Editor's note: The Design Solutions department of the June 2010 edition of HPAC Engineering’s biannual Boiler Systems Engineering special section included a case study on transport-membrane-condenser technology. In response to reader interest, HPAC Engineering invited the authors to explain in greater detail how the technology works.

An upgrade to industrial and commercial high-pressure steam boilers, a transport-membrane-condenser (TMC) system enables the capture, recovery, and re-use of all sensible and latent waste heat, as well as water vapor from exhaust/flue gas.

Tubes made of a nanoporous ceramic membrane capture water via capillary condensation. A partial vacuum inside of each tube aids the transport of water through the tube wall.

A TMC system typically is applied in conjunction with an economizer. In a typical boiler steam loop, a TMC water-recovery unit operates in two stages to maximize its ability to recover both water and heat; thus, two separate cooling-water streams are used.

On the water side, first-stage TMC inlet water is obtained from the makeup water through a softener going to the boiler system. TMC outlet water with recovered water vapor and associated latent heat from flue gas goes to the deaerator for boiler-water makeup. Second-stage TMC inlet water comes from the deaerator system; it goes to the boiler with extra water recovered from the flue gas.

On the flue-gas side, the TMC typically is situated between the boiler exhaust and stack. However, it also can be mounted off of the boiler, in a side stream of the boiler exhaust.

The system is
designed to recover most of the remaining heat in flue gas. This includes latent heat
associated with water vapor, which accounts for two-thirds of the waste heat from natural-gas combustion. During this process, flue gas enters high- and
low-pressure economizers. These components recover sensible heat from the flue gas to
provide preheated water back to the system and cool flue gas before it enters the
TMC.

Flue gas enters the TMC from the low-pressure economizer. The nanoporous-ceramic-membrane material condenses water vapor with the remaining
sensible and latent heat for use as boiler feedwater, exhausting cooler, drier flue gas to
the stack. The hot-water vapor extracted from the flue gas combines with incoming cold
makeup water to expand and heat the water supplied to the system. The warm water is
pumped out of the TMC through the low-pressure
economizer, where it picks up additional heat from the flue gas on its way to the
deaerator/makeup tank, shown at the top of Figure 1.