A plate-fin heat exchanger to exchange heat from a first fluid to a second fluid, the heat exchanger comprising: a center manifold to receive the first fluid; first and second nested loops configured to receive the first fluid from and exit the first fluid to the center manifold; the first nested loop comprising: a first inner loop having a plate-fin constitution and having an inner loop inlet and an inner loop outlet; and a first outer loop having a plate-fin constitution and disposed around the first inner loop, the first outer loop having an outer loop inlet and an outer loop outlet, wherein the inner loop inlet and the outer loop inlet are adjacent, and the inner loop outlet and the outer loop outlet are adjacent, and wherein the cross-sectional flow area of the outer loop from the outer loop inlet to the outer loop outlet is larger than the cross-sectional flow of the inner loop from the inner loop inlet to the inner loop outlet; said second nested loop comprising: a second inner loop having a plate-fin constitution and having an inner loop inlet and an inner loop outlet; and a second outer loop having a plate-fin constitution and disposed around the second inner loop, the second outer loop having an outer loop inlet and an outer loop outlet, wherein the inner loop inlet and the outer loop inlet are adjacent, and the inner loop outlet and the outer loop outlet are adjacent, and wherein the cross-sectional flow area of the outer loop from the outer loop inlet to the outer loop outlet is larger than the cross-sectional flow of the inner loop from the inner loop inlet to the inner loop outlet; and wherein the first outer loop outlet is adjacent to the second outer loop outlet of the second outer loop. a center manifold to receive the first fluid; first and second nested loops configured to receive the first fluid from and exit the first fluid to the center manifold; the first nested loop comprising: a first inner loop having a plate-fin constitution and having an inner loop inlet and an inner loop outlet; and a first outer loop having a plate-fin constitution and disposed around the first inner loop, the first outer loop having an outer loop inlet and an outer loop outlet, wherein the inner loop inlet and the outer loop inlet are adjacent, and the inner loop outlet and the outer loop outlet are adjacent, and wherein the cross-sectional flow area of the outer loop from the outer loop inlet to the outer loop outlet is larger than the cross-sectional flow of the inner loop from the inner loop inlet to the inner loop outlet; a first inner loop having a plate-fin constitution and having an inner loop inlet and an inner loop outlet; and a first outer loop having a plate-fin constitution and disposed around the first inner loop, the first outer loop having an outer loop inlet and an outer loop outlet, wherein the inner loop inlet and the outer loop inlet are adjacent, and the inner loop outlet and the outer loop outlet are adjacent, and wherein the cross-sectional flow area of the outer loop from the outer loop inlet to the outer loop outlet is larger than the cross-sectional flow of the inner loop from the inner loop inlet to the inner loop outlet; said second nested loop comprising: a second inner loop having a plate-fin constitution and having an inner loop inlet and an inner loop outlet; and a second outer loop having a plate-fin constitution and disposed around the second inner loop, the second outer loop having an outer loop inlet and an outer loop outlet, wherein the inner loop inlet and the outer loop inlet are adjacent, and the inner loop outlet and the outer loop outlet are adjacent, and wherein the cross-sectional flow area of the outer loop from the outer loop inlet to the outer loop outlet is larger than the cross-sectional flow of the inner loop from the inner loop inlet to the inner loop outlet; and a second inner loop having a plate-fin constitution and having an inner loop inlet and an inner loop outlet; and a second outer loop having a plate-fin constitution and disposed around the second inner loop, the second outer loop having an outer loop inlet and an outer loop outlet, wherein the inner loop inlet and the outer loop inlet are adjacent, and the inner loop outlet and the outer loop outlet are adjacent, and wherein the cross-sectional flow area of the outer loop from the outer loop inlet to the outer loop outlet is larger than the cross-sectional flow of the inner loop from the inner loop inlet to the inner loop outlet; and wherein the first outer loop outlet is adjacent to the second outer loop outlet of the second outer loop.