Abstract: In a circuit board installation in which circuit boards are slidably mounted in opposed guide rails of a rack in a cabinet, an efficient and relatively inexpensive flat cooling module is slidably mounted in opposed guide rails between two circuit boards. The cooling module comprises a pair of flat side walls spaced apart and sealed together around their edges to form a sealed chamber, an inlet port in the cooling module connected to a source of pressurized cooling fluid and, in each of the side walls, mutually spaced orifices in which nipples may be fitted and through which pressurized cooling fluid is directed towards components of the adjacent circuit boards.