NOVELTY - The heat exchanger comprises a heat exchanger module having first and second piping bundles with tubular strands (1a, 1b) for the first and the second medium. The tubular strands of the first medium lead to an end in a common first inlet collection chamber (2a) and to its opposite end into a common first outlet-collection chamber (3a). The tubular strands of the second piping bundle lead to an end in a second inlet-collection chamber (2b) and to its opposite end in a second outlet-collection chamber (3b).    USE - Heat exchanger for heat exchanging of a first medium e.g. digested sludge into a second medium e.g.raw sludge, useful for waste water- or sewage treatment plant.    ADVANTAGE - The heat exchanger can be simply, easily and economically constructed with high heat transfer efficiency.    DETAILED DESCRIPTION - The heat exchanger comprises a heat exchanger module having first and second piping bundles with tubular strands (1a, 1b) for the first and the second medium. The tubular strands of the first medium lead to an end in a common first inlet collection chamber (2a) and to its opposite end into a common first outlet-collection chamber (3a). The tubular strands of the second piping bundle lead to an end in a second inlet-collection chamber (2b) and to its opposite end in a second outlet-collection chamber (3b). The tubular strands of the first and second piping bundles run within a module parallel to each other and linearly without crescent-shaped deflecting sections. The collecting chambers for the first and second medium are arranged to each head of the heat exchanger modules. The first inlet collection chamber and the second outlet-collection chamber are arranged directly adjoining to each other and separately arranged from one another by a separation wall (7) to the head of a module, and/or the second inlet collection chamber and the first outlet collection chamber are arranged directly adjoining to each other and separately arranged by a separation wall to the head of the module. The separation wall is arranged vertically to longitudinal direction of the heat exchanger module. The collection chambers are sequentially arranged to the head in longitudinal direction. The tubular strands of the first piping bundle are arranged over first intermediate tube (4a) through the second outlet-collection chamber and the intermediate tube ends in the first inlet-collection chamber. The tubular strands of the second piping bundle are arranged over second intermediate tube (4b) through the first outlet-collection chamber, and the intermediate tube ends in the second inlet-collection chamber. The tubular strands are formed as box-shaped tubular strands with quadratic or rectangular cross-section, whose tube walls form the heat exchanger areas. The pairs of adjoining tubular strands flow through by the first and the second medium. The intermediate tube has round or oval cross-section as the tubular strands. The intermediate tube has a round cross-section with a diameter, which corresponds to the edge length of the box shaped tubular strands or is smaller than the tubular strands. The separation wall is arranged parallel to longitudinal direction of the heat exchanger module. The collection chambers are arranged side by side and/or one upon the other to the head. The tubular strands for the medium are formed from rectangular shaped box profiles, which are arranged with predetermined interval to each other, and the tubular strands for the other medium are formed from the intervals between two adjoining box profiles. The tubular strands stretch itself over the total height and/or width of the heads and consequently over the height and/or width of both collection chambers separated through the separation wall. Guide pieces with obliquely passing guiding surfaces are inserted into the tubular strands, block the access of the respective tubular strands to the collecting chamber and release the access to the adjoining collecting chamber. The heat exchanger module has a quadratic or rectangular cross-section in the area of the piping bundle. The collection chambers have a round or rectangular cross-section. The collection chamber and/or each collection chamber have a lockable maintenance opening. The end side external collection chamber has a module or all collection chambers of front side maintenance opening lockable over a lockable flange or a cover, and the interior collection chamber has a module of a circumferential side of maintenance access e.g. a maintenance opening lockable with a lockable cover or a lockable flap. The heat exchanger modules are sequentially turned in row and successively flow by the first and the second medium in counter flow. The outlet-collection chamber of the module is connected over a connection line with the inlet collection chamber of the subordinated module. The first and the second medium are guided in the counter flow or direct current.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of a heat exchanger and/or heat exchanger module.    Tubular strands (1a, 1b)    First and second inlet-collection chamber (2a, 2b)    First and second outlet-collection chamber (3a, 3b)    Intermediate tube (4a, 4b)    Separation wall. (7)