Design Input needed - Keg Double Boiler (Bain Marie style)

[VARocketry]

After reading many all GRAIN mash recipes and process write-ups, I accept the advice that a DOUBLE BOILER is needed. So I've developed a Bain Marie style Double boiler design using a regular half barrel keg and 100 QT SS pot from EBAY.

After much reading, it seems it's much more efficient to use GLYCOL as the bath medium to increase the thermal transfer capacity when approaching water's boiling temperature.

DoubleBoilerDesign.jpg

I'd appreciate any advise anyone might offer as a fellow and more experienced builder, but here's what I was planning.

1. Acquire a regular 15.5 gal SS Half barrel keg as the core vessel for the boiler.

2. Buy a 100QT SS pot with lid from EBay, Amazon, or other source. In this design the SS Pot will be the outer vessel, sliding the keg inside. Kegs are 16.5 inches wide and these 100 Quart SS Pot are typically 20 inches wide and tall.

3. Using your cut-off tool of choice, Cut off the bottom skirt completely. Some people trim of portions of the top skirt, too, to aid access or to install a fill port.

4. The SS Pot Lid will be the apron to close the top gap between the two vessels. I am thinking of using an angle grinder to cut a 16.5" hole in the SS pot cover, much like you cut the top off a keg to make a keggle. The lid now becomes a skirt that I'll braze or weld to the keg and the pot to seal the 2" gap between the two.

5. Cut holes for two 1" or 3/4" copper pipes - one to act as a vent, one with a 2: 3/4 reducer to act as fill spout.

6. The planned clearance on the bottom is 4 inches so I had thought I'd install the welding spud for the heater element and check clearances. Cut a 1 inch hole in the side of the 100QT SS Pot near the bottom located to leave a inch or more of clearance to the bottom.

7. I like the looks and discussion of using a 1 inch welding spud for the threaded connector for the electric heater element. Braze or weld the welding spud (or tri-clamp ferrule if that's your perference) onto the SS pot.

[ Watch this videos - https://www.youtube.com/watch?v=RhZM6yG ... iOm7EshI3g and
https://www.youtube.com/watch?v=LIkvP55UbxM ]


8. Then place a couple pieces of styrofoam cut to four inches height and put them on the bottom of the SS pot. The keg will seat nicely on the styrofoam at the correct height so the brazing or welding of the lid top can be easily done.

9. I'm guessing the lid should be first brazed/welded to the keg.

10. The copper pipes can be brazed to the lid.

11. The lid can then be brazed/welded to the outside SS Pot to seal the top.

Once complete, a cup of ACETONE poured into the now sealed inner vessel through the heater element spud will instantly dissolve the styrofoam and clear it out. Easy peasy.




Post by myles:

"Using boiling water as your thermal transfer fluid is very slow, there just is not enough of a thermal gradient between water boiling at 100 deg C ant the boiling point of your wash. It will take ages to heat up. You might start producing vapour OK but as your alcohol is depleted and the wash temperature rises the vapour production rate will slow down, due to the decreasing thermal gradient. When using boiling water a lot of the energy you put in is used to turn the water to steam, and if that is escaping then that energy is not going into your inner pot. Please try it with some small saucepans first. I use a glycol double boiler and when I get it back in use will be running it at 115 deg C to maximise the life of the glycol.

With the propylene glycol which has a high boiling point you never get it anywhere near boiling temperature. However, it does start to oxidise and degrade at lower temperatures. In commercial thermal transfer fluid they get round this by using additives to buffer out ph changes etc. In reality though for our use you probably don't want to be that hot anyway or you are going to burn your wash, which is why you are using a double boiler in the first place.

Now as for the oils you should never get close to the smoke point or you will be too hot. It is a good idea to use one with a high smoke point but you also need to think about the odour. The oil will give off fumes at temperatures well below the smoke point and some of the oils can be fairly unpleasant.

If you are using immersion elements you do need to take care with your power. Most importantly during warm up when the oil is still cold. What happens is that the surface temperature of the element is surprisingly high and before the oil starts to circulate you tend to get localised burning of the oil. Over time the oil degrades, even if you keep the temperature well below smoke point. It might be a good idea to incorporate some sort of level tube or sight glass so that you can monitor the colour of the oil. Periodic filtering of any oil that you use might be required to keep it clean and to protect the element.

You can bulk out the oil with fillers if required but it might not be necessary. Provided you insulate the outer boiler well the oil acts as a thermal store that can be very useful. For example if you are doing a strip run on the grain, you can turn off the power part way through the run and finish it off on the stored energy that is retained within the oil. Well insulated the oil keeps hot for a considerable length of time."

[VARocketry]

Double BoilerLeg-Design-2.jpg

Updated thoughts:

I'd appreciate any advise anyone might offer as a fellow and more experienced builder, but here's what I was planning.

1. Acquire a regular 15.5 gal SS Half barrel keg as the core vessel for the boiler.

2. Buy a 100QT SS pot with lid from EBay, Amazon, or other source. In this design the SS Pot will be the outer vessel, sliding the keg inside. Kegs are 16.5 inches wide and these 100 Quart SS Pot are typically 20 inches wide and tall.

3. Deleted

4. [EDIT thanks to ZOMBIE's comment] The SS pot lids are THIN and won't support the weight of your boiler when filled with 12 gals mash/low wines. The original Keg skirt does not provide four inches of clearance so we'll have to support the boiler inside the SS pot (ie., legs). Take care to position so not to interfere with the heater element.

5. The SS Pot Lid will be the apron to close the top gap between the two vessels. I am thinking of using an angle grinder to cut a 16" hole in the SS pot cover, much like you cut the top off a keg to make a keggle. The lid now becomes a skirt that I'll braze or weld to the keg and the pot to seal the 2" gap between the two.

Question: Will a butt joint connection between the thin lid material and keg boiler vessel be sufficiently strong and not leak?
I've wondered if it's necessary or advisable to bend a 90* flange on the lid around the opening. I'm not sure whether the SS lid material can be easily formed into a 90 degree flange. I suppose you could cut a smaller diameter hole in the lid and cut tabs out to the ~16 inch diameter that could be bent up to form additional flange connection surface to braze or weld. Alternatively a copper material cap could be readily formed in exactly the way you suggest. There are many sites that describe how to form a cone from a flat sheet of material.

6. Cut holes for two 1" or 3/4" copper pipes - one to act as a vent, one with a 2: 3/4 reducer to act as fill spout.

7. The planned clearance on the bottom is 4 inches so I had thought I'd install the welding spud for the heater element and check clearances. Cut a 1 inch hole in the side of the 100QT SS Pot near the bottom to mount the welding spud and leave a inch or more of clearance to the bottom.

8. I like the looks and discussion of using a 1 inch welding spud for the threaded connector for the electric heater element. Braze or weld the welding spud (or tri-clamp ferrule if that's your perference) onto the SS pot.

[ Watch this videos - https://www.youtube.com/watch?v=RhZM6yG ... iOm7EshI3g and
https://www.youtube.com/watch?v=LIkvP55UbxM ]

9. With legs attached to the bottom of the keg vessel, this will support the weight when filled. The lid or cone cap can be fitted for braze or welding to the keg and stainless pot.

10. I'm guessing the lid should be first brazed/welded to the keg.

11. The copper pipes can be brazed to the lid.

12. The lid can then be brazed/welded to the outside SS Pot to seal the top.

13. The lid can be brazed to the outside pot ... Or, alternatively, a silicone /PTFE gasket can used on the pot lid to seal it without brazing as it's not pressurized.