(EP991465-A1)      An active microchannel heat exchanger to heat a working fluid comprises: (i) an exothermic reaction chamber having a first shell which defines exothermic reactant flow and exhaust flow paths and which has at least one inlet and an exhaust outlet; (ii) an exhaust chamber which has a second shell and a containment wall defining an exhaust flow path connected to the exhaust outlet, the second shell has an outlet and the chamber has microchannels through which an exhaust flows; and (iii) a heat exchanger chamber which thermally contacts the exhaust chamber and has a third shell through which a working fluid flows via an inlet and an outlet, and such that heat from the reaction chamber is convected by the exhaust through the exhaust chamber and by conduction through the containment wall to heat the working fluid. Also claimed is a liquid vaporiser comprising a heat exchanger as above where: (i) a porous material is placed in the exothermic reaction chamber; (ii) the exhaust chamber does not have microchannels; and (iii) heat convected through the exhaust chamber and conducted through the containment wall vaporises the liquid in the heat exchanger chamber. Also claimed is a chemical processor comprising a heat exchanger as above wherein the working fluid flow path contains microchannels.    USE -   The heat exchanger is useful as a liquid fuel vaporiser and/or steam generator for fuel cell power systems, and as a heat source for sustaining endothermic chemical reactions and initiating exothermic reactions.    ADVANTAGE -   The heat exchanger is small, lightweight and efficient.    (US6200536-B1)      An active microchannel heat exchanger to heat a working fluid comprises: (i) an exothermic reaction chamber having a first shell which defines exothermic reactant flow and exhaust flow paths and which has at least one inlet and an exhaust outlet; (ii) an exhaust chamber which has a second shell and a containment wall defining an exhaust flow path connected to the exhaust outlet, the second shell has an outlet and the chamber has microchannels through which an exhaust flows; and (iii) a heat exchanger chamber which thermally contacts the exhaust chamber and has a third shell through which a working fluid flows via an inlet and an outlet, and such that heat from the reaction chamber is convected by the exhaust through the exhaust chamber and by conduction through the containment wall to heat the working fluid. Also claimed is a liquid vaporiser comprising a heat exchanger as above where: (i) a porous material is placed in the exothermic reaction chamber; (ii) the exhaust chamber does not have microchannels; and (iii) heat convected through the exhaust chamber and conducted through the containment wall vaporises the liquid in the heat exchanger chamber. Also claimed is a chemical processor comprising a heat exchanger as above wherein the working fluid flow path contains microchannels.    USE -   The heat exchanger is useful as a liquid fuel vaporiser and/or steam generator for fuel cell power systems, and as a heat source for sustaining endothermic chemical reactions and initiating exothermic reactions.    ADVANTAGE -   The heat exchanger is small, lightweight and efficient.