Nanoporous Membrane Tube Condensing Heat Transfer Enhancement Study

A Transport Membrane Condenser (TMC), made from nanoporous membrane tube bundles, was developed by Gas Technology Institute (GTI) to recover the water vapor and its significant amount of latent heat from boiler flue gases to improve boiler efficiency and save water. Water vapor condensing phenomenon inside membrane pores is different in two aspects from surface condensation when the membrane pore size is in the nanometer scale. First, based on the pore capillary condensation mechanism—Kelvin equation, pore condensation can occur when local gas stream relative humidity is well below 100%, so more water condensation is possible at the same surface temperature compared with surface condensation. Second, as membrane heat exchanging surface continues evacuating condensed water to the permeate side, no water will be accumulated on the condensing surface, which eliminates the additional heat transfer resistance caused by the condensed liquid film (or droplets) for a conventional impermeable condensing surface. Experiments have been carried out to study the phenomena for both a nanoporous membrane tube bundle and an impermeable stainless steel tube bundle with the same characteristic dimensions. Flue gas streams with a water vapor mass fraction 11.3%, temperature ranges from 65°C to 95°C were used for the experimental study, which covers the typical TMC waste heat recovery application parameter range. Results show the convection Nusselt number of the membrane tube bundle is 50 to 80% higher than that of the impermeable stainless steel tube counterpart at typical condensation heat transfer conditions. More parameter study was also done to study a wider range of parameters. The condensing heat transfer enhancement effect gives a good perspective for using nanoporous membrane surface to design high efficiency condensing heat exchangers to recover both water vapor and its substantial amount of latent heat from high moisture content low grade waste heat streams.