The entrance channel (10) for leading the flow of reagent into passages (13) between two or more foils (6-8) is in direct thermal contact with the transition region between the channel and passages. The flow of reagent reaches the transition region after passing through the entrance channel first. The foils are heated by supplying thermal energy in order to convert a liquid reagent flowing through passages between them into a vapour. The foils have at least one entrance channel extending essentially transverse to the reagent flow direction and extending over at least the majority of their length. The passage volume increases significantly in a transition region between the entrance channel and flow passage.
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