NOVELTY - Processing unit, preferably an ion exchanger (10) comprises: a cooling device of a functional system, preferably a fuel cell system of a motor vehicle; a container (12) with at least one inlet (14) for cooling fluid to be processed and at least one outlet (16) for the processed cooling fluid, where a granular ion exchange medium (18) is arranged between the inlet and the outlet, in a flow path of the cooling fluid; and a compression device for compressing the ion exchange medium, and which comprises at least one elastic porous compression element, preferably an open-celled foam element.    USE - The processing unit is useful for processing a cooling fluid.    ADVANTAGE - The processing unit: has a simple- and highly compact structure; ensures enhanced processing of the cooling fluid, and balanced positive and negative changes in volume of the ion exchange medium relative to the container; and makes use of compression element that achieves improved flow of the ion exchange medium enhancing the ion-exchange efficiency and ion exchange capacity, thus it reduces a loss of pressure in the container.    DETAILED DESCRIPTION - Processing unit, preferably an ion exchanger (10) comprises: a cooling device of a functional system, preferably a fuel cell system of a motor vehicle; a container (12) with at least one inlet (14) for cooling fluid to be processed and at least one outlet (16) for the processed cooling fluid, where a granular ion exchange medium (18) is arranged between the inlet and the outlet, in a flow path of the cooling fluid; and a compression device for compressing the ion exchange medium, and which comprises at least one elastic porous compression element, preferably an open-celled foam element, which is arranged between the inlet and the outlet, for the cooling fluid to flow through in the flow path.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic partial longitudinal section of an ion-exchanger of a cooling device with a foam element.    Ion exchanger (10)    Container (12)    Inlet (14)    Outlet (16)    Ion exchange medium (18)