Limiting oxygen exposure once yeast has been pitched is a critical part of producing good beer. There are a lot of ways to avoid oxygen throughout the brewing process, but I’ve always struggled with purging kegs. A number of different methods are used for this – you can blow CO2 through your keg, as one example. Or you can fill a keg with water, and then displace that with CO2. But I have issues with these methods – they waste CO2. So I’ve designed a basic 3D printed keg purging system. This system kills two birds with one stone – it serves as a blow-off, and allows you to purge your kegs with CO2 captured from your fermentation.
The concept behind this system is quite simple. A blow-off tube directs the CO2 of your ferment into a wide-mouth mason jar. The jar collects any material that gets blown off, while an outlet on the jar collects the clean CO2. This CO2 is directed to the liquid-out port of the keg, thus “filling” the keg from the bottom with CO2. An airlock attached to the gas-in port allows gas to vent while keeping room-air out. Making all of this possible is four 3D printed parts, which can be produced for less than a dollar.
The embedded video, below, shows the system and goes over how to put it together. This companion article gives additional assembly details and described a bit more of the underlying concept. Now, without further ado, the video:
The average gravity beer will produce roughly twenty times the volume of CO2 as there is wort. For example, 20L (5 US gallon) of wort at 1.055 gravity will produce roughly 400 L (105 gallons) of CO2! Passed through a keg, this volume of CO2 will render the keg nearly oxygen free.
CO2 is more dense than air, so passing CO2 into the keg via the liquid-out port should, in theory, create an expanding layer of CO2 at the bottom of the keg. This expanding layer then displaces the air upwards and out of the gas-in connector. In reality this neat layering of gas will not occur, with both mixing and diffusion limiting the layering effect. But even the worst-case scenario – even mixing of CO2 and air in the keg – would still reduce the oxygen concentration dramatically.
For example, 20 L/5 gallons of a 1.035 bitter would reduce the CO2 concentration in a 20 L/ 5 gallon keg to less than 2%. For an average gravity ale (1.055), the final oxygen concentration is less than 1%. And that is the worst case scenario – i.e. assuming there is zero layering of CO2 in the keg. In reality some displacement will occur, giving an even lower final oxygen concentration.
I am relatively new to the 3D printing community, but like the brewing community, it is a very friendly and open group of people who share their designs freely. My design is based on the Canning Jar Lids plans by AlexEnglish, and the Hose Barbs plans by Hamano. All of the .stl files needed to print my design can be found at this link, within my account at Thingiverse.
This 3D printed blow-off/purging system is comprised of three main components. You must use a carboy or fermentation pail/conical that can be sealed with a rubber bung or otherwise can be attached to 3/8″ tubing. This tubing runs to a 3D printed lid on a large (at least a 2 L/0.5 gallon) wide-mouthed mason jar. This dumps the CO2 (and any blow-off) into the jar.
A 7/16″ hose barb is also incorporated into the lid. This passes CO2 from the jar into standard 7/16″ beer line. This beer line in turn is connected via a liquid-out disconnect to the keg. A 3D printed adapter is used to connect an airlock to the gas-in port of the keg. This allows the keg to purge without letting room-air in.
For each blow-off, print one of each of the following:
- Adaptor list (Lid.stl)
- 10 mm (3/8″) lid barbs (LidBarb.stl)
- 4 mm (7/17″) lid barb (4mmLidBarb.stl)
- 10 mm (3/8″) airlock elbow (AirElbow.stl)
I recommend printing in PLA or PETG (food grade if you have it), using 100% infill. The airlock elbow and 10 mm lid barb will need supports (make sure to use build plate supports only). Print everything else without supports.
In addition to the 3D printed parts you will also need a few additional items:
- A #6.5 drilled bung
- A 10 mm (3/8″) hose barb. Buy one, or print one yourself
- A 2 L (2 quart) wide-mouth jar
- 1 m (~3′) of 3/8″ ID vinyl tubing (CO2 tubing or other)
- 1 m (~3′) of 7/16″ tubing (beer line or other)
- A liquid-out keg disconnect (pin-lock or ball-lock, to match your kegs)
- A gas-in keg disconnect (pin-lock or ball-lock, to match your kegs)
- An airlock
Assembling the 3D Printed Keg Purger
This is shown in the video, although I did forget to mention one important point – highlighted below in bold. In short:
Assembling the blow-off trap/CO2 separator:
- Print everything and remove any support material.
- Push the small lid barb through one of the square openings in the lid such that the barb is above the lid. It should fit air-tight, but can be sealed with glue if necessary. This part is delicate, so if you can attach it without glue it makes future replacement easy.
- Push the large double-ended barb into the lid and glue into place.
- Missing in the video: Run a ring of food-grade silicone or food-grade plumbers putty around the inside edge of the lid to help improve the seal.
- Attach a short length of 3/8″ tubing to the hose barb inside of the lid. THis should reach 2/3rds of the way to the bottom of the jar.
- Attach a longer length of 3/8″ tubing to the other side of the 10 mm barb, long enough to reach the attachment point on your fermenter. If using a carboy or pail, attach a bung with a 10 mm 3/8″ barb.
- Attach a length of 7/16″ beverage line to the smaller hose barb. This needs to be long enough to reach the liquid-out port of your keg. To the other end of this line, attach a liquid-out disconnect.
Assembling the airlock connector:
- Cut two short (5 cm/2″) lengths of 3/8″ tubing. Push this tubing onto either side of the angled barb.
- Attach an airlock onto the tubing attached to the barb that is in-line with the rectangular part of the angled barb.
- Push a gas disconnect onto the tubing attached to the other barb.
Using the Purger
- Clean all tubing and connectors with sanitizer. Leave a small amount in the bottom of the mason jar.
- Attach the bung (or other attachment device) to your fermenter immediately after pitching yeast. If using a carboy I recommend using a carboy clamp, like this one from TheWalrus, to ensure a good seal.
- Attach the liquid disconnect to the liquid-out port of a sanitized and sealed keg.
- Attach the airlock to the gas-in port of the keg
- Disconnect the keg once bubbling has slowed. I recommend adding a small amount of pressure from your CO2 system to ensure a stable seal.
- Disconnect the system from the fermenter and replace with a conventional airlock until the beer is ready for packaging.
- Transfer the finished beer into your purged keg, carbonate and enjoy!