Most air conditioning systems inefficiently control humidity with refrigerant-based cooling, where air flows over a cold finned-tube coil with cold refrigerant inside. Because water vapor condenses from the air onto the cold surface, the air is dehumidified and then subsequently reheated for recirculation. A few cooling applications use refrigerant-absorber systems where refrigerant cools an absorbent liquid, which improves humidity control with a low vapor pressure. Existing three-fluid heat exchangers with refrigerant, absorbent, and air have inefficiencies like dead zones, and the absorbents can become entrained in the air and carryover into the building. To overcome these deficiencies, researchers at NREL in collaboration with Oak Ridge National Laboratory and Xergy, Inc. have developed a three-fluid heat exchanger with co-axial pervaporative/structural tube configurations, which prevent absorbent carryover or leakage and ensure even distribution on the heat exchanger.
The tube-in-tube heat exchanger directs an air flow over a selectively permeable tube, which contains the absorbent solution cooled or heated by refrigerant inside an inner non-porous tube. The refrigerant cools the absorbent to absorb water through the membrane from the air or heats it to release water through the membrane and into the air. The system may be configured as a shell and tube design, with the third fluid free to flow on the outside of the shell, or as a shell and tube-in-tube, with a central tube containing a first fluid, a second tube containing a second fluid, and an outer shell containing the third fluid. This particular geometry mitigates losses from dead zones and viscous drag, simplifies exchanger manufacturing by reducing the number and length of seams that can leak, and may improve exchanger reliability.
To learn more about Tube-in-Tube Ionic Liquid Heat Exchanger Employing a Selectively Permeable Tube, please contact Erin Beaumont at:
Applications and Industries
Refrigeration cycles powered by a heat source, such as industrial waste heat, combustion, or stored solar heat, may include those for
- Heating, ventilation, and air conditioning,
- Refrigeration, and
- Heat pumps.
The three-fluid heat exchanger
- Uses a co-axial pervaporative/structural tube configuration to reduce energy inefficiencies associated with refrigerant-absorber heat exchangers,
- Simplifies manufacturing by reducing the number of and length of seams prone to leakage, and
- May improve refrigerant-absorber system reliability.