Methods and apparatus for uniform distribution of liquid desiccant in membrane modules in liquid desiccant air-conditioning systems

1. A heat exchanger for use in a desiccant air conditioning system, comprising:

a plurality of membrane-covered structures facing each other in a generally parallel arrangement and being spaced apart to define air channels therebetween through which an air flow to be treated by the desiccant air conditioning system can flow, each of said membrane-covered structures comprising:

a structure having a hollow interior portion defining a heat transfer fluid channel through which a heat transfer fluid can flow, said structure also having one or more outer hydrophobic polymer surfaces; and

one or more hydrophobic polymer membranes covering the one or more outer surfaces of the structure to define a liquid desiccant gap therebetween through which a liquid desiccant can flow, each membrane being heat sealed to the outer surface of the structure at discrete locations spaced apart by less than 50 mm on the outer surface to define liquid desiccant flow channels, wherein the liquid desiccant flow channels have a channel height measured from the outer surface to the membrane not exceeding 0.5 mm; and

a liquid desiccant pressure regulator connected to a liquid desiccant inlet of the heat exchanger, said pressure regulator configured to maintain a positive pressure at the liquid desiccant flow channels not exceeding a given preset value at a flow rate sufficient to fill each of the liquid desiccant flow channels across a given range of liquid desiccant temperature and concentration conditions, said liquid desiccant pressure regulator configured to divert a portion of the liquid desiccant flowing to the heat exchanger to a liquid desiccant tank in order to maintain a constant liquid desiccant pressure at the liquid desiccant flow channels.

2. The heat exchanger of claim 1 , wherein each membrane is heat sealed to the outer surface of the structure at discrete locations spaced apart by less than 30 mm on the outer surface to define liquid desiccant flow channels.

3. The heat exchanger of claim 1 , wherein the liquid desiccant flow channels have a channel height measured from the outer surface to the membrane not exceeding 0.3 mm.

4. The heat exchanger of claim 1 , wherein the flow rate in the liquid desiccant flow channels can be further maintained by a constant flow device between a pump and the liquid desiccant pressure regulator.

5. The heat exchanger of claim 1 , wherein the pressure regulator comprises an overflow tube or a spring-activated back pressure regulator.

6. The heat exchanger of claim of 1 , wherein the pressure regulator is configured to limit the flow rate of the liquid desiccant flowing into the liquid desiccant flow channels to 2-10 mm/sec while maintaining a positive pressure at the inlet of each liquid desiccant flow channel across a range of temperatures and liquid desiccant concentration levels to improve uniformity of the wetting out of the membranes and the uniformity of the liquid desiccant flowing through the liquid desiccant flow channels.

7. The heat exchanger of claim 1 , wherein the pressure regulator is configured to maintain the flow rate of the liquid desiccant at ⅕- 1/20th of the flow rate of the air in lbs./min. over a temperature range 50-140° F. during cooling operations and 0-100° F. during heating operations for desiccant concentrations ranging from about 15-40% when the liquid desiccant comprises LiCl.

8. The heat exchanger of claim 1 , wherein the pressure regulator is configured to maintain the flow rate of the liquid desiccant at less than ⅕th of the flow rate of the air flow in lbs./min. over a range of liquid desiccant temperature and concentration levels needed to condition the air by humidification, dehumidification, cooling, or heating.

9. The heat exchanger of claim 1 , wherein the discrete locations at which the membrane is heat sealed to the outer surface of each structure comprise a pattern of lines or dots.

10. The heat exchanger of claim 1 , wherein the outer surface of each structure includes a plurality of raised features at the discrete locations at which the membrane is heat sealed.

11. The heat exchanger of claim 10 , wherein the raised features comprises less than 15% of the surface area of the outer surface of each structure.

12. The heat exchanger of claim 1 , wherein the heat transfer fluid gap in each structure has a thickness of 0.5 mm to 2 mm.

13. The heat exchanger of claim 1 , wherein each structure comprises a panel, and wherein the panels have a vertical orientation.

14. The heat exchanger of claim 1 , wherein each structure comprises a panel, and wherein the panels have a horizontal orientation.

15. The heat exchanger of claim 1 , wherein the hollow interior portion of each structure is defined by a metal layer, and wherein the heat transfer fluid comprises a refrigerant.

16. The heat exchanger of claim 1 , wherein each structure has a tubular shape.

17. The heat exchanger of claim 1 , wherein each structure has a planar construction.

18. The heat exchanger of claim 1 , wherein each structure comprises two polymer plates that are sealed together around their periphery.

19. The heat exchanger of claim 1 , wherein each structure has a curved plate shape.

20. The heat exchanger of claim 1 , wherein each structure is vacuum formed to form the heat transfer fluid gap.

21. The heat exchanger of claim of 1 , wherein the pressure regulator is configured to limit the flow rate of the liquid desiccant flowing into the liquid desiccant flow channels to 5-10 mm/sec.