Stainless steel conical beer fermenter Pt.2

The next step in the beer fermenter project is to mount some copper blocks to the outside of the stainless tank. The purpose of these blocks will be to thermally couple two peltier devices to the surface of the tank. The tank is cylindrical (near the top) and the peltiers are flat, so the copper blocks must be curved on one face, and flat on the other. I started by cutting off some chunks of copper and milling the edges square.



Next, I used a long end mill to profile the sides with a radius that matches the outside of the stainless tank. My plan was to silver solder the copper blocks to the stainless tank exterior. This turned out to be a very bad idea. I started by fluxing the copper and stainless, then separately covering them with a thin layer of silver solder. No problem yet. I put the semi-cooled block onto the tank, and planned to heat the tank and block and let the two solder-covered surfaces melt together. This started working, but then the stainless expanded dramatically under the block. The copper was lifted a clear 1/4" off the surface near the edges, while the center was making contact. I removed the heat, and looked inside the tank to find this new disaster:

A monster crack had developed in the wall of the tank! I am still not sure why this happened. It obviously has something to do with the metals' differing rates of expansion, but I had no idea the consequences could be so damaging. Perhaps this has something to do with the stresses in the metal from the spinning (cone-forming) operation?

Luckily it wasn't too difficult to repair the crack. Since the crack went clear through the tank wall, I had to fight the silver solder which was molten and trying to flow into the weld puddle. The copper blocks must be making good thermal contact, since I maxed my TIG machine out at 200A, and it was just enough to comfortably weld.

There are two copper blocks (each made of two pieces of bar). In order to avoid the silver-soldering nightmare again, I used silver epoxy to join the copper to the tank. After both blocks were attached, I mounted the assembly in the milling machine, and flattened the faces.

If I were going to do this again, I would profile the copper backside, flatten the face, then use silver epoxy to attach it to the tank.

UPDATE: This method did NOT work. I was not able to hold the glass without rocking it very slightly. This created a very smooth, but curved surface on the copper. It was unsuitable to mount the peltier.

Here, I am removing the milling marks with sandpaper mounted on a 1/4" thick glass plate. This took a very long time. In fact, I am still "going up through the grits" right now. The next step will be to weld a small threaded boss on either side of the copper blocks. This will serve as an attachment point for the peltier heatsink. The peltier itself will be pinched between the copper and the heatsink.

Three-way flow regulator for argon shielding gas

The serious TIG welders have two flow regulators on their argon supply -- one for the torch, and the other for the purge argon on the backside of the weld. I found some cheap flow regulators on eBay, and I made a manifold for the two I bought plus the original regulator. I've imagined situations where three regulators might be helpful like welding a gusset on the outside of a tank. The gusset would need backing gas in addition to the torch side, and the inside of the tank would need to be purged as well. Who knows.
I made the manifold itself out of a block of white Delrin with four 1/4-18 pipe threads cut into it. The original regulator (on the left) has 1/4-18 straight threads. I just ran my tap really deep into the Delrin so that the regulator would fit in. It's very easy to make metal fittings seal with Delrin and no Telfon tape is necessary. The Chinese argon pressure regulator also had a 1/4-18 female straight thread which was intended to seal on the end of the pipe. A regular 1/4" brass pipe thread nipple worked just fine and presumably bottomed out and sealed on its end.

DIY stainless steel conical beer fermenter Pt.1

Please search my blog for "fermenter" to find all of the posts regarding this project.

I am building a stainless steel tank that will eventually become a very unique beer-brewing vessel. My idea is to make a tank such that the entire process can take place without ever having to transfer the beer from one tank to another. This vessel will boil the wort, chill the wort, provide a temperature-controlled fermentation period, allow the trub to be removed, and provide a secondary fermentation. This tank was designed with my experience in brewing about 30 5-gallon batches of beer using the extract process. I don't have much inspiration to do all-grain brewing yet.

Having said all of that, I am also learning to TIG weld, and this project will provide many different welding setups -- all in stainless steel.


I bought a stainless steel conical hopper, model TMS14514 from
http://www.toledometalspinning.com/products/hoppers/priceList.asp

Toledo Metal Spinning sent the item very quickly, and I am impressed with the quality. The edges are extremely flat, and the overall finish and dimensional tolerances are great.

It holds 6.4 gallons total, so a 5 gallon batch of beer should fit pretty well. The hopper is a continuous piece with no hole in the bottom. I will be mounting a butterfly valve at the apex, so I need to cut the tip off to match the diameter of the valve housing. I knew before I ordered the hopper that I would only need to slice off about 1/8" off the end.

I used a slitting saw in my milling machine to do the job. This left me with a super flat clean edge. 70 RPM, 0.5 inches per minute, however the feed rate is measured at center of the saw, and I programmed a G2 circular path. This means the feed at the cutting point is probably lower. I had problems with chatter, thus necessitating this low feed rate.

This is one half of the butterfly valve housing after I welded it to the cone. The blue hose is silicone, and is carrying argon to the backside of the weld. In addition to the foil on top, I have made a dam with aluminum foil and tape inside the neck of the cone to trap the argon in the space around the weld.

Since the first weld went so well, I decided to weld on the inside of the fitting as well. Ultimately, this was not a great idea, but the weld itself went well. I used a copper tube with a line of tiny holes drilled in it to disperse backing argon to the outside of the cone. I had a fair bit of room inside the valve fitting for the TIG torch and filler rod.

I used a die grinder to smooth out the interior weld. After putting it together, it leaked! I had used the die grinder too much, and made the metal thin enough where a tiny pinhole in the weld made it all the way through the metal. I re-welded the outside bead, and then realized that I should have just made a couple of passes on the exterior to build up material. Then I could die-grind away the inside until I ground into the weld bead. No need to weld the interior. This would provide a nice smooth surface inside the tank and ensure there was enough material to keep it structurally sound.


It looks good now.


I originally started to cut this hole with a high-quality hole-saw in a corded drill. After a few seconds, I realized it was probably not going to work. Stainless is just such a tough metal, cutting tools just bounce off it. I used a free-hand plasma cutter to make the hole.

I made another aluminum foil/tape dam around the wall on the interior.


The weld went pretty well.



This time, I learned how to do it. Instead of welding on the inside, I just built up a nice bead, then used the die-grinder on the interior until I ground into the bead. It's nice and smooth on the inside.

Repairing a stainless butterfly valve

I am preparing to build a stainless steel beer-brewing tank. In preparation, I've been buying various stainless valves and tubes from eBay. The bottom of the brew-tank needs a large valve -- preferably a butterfly valve. I found one on eBay for less than $20 shipped. The seller said it was a "great valve". As it turned out, the only thing great about it was the price.
This thing has had a very hard life. Luckily, there is nothing missing, and all of the pieces are in fair shape except for the rubber seal.


Not only does the seal look bad, but it smells bad too. It has a strange sweet smell like antifreeze. I don't want my beer touching this thing even if it didn't leak. I decided to make a replacement since there is no manufacturer listed on the valve and buying a replacement seemed like a long-shot. First, I sandblasted the metal parts.


It's mostly OK, but there is some pitting. It may not cause any leaks.

I have some Dow MDX4-4210 "biomedical" elastomer left over from a work project. This stuff is basically liquid silicone that, when mixed with a catalyst, cures into a solid piece. The cure time is almost a day at room temperature, but only a couple hours at 130*F.
I machined a two-part mold out of 1" thick aluminum.

After mixing the silicone and catalyst, there are thousands of trapped air bubbles in the mixture. This will create a part that is more like a sponge than a solid piece of rubber. The product information sheet suggested de-gassing the mixed silicone in a vacuum chamber for 30 minutes, which is what I did. It works great, and the cure time is so long at room temperature, there is no danger of running out of "pot life".

I used a custom-made punch to cut the shaft holes after the piece was cured and removed from the mold. The replacement seal is a lot softer than the original. I don't know if/how that would affect the valve's operation. I'll probably test this valve before I weld it onto the brew tank.

Rust in the aquarium / Passivating stainless steel

OK, so I owe the titanium fanboys an apology. In my original post about aquarium chillers, I asserted that a stainless steel chiller coil would not rust in a saltwater aquarium. The coil in my tank rusted after about 6 months. Take a look:

I decided to sandblast the part so that I could see how extensive the damage was.

Yeow! The metal must have been exceptionally thin, and the sandblasting blew right through it!

I used 316 SS filler rod, and TIG welded the hole shut. The blue tube is an argon hose to protect the inside of the stainless tube from oxidation while welding.


Now it's all patched up. The heavy corrosion was localized around the area where the stainless coil entered the plastic filter box (see top picture) . My guess is that vibration from the pump caused the rough surface of the plastic to abrade the passivated layer on the stainless, thus causing a localized spot where the metal was unprotected. I've heard that passivated the stainless in an acid bath can create a stronger, more uniform passive layer. There are two common acids used to passivate stainless: nitric and citric. Nitric acid is nasty stuff, and it's possible to damage the stainless parts if the procedure is done incorrectly. Citric acid seems to be just as effective as nitric, and it's non-toxic. Here is the best reference on citric acid passivation:

http://www.astropak.com/downloads/technical_papers/boeing_passivation.pdf

I mixed a %15-by-weight solution of citric acid in water. I got 2lbs of citric acid on eBay for under $10 shipped. I submerged the stainless coil in the solution for 2 hours at room temperature. I could see the surface changed a bit -- the shade of gray was a little different. Hopefully this means it built up a nice strong oxide layer.

I've put the coil back into the aquarium, and added some silicone pads to the filter box so that the plastic would not scrape away at the stainless coil. I'll keep you updated to see how effective it is.