Refrigeration cycle

A refrigeration cycle comprising:

• a compressor;
• a first heat exchanger connection with said compressor;
• and
• a second heat exchanger connected, at one end, with said first heat exchanger through an expansion means, and at another end, with said compressor;
• wherein at least one heat transfer tube of at least one of the first heat exchanger and the second heat exchanger is provided on an inner surface with a plurality of helical ridges which extend at a helical angle .alpha. of 10 degrees to 20 degrees with respect to the axis of the tube, said ridges having a pitch Pf1 such that when an inner diameter of the heat transfer tube is denoted by Di, a ratio Pf1/Di is within a range of 0.05 to 0.1, and secondary grooves which cross the ridges, said secondary grooves having a depth Hf2 which is set within a range of 40 to 100% of a height Hf1 of said ridges, a width Wf2 which is set between a top width Wt of said ridges and a bottom width Wb of said ridges, and a cross-sectional shape which provides said ridges with said secondary grooves with a heat transfer area which is not less than a heat transfer area without said secondary grooves.