How to Build a Conical Fermentor Cheap

By Scott Northuis



1.    Decide on size of vessel needed and size components.

For my set up, I wanted a 15-gallon model.  I needed to figure the size of the two components, the cone and the cylinder, to get my desired volume.  I tried to make the unit look symmetrical in that the height of the cylinder equals the height of the cone.  The formulas needed to figure the volume and heights of the conical is as follows:

·         Volume of Cylinder = Pi x radius squared x height

·         Volume of Cone = 1/3 x Pi x radius squared x height

·         Height of Cone = Square Root [Diameter squared – (Diameter/2) squared]

·         Circumference of a Circle = 2 x Pi x radius

·         1 cubic foot = 1728 cubic inches

·         1 cubic foot holds 7.38 gallons of water

To start, I guessed at a diameter of 16 inches.  This gave a height of cone around 13.86” and a volume of 3.97 gallons.  I then sized a cylinder section with a 16” diameter to get around 11 gallons.  This worked out to be a height of about 13”.  Since my heights were fairly close, I decided to run with this.

2.Figure sheet material needed. (also size of ball valves and fittings)

Now I was ready to figure out the size of pieces that I needed to build the fermentor.  The cylinder section is easy to figure out.  I know the height from above, so now I need to figure the circumference of a 16” diameter circle.  The Circumference was approximately 50.27”.  So I needed a sheet of copper 13” x 50.27” for the cylinder.

For the cone, I only need to know the diameter to figure the sheet size.  To make a cone shape, I need to draw a semi-circle with a radius equal to the diameter of the cone, 16”.  In terms of a flat sheet of copper, I will need a sheet 32” long x 16” to make the cone.  For the top I needed a 16” diameter circle plus another circle for the cover.  So the top needs a 16” x 16” piece of copper.  I’ll assume that my cover will be slightly smaller than the top, so I’ll make that about 13” in diameter, which gives a 13” x 13” sheet of copper.

Totals: 13” x 50.27 sheet, 16” x 32” sheet, 16”x16” sheet, and a 13” x 13” sheet.  Luckily my local scrap guy had some sheets lying around that fit the bill without too much waste.  I used approximately a 1/32” thickness sheet.   I do not know the gauge, but this is fairly thin stuff.  It was easy to work with and seems to be pretty strong when brazed.  If you have access to better tools for cutting, then try going a little heavier gauge.

The fittings I used for the bottom drain and side port was a 7/8” coupling and a 1/2” coupling respectively.   This is because I had a 7/8” (or Ύ”) ball valve and a ½” ball valve lying around.

3.    Sketch and cut shapes (grind parts)

Now it is time to cut out my shapes.  For the top, I just drew a 16” diameter circle and cut it out.  I also cut a 14” diameter circle for a cover (to fit over the top after a hole has been cut in it).  The cylinder section is easy, just sketch a 13”x50.27” rectangle and cut it with tin snips.  Try to be as accurate as possible so that fitting the parts together will go smoothly.  Take a compass and sketch the 16” diameter and 13” diameter circles and cut with tin snips.  Now the hard part, rig up a string or straight edge so that you can sketch a 32” diameter half-circle on the 16”x32” piece of copper.  Make sure you take your time and get this accurate.  Also sketch a small 7/8” radius half circle with the same center mark as the large half circle.  This is the hole for you bottom port.  Cut with tin snips and grind/file all edges smooth on all pieces. 

A picture of the cone sketch is shown below.


4.    Form parts and braze

Okay, now it’s time to form the cone and cylinder sections, then braze them together.  The cylinder section is fairly easy.  Just roll it until the edge meet.  I rolled it so that it was slightly smaller than the actual diameter at first, so I could hold the shape.  Once the shape is formed, I took a green scotchbrite pad and cleaned the edges.  I then used vice grips and clamped the meeting edges together.  I did not overlap the edges.  Once clamped, I fluxed the joining parts and then spot brazed a few spots with a propane torch and standard household solder to hold it together.  Take your time, because if you get it too hot, it will buckle.  When I felt comfortable that it would hold shape, I finished brazing the entire outside meeting edge (you can do the inside, but it was easier to work on the outside).  You should only have to do one side because the solder should seep through.  Check your brazing to make sure it settled smooth.  If it doesn’t, you will need to grind and polish later or touch it up with the torch.

For the cone section, I took a piece of 7/8” copper tubing to help bend the bottom tightly.  The top (or larger portion) of the cone will follow.  Try to get the bottom as tight as possible because this will help hold its shape.  Take your time on this, it’s not a race. After I formed the cone, I cleaned the edges with a scotchbrite pad and clamped the edges together (do not overlap).  I then spot brazed a few spots to hold it together.  It might help to have a friend hold the cone and push towards the middle of the cone so you can get the seam to stay butted.  Once it was held together, I finished brazing the outside of the cone.  Remember to take your time so that you don’t have to grind the solder later.  The last bit of brazing involves the 16” circle cut for the top.  I set this inside the cylinder, approximately 1/8” down from the top all the way around.  This gives plenty of room to solder it in place.  Once I got it to fit, I soldered all the way around.

5.    Build cover

Once everything is soldered, I cut out a 12” diameter hole in the cover and ground the edges smooth.  The picture below shows what it looked like at this point. 


Now what I did from here can be changed or modified, but this is what I did on this prototype:

I brazed two pieces of ½ copper to the sides of the fermentor.  I then put together a bracket that is brazed to the top of the 14” lid.  I drilled a hole in each piece of ½” copper so that I can put a cotter pin through it and the bracket to hold it down on the fermentor.  I also added a ring of food grade silicone under the lid to seal it to the fermentor (let it dry before attaching it to the fermentor!)

6.    Fermentor Stand

I built the fermentor stand out of 1”x1” box and 1” x 3/16” flat stock.  I took the flat stock and formed it into a 15” diameter circle and welded it end to end.  I then drew up a template on cad to build it with 3 legs at a 120 degree angle from each other, with a base piece to hold it together.  I made the legs about 30” tall.  I would recommend checking the size of your kegs first, before cutting the legs.  Here is a drawing with some dimensions:


7.    Ferment away

That’s really all there is to it.  If you don’t want to build one for yourself, send me an email and we might be able to work something out.

Email Address: snorthuis at hotmail dot com