JPH07111287B2 - Heat transfer tube for absorber - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improvement of a heat transfer tube used for an absorber such as an absorption refrigerator and an absorption heat pump.

[Prior Art and Problems] An absorber used in an absorption refrigerator, an absorption heat pump, or the like is configured by arranging a large number of heat transfer tubes horizontally or vertically in a closed container. In this case, an absorbing liquid, for example, a LiBr aqueous solution (concentration of about 60% by mass) is dropped and sprinkled on the outside of the heat transfer tube to absorb the vapor of the refrigerant generated in the evaporator, and at the same time, the latent heat at the time of absorption is cooled in the tube. It is operated so as to be removed by water.

Absorption is caused by the pressure difference between the evaporation pressure in the evaporator and the saturated vapor pressure of the absorbing liquid dropped on the surface of the heat transfer tube. If this pressure difference is large, the capacity is improved. Further, the lower the temperature or the higher the concentration of the absorbing liquid, the lower the saturated vapor pressure and the larger the pressure difference, which contributes to the improvement of the absorbing capacity.

On the other hand, looking at the behavior on the outer surface of the heat transfer tube, a mass transfer in which the condensed water diffuses into the absorption liquid and a heat transfer for the generation and removal of heat coexist, showing a very complicated aspect. However, the current situation is that the phenomenon has not been fully clarified, and the smooth tubes have dominated the heat transfer tubes used therefor.

In order to improve the performance of the heat transfer tube, it is necessary to improve both of the above mass transfer and heat transfer, but from the viewpoint of absorption capacity, it is important to improve the mass transfer performance as well as the heat transfer. That's natural.

The arrangement of the heat transfer tubes in the absorber today is mainly horizontal, and a method of dripping and spraying the absorbing liquid from above to the horizontal tube group is adopted. At this time, the absorbing liquid flowing on the surface of the tube becomes a thin film, and further knowledge of reducing the heat transfer resistance to improve the efficiency of the device is moving toward a further thin film. However, even in the current thin-film flow-through system, a dramatic improvement in absorption performance cannot be expected unless the mass transfer is promoted rather than the heat transfer is promoted as described above.
For this reason, recently, attempts have been made to use finned tubes such as low fin tubes for the purpose of increasing the heat transfer area and at the same time making the absorption liquid into a thin film. Has not yet improved its absorption capacity.

In addition, in the actual machine, a small amount of surfactants such as n-octyl alcohol and diethylhexanol are added to the absorption liquid, which causes severe interface disturbance based on the so-called Marangoni effect in the solution at the time of absorption, and the absorption capacity jumps. A method is being used to improve it.

Since the absorber is an important component that influences the performance of the equipment, it is a major issue to improve the performance of the absorber in order to miniaturize and improve the performance of the equipment in the future.

Therefore, the improvement of the performance of the heat transfer tube is an important point, and in particular, the invention of a heat transfer tube capable of promoting the mass transfer in the absorption process and the elucidation of the outstanding role of the above-mentioned surfactant against it It is desirable to select the tube shape based on it.

[Object of the Invention] The present invention has been made in view of the above-mentioned circumstances. In particular, it is intended to provide a novel heat transfer tube for an absorber having a surface shape that can be an optimum condition for the behavior on the surface of the heat transfer tube due to the addition of a surfactant.

[Summary of the Invention] That is, the gist of the present invention is to provide a plurality of grooves extending in the longitudinal direction on the outer surface of a heat transfer tube and a large number of fins with a fine pitch between the grooves, and these grooves and fins. The optimum conditions are defined in the configuration of (1) above, whereby a heat transfer tube capable of dramatically improving the absorption effect in the absorber compared to the conventional example can be supplied to the market.

[Examples] The present invention will be described below based on Examples.

The inventors have conducted detailed research experiments on heat transfer performance and mass transfer performance. As a result, it was found that when convection occurs in the absorbing liquid film on the surface of the heat transfer tube, mass transfer is greatly promoted along with heat. The solution on the surface of the heat transfer tube absorbs the refrigerant vapor on the surface in contact with the refrigerant vapor to have a low concentration, but the movement of the liquid film in the depth direction does not progress much only by diffusion. Therefore, if convection is generated in this solution, agitation occurs in the liquid film, the refrigerant progresses in the depth direction, only the solution surface does not lower the concentration and suppress absorption, and the mass transfer performance is improved. Greatly improves.

On the other hand, when a water-LiBr system or the like is used as the refrigerant-absorbent, a surfactant is added to the absorbing solution, but when this addition absorbs water vapor, the interfacial tension of the solution surface changes, which is severe. Marangoni convection occurs. As a result, the absorption capacity is greatly improved as compared with the case where no surfactant is added.

The present invention focuses on the convection phenomenon caused by the surfactant, and has found the optimum configuration of the surface shape of the heat transfer tube that can further forcibly stir the solution when this convection occurs.

According to the heat transfer tube of the present invention, a plurality of parallel grooves having a large depth are provided in the longitudinal direction on the surface of the tube, whereby the dripped solution is actively stirred and simultaneously flows in the tube axial direction. Was generated to cause the solution to spread.
Further, a large number of fins are provided between the parallel grooves, which increases the heat transfer area and improves the wettability of the solution on the surface by optimizing the fin size, greatly increasing the effective heat transfer surface. I understood it. Furthermore, it was also found that the solution can freely move in the tube axis direction by the parallel grooves even with respect to the Marangoni convection generated by the surfactant, and the convection acts more actively. It has been clarified by a number of experiments that a certain optimum shape or size is required to make the effect of the groove and the fin more reliable.

FIG. 1 is a cross-sectional view showing a specific example of the conduction tube 1 according to the present invention, and FIG. 2 is an explanatory front view thereof. The example shown is
A regular size (outer diameter 19 mm) copper tube for heat transfer tube is used, parallel grooves 2 and 2 with the number of grooves N and groove depth V are formed on the surface evenly on the circumference, and between the grooves 2 and 2. Fin height hf, fin pitch pf
In order to investigate the effect of this on the absorption performance, various fins N, groove depth V, fin height hf, and fin pitch pf were variously changed. A test material was prepared and the performance was measured.

This performance measurement was carried out by incorporating 24 test heat transfer tubes into a performance measuring device 4 imitating an absorber as shown in FIG. 3 in 3 rows and 8 stages with an effective length of 300 mm.

In the experiment, the absorption solution 6 consisting of LiBr aqueous solution having a concentration of 58% by mass and a temperature of 40 ° C. was dropped from the nozzles 7a, 7a of the dropping pipes 7, 7, and 28 ° C. in the absorber heat transfer tubes 1, 1 at a flow rate of 1 m / sec. While the cooling water 8 is supplied, water (refrigerant) 1 is supplied to the cooling heat transfer tubes 11 and 11 in the evaporator section 10.
2 was dropped from the nozzles 15a, 15a of the dropping pipes 15, 15 and the flow rate of water 13 flowing into the heat transfer pipes 11, 11 of the evaporator section 10 was controlled so that the evaporation temperature was constant at 10 ° C. In addition, 9 in FIG.
Is a low-concentration absorbing liquid that has absorbed the refrigerant vapor.

In this experimental method, if the heat transfer performance and mass transfer performance of the absorber section 5 are good, the amount of water vapor 14 absorbed will increase and the evaporator section will
The cooling capacity of water 13 at 10 is improved. In addition, in the first experiment, in order to investigate only the effect of the tube shape, the measurement was performed without adding a surfactant.

FIG. 4 shows the relationship between fin pitch and refrigerating capacity, FIG. 5 shows the relationship between fin height, parallel groove depth and refrigerating capacity, and FIG. 6 shows the relationship between parallel groove number and refrigerating capacity.
Each value is a numerical value when the liquid film flow rate of the LiBr aqueous solution (mass flow rate per unit length flowing on the tube side) is 0.016 kg / mS.

In the above, the width of the groove itself does not show a great influence as much as other factors, but here, the groove width is about 1.5 to 2 mm.

As can be seen from Fig. 4, when the fin pitch is 1.5 mm or more, the performance deterioration becomes clear. This is a parameter that determines the wettability of the solution, and it can be seen that the range of 0.4 to 1.5 mm is good.

Further, it can be seen from FIG. 5 that there is a kind of relative relationship between the fin height and the groove depth. That is, unless the groove depth is made equal to or larger than the fin height, the stirring of the solution does not act so much and the performance is deteriorated. Further, if the fin height is low, the heat transfer area is reduced, and if it is high, the wettability is lowered and the performance is lowered. Therefore, considering the relationship between the two, it is clear that the fin height is 0.5 to 1.5 mm and the groove depth is 0.5 to 2 mm.

Further, the number of grooves is also closely related to the performance. If the number of grooves is small, the stirring effect of the solution is small, and if the number of grooves is large, the heat transfer area is reduced and the performance is degraded. It can be seen from Fig. 6 that 10 to 20 articles are good, which indicates that the circumferential groove spacing is 3 to
It turns out that 6 mm is suitable.

Furthermore, considering that the heat transfer tube according to the present invention is horizontally arranged, it is not preferable that the angle formed by the tube axis and the groove is large, and the angle of the groove with respect to the tube axis is limited to about −10 ° to + 10 °. Is desirable.

FIG. 7 is a diagram showing the results of performance measurement of the heat transfer tube of the present invention processed based on the optimum conditions obtained from the above examination results, and the low fin tube and the smooth tube as comparative examples. In the figure, the measurement results with and without addition of 250 ppm by weight of n-octyl alcohol as a surfactant (black coating) are shown as comparison data.

As is clear from FIG. 7, in the case where no surfactant is added, the heat transfer tube of the present invention shows a significant improvement in refrigerating capacity of about 1.7 times as compared with the conventional smooth tube, and further has the same processing form. The improvement is about 1.45 times that of the low fin tube, which clearly shows that the effect of the tube shape defined in the present invention is effectively exerted. Also, when a surfactant is added, the refrigerating capacity is
About 1.4 times, about 1.22 times that of the low fin tube, it can be seen that the effect of Marangoni convection is effectively exerted. This was also confirmed by visualization of the solution on the tube surface when the surfactant was added. That is, the heat transfer tube of the present invention,
Upon absorption, vigorous solution movement could be observed along the parallel grooves. In addition, when the surfactant is added, the substantial improvement rate is lower than that in the case where the surfactant is not added, but in the case of the heat transfer tube of the present invention, this is the theoretical value of the maximum refrigerating capacity given from the experimental conditions. Because the values are quite close,
It has been confirmed by another experiment that the refrigerating capacity can be further improved by using a machined surface with grooves and protrusions not only for the outer peripheral structure but also for the pipe inner surface structure.

[Effects of the Invention] As described above in detail, the heat transfer tube according to the present invention exhibits the effect of promoting the Marangoni convection by the surfactant as well as the effect of stirring the solution due to the tube shape, the increase of the heat transfer area, and the like. It is possible to greatly improve the transfer of heat and both sides of the material, and improve the performance of absorbers such as absorption chillers and absorption heat pumps that use it, which contributes to downsizing of equipment and higher efficiency. Has something to admire.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a heat transfer tube according to the present invention,
FIG. 2 is an explanatory front view thereof, FIG. 3 is an explanatory view showing an outline of a performance measuring device, and FIGS. 4 to 7 are diagrams showing performance measurement results. 1: Heat transfer tube, 2: Groove, 3: Fin.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tokio Miyauchi 3550 Kidayo-cho, Tsuchiura-shi, Ibaraki Hitachi Cable Co., Ltd. Tsuchiura factory (72) Inventor Kiyoshi Oizumi 3550 Kida-yocho, Tsuchiura-shi, Ibaraki Hitachi Cable Co., Ltd. Tsuchiura Factory (56) Reference JP 62-206356 (JP, A) JP 63-306370 (JP, A)

Claims (3)

[Claims] 1. A heat transfer tube for an absorber in which an absorbing liquid is dripped on the outside and cooling water is allowed to flow on the inside, a plurality of grooves extending in the longitudinal direction on the outer surface, and a fine pitch provided between the grooves. The fin height is 0.5 to 1.5 mm, the fin pitch is 0.4 to 1.5 mm, the groove depth is 0.5 to 2 mm, and the groove spacing is 3 to 6 mm in the circumferential direction. Heat transfer tubes for absorbers each configured as follows. 2. The heat transfer tube according to claim 1, wherein the bottom surface of the groove is formed to be substantially equal to or deeper than the fin bottom. 3. The heat transfer tube according to claim 1 or 2, wherein the angle of the groove with respect to the tube axis is within the range of -10 ° to + 10 °.