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Daox 06-21-11 04:05 PM

Yes, you are correct. However, to keep things simple for me, and to build in a bit of safety factor, I kept the load uniform on the sidewalls. So, the same amount of stress that is on the bottom is also on the top. Its a bit of overkill, but thats not a bad thing and it simplified things for me.

I still have a few more iterations I want to run. I might take the time to make a full nice and proper model.

Daox 06-21-11 06:19 PM

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I ran a few more iterations since the last posting. I'm really not sure how well wood ages strength wise, so I was looking for a little added assurance. I'd like the tank to last as long as everything else, so I'm looking at may years of service. Also, if I happen to get a small leak and the wood gets wet... There are many things that could compromise the tank, so a little extra beef isn't bad.

Anyway, I tweaked my pressure setting for a deeper tank which I forgot to do, and I also added some additional bracing. By adding a couple more 2x4s it is really looking good ($10 of wood maybe). The only high stress point is right under the bar that goes across the tank in the middle. Tomorrow I'll try running a simulation with correct loading on the sidewall and see how that effects things. It should reduce the stress on the cross bar. Until then, this is the current design using 15/32 plywood sides/bottom.

Daox 06-22-11 09:13 AM

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This morning I ran one more simulation. I still couldn't get a nice gradual pressure change over the height of the sidewalls. So, instead I split them into three different sections. I applied 1.7 psi to the bottom, 1.1 psi to the middle, and .56 psi to the top. You can see the divided sections here.

I then ran the simulation and here is what resulted, 1800 max psi. This is roughly 1/3rd what the plywood can take so I should have plenty of safety factor.

The end result of all of this design work is that I'll be saving around $55 in materials cost. Considering my total estimated cost for the tank is now around $300 (I haven't figured out all the insulation costs yet), that is a substancial percentage of the cost of the tank.

S-F 06-22-11 12:30 PM

Just find some recycled polyiso. I just got 80 sheets @ 4' x 8' x 2.25" for $14 a sheet delivered.

AC_Hacker 06-22-11 12:48 PM


Originally Posted by Daox (Post 14199)
This morning... I split them into three different sections...

Looks like a fair approximation...


Daox 06-22-11 01:19 PM

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I emailed Gary to get his input on the tank design. He recommended not exceeding 1800 psi. So, I reran my simulation with 19/32 plywood (an extra $9), and the stress dropped to just under 1400. I'll probably go with that just because a little extra won't hurt. $9 for peace of mind is more than worth it.

TimJFowler 06-22-11 05:44 PM

How are you planning on fastening the wood and sealing it?

Daox 06-22-11 05:58 PM

The wood frame will be screwed and glued together.

To line the tank, I'll be using a paint on liner material called permaflex. It says it holds up to 250F temperatures. It is also less expensive than a pond liner like Gary used on his tank. We'll see, but I think it might also be easier to install as well. One person who submitted his project to builditsolar is using it. I've tried to contact him to verify that its all working well for him, but haven't gotten a reply.

greif 06-23-11 08:06 PM

Paint on Waterproof Liner For Solar Heat Storage Tanks

I highly suggest to use 2 x 6 supports and run a few more around the preimeter like I did, maybe every foot up as that is alot of pressure on those walls, I have 1.25 thick walls and as you can see I have lots of support and still I get about 3/4" bowing about 2' up, I use 2 x 4 but wish I would have did 2x6, I will be adding more this summer.

Gary Reif

Daox 06-24-11 09:53 AM

Thanks for the info Gary, I really appreciate it!

As I recall your tank is very tall. The height of the water directly effects the pressure at the bottom and thus the gradient of pressure that the wall sees, the bottom seeing the most. Since my tank is so much shorter, it shouldn't need the extra reinforcement that a taller tank would. According to all the engineering data I can find, this should tank should hold up fine. Will it bow? I'm sure it will, but wood bends a lot before it breaks.

Moving forward, I have to get measurements of the exact envelope that I'm working in. I'll be designing another frame around the tank to hold the cellulose. It will be a very low cost design being framed in 2x4s or 2x2s and covered in construction plastic as much as possible. I'll then blow cellulose into this framing and that will give me my inexpensive insulation.

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