in a given row in order to vary the distances thereby and it is possible to vary the distances between adjacent longitudinal edges 20 at random fig1 shows the connections of the internal fluid for the array 10 with the elements of row 12 connected in series and the elements of row 14 connected in parallel the flow directions of fluid have been indicated by arrows the elements of each row can be connected in parallel or series as desired fig1 including an example of each for purposes of illustration the individual elements shown in fig1 are of generally the same type as that shown in fig6 c with an insulating partition wall in each element which separates the elements into two chambers any of the other embodiments of the elements could of course be used as well with the fluid connection arranged accordingly as shown fluid flows from hot fluid source 70 through feed line 73 into the lower end of one chamber of element 71 the fluid flows upward and emerges from the top of the chamber flowing through line 74 to the adjacent element where it flows downward at the lower end of the adjacent element the fluid flows through line 76 to the next adjacent element this connection and flow pattern is continued through the remaining elements in the array similarly fluid emerging from cold fluid source 72 flows through feed line 78 to the second chamber of element 71 upward through the second chamber to line 75 which conveys it to the second chamber of the adjacent element where it flows downward at the lower end of the adjacent element.