A fluid cooler packet for a plurality of electronic components comprising: a plurality of individual cooling circuits for receiving a supply of cooling fluid from a supply header and delivering that cooling fluid to an associated electronic component, said plurality of cooling circuits each including a return passage for receiving a return fluid after having cooled the associated electronic component, and returning the return fluid to a return header; and a volume of the supply header decreasing in a downstream direction as it passes over the plurality of individual cooling circuits, and a volume of the return header increasing as it moves in a downstream direction over the plurality of individual cooling circuits, wherein said plurality of individual cooling circuits are assembled in an array of rows and columns, and there are a plurality of parallel supply header portions (611;413) associated with each of said rows, and a plurality of parallel return header portions (613;411) associated with each of said rows, with a downstream direction being defined across said plurality of columns, and characterized in that said parallel supply header portions have nested portions which are positioned outwardly of a face of said parallel return header portions spaced away from at least some of the individual cooling circuits, with a volume of said nested portions of said parallel supply header portions decreasing as said volume of said return header parallel portions increases in a downstream direction, and said parallel return header portions have nested portions which are positioned outwardly of a face of said parallel supply header portions spaced away from at least some of the individual cooling circuits, with a volume of said nested portions of said parallel return header portions increasing as said volume of said supply header parallel portions decreases in a downstream direction. a plurality of individual cooling circuits for receiving a supply of cooling fluid from a supply header and delivering that cooling fluid to an associated electronic component, said plurality of cooling circuits each including a return passage for receiving a return fluid after having cooled the associated electronic component, and returning the return fluid to a return header; and a volume of the supply header decreasing in a downstream direction as it passes over the plurality of individual cooling circuits, and a volume of the return header increasing as it moves in a downstream direction over the plurality of individual cooling circuits, wherein said plurality of individual cooling circuits are assembled in an array of rows and columns, and there are a plurality of parallel supply header portions (611;413) associated with each of said rows, and a plurality of parallel return header portions (613;411) associated with each of said rows, with a downstream direction being defined across said plurality of columns, and characterized in that said parallel supply header portions have nested portions which are positioned outwardly of a face of said parallel return header portions spaced away from at least some of the individual cooling circuits, with a volume of said nested portions of said parallel supply header portions decreasing as said volume of said return header parallel portions increases in a downstream direction, and said parallel return header portions have nested portions which are positioned outwardly of a face of said parallel supply header portions spaced away from at least some of the individual cooling circuits, with a volume of said nested portions of said parallel return header portions increasing as said volume of said supply header parallel portions decreases in a downstream direction.