we almost need a hvac section !

[bobhenry]

I just noticed that the heating ventilation and air conditioning topics are everywhere.

I had a question with regard to my soon to be constructed solar (beer can) furnace and couldn't find it a fitting home.

Well anyway here is the question......

I was brain storming air flow and not being as smart as many of our engineer types I was kinda in a fog as to how to divide and equilize the pressurized air flow thru the solar furnace tubes.

Here are the parameters, the furnace is 29 wide by 87 tall that's 10 cans wide and 16 cans tall. There is an 1 1/2" hole in the bottom of each can and with the entire top cut out with a can opener the top is a 2" opening. The flow will be as in the drawing below. The unit will be fed with a small yet to be determined fan.



This being said I am trying to determine how to equalize the air flow thru the tubes. I am smart enough to know the air flow will take the path of least resistance and do not want 90% of the air flow to go thru the first 2 or 3 tubes. This would greatly reduce the effeciency and leave the heat in the last several tubes. In the picture I have added an inclined baffle that will raise the compression in the entrance chamber the further back the air flows. Am i off base here? My other thought was to let the entrance holes in the bottom support board get progressively larger as they step to the rear (left side). To me the inclined baffle make the most sense but does it work in actuality?

I would appreciate any fluid dynamics engineer type or perhaps an HVAC designers opinion and am open to any good ideas to make this little unit more effecient after all it's free heat and you all know I LOVE FREE !

[Shadow Catcher]

Bob, Not an expert (x the unknown factor, spurt drop of water under pressure, expert = unknown drip under pressure) why not let convection take care of the issue. No fan needed, it will not function when there is no heat gain though you will want a damper to close to keep from getting reverse flow during cold weather/night that exceeds heat gain. Do a search on passive solar heater.

[bobhenry]

Tom the air is leaving the caboose and being sent to the solar furnace I didn't feel that the thermal lift would provide enough suction to pull the ambient inside air from the building and push it back into the building. I have 2 electrical needs a thermostat to prevent operation below 75 degrees and I am thinking a timer to pulse the fan now and then to reduce the electrical drain. ( be it 12 volt DC or 120 volt AC )

[pete42]

Mr. Bob
I would think like a car or motorcycle exhaust manifold once the air is forced into the collector tubes
by the fan and the collector is full of air it would come out of each tube the same amount due to the back pressure
creatated by the exit openings size vs the input openings size.

even if both openings are the same I believe there will more air going in than can escape
creating a slight back pressure causing all the tubes to be filled with air.

or it won't work at all and you need to just buy a heater...............

one other thought could you make the tubes all one long tube one way in one way out with the air forced through each tube?

pete

[Shadow Catcher]

Bob from what I saw the pure convection should work fine, there are a lot of collectors out there doing it. Once inside the caboose yes you will want a fan to help distribute it. At most just using a low power draw suction fan should be sufficient. see http://www.motherearthnews.com/Renewabl ... eater.aspx

[Forrest747]

No need for teh incline just add some loose springs and the pressure difference will be fine.

[bobhenry]

No need for teh incline just add some loose springs and the pressure difference will be fine.

loose springs ??????

[Forrest747]

No need for teh incline just add some loose springs and the pressure difference will be fine.

loose springs ??????

http://dsc.discovery.com/tv-shows/mythbusters/videos/drain-disaster.htm They explain it better than i could.

[48Rob]

Bob,

As the sun warms the assembly, the air in the individual "tubes" will get warmer, which will cause it to rise.
As it rises, it will draw an equal amount of cold air from the bottom.
Equalizing the draw from the front to the back of the intake chamber should not be an issue to consider, unless your intake is smaller than the combined area of all the tube openings combined.

Rob (who is not an engineer... )

[Forrest747]

Sorry didn't know much of anything until today. Was looking at the ramp and thinking easier to add obstructions to even out the air flow. Now i understand

[rowerwet]

make the outlet be on the opposite side of the collector as the inlet, instead of the same side. you could move the outlet, or put the baffle across the top with the opening on the far side from the inlet, either way would make the distance through each tube about equal. Keep in mind hotter air or water has more volume than cold, make the outlet plumbing larger than the inlet.
a great forum for DIY heating and solar is http://www.hearth.com/talk/ I have the same name there.

[bobhenry]

Well I needed a rainy day project so I layed out the frame for the solar furnace.
Bud cans are 2 5/8 in diameter and just a dab over 4 3/4" tall. I used 4 3/4 because they set inside each other just a bit.

I fiddled with a hole saw and all kinds of manners to drill the pathway thru each can and then found a site where a guy was making one and simply used a hand held can opener

DUH !

The bottom is a bit tricky as you have to hold the opener at a weird angle.

a 27" board gives you 10 cans wide with centers laid out 1 5/8" from the end then 2 5/8" center to center. I figured mine at 16 cans tall which is 76 inches and added a 1/2" just in case! If it turns out a bit sloppy a layer of foam could be added to snug up the stack.

I figured overall height at 92 1/2" inside giving a 8" +/- entry and an 8" +/- exit plenum I cut the frame 3 1/4" deep and the plexy and 7/16 osb surface mount.This way a layer of 1/2 foam can be added inside under the cans to snug things up. Since the cavity for the cans is all but exact I saw no need to tape or seal the cans as others have done. After all where can they go. The 28 1/2" x 94" unit should be light enough to be easily handled. They could be made in modules and hooked in series for more heat