A direct expansion evaporator includes an inner guiding duct defining a feeding channel for guiding raw material, and an outer guiding duct enclosing the inner guiding duct therewithin to form a heat exchange channel between the outer and inner guiding ducts for guiding refrigerant flowing along the heat exchange channel to heat-exchange with the raw material along the feeding channel, wherein a helix indention is formed at the outer guiding duct to form the heat exchange channel partitioned by a helix partition, wherein a peak of the helix partition is biased against an outer surrounding wall of the inner guiding duct to conceal the heat exchange channel along the inner guiding duct in a weld-less manner.