Plate fin heat exchangers consist of a block of alternating layers of corrugated fins. The layers are separated from each other by parting sheets, and sealed along the edges by means of sidebars. In most plate fin-exchangers, the central effective length of the block consists of fining laid parallel to the block axis, to give true counter flow heat exchange among the streams. These direct the flow to gaps in the side bars so as to connect the heat transfer passages with the headers and nozzles, which are welded onto the outside of the block. Large plate fin heat exchangers are made of aluminum and can be up to 1.2m wide, 1.2m deep and 6.2m long. They are most frequently used in low temperature applications, where they give the benefit of a multi-stream capability, ensuring that all the cold streams produced in a process can be used to cool the incoming warm streams.