HEAT EXCHANGER

Heat exchange apparatus (100) such as a condensing boiler may comprise a gas heating stage comprising a liquid source and an evaporator heat exchanger (135) configured to heat a first gas stream (e.g. boiler inlet air) flowing from a first gas inlet to a first gas outlet, the evaporator heat exchanger being configured to evaporate liquid from the liquid source (139) to add vapour to the first gas stream, and, a gas cooling stage comprising a condenser heat exchanger (235) configured to cool a second (e.g. different) gas stream(e.g. boiler exhaust gas) flowing from a second gas inlet to a second gas outlet, the condenser heat exchanger being configured to condense vapour present in the second gas stream, and preferably to direct condensed vapour into a or the liquid source (239). The evaporator heat exchanger (135) transfers thermal energy from the liquid source to the first gas stream. The condenser heat exchanger (235) transfers thermal energy from the second gas stream to the liquid source. In each heat exchanger, the liquid source may be a liquid stream that flows in counter-flow to the gas stream, and the same liquid stream may flow through both heat exchangers in an open or closed circuit (140, 142) such that such that the liquid stream acts as a heat (or heat and mass) transfer fluid between the two heat exchangers (135, 235). In this way, thermal energy may be transferred between the two respective gas streams by the same liquid stream. The apparatus thus acts as a gas-to-gas heat exchanger comprising a gas-to-liquid heat exchanger configured to cool a first gas stream and a further gas-to-liquid heat exchanger configured to heat a second gas stream. Each of the evaporator heat exchanger (135) and condenser heat exchanger (235) may comprise a porous structure such as an open-cell foam structure (130).The porous structure may be rotatably mounted and partially submerged in the liquid stream, which may flow in counter-flow to the gas stream above it. The structure may comprise an elongate porous structure rotatable about its lengthwise axis, the liquid and gas streams each flowing parallel with that axis such that the heat exchange process causes a temperature gradient to develop along the length of the porous structure. The porous structure may have an average effective pore diameter of at least 3mm,such large pores being resistant to blocking, such that an annular seal against the circumferential outer wall/perimeter of the porous structure is not r