Experimental Investigation of a Novel Membrane-based Condensing Heat Exchanger used for High Efficiency Furnaces

Building space heating consumes approximately a third of all global natural gas end use, contributing significantly to global warming. Higher efficiency (aka, condensing) furnaces hold only about 25% of the furnace market in US buildings. One reason for this is that the condensing heat exchanger must use highly expensive, needs corrosion-resistant materials due to acidic components in the furnace flue gas stream. Increasing the market share of high efficiency furnace is beneficial to reducing greenhouse gas emissions. This study developed and tested a benchtop prototype of a novel membrane-based condensing (heat recovery) heat exchanger for high efficiency furnace to achieve non-acidic condensation via nano-porous membranes. Test results show that both sensible and latent heat are recovered with a fraction of latent heat recovery varies from 39% to 73%. The amount of water condensed through the membrane heat exchanger increases with the increase of flue gas flow rate while it decreases by increasing coolant temperature. The fraction of latent heat recovery decreases with the increase of flue gas flow rate and coolant temperature. The pH value of condensed water was only mildly acidic, varying from 5.0 to 6.3 without any additional treatment. It achieves significant improvement when compared with the conventional condensing furnace. Therefore, feasibility of the membranebased condensing heat exchanger has been experimentally verified, and it has potential to enable wider market penetration of highly energy-efficient condensing furnaces by reducing costs for dealing with the acid condensation.