A modular heat exchanger comprising: 1) a frame; 2) a first plate assembly comprising; a) a first plate including a first plurality of channels; b) a first aluminum alloy distributor that receives a first fluid and distributes it to each of the first plurality of channels, wherein the first plate and the first distributor are joined via a first friction-stir welding joint, the first distributor including an inlet that is detachably connectable with an input port for the first fluid; and c) a second aluminum alloy distributor that receives the first fluid from each of the first plurality of channels, wherein the first plate and the second distributor are joined via a second galvanic-corrosion-free joint, the second distributor including an outlet that is detachably connectable with an output port for the first fluid; and 3) a second plate assembly comprising: a) a second plate including a second plurality of channels; b) a third aluminum alloy distributor that receives the first fluid and distributes it to each of the second plurality of channels, wherein the second plate and the third distributor are joined via a third galvanic-corrosion-free joint, the third distributor including an inlet that is detachably connectable with the input port; and c) a fourth aluminum alloy distributor that receives the first fluid from each of the second plurality of channels, wherein the second plate and the fourth distributor are joined via a fourth galvanic-corrosion-free joint, the fourth distributor including an inlet that is detachably connectable with the output port; wherein each of the first plate assembly and second plate assembly is demountably attachable to the frame and removable from the frame independent of each of the other first and second plate assemblies (304-1, 304-2), and wherein the first plate assembly and the second plate assembly collectively define a first conduit between the first and second plate assemblies (304-1, 304-2) for conveying a second fluid substantially parallel to flow of the first fluid within the first and second plurality of channels when the first plate assembly and second plate assembly are attached with the frame, and further wherein the first conduit is fluidically isolated from the first plurality of channels 504 and the second plurality of channels. 1) a frame; 2) a first plate assembly comprising; a) a first plate including a first plurality of channels; b) a first aluminum alloy distributor that receives a first fluid and distributes it to each of the first plurality of channels, wherein the first plate and the first distributor are joined via a first friction-stir welding joint, the first distributor including an inlet that is detachably connectable with an input port for the first fluid; and c) a second aluminum alloy distributor that receives the first fluid from each of the first plurality of channels, wherein the first plate and the second distributor are joined via a second galvanic-corrosion-free joint, the second distributor including an outlet that is detachably connectable with an output port for the first fluid; and a) a first plate including a first plurality of channels; b) a first aluminum alloy distributor that receives a first fluid and distributes it to each of the first plurality of channels, wherein the first plate and the first distributor are joined via a first friction-stir welding joint, the first distributor including an inlet that is detachably connectable with an input port for the first fluid; and c) a second aluminum alloy distributor that receives the first fluid from each of the first plurality of channels, wherein the first plate and the second distributor are joined via a second galvanic-corrosion-free joint, the second distributor including an outlet that is detachably connectable with an output port for the first fluid; and 3) a second plate assembly comprising: a) a second plate including a second plurality of channels; b) a third aluminum alloy distributor that receives the first fluid and distributes it to each of the second plurality of channels, wherein the second plate and the third distributor are joined via a third galvanic-corrosion-free joint, the third distributor including an inlet that is detachably connectable with the input port; and c) a fourth aluminum alloy distributor that receives the first fluid from each of the second plurality of channels, wherein the second plate and the fourth distributor are joined via a fourth galvanic-corrosion-free joint, the fourth distributor including an inlet that is detachably connectable with the output port; a) a second plate including a second plurality of channels; b) a third aluminum alloy distributor that receives the first fluid and distributes it to each of the second plurality of channels, wherein the second plate and the third distributor are joined via a third galvanic-corrosion-free joint, the third distributor including an inlet that is detachably connectable with the input port; and c) a fourth aluminum alloy distributor that receives the first fluid from each of the second plurality of channels, wherein the second plate and the fourth distributor are joined via a fourth galvanic-corrosion-free joint, the fourth distributor including an inlet that is detachably connectable with the output port;