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A heat exchanger finned tube 10 for use in fabricating a heat exchanger 1 useful as the evaporator for refrigerators or the like wherein a hydrocarbon refrigerant is used. Two tube insertion holes spaced apart from each other are formed in each of plate fins 12, and two straight tube portions 11a of

A heat exchanger finned tube 10 for use in fabricating a heat exchanger 1 useful as the evaporator for refrigerators or the like wherein a hydrocarbon refrigerant is used. Two tube insertion holes spaced apart from each other are formed in each of plate fins 12, and two straight tube portions 11a of a hairpin tube 11 are inserted through the respective holes of each plate fin to arrange the plate fins 12 in parallel into a plurality of fin groups 13 spaced apart on the straight tube portions 11a longitudinally thereof. The hairpin tube 11 is enlarged with use of a fluid to fixedly fit the plate fins 12 of each tin group 13 around an enlarged tube portion 14 of the hairpin tube 11 and provide a finless part 19 between each pair of adjacent fin groups 13 on each of the straight tube portions 11a. A restrained small-diameter portion 15 is provided in each of the finless parts 19 of each straight tube portion 11a. The heat exchanger 1 fabricated using the finned tube 10 exhibits the desired refrigeration performance with the leakage of refrigerant diminished.

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1. A finned tube for use in heat exchangers which comprises a hairpin tube having two straight tube portions, and a plurality of fin groups arranged on the straight tube portions longitudinally thereof at a spacing, each of the fin groups comprising a plurality of parallel plate fins extending acros

1. A finned tube for use in heat exchangers which comprises a hairpin tube having two straight tube portions, and a plurality of fin groups arranged on the straight tube portions longitudinally thereof at a spacing, each of the fin groups comprising a plurality of parallel plate fins extending across and fixed to the two straight tube portions. each of the plate fins having two tube insertion holes spaced apart from each other, the plate fins being fixedly fitted around an enlarged tube portion of the hairpin tube by inserting the two straight tube portions through the respective holes of each plate fin and enlarging the hairpin tube with use of a fluid, the straight tube portions each having a finless part between each pair of adjacent fin groups thereon, at least one of all the finless parts of each straight tube portion having a restrained small-diameter portion smaller than the enlarged tube portion in diameter and having a predetermined length.2. A finned tube for use in heat exchangers according to claim 1 wherein the restrained small-diameter portion is provided in each of the finless parts of each straight tube portion of the hairpin tube.3. A finned tube for use in heat exchangers according to claim 1 wherein each of opposite ends of the restrained small-diameter portion is made integral with the enlarged tube portion by a flaring portion formed therebetween and increasing in diameter toward the enlarged tube portion.4. A finned tube for use in heat exchangers according to claim 1 wherein the restrained small-diameter portion is an unenlarged tube portion.5. A finned tube for use in heat exchangers according to claim 1 wherein the hairpin tube is integrally provided on an inner peripheral surface thereof with inner fins extending longitudinally thereof and arranged at a spacing circumferentially thereof.6. A finned tube for use in heat exchangers according to claim 5 wherein the hairpin tube has high and low two kinds of inner fins alternately arranged circumferentially thereof and projecting from the inner peripheral surface of the tube to different heights, the high inner fins being 0.7 to 1.7 mm in height from the surface of the hairpin tube, the low inner fins being 0.4 to 1.2 mm in height from the surface.7. A finned tube for use in heat exchangers according to claim 5 wherein all the inner fins are equal in height and are 0.7 to 1.2 mm in height from the inner peripheral surface of the hairpin tube.8. A finned tube for use in heat exchangers according to claim 6 wherein the pitch of the inner fins is 0.4 to 1.6 mm.9. A finned tube for use in heat exchangers according to claim 6 wherein the hairpin tube is 6 to 10 mm in outside diameter and 0.4 to 0.8 mm in the wall thickness of a circumferential wall thereof.10. A heat exchanger comprising a heat exchanger finned tube according to claim 1 and formed in a zigzag shape in its entirety by bending the straight tube portions of the hairpin tube in the same direction at each pair of finless parts located in the same position with respect to the longitudinal direction of the straight tube portions, each pair of finless parts adjacent to each other longitudinally of the straight tube portions being bent in different directions.11. A heat exchanger according to claim 10 wherein each finless part of each of the straight tube portions of the hairpin tube has a restrained small-diameter portion, and the heat exchanger finned tube is bent at the restrained small-diameter portion of each finless part.12. A heat exchanger according to claim 10 wherein the hairpin tube is integrally provided on an inner peripheral surface thereof with inner fins extending longitudinally thereof and arranged at a spacing circumferentially thereof.13. A heat exchanger according to claim 12 wherein the hairpin tube has high and low two kinds of inner fins alternately arranged circumferentially thereof and projecting from the inner peripheral surface of the tube to different heights, the high inner fins being 0.7 to 1.7 mm in height from the surface of the hairpin tube, the low inner fins being 0.4 to 1.2 mm in height from the surface.14. A heat exchanger according to claim 13 wherein the pitch of the inner fins is 0.4 to 1.6 mm.15. A heat exchanger according to claim 13 wherein the hairpin tube is 6 to 10 mm in outside diameter and 0.4 to 0.8 mm in the wall thickness of a circumferential wall thereof.16. A heat exchanger according to claim 12 wherein all the inner fins are equal in height and are 0.7 to 1.2 mm in height from the inner peripheral surface of the hairpin tube.17. A heat exchanger according to claim 16 wherein the pitch of the inner fins is 0.4 to 1.6 mm.18. A heat exchanger according to claim 16 wherein the hairpin tube is 6 to 10 mm in outside diameter and 0.4 to 0.8 mm in the wall thickness of a circumferential wall thereof.19. A refrigerator which is provided with a refrigeration cycle having a compressor, a condenser and an evaporator, the evaporator being a heat exchanger according to claim 10, and wherein a hydrocarbon refrigerant is used as the refrigerant and circulated at a rate of 1 to 9 kg/h.20. A refrigerated showcase which is provided with a refrigeration cycle having a compressor, a condenser and an evaporator, the evaporator being a heat exchanger according to claim 10, and wherein a hydrocarbon refrigerant is used as the refrigerant and circulated at a rate of 1 to 9 kg/h.21. A process for producing a finned tube for use in heat exchangers which process comprises preparing a hairpin tube having two straight tube portions, and a multiplicity of plate fins each having two tube insertion holes spaced apart from each other, inserting the two straight tube portions through the respective holes of each plate fin to arrange the plate fins in parallel into a plurality of fin groups spaced apart on the straight tube portions longitudinally thereof and provide a finless part between each pair of adjacent fin groups on each of the straight tube portions, restraining at least one of all the finless parts of each straight tube portion by a restraining die having a cylindrical restraining portion with a diameter smaller than the inside diameter of the tube insertion holes of the plate fins, and introducing a fluid into the hairpin tube in this state to enlarge the tube and fixedly fit the plate fins of each fin group around an enlarged tube portion of the hairpin tube.22. A process for producing a finned tube for use in heat exchangers according to claim 21 wherein each of the finless parts of each straight tube portion of the hairpin tube is restrained by the restraining die.23. A process for producing a finned tube for use in heat exchangers according to claim 21 wherein the restraining die has a cavity comprising a cylindrical restraining portion having a predetermined length, two flaring portions extending from respective opposite ends of the restraining portion and increasing in diameter outward longitudinally of the restraining portion, and tube enlargement permitting portions extending from respective larger ends of the flaring portions and having an inside diameter not smaller than the inside diameter of the holes of the plate fins.24. A process for producing a finned tube for use in heat exchangers according to claim 21 wherein the restraining portion has an inside diameter equal to the outside diameter of the hairpin tube before enlargement.25. A process for producing a finned tube for use in heat exchangers according to claim 21 wherein the hairpin tube is integrally provided on an inner peripheral surface thereof with inner fins extending longitudinally thereof and arranged at a spacing circumferentially thereof.26. A process for producing a finned tube for use in heat exchangers according to claim 25 wherein the hairpin tube has high and low two kinds of inner fins alternately arranged circumferentially thereof and projecting from the inner peripheral surface of the tube to different heights, the high inner fins being 0.7 to 1.7 mm in height from the surface of the hairpin tube, the low inner fins being 0.4 to 1.2 mm in height from the surface.27. A process for producing a finned tube for use in heat exchangers according to claim 25 wherein all the inner fins are equal in height and are 0.7 to 1.2 mm in height from the inner peripheral surface of the hairpin tube.28. A process for producing a finned tube for use in heat exchangers according to claim 26 wherein the pitch of the inner fins is 0.4 to 1.6 mm.29. A process for producing a finned tube for use in heat exchangers according to claim 26 wherein the hairpin tube is 6 to 10 mm in outside diameter and 0.4 to 0.8 mm in the wall thickness of a circumferential wall thereof.30. A process for fabricating a heat exchanger comprising a heat exchanger finned tube produced by a process according to claim 21, the heat exchanger being formed in a zigzag shape in its entirety by bending the straight tube portions of the hairpin tube in the same direction at each pair of finless parts located in the same position with respect to the longitudinal direction of the straight tube portions, each pair of finless parts adjacent to each other longitudinally of the straight tube portions being bent in different directions.31. A process for fabricating a heat exchanger according to claim 30 by using a heat exchanger finned tube wherein the finned tube is bent at the portion of the finless part restrained by the restraining portion of the restraining die.32. A process for fabricating a heat exchanger according to claim 30 wherein the hairpin tube of the heat exchanger finned tube is integrally provided on an inner peripheral surface thereof with inner fins extending longitudinally thereof and arranged at a spacing circumferentially thereof.33. A process for fabricating a heat exchanger according to claim 32 wherein the hairpin tube has high and low two kinds of inner fins alternately arranged circumferentially thereof and projecting from the inner peripheral surface of the tube to different heights, the high inner fins being 0.7 to 1.7 mm in height from the surface of the hairpin tube, the low inner fins being 0.4 to 1.2 mm in height from the surface.34. A process for fabricating a heat exchanger according to claim 32 wherein all the inner fins are equal in height and are 0.7 to 1.2 mm in height from the inner peripheral surface of the hairpin tube.35. A process for fabricating a heat exchanger according to claim 33 wherein the pitch of the inner fins is 0.4 to 1.6 mm.36. A process for fabricating a heat exchanger according to claim 33 wherein the hairpin tube is 6 to 10 mm in outside diameter and 0.4 to 0.8 mm in the wall thickness of a circumferential wall thereof.