| CPC F24H 1/103 (2013.01) [F24H 9/0021 (2013.01)] | 21 Claims |

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1. A heater for a fluid, the heater comprising:
a heater shell; and
a heating device arranged inside of the heater shell and comprising a water inlet and an internal passage,
wherein a front segment of the heating device is installed with a metal elastic device, and a portion of an inner wall of the heater shell corresponding to the heating device with no metal elastic device installed is partially or wholly provided with a helical groove structure, the front segment with the metal elastic device providing a higher rate of heat transfer from the heating device to the fluid than the portion with the helical groove structure, the front segment with the metal elastic device configured to heat the fluid before the fluid is mixed in the portion with the helical groove structure,
wherein the fluid flows in a first direction at the heating device and in a second direction at the heater shell,
wherein the fluid flows in the first direction through the water inlet and the internal passage, and
wherein the portion of the inner wall of the heater shell with the helical groove structure and the heating device cooperate to form a helical flow path around the heating device.
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