This invention relates to a method of and an apparatus for the fabrication of a spiral fin wherein a strip of material of rectangular cross section is formed into a spiral shape and wound on the outer periphery of a tube in one operation.
One constituent element of a heat exchanger is a finned tube having a spiral fin wound on the outer periphery of the tube. The finned tube is produced by what is generally referred to as a rolling and bending operation whereby a strip of metal of rectangular cross section payed out of a supply reel is rolled by a roll of a larger diameter, referred to as a rolling pan (or a pan roll), and a roll of a smaller diameter, referred to as a spindle roll, arranged in predetermined spacing relation with respect to the former roll with the rolling reduction being increased linearly in going from the inner edge of the spiral fin in contact with a tube toward the outer edge thereof, so that the strip is bent spirally and at the same time wound on the tube.
It is known that the inner diameter of a spiral fin is determined by the difference in rolling reduction between the inner edge portion and the outer edge portion of the fin when the width of the strip is constant. Thus, to fabricate a spiral fin of a small inner diameter by using a strip of a constant width, it is necessary that the difference in rolling reduction between the inner edge portion and the outer edge portion of the strip be increased. That is, the outer edge portion should have a larger rolling reduction.
Generally, in a rolling operation, the strip can be bitten well by the rolls and drawn quickly between the rolls when a rolling reduction is small. However, as a rolling reduction increases, slip would occur between the strip and the rolls, so that the strip would not be bitten well by the rolls and would be drawn with difficulty between the rolls.
In rolling and bending a strip to fabricate a spiral fin, the strip is drawn between the rolls at a speed close to the drawing speed at which the widthwise central portion of the strip, having a substantially mean rolling reduction, is drawn and rolled by the rolls, because the inner edge portion and the outer edge portion of the spiral fin have different rolling reductions. Because of this, slip has tended to occur between the rolls and the strip in a portion thereof corresponding to the inner edge portion of the spiral fin. Meanwhile, the strip has a tendency to be subjected, in a portion thereof corresponding to the outer edge portion of the spiral fin, to a compressive force which forces the strip between the rolls, due to the fact that the strip is fed between the rolls in this portion at a speed higher than the rolling speed of the strip as a whole.
When the spiral fin to be fabricated has a small inner diameter as compared with the width of the strip used, the compressive force applied to a portion of the strip corresponding to the outer edge portion of the spiral fin, as described hereinabove, would increase in intensity, so that partial buckling would occur in the strip immediately before being drawn between the rolls. Thus, the fabricated spiral fin would have riffles in its outer edge portion which would render the spiral fin unacceptable for specifications. When such riffles occur in the outer edge portions, processing of the spiral fin in the following steps would be interferred with, thereby reducing the operation efficiency in the fabrication of the spiral fin.
When the difference in rolling reduction between the inner edge portion and the outer edge portion of the spiral fin is increased to obtain a small inner diameter in the spiral fin, the compressive force applied to the outer edge portion of the spiral fin is increased in intensity as aforesaid, and this would increase the thickness of the strip before it is drawn between the rolls. Combined with deformation of the rolls due to an increase in rolling load, this would result in the strip being bitten intermittently by the rolls or the strip not being bitten at all. When the strip is intermittently bitten by the rolls, rupture would occur in the outer edge portion of the fabricated spiral fin. When the strip is bitten by the rolls, it would be impossible to continue the rolling and bending operation.
Under these circumstances, it has not been the performance which a finned tube of a heat exchanger is required to show but the limitations placed by the operation to be performed on a strip for fabricating a spiral fin that have been a predominant factor in deciding the dimensions of the spiral fins and tubes. Thus, the present tendency is to use tubes and spiral fins of larger dimensions than are necessary, raising the serious problem of wasting material from the point of view of conserving natural resources and reducing production cost.