Buckled Aluminium Pot While Cooling

[String]

Did my first brew in a 40 L crab cooker pot today.

All went well until I wanted to chill the pot down to pitching temp.
As I don't have my immersion chiller yet, I placed the pot in a large eathernware planter pot full of water.
I placed a couple of bricks in the bottom, to keep the hot aluminium from touching the pot.
Turned the tap on to have water just dribbling over the side of the planter pot.

When I pulled the aluminium pot out, it had buckled aross the bottom, sinking in by about 30 mm.

I just heated the empty pot for a minute, then stood in the pot to straighten the bottom out. But I'm not sure this is a long term fix.

Does anyone else have these issues?
Would letting the pot sit for 20 mins or so before putting it in water stop it from happening again?

 

[manticle]

I know from when I worked in kitchens that aluminium oven trays could sometimes buckle but they were easy to reshape.

Is no chill an option till you get your plate chiller?

 

[String]

No chill is an option, but after reading so much on the chill/no chill debate I wanted to try it.
I've no chilled the last 2 brews, including my last version of Dr Smurto's Golden Ale.
Just wanted to see what difference it made.

 

[pk.sax]

It would be the stress on the inside of the pot considering that the outside would chill quicker. Only option I see is to use an immersion chiller. Aluminium pots have to be thicker than the corresponding steel pot so this is bound to happen if you chill like you did.

Then again, that's the theory. Shape + constraints + thermal stress between inside and outside. Maybe if someone that uses an aluminum pot and an immersion chiller can confirm this.

 

[gavinl]

I have only used my immersion chiller once, but I doubt there'd be a chance of that happening as it takes ~15mins to cool down to 25°

Using a Robinox, however if you got your crab pot from Rays Outdoors I don't think there's much difference in thickness. That's my experience anyway.

 

[String]

Yeah, it's a Ray's Outdoors one.

I'm waiting on some copper tubing from Polyaire for the immersion chiller, so I'll hold out on the chilling until then.

 

[Synthetase]

That's interesting. I used to BIAB on the stove using a 15 L aluminium pot and chuck it in a bath full of iced water and never had anything like that happen. How thick is the pot?

 

[pk.sax]

It's kind of the same thing as trying to bend a long rod bs trying to bend a short rod. The net force would act through the centre, the wider the pot - the greater the resultant moment applied to the centre point when shifting the distributed load to that point. A big flat plate will warp a lot easier than a small plate.

 

[String]

That's interesting. I used to BIAB on the stove using a 15 L aluminium pot and chuck it in a bath full of iced water and never had anything like that happen. How thick is the pot?

It's only maybe 5-8mm thick.

I did the same with the 19 litre Big W pot I have for my first brew, and it didn't warp at all.

I thought I had bent it from sitting it on the bricks, they didn't give the pot support across the whole base.
But reading the replies, maybe it didn't matter what I sat the pot on, it was going to warp anyway....

 

[joshuahardie]

Wow

I have the same pot, which has probably had 30 brews to its name.

if I am not no chilling, I am placing the pot and lid into the bathtub full of cold water.
Never had a hint of this.

One thing I can think of
I assume that you are heating via direct heat to the bottom of the pot. maybe this has superheated the metal way more than my electric system.
Would then letting the pot naturally cool for 10 mins for some of the heat on the base to dissipate, stop this from happening again?

 

[manticle]

It's only maybe 5-8mm thick.

I did the same with the 19 litre Big W pot I have for my first brew, and it didn't warp at all.

8mm is a pretty hefty pot.

 

[Markbeer]

8mm is a pretty hefty pot.

5 mm is a hefty pot too

 

[pk.sax]

You have to realise the difference between steel and aluminium.

Aluminium's plastic behaviour kicks in much sooner, so - the thermal stress wanting to make it deform will suddenly push it over the limit and the stress would be gone because the shape changed (i.e. work was done/energy dissipated). With steel, it would still be well within its elastic region and although, it would deform (less than aluminium would) it would bounce right back as the whole process is done within the elastic limit.

Direct heat on the bottom from a burner obviously exacerbates this. Putting it on the bricks prolly didn't help because the weight of the wort added to the force in the centre of the pot upwards. It can and will probably be a few different factors adding up to make it buckle as I'm sure the pot is made strong enough to handle boiling on a direct flame. Hence I suggested using an immersion chiller to take away a contributing factor.

PS: Yea... 8 mm is a friggin hefty pot!!! 4.5 mm aluminium for a frypan is pretty strong already, pretty hard to bend by hand.

 

[String]

Looking at the pot now, it's more like 3mm.

Will try to give it a solid base to sit on while it's hot and see if it makes a difference.