the 
liquid. It should be noted that the book "Handbuch der Kaltetechnik", 
Volume VII, pp. 391-393 of R. Plank (published in 1959) discloses a 
heat-mass-exchanger used in a so-called drop absorber, in which a solution 
of a work medium and a solvent is conducted along the outer side of a 
number of helical pipes which are vertically arranged in a holder and 
along which also the gas flows which is to be absorbed in the solution. 
The liquid to be heated flows in the pipes. Since the turns of the pipes 
are located close to each other, a continuous film of liquid solution is 
often formed on the pipes. The desired drop formation occurs only 
incidentally. A weak superficial mixing with the gas to be absorbed 
(enrichment) takes place in the film. It has also been suggested to 
enlarge the surface area available for the heat-mass-exchange by providing 
the pipes with lamellae. However, there is again formed on the lamellae a 
film of the solution, which is mixed only to a small extent with the gas 
to be absorbed flowing along the lamellae. 
The present invention has for its object to provide a heat exchanger acting 
as a heat-mass-exchanger, in which an optimum mixing between gas and 
liquid takes place. Such a heat exchanger can be used both in the process 
industry and in absorption heat pumps. 
The invention is for this purpose characterized in that each of the 
lamellae is provided with a capillary passage, which, with a vertically 
arranged pipe, is located above the surface of an underlying lamella in a 
shifted position with respect to the capillary passage of the said 
underlying lamella while the lamellae are arranged so as to be clear of 
the inner wall of the holder. 
The capillary passages in the lamellae promote to a great extent the 
formation of a regular pattern of drops. Since vapour can flow on all 
sides around these falling drops, the drops are in themselves thereby 
enriched by the gaseous work medium with the use of the heat exchanger in 
an absorber. Furthermore, the drops falling on an underlying lamella cause 
an intensive mixing of the comparatively hot upper layer in the film of 
the solution on the lamella already considerably enriched by the flowing 
gas with the poorer comparatively cold lower layer in this film. With the 
use of the heat exchanger in a generator for an absorption heat pump, the 
falling drops cause an intensive mixing of the already impoverished lower 
layer in the film with the richer upper layer in this film. The 
impoverishment of the solution desired in the generator is thus 
considerably intensified. The hot gaseous work medium ascending in the 
generator moreover gets into contact with the falling drops and the richer 
upper layer of the film so that an additional quantity of rich gas is 
generated. 
A particular embodiment of the heat exchanger, which provides a compact 
absorber that can be mass-produced in a simple manner, is further 
characterized in that the finned pipe comprises a first pipe, which is 
closed at its upper end and its lower end and within which extends a 
coaxial second pipe, which is open at both ends and is in open 
communication at its upper end with an inlet of the first pipe and is 
connected at its lower end to an outlet passed through the lower end of 
the first pipe, while the space between the holder and the first pipe is 
connected near the upper end of the holder to a first inlet and a second 
inlet of the holder and is connected near the lower end of the holder to 
an outlet. 
A further particular embodiment of the heat exchanger, which provides a 
compact generator that can be mass-produced in a simple manner, is 
characterized in that the finned pipe is closed at its upper end passed 
into the holder and is connected near its lower end to a vessel arranged 
outside the holder, while the holder is provided near the upper end of the 
pipe with an inlet and a first outlet and is provided near its lower end 
with a second outlet. 
A preferred embodiment of the heat exchanger, in which liquid flows in a 
radial direction over the lamellae, is characterized in that the lamellae 
of the vertically arranged pipe are downwardly directed with their free 
ends directed radially outwards.

The heat exchanger shown in FIG. 1 comprises a circular metal pipe 1 of 
thermally good conducting material, such as, for example, steel. The pipe 
1 is surrounded by a coaxial circular-cylindrical metal holder 3. To the 
outer wall of the pipe 1 is welded a metal fin which consists of a 
plurality of interconnected rectangular lamellae 5, the metal fin being 
helically wound around the pipe 1, the adjacent lamellae 5 of successive 
turns of the resulting helix overlapping each other in part, viewed in the 
circumferential direction (tangential direction) of the pipe 1 (see also 
FIG. 2). Each of the lamellae 5 is provided with the pipe 1 capillary 
passage 7, which, with a vertically arranged, is located above the surface 
of an underlying lamella 5. The capillary passage of the overlying lamella 
is shifted in position with respect to the capillary passage of the 
adjacent underlying lamella. The lamellae 5 are arranged so as to be clear 
of the inner wall of the holder 3 and are directed slightly downwards at 
their free ends. The diameter of the pipe 1 is comparatively large with 
respect to the length (viewed in a radial direction) of the lamellae 5. 
In the present case, the rectangular lamellae 5 have a length of 20 mm, a 
width of 9 mm and a thickness of, for example, 1 mm. The outer diameters 
of the pipe 1 and the holder 3 are 60 mm and 125 mm, respectively, while 
the wall thicknesses are 2.5 mm and 2.5 mm. Due to the pitch angle (0.05 
radian) of the helical line formed, the lamellae 5, viewed over their 
width (tangential direction), are directed downwards (see FIG. 2). As 
stated, the lamellae 5, viewed in radial direction, are also slightly 
directed downwards (angle of 0.05 radian). The lamellae 5 consequently 
hang downwards effectively in two orthogonal directions. The vertical 
distance between two adjacent lamellae 5 in successive turns is about 9 
mm. The capillary passages are located approximately at the centre 
(tangential direction) in the front part (radial direction) of the 
lamellae. 
The heat exchanger constituted by the pipe 1 and the holder 3 is used for 
transferring heat between a gaseous or liquid first medium flowing in the 
pipe 1 and a liquid second medium flowing and dripping downwards in a the 
cascase over lamellae 5 in the space between the pipe 1 and the holder 3 
as well as for transferring gas mass from and to the second medium. The 
liquid second medium consists of a solution of a so-called work medium and 
a solvent. Essentially, two cases should be distinguished, i.e. the use of 
the heat exchanger as a so-called absorber and its use as a so-called 
generator. In both applications, the heat exchanger becomes a so-called 
heat-mass-exchanger, in which besides a heat transfer also a gas mass 
transfer takes place. Depending upon the kind of solution used, the 
diameter of the capillary passages varies. If as a work medium ammonia and 
as a solvent water are used, the said diameter of the capillary passages 7 
is preferably 2 mm, while the thickness of the lamellae 5 is preferably 1 
mm. 
In the case of the absorber 9 shown in FIG. 4, a coaxial second pipe 13 is 
situated within the first pipe 1, which is closed at both ends. The pipe 
13 is in open communication at its upper end with an inlet 15 of the first 
pipe 1 via the annular space between the two pipes. The open lower end of 
the second pipe 13 is connected to an outlet 17 passed through the lower 
end of the first pipe 1. The space between the closed-end holder 3 and the 
first pipe 1 is connected to a first inlet 19 and an outlet 21. 
Furthermore, the latter space is connected to a second inlet 23, which is 
located at the upper side of the holder 3. The operation of the absorber 9 
is as follows. 
A liquid solution of work medium and solvent poor in work medium, for 
example ammonia and water, at a comparatively high temperature is sprayed 
into the space between the holder 3 and the first pipe 1 through the 
second inlet 23. This solution flows downwards under the influence of the 
force of gravity in a the cascade over lamellae 5. Comparatively cold 
gaseous work medium (ammonia gas) is passed into the space between the 
holder 3 and the first pipe 1 through the first inlet 19. A liquid or 
gaseous cooling medium (for example water or air) at a comparatively low 
temperature flows through the inlet 15 into the space between the first 
pipe 1 and the second pipe 13, enters the second pipe near the upper end 
of the first pipe 1 and leaves this second pipe via the outlet 17. The 
solution flowing downwards forms a liquid film 25 on each of the lamellae 
5 (see FIGS. 2, 3 and 6). This liquid film 25 can be assumed to be 
composed of a comparatively cold lower layer 27 and a comparatively hot 
upper layer 29. The temperature in the liquid film on the lamella 5 
increases, as indicated by the arrow 31 in FIG. 6. Drops 33 are formed 
below the capillary passages 7 and fall on the film 25 of an underlying 
lamella 5. The various stages of the formation of the drops 33 are shown 
in FIG. 2 with reference to four lamellae 5. The gaseous work medium 
present in the space between the holder 3 and the first pipe flows around 
the film 25 and the drops 33. The upper layer 29 of the film 25 and the 
drops 33 then absorb ammonia gas. In the film 25, an intensive mixing of 
the already considerably enriched comparatively hot upper layer 29 with 
the poorer comparatively cold lower layer 27 takes place throughout the 
film 25. This mixing is shown more clearly on the lower-most lamella 5 
with reference to a drop 35 falling on it. Thus, the film 25 is cooled 
throughout its thickness so that the absorption of ammonia is promoted and 
is more uniform in the whole film 25, while at the same time a homogeneous 
concentration of work medium is obtained in the film 25. Since the 
lamellae 5 are directed downwards with their free ends and one 
longitudinal edge, the film 25 flows just in front of the relevant edges 
from the lamella 5 downwards in the form of drops 37 (see FIG. 3). Due to 
the downwardly directed lamellae 5, it is achieved that the largest 
possible quantity of solution flows over the lamellae, while it is 
moreover avoided that an excess quantity of solution flows along the outer 
wall of the first pipe 1. The drops 33 fall on the film 25 of an 
underlying lamella 5 at an area which is shifted with respect to the 
underlying capillary passage 7. As a result, the mixing process is 
promoted, while the formation of drops below the capillary passages is not 
or substantially not disturbed. Moreover, with capillary passages arranged 
vertically below each other, effectively a continuous flow without drops 
would be obtained, which reduces the absorption effect. The liquid flowing 
through the capillary passages ensures that a comparatively thin liquid 
film is formed on the lamellae so that a good heat transfer is realized. A 
quantity of enriched liquid solution 39 is collected in the lower part of 
the holder 3 and is then discharged via the outlet 21. 
In the case of the generator 11 shown in FIG. 5, the pipe 1 passed into the 
closed end holder 3 is closed at its upper end and is connected at its 
lower end passed to the outside through the bottom of the holder 3 to a 
vessel 41. The holder 3 is provided near the upper end of the pipe 1 with 
an inlet 43 and is provided at its lower end with a first outlet 45. The 
inlet 43 and the first outlet 45 are both connected to the space between 
the holder 3 and the pipe 1. Furthermore, a second outlet 47 is connected 
to the said space near the upper end of the holder 3. The vessel 41 
contains an evaporation and condensation medium 49, such as, for example, 
water, which is heated by a heat source not shown, for example a gas 
burner. The evaporated water ascends in the pipe 1 and condenses on the 
comparatively cold inner wall of such paper. A condensation film 51 is 
then formed on the inner wall of the pipe 1. The condensate flows under 
the influence of the force of gravity back to the vessel 41. The pipe 1 
and the vessel 41 consequently act as a kind of heat pipe (thermo-siphon). 
The condensing water vapour gives off heat to the comparatively cold rich 
solution which is supplied through the inlet 43 and flows and drips 
downwards in a cascade over lamellae 5. There is formed on the lamellae 5 
a film layer 53 of a solution comprising a comparatively hot lower layer 
55 and a comparatively cold upper layer 57 (see FIG. 7). Viewed over a 
longitudinal sectional view of a lamella 5, the temperature in the liquid 
film 53 on the lamella 5 increases in the direction of the arrow 59. The 
lower layer 55 already impoverished by boiling is mixed with the richer 
upper layer 57. In the case of film evaporation, the impoverished heavier 
upper layer 57 sinks into the lower layer 55. A further mixing is obtained 
in that a drop 61 formed laterally below the capillary passage 7 falls 
from the overlying lamella onto the film of the adjacent underlying 
lamella. Due to this intensive mixing, a more uniform temperature increase 
in the film 53 is obtained, which leads to a strong expulsion of gaseous 
work medium from the film 53. Drops 63 are also formed just in front of 
the downwardly directed edge and the free end of the lamella 5 and these 
drops fall downwards on the film 53 of the underlying lamella and cause a 
further mixing in the film 53. Moreover, already expelled gaseous work 
medium flows around both the falling drops 61 and the drops 63 so that an 
additional quantity of gaseous work medium is obtained from the falling 
drops. The gaseous work medium is discharged through the second outlet 47, 
for example, to a condenser. Impoverished solution 65 is collected in the 
lower part of the holder 3 and is discharged through the first outlet 45, 
for example to an absorber, in order to be enriched again and then to be 
conducted back to the generator 11. 
The absorber 9 and the generator 11 as described may be used in the process 
industry, where there is a strong need for devices for enriching and 
impoverishing solutions which contain constituents having a comparatively 
low boiling point and a comparatively high boiling point, respectively. 
Due to the use of the heat exchanger described, which permits of obtaining 
a maximum mixing in film layers, an optimum enrichment or impoverishment 
of such a solution may be obtained. The heat exchanger is of particular 
advantage when used in absorbers or generators of so-called absorption 
heat pumps. Due to the comparatively simple construction of the heat 
exchanger, the latter can be mass-produced at low cost. This is of 
particular importance for heat pumps which are used for heating or cooling 
private houses. 
It should be noted that the medium passed through the inlet 15 into the 
pipe 1 of the absorber 9 may also be passed entirely in a flow direction 
opposite to that of the solution to be enriched through the pipe 1. The 
second pipe 13 may then be dispensed with. However, it is then necessary 
to provide an additional outlet in the holder 3. The holder 3 of the 
absorber 9 and the generator 11 are each preferably provided with a 
thermally insulating coating. The second pipe 13 is preferably also 
provided with such a coating or is entirely made of thermally insulating 
material. The vessel 41 in the generator may be heated by a gas or oil 
burner, but electrical heating is also possible. Furthermore, heating may 
be effected by means of flue gases or waste heat, for example with the aid 
of a heat exchanger arranged in the vessel 41. 
Although the invention is described with reference to finned pipes, in 
which a considerable drop formation takes place near the free ends and the 
downwardly directed longitudinal edges of the lamellae, it is not limited 
thereto. The extent to which the said drop formation takes place depends 
upon a large number of parameters. Important parameters in this connection 
are: 
the liquid flow rate to be processed, 
the mass pair used (surface tension/extent of wetting), 
the sharpness of the edges of the lamellae, 
the value of the angle at which the free ends of the lamellae are directed 
downwards, 
the value of the pitch angle of the helical line, 
the diameter of the capillary passages. 
For example, in the case of sharp edges of the lamellae and substantially 
horizontal lamellae, a comparatively small number of drops will be formed 
near the free ends and the longitudinal edges of the lamellae. The drop 
formation then takes place mainly below the capillary passages, which 
process substantially the whole quantity of liquid. In the case of 
lamellae hanging strongly downwards, both kinds of drop formation occur. 
The drops formed below the capillary passages then slide along the lower 
side of the lamellae slightly towards the free ends and the longitudinal 
edges. In practice, it will have to be ascertained empirically, which 
combination of parameters is to be preferred.