A direct expansion evaporator for making frozen product from raw material, comprising: a feeding channel having a feeding end and a dispensing end for said raw material feeding through said feeding channel; a heat exchange channel (20A, 20B, 20C) thermally communicating with said feeding channel for guiding refrigerant passing through said heat exchange channel (20A, 20B, 20C) to heat-exchange with said raw material within said feeding channel, wherein said heat exchange channel (20A, 20B, 20C) has a pre-cooling portion (28A, 28B, 28C) formed toward said feeding end of said feeding channel and a freezing portion (26A, 26B, 26C) formed toward said dispensing end to thermally communicate with said feeding channel, wherein said feeding channel is arranged for guiding said raw material to flow from said pre-cooling portion (28A, 28B, 28C) of said heat exchange channel (20A, 20B, 20C) to said freezing portion (26A, 26B, 26C) so as to initially pre-cool said raw material when entering into said feeding end of said feeding channel and to substantially freeze said raw material to form said frozen product before said frozen product is dispensed at said dispensing end of said feeding channel; an outer guiding duct and an inner guiding duct coaxially enclosed within said outer guiding duct to define said feeding channel within said inner guiding duct and said heat exchange channel (20A, 20B, 20C) between said outer and inner guiding ducts (30A, 32A), wherein said heat exchange channel (20A, 20B, 20C) has a helix path configuration extended from said dispensing end of said feeding channel to said feeding end thereof, and wherein a helix indention is formed at said outer guiding duct to form said heat exchange channel (20B, 20C) partitioned by a helix partition, wherein a peak of said helix partition is biased against an outer surrounding wall of said inner guiding duct to conceal said heat exchange channel (20B, 20C) along said inner guiding duct in a weld-less manner, characterised in that the direct expansion evaporator further comprises first and second retention members (41A, 42A) to retain the inner guiding duct within the outer guiding duct. a feeding channel having a feeding end and a dispensing end for said raw material feeding through said feeding channel; a heat exchange channel (20A, 20B, 20C) thermally communicating with said feeding channel for guiding refrigerant passing through said heat exchange channel (20A, 20B, 20C) to heat-exchange with said raw material within said feeding channel, wherein said heat exchange channel (20A, 20B, 20C) has a pre-cooling portion (28A, 28B, 28C) formed toward said feeding end of said feeding channel and a freezing portion (26A, 26B, 26C) formed toward said dispensing end to thermally communicate with said feeding channel, wherein said feeding channel is arranged for guiding said raw material to flow from said pre-cooling portion (28A, 28B, 28C) of said heat exchange channel (20A, 20B, 20C) to said freezing portion (26A, 26B, 26C) so as to initially pre-cool said raw material when entering into said feeding end of said feeding channel and to substantially freeze said raw material to form said frozen product before said frozen product is dispensed at said dispensing end of said feeding channel; an outer guiding duct and an inner guiding duct coaxially enclosed within said outer guiding duct to define said feeding channel within said inner guiding duct and said heat exchange channel (20A, 20B, 20C) between said outer and inner guiding ducts (30A, 32A), wherein said heat exchange channel (20A, 20B, 20C) has a helix path configuration extended from said dispensing end of said feeding channel to said feeding end thereof, and wherein a helix indention is formed at said outer guiding duct to form said heat exchange channel (20B, 20C) partitioned by a helix partition, wherein a peak of said helix partition is biased against an outer surrounding wall of said inner guiding duct to conceal said heat exchange channel (20B, 20C) along said inner guiding duct in a weld-less manner, characterised in that the direct expansion evaporator further comprises first and second retention members (41A, 42A) to retain the inner guiding duct within the outer guiding duct.