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Irishvoyageur

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Reply with quote  #11 
What make and model 12V fans are you using or considering? I plan to run my fan(s) with a solar panel. I am still in the design stage and plan to install a long narrow air heater below my porch windows next fall.

KevinH

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Posts: 565
Reply with quote  #12 
The duct fans are usually rated based on the "boosted" air flow, not what the fan by itself does.  For example, I saw one at Menards rated at 250 CFM boosted (that is the total air flow from the furnace fan and booster fan combined).  The duct fan by itself is only 140 CFM in free air.  In a collector with its air flow resistance that 140 CFM would drop a lot.

Kevin H

gbwillson

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Reply with quote  #13 
Welcome Irish-

We need more Irish! How large is the collector you will be building? How did you determine the desired 120-150CFM output for your collector? For a great selection of 12 volt fans of all shapes, sizes and capacities, check out Digikey.com. You can use more than one fan as well. Just remember that a fans CFM rating is in free, unrestricted, air so you will want fans with a total CFM capacity that exceeds the desired CFM output of the collector.

Greg in MN



https://www.digikey.com/products/en/fans-thermal-management/dc-fans/217
Irishvoyageur

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Posts: 31
Reply with quote  #14 
I estimated my fan requirements by multiplying the collector size by 3 and added 20%. I am looking at building a long narrow screen collector. It will be vented and exhausted directly through a wall into a sun porch (no long venting pipes are needed). I would like to use a 6 inch 12 volt DC fan that could push air through the collector and run off a solar panel. I built a small test screen collector this winter (about 12 sq ft). Now planning to build a large one next summer.
Irishvoyageur

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Posts: 31
Reply with quote  #15 
Sorry, just saw your post. Very useful information. Can you provide more details on the collector? Are you passing air first through a pack pass, then to the front of the collector between two screens? Regarding my fan selection, I will likely go with a push and pull design (one pushing the intake and one pulling the exhaust). I have not considered a filter. I would need substantially more fan power if a filter was used. Right now, I am considering two different Noctua 140mm fans that are rated at 110 and 150 cfm in free air.
gbwillson

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Reply with quote  #16 
My design had the air entering the collector on top of the plate between screen layers, before returning under the plate, between the fins. I think the idea of two fans, one pushing, one pulling is a good idea. I do wonder if two DC computer fans will be enough to keep the collector efficient. I would suggest a far more powerful fan be available when you first operate the collector just in case the small fans aren't enough. It would buy you time to with buy upgraded fans or make other changes so the collector doesn't overheat. It's always better to have far more fan capacity than needed. You can easily reduce the airflow for maximum efficiency. A formula for calculating proper airflow plus a bit extra might get you started. But with an unknown design, you could easily underestimate your airflow needs. The highest efficiency heaters collect a LOT of heat, and thus, need a LOT of airflow!

A simple, cheap pleated furnace filter can be used. Also, an air filter for a car or truck would work quite nicely too. Just make sure you don't skip the filter or use a cheap spun fiberglas filter. In either case, you will end up regretting your decision and have to open up your collector for cleaning.

Greg in MN
Irishvoyageur

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Posts: 31
Reply with quote  #17 
I figure I can always go with a higher rated fan if desired. Do people clean their collectors periodically to remove fine particulates that collect on surfaces and reduce the efficiency or flow rate of the collector? A filter would help, but that would also drastically reduce the air flow and hence require a fan with substantially higher flow capacity. Does a filter eliminate the need to open up the collector for cleaning?
gbwillson

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Reply with quote  #18 
The air entering a collector generally comes from within your home, so if you feel you have a very clean home, perhaps you could skip the air filtration or try the collector without filtration to check airflow. But a cheap, basic pleated air filter adds very little resistance unless it starts to load up with dust, dirt, pollen and hair. And, it's a heck of a lot easier to use a filter than to open a collector as needed for cleaning. Dust can collect on the absorber, diminishing heat transfer. It can also stick to the underside of the plastic glazing as the fast moving air can create a mild static charge attracting dust. Again, an air filter is not needed in some cases, but unless your home is free of pets, dust, pollen, etc., it's a small feature worth having. 

Greg in MN
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