Title of Invention

FALLING FILM TYPE HEAT EXCHANGER TUBE

Abstract

A falling film type heat exchanger tube for heat exchange between a liquid film on an external surface of a tube and a liquid flowing through inside the tube, comprising ribs formed in protrusion on an internal surface of the tube and extending spirally with a suitable distance between adjacent ribs; concavities formed on the external surface of the tube and extending spirally with a suitable distance between adjacent concavities; and a plurality of independent projections formed on the external surface of the tube and laid out spirally, said projection having a recess formed on its upper surface in such a way that an area aligned with said ribs on the internal surface of the tube is lower than an area aligned with an area between the ribs.

Full Text

Field of the Invention The present invention relates to a falling film type heat exchanger tube, such as a heat exchanger tube for a falling film evaporator for performing a heat exchange between a falling film of refrigerant {wat0r) formed on an external surface of a tube and a water flowing inside this tube to evaporate this refrigerant, and a heat exchanger tube for a falling film absorber for performing a heat exchange between an absorption liquid film dripped or dispersed on an external surface of a tube and a fluid flowing inside this tube to cool the absorption liquid. Description of the Prior Art Conventionally/ an absorption type heat exchanger such as an absorption type chiller has been used in such a way that the inside of the heat exchanger is kept in a vacuum state and a refrigerant on the outer surface of the tube is evaporated at a low temperature to obtain cold water in the tube by extracting an evaporation latent heat from the water in the tube. This cold water obtained is used for an air-conditioner or the like. According to this heat exchanger, an absorber and an evaporator are accommodated together inside one body. In order to obtain evaporation continuously, a refrigerant vapor generated by the evaporator is absorbed into an absorption liquid dispersed on the surface of a heat exchanger tube, and the inside of the body is maintained at a constant degree of vacuum. Accordingly, in order to improve the refrigeration capacity of ‘n absorption type chiller, it is necessary to increase the quantity of the refrigerant vapor generated in the evaporator and to increase the absorption quantity or the absorption capacity. Improving the performance of the heat exchanger tube is the most effective means for increasing the absorption capacity-For this purpose, the applicant of the present invention proposed a heat exchanger tube having formed independent fins by providing grooves and hills extending in a tube axial direction on an external surface of the tube (Japanese Patent Application Laid-open Public No. 9-113066). Further, according to a falling film type evaporator such as an absorption type water cooler, there has been performed a heat exchange between a refrigerant that flows down on an external peripheral surface of a heat exchanger tube and a liquid such as water that flows through inside this tube, thereby to cool the water within the tube. The refrigerant which flows down on the heat exchanger tube spreads out the surface of the heat exchanger tube, and is then evaporated at a low pressure while taking heat, at the and cold water is passed through inside the tube. Then, a liquid film of the refrigerant is formed on the external , surface of the tube. When this refrigerant evaporates/ the cold water flowing inside the tube is cooled* in this case’ at the time when the refrigerant wet and spread on the surface of the heat exchanger tube evaporates, the latent heat of vaporization is deprived from the heat transfer surface. Therefore, in order to efficiently cool the; water inside the tube, it is necessary to increase as far as possible the contact area between the heat exchanger tube and the refrigerant, that is, the area of the heat transfer surface (external surface of the tube). For providing a falling film type heat exchanger tube that meets this requirement/ the applicant of the present invention proposed a heat exchanger tube provided with a large number of fins on the external surface of the tube (Japanese Patent Application Laid-open Public No. 7-71889). According to this conventional heat exchanger tube, there are provided fins extending in a direction to be orthogonal with or in a spiral fashion with respect to a tube axial direction, on the external surface of the tube, and there are also provided grooves on the tops of the fins along with these fins- Further, there are provided concavities crossing an upper half portion of each fin in predetermined pitches• An angle formed between both side walls of each groove is within a range from 70 to 150*’. This heat exchanger tube has an advantage that the spreading property of the refrigerant is excellent, with a large surface area of heat transfer, resulting in a superior heat transfer performance to that of the prior art. The above-explained conventional heat exchanger tube for an absorber described in Japanese Patent Application Laid-open Public No. 9-113066 has concavities on the external surface of the tube at the rate of 3 to 25 (concavities/tube circumferential length). Therefore, this tube has sufficient spreading property of the absorption liquid in a tube circumferential direction. However’ on the other hand, in the tube axial direction’ the spreading property is so poor that the absorption liquid leaves the surface of the tube before the absorption liquid absorbs the vapor generated by the evaporator, with a result of performance reduction• The above-mentioned conventional heat exchanger tube for an evaporator described in Japanese Patent Application Laid-open Public No, 7-71889 has achieved the initially intended object. However, the heat transfer performance of this tube has come insufficient as a heat exchanger tube for an evaporator for which higher performance has been required increasingly in recent years, as explained below* According to this conventional heat exchanger tube, grooves are provided in a longitudinal direction of fins, and the upper half portion of each fin is divided into two in a Y shape as viewed from the cross section orthogonal with the longitudinal direction of the fins, with the division angle of each fin being within a range from 70 to 150*". Since, these divided portions close the grooves formed between the fins in the end a spreading property of the refrigerant to the grooves between the fins is poor and thick liquid film is formed, thus lowering the evaporation performance• Further, the fins are disconnected at concavities extending in a direction orthogonal with the longitudinal direction of the fins. Since, the concavities have a smaller deepness than the height of the fins, thus providing insufficient spreading property of the refrigerant in the tube axial direction. As a result, a liquid film is formed in a large thickness, which lowers the evaporation performance. SUMMARY OF THE INVENTION It is an object of the present invention to provide a falling film type heat exchanger tube, including a heat exchanger tube for a falling film absorber with improved spreading property of the absorption liquid in the tube axial direction and a heat exchanger tube for a falling film type evaporator with high evaporation performance of the thinner refrigerant and excellent evaporation heat exchange performance. A falling film type heat exchanger tube according to the present invention comprises ribs formed in protrusion on an internal surface of the tube and extending spirally with a suitable distance between adjacent ribs/ concavities formed on an external surface of the tube and extending spirally with a suitable distance between adjacent concavities, and a plurality of independent projections formed on the external surface of the tube and laid out spirally, said projection has a recess formed on its upper surface in such a way that aligned with the ribs on the internal surface of the tube is lower than an area aligned with an area between the ribs. In this falling film type heat exchanger tube/ it is preferable that the concavities on the external surface of the tube and the ribs on the internal surface of the tube are formed at positions mutually aligned with each other• Each projection is formed in a quadrangular pyramid having a height of, for example, 0.20 to 0.40 mm Further, it is preferable that each projection has an area rate (A) within a range of 0,25 S, A’ 0.40 as the rate of the area of the upper surface to the area of the bottom surface. Further, from the viewpoint of the cross section orthogonal with the tube axis, it is desirable that a pitch (P) of the concavities on the upper surface of the independent projections is within a range of 5.75 ‘ P ‘ 6.75 mm. Further, it is desirable that an angle 6 formed by the rib and the tube axial direction is within a tange of 40"*’ (9’ 44'. Further, it is preferable that a pitch PF of the projections in the tube axial direction is within a range of 0.89 ‘ PF ‘ 1.12 mm. According to the present invention, the independent projections having a quadrangular pyramid shape, for example, are disposed spirally on the external surface of the tube, and the upper surface of the projection has a recess:’ corresponding to an area of the rib on the internal’ surface portion is pulled into the low’ portion by the surface tension, with a resultant reduction in the film thickness of the refrigerant at the high portion of the projection, which improves the evaporation heat transfer performance* Further, when the dispersed refrigerant flows along an area between the projections disposed spirally, the refrigerant is induced to the concavities formed on the external surface of the tube, thus reducing the thickness of the refrigerant existing at other portions, which improve? the evaporation heat transfer performance. According to the present invention the projections provided mutually independent of each other on the external surface of the tube are formed to have their edge extending in the tube axial direction♦ Accordingly, the distance between the projections in the tube axial direction changes in a tube circumferential direction, so that the size of space sandwiched between the projections changes. As a result, a liquid dripped or dispersed on the external surface of the heat exchanger tube does not flow smoothly in the tube circumferential direction smoothly in the tube axial direction. Thus, the spreading property of the liquid in the tube axial direction improves. The heat exchanger tubes are usually made of copper or copper alloy, but they can also be made of aluminum, aluminum alloy, steel, titanium or the like. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view for showing a part of a falling film type heat exchanger tube relating to an embodiment of the present invention; Fig. 2 is a cross sectional view for explaining a pitch (P) of concavities; Fig. 3 is a cross sectional view for explaining a lead angle of ruby Fig. 4 is a perspective view for showing a part of an absorption type heat exchanger tube relating to an another embodiment of the present absorption type heat exchanger tube shown in Fig# 4, including a tube . axis; Fig. 6 is a view for explaining an area rate A; Fig. 7 is a top plan view of projections; Fig» 8 is a cross sectional view of a surface orthogonal with a tube dis; Fig. 9 is a diagram for showing a testing apparatus to be used for testing the performance of heat exchanger tubes; Fig. 10 is a graph for showing a relationship between an overall heat transfer coefficient and a pitch of projections; p Fig. 11 is a graph for showing a relationship. between an overall heat transfer coefficient and the area rate A; Fig. 12 is a graph for showing a relationship between an overall heat transfer coefficient and a pitch P of concavities; Fig. 13 is a graph for showing a relationship between an overall heat transfer coefficient and a lead angle of ribs0; Fig. 14 is a graph for showing a relationship between an overall heat transfer coefficient and a projection height FH; Fig* 15 is a graph for showing a relationship between an overall heat transfer coefficient and an angle 0 formed by concavities on an external surface of a tube with respect to a tube axis; Fig. 16 is a graph for showing a relationship’ between an overall heat transfer coefficient and an area rate AF which is a rate of an area AFl of an extended part of an edge portion of projections to an area AF2 of a space sandwiched between the Fig. 17 is a graph for showing a relationship between an overall heat transfer coefficient and a pitch PE of a projection 4 in a tube circumferential direction; Fig. 18 is a graph for showing a relationship between an overall heat transfer coefficient and an area rate A which is a rate of an area of an upper surface of a projection to an area of a bottom surface of the projection; Fig. 19 is a graph for showing a relationship between an overall heat transfer coefficient and a circumferential length pitch P of the concavities on the external surface of the tube; and Fig 20 is a graph for showing a relationship between Q: cooling capacity of the evaporator (kcal/h) G: flow quantity of the water (kg/h) in evaporator Cp: specific heat of the water (kcal/kg • **€) Tin: temperature of the water at the inlet (°C) Tout; temperature of the water at the outlet (°C) ATmx algorithmic average temperature difference of Tin and Tout (‘‘C) Te: evaporation temperature of the refrigerant (°C) Ko: overall heat transfer coefficient (kcal/m’h'‘C) AQ: standard external surface area of the original tube (m’) D(j: external diameter of the original tube (ra) L; effective length of the tube (m) N: number of tubes (piece) Fig. 10 is a graph for showing a relationship between an overall heat transfer coefficient obtained from the above equation (1) and the pitch of the projections PF. Fig, 11 is a graph for showing a relationship between an overall heat transfer coefficient and the area rate A. Fig. 12 is a graph for showing a relationship between an overall heat transfer coefficient and the pitch P of concavities• Fig. 13 is a graph for showing a relationship between an overall' heat transfer coefficient and the lead angle 6 of the ribs'. And Fig. 14 is a graph for showing a relationship between an overall heat transfer coefficient and the height FH of the projections. As shown in Figs. 10 to 14 and in Table 1, the overall heat transfer coefficients of the examples hi to A13 were higher than the overall heat transfer coefficients of In Tables 2 and 3, each mark denotes following size Do: external diameter of the original tube (mm) T: wall thickness of the original tube (mm) DF; external diameter of the fin fabricated part (mm) FW: thickness of the bottom wall (mm) PF: pitch of the projection in tube axial direction (mm) A: area rate of the projection P: Pitch of the concavities (mm) PR: pitch of the projections in the tube circumferential direction (mm) AF: A rate AF which is a rate of an area AFl of an extended part of an edge portion of projections to an area AF2 of a special Sandwiched between the projections ‘s an angle came by the concavities 33 on the external surface of the tube with respect to the tube axe. Test conditions are set as follows. Pressure in the vessel ‘6.0 mmHg Density of the LiBr water solution at the inlet: 63% by weight Temperature of the LiBr water solution at the inlet: 46*'C Flow speed of the cooling water: 1.50 m/sec Temperature of the cooling water at the inlet: 32*’0 Flow quantity of the LiBr water solution: 0.017 to 0’035 kg/ms Surfactant: 2-ethylhexanol-added Layout of the tubes: 1 row x 6 stages (stage pitch 26 mm) Number of paths: 6 paths The flow quantity of the cooling water is set based on the cross section of the end portion of the tube (original tube). Further, flow quantity of the LiBr water solution is the quantity of the absorption liquid flowing down along one side of the tube. An overall heat transfer coefficient K’, was calculated from the measured value obtained, based on said equation (1) Fig. 15 is a graph for showing a relationship between an overall heat transfer coefficient obtained from the equation (1) and an angle 6 formed by concavities 33 on an external surface of a tube with respect to a tube axis. Fig. 16 is a graph for showing a relationship between an overall heat transfer coefficient and an area rate AF which is a rate of an area AFl of an extended part 35 of an edge portion of the projections to an area AF2 of a space sandwiched between the projections. Fig. 17 is a graph for showing a relationship between an overall heat transfer coefficient and a pitch PR of a projection 34 in a tube circumferential direction* Fig. 18 is a graph for showing a relationship between an overall heat transfer coefficient and an area rate A which is a rate of an area of an upper surface of a projection 34 to an area of a bottom surface of the projection 34. Fig. 19 is a graph for showing a relationship between an overall heat transfer coefficient and a circumferential length pitch P of the concavities 33 on the external surface of the tube. Fig. 20 is a graph for showing a relationship between an overall heat transfer coefficient and a pitch PF of projections 34 on a cross section orthogonal With a tube axis. As shown in Figs. 15 to 20 and in Tables 2 and 3, the overall coefficients of heat transfer of the examples CI to C14 that satisfy claims 9 to 15 of the present invention were higher than the overall coefficients of heat transfer of the comparative examples Dl to D17, AS explained above, according to the present invention’ since the edge of the independent projections extend in the tube axial direction to form extended parts and since concavities are provided on the external surface of the tube, there is exhibited improved spreading property of the absorption liquid in the tube circumferential direction and in the tube axial direction, resulting in an improved absorption heat transfer performance. This makes it possible to provide a compact apparatus with high performance, and to reduce the quantities of materials for structuring the heat exchanger tube 1. A falling film type heat exchanger tube for promoting a heat exchange between a liquid film on ah external surface of a tube and a liquid flowing through inside the tube, comprising: Ribs formed in protrusion on an internal surface of the tube and extending spirally with a suitable distance between adjacent ribs; Concavities formed on the external surface of the tube and extending spirally with a suitable distance between adjacent concavities; and a plurality of independent projections formed on the external surface of the tube and laid out spirally’ said projection having a recess formed on its upper surface in such a way that an area aligned with said ribs on the internal surface of the tube is lower than an area aligned with an area between the ribs. 2. A falling film type heat exchanger tube according to claim 1, wherein the concavities on the external surface Of the tube and the ribs on the internal surface of the tube are being formed at positions mutually aligned with each other. 3. A falling film type heat exchanger tube according to claim 1 or 2, wherein each projection is formed in a quadrangular pyramid shape. 4. A falling film type heat exchanger tube according to claim 3, wherein the height of each projection is within a range from 0.20 to 0.40 rate. 5. A falling film type heat exchanger tube according to any one of claims each projection has panel area rate (A) within a as the rate of the area of the upper surface to the area of the bottom surface* 6. A falling film type heat exchanger tube according to any one of claims 1 to 5’ wherein from the viewpoint of the cross section orthogonal with the tube axis, a pitch (P) of the concavities on the upper surface of the independent projections is within a range of 5.75 mm. 7. A falling film type heat exchanger tube according to any one of claims 1 to 6, wherein an angle 0 formed by the ribs with the tale axial direction is within a range of 8. A falling film type heat exchanger tube according to any one of claims 1 to 7, wherein a pitch PF of the projections in the tube axial direction is within a range of 0.89 ‘ PF ‘ 1.12 mm. 9. A falling film type heat exchanger tube according to claim 1, wherein the edge of said projections are extended to the tube axial direction, and the heat exchanger tube is used for an absorber. 10. A falling film type heat exchanger tube according to claim 9, wherein each projection has an area rate (A) within a range of 0.25 ‘ A’ 0.40 as the rate of the area of the upper foreface to the area of the bottom surface. 11. A falling film type heat exchanger tube according to claim 9 or 10, wherein from the viewpoint of the cross section orthogonal with the tube axis, a pitch (P) of the concavities on the upper surface of the independent projections is within a range of 5.75 ‘ P ‘ 6,75 mm. 12. A falling film type heat exchanger tube according to any one of claims 9 to 11, wherein an angle 6 formed by the concavities on the external surface of the tub with respect to the tube axial direction is within a range of 30' 13. A falling film type heat exchanger tube according to any one of claims 9 to 12, wherein a pitch PF of the projections in the tube axial direction is within a range of 0.62 ‘ PF ‘ 1.33 mm. 14. A falling film type heat exchanger tube according to any one of claims 9 to 13, wherein a pitch PR of the projections in the tube circumferential direction is within a range of 0.50 ‘ PR ‘ 1.20 mm. 15. A falling film type heat exchanger tube according to any one of claims 9 to 14, wherein an area rate (AF), which is a rate of an area (AFl) of the extended part of the edge portion of the projections to a cross sectional area (AF2) of the space sandwiched between the projections, is within a range of 0,05 ‘ AF ‘ 0.65. 16. A falling film type heat exchanger tube substantially as described hereinabove and illustrated with reference to the accompanying drawings.

Documents:

290-mas-1999-abstract.pdf

290-mas-1999-claims filed.pdf

290-mas-1999-claims granted.pdf

290-mas-1999-correspondnece-others.pdf

290-mas-1999-correspondnece-po.pdf

290-mas-1999-description(complete)filed.pdf

290-mas-1999-description(complete)granted.pdf

290-mas-1999-drawings.pdf

290-mas-1999-form 1.pdf

290-mas-1999-form 26.pdf

290-mas-1999-form 3.pdf

290-mas-1999-form 4.pdf

290-mas-1999-form 5.pdf