The all-welded plate and frame heat exchanger is based on the advantages of a detachable plate heat exchanger to make up for the lack of a new tube type of heat exchanger. Similar to the semi-welded heat exchanger, there are no gaskets and can be used in processes requiring higher pressure and temperature. It retains the full countercurrent heat exchange characteristics of the detachable heat exchanger, which is 3-5 times more efficient than the traditional shell and tube type of heat exchanger. Depending on the applications, the plate and frame are divided into three types, undetachable, unilaterally and completely detachable.