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[MichaelS]

Aloha,

 

Having recently joined a team working on an off-grid Solar-Driven Humidifcation-DeHumidification Desalination project, I am having my trial by fire with the art and science of heat exchanger fundamentals. To help illustrate the situation I am up against, and to help assess the feasibility/practicality of the current design the first PDF attached is a "Simplified HEX Diagram"

 

Overview: A very limited number of solar thermal collectors bring energy into the system, and via heat exchanger #1, seawater is heated to the high 80's to mid 90's C (but keeping seawater temperature to less than 100 C). I have the option of running the water (or depending on the final temperature selected, oil) in the collector system at 90-270 degrees C, with a flow rate from 6-12 GPM. (Note: the tradeoff for running a higher solar temperature is a few percent lower Watts thermal output per panel.) The heated seawater evaporates, is transported via a saturated airstream, is condensed, and we have freshwater. The trick is to conserve and re-use as much heat as practical, so heat exchanger #2 is introduced to: a) recapture the latent heat of condensation of the fresh water to heat the incoming seawater before this seawater is further heated via heat exchanger #1, and cool the freshwater to be sent back into the condenser to assist with the condensation process. 

 

Questions:

 

1) What type of heat exchangers would be best for this system?

2) What materials would be adequate for the heat exchangers?

3) Is this current design viable? 

4) Currently heat exchanger #1 has a temperature cross; would it be more efficient to raise the incoming hot temperature to remove this cross?

5) What realistic values can I expect for the hot seawater out, and the cold freshwater out of heat exchanger #2?

 

 

Another source of heat I am trying to decide what to do with is the 45 C seawater exiting the evaporator. An option I came up with is tapping a portion of the incoming cold ocean water, maybe 1 to 2 GPM of the 8.5 GPM, and running this cold ocean water through a heat exchanger number 3 (represented in second attachment, "Simplified HEX Diagram 2.1") with the hot 45 C evaporator seawater before rejoining the stream with the additional 6.5 to 7.5 GPM of heated seawater from heat exchanger #2.

 

6) Would heat exchanger #3 assist or hamper heat exchanger #2 with its heat transfer duties?

7) Other suggestions on how or where to utilize the 45 C seawater from the evaporator before returning it to the ocean?

8) Do you have any other suggestions on how to make this system more efficient (without raising the seawater temp over 100 C)?

9) Any other advice and guidance for a newbie way out of his league would be much appreciated?

 

Please feel free to address any question by referencing its number, or just provide your thoughts in general. Thank you for your time and consideration.

Attached Files

•  Simplified HEX Diagram.pdf   523.55KB   326 downloads
•  Simplified HEX Diagram 2.1.pdf   532.01KB   380 downloads