In recent decades, the refrigeration industry has been developed rapidly, thus a heat exchanger, as one of the four main components of the air conditioner, is also required to be improved to optimize the design according to the market requirements.  A parallel flow heat exchanger has characteristics, such as a high cooling efficiency, a small size and a light weight, thus can meet the market requirements quite well, and in recent years, it has been increasingly applied in automotive air conditioning systems and other systems requiring the heat exchanger, such as household appliances. 
The parallel flow heat exchanger mainly includes circulating tubes, fins and headers, and the circulating tubes are generally micro-channel flat tubes.  The headers are provided at both ends of the micro-channel flat tubes to distribute and collect refrigerant.  The corrugated fins or louvered fins are provided between adjacent micro-channel flat tubes to improve the heat exchange efficiency between the heat exchanger and the air.  A baffle is provided inside the header to divide all of the micro-channel flat tubes into a plurality of flow paths, and with reasonable distribution of flat tubes in each flow path, a better heat exchange efficiency may be realized. 
A circular header is usually employed in the parallel flow heat exchanger to obtain a high pressure resistance.  The header of the parallel flow heat exchanger composes of multiple parts and needs multiple manufacturing procedures, such as flanging and welding. 
A conventional heat exchanger generally includes an inlet header, an outlet header, fins and flat tubes.  The inlet header and the outlet header are arranged in parallel.  The flat tubes in the same layer are each formed integrally by bending, and each includes a first horizontal portion connected to the inlet header, a second horizontal portion connected to the outlet header, and a bent portion which is twisted.  The first horizontal portion and the second horizontal portion are arranged in parallel. 
Referring to U.S. Pat.  No.  5,531,268 issued on Jul. 2, 1996, the conventional bent heat exchanger is made on basis of a single-layer heat exchanger, in detail, an arc-shaped surface is formed at the middle of each straight flat tube and then the whole heat exchanger is twisted by a certain degree along a center line of the arc-shaped surface, therefore the single-layer heat exchanger is bent to a double-layer heat exchanger, and after being bent, the arc-shaped surface at the middle of the straight flat tube is the bent portion. 
However, such bent portion has the following defects. 
Firstly, the bending process of the flat tube has a low precision, which is apt to cause a partial deformation of the heat exchanger and damage structures of the fins nearby, thus the heat exchange efficiency is reduced. 
Secondly, the external dimension of the heat exchanger cannot be accurately controlled due to the bending of the flat tube, which in turn results in installation difficulties. 
Therefore, it is necessary to improve the conventional technology to solve the above technical problems.