in fig1 the plate walls are represented by inwardly concave entry area n by a central inwardly convex area o and by a substantially planar exit area p in these two embodiments better efficiency of operation will be attained by assuring that the flow of gas or air is from the side of the device which represents bottom portions of fig1 and 18 in the embodiments of fig1 and 19 the direction of flow may be from either side in fig1 it will be seen that inwardly concave sides k provide convex adjacent areas and inwardly convex side walls m provide concave adjacent areas central walls l provide planar central areas in the embodiment of fig1 the sides of the plate walls q are of uniform planar construction along their full through extent in the previous embodiments i have shown a pair of resin stretch supporting spreader or mounting bars 47 47 39 as having a pair of side rails for mounting two tandem wire rows w in fig2 a variation is shown in which an opposed pair of insulating or resin bars 67 is provided with three rails represented by two side and one central rail for mounting three tandem wire rows w to thus provide three sets of wire stretches 62 compare the.