Technical Mash Tun Question involving thermodynamics.

[CyberAle]

Hi fellas,

I am brand new here and this is my frist post so please excuse me for any mistakes I do. Anyway I will start by describing my new electric mash tun I have created. It is a 50L stockpot with a 2.2 kW heating element inserted into the side with a false bottom about 20mm above the heating element. The stockpot is 40 mm in diameter, unfortunately I was not given the height in the spec but I could measure it if needed. I have got an STC 1000 wired up to control the heating element to keep the mash tun temperature at an accurate temperature setting. My question is, will the heat transfer rate from the heating element to the water be fast enough to be able to assume that the grain and water are the same temperature at every point inside the mash tun? and if so does that mean I can put my temp sensor anywhere in the mash tun and effectively keep an accurate temperature? I have done some basic thermodynamics in 1st year Physics but I think this is beyond me.

Thanks in advance for any replies!

 

[barls]

id put your sensor at the return so you don't over heat it.

 

[TheWiggman]

Welcome to the forum.

Will the heat transfer rate from the heating element to the water be fast enough to be able to assume that the grain and water are the same temperature at every point inside the mash tun?
Once mashing, no. When the water is heating the convection currents will cause the water to stir but there will be voids of different temps within the water during heating. Once the temp has settled, the water will be a uniform temp give or tank half a °C. Once you 'strike' (add the grist), same story - uniform temp. When you're heating without flow however the situation is different. The grain has a higher specific heat than water does, which means it takes more energy to heat it to a given temperature. The heat also needs to transfer through the grain so you will have a gradient of heat from the element out, the element being the hottest point.
Moral of the story - the liquid needs to move to carry the heat/energy effectively.

and if so does that mean I can put my temp sensor anywhere in the mash tun and effectively keep an accurate temperature?
You can, but you want it as close to the element as possible.

Thanks in advance for any replies!

When the element's on it's a good idea to either thrash the water around with a big spoon or have a recirculation pump running. This is only important for doing temp steps, and you can make decent beer without getting all fancy with pumps.

 

[SBOB]

Its unlikely you will have exactly the same temp everywhere, but measuring the temp at the bottom outlet of your pot should be the 'hottest' reading and the one I would use for my control loop

 

[MastersBrewery]

The question is a bit general as the answer is system dependant. Target temps are aimed at specific enzime activity, this activity takes place in the wort, not the actual grain; therefore temp should be taken where wort is hotest, some systems that is the exit of the tun(direct fired), others it will be the exit of the HX (herms/rims), my first system I used an over the side element in the mash to raise temp stirring constantly with the element in that case the mash itself was the hotest point.

2cents

MB

 

[CyberAle]

Thanks for all te quick replies!

Yeah I can't get the sensor probe that close to the element because o the false bottom but i mean I can get within 40 mm or so. So basically as long as the wort itself where the temperature probe is, is at the desired temperature within say 0.5 C it will be close enough to the desired temp to create the effects I need in the beer? The reason I say this is because down the track once I get a bit more confortable with my new setup I want to be able to do some fine tuning with mash tun temperatures

Cheers (I am trying to respond to everyone here)

 

[MastersBrewery]

Yep 40mm is fine mate at that you'd have pretty tight control.

 

[CyberAle]

Thanks Masters. You have pretty much answered my exact question!

 

[Ducatiboy stu]

Look....in all honesty, you dont need a temp controller heated mash tun

You would be much better working on insulation than temp control

Mash will hold temperature very, very well with minimal insulation due to its thermal mass

I had mash temps drop 2*c in 1 hr in a shity old blue plastic willow esky covered in a blanket

Remember, most mash conversion using modern malts happens in the first 20-30mins, a 1 hr mash gives lots of insurance and max eff %

 

[Mardoo]

...and if you want to step mash in an esky you can use an over the side element.

However, I well understand the thrill of the build, if that's part of what you want.

 

[TSMill]

Ah, most answers seem to assume the mash liquor is being circulated during the mash, is it?

 

[TSMill]

Echo

 

[klangers]

My question is, will the heat transfer rate from the heating element to the water be fast enough to be able to assume that the grain and water are the same temperature at every point inside the mash tun?

No.

You'll need to stir or recirculate. The issue is that a mash is orders of magnitude more viscous than plain water, so convection is almost non-existent.

 

[CyberAle]

Ducatiboy stu, yeah I used to have a 50L esky with copper piping, which worked fine for me, go a decent 20 plus brews out of it. It was just getting hard to maintain as it was hard to clean the inside and I couldn't really get inside the pipe with out taking the whole thing apart and it was also leaking a bit. And as Mardoo said, I just wanted to build something fun and new and real easy to take apart and clean.

Klangers. It is not circulated with a pump or anything but I do intend to stir it pretty regularly with a big spoon every 5 minutes or so.

 

[Ducatiboy stu]

False bottom...FTW

 

[klangers]

Klangers. It is not circulated with a pump or anything but I do intend to stir it pretty regularly with a big spoon every 5 minutes or so.

To get an even heating and therefore avoid frying enzymes, the power (watts) you can apply to your mash is dependent on the rate of stirring/recirculation. If you are heating without stirring, there will a massive temperature gradient within the mash that will fry the enzymes near the element and leave the rest of the mash cooler.

Heat transfer via conduction requires temperature differential. Without stirring (forced convection), you rely on conduction which is just not going to transfer 2000w throughout the mash.

I'd recommend to stir the mash continuously whenever you are heating it.

 

[technobabble66]

^^ +1
Without recirculating with a pump, you'll need to stir while you're heating. It's the only way.

I do something similar most of the time - just stir and have a temp probe sitting in the wort near the top. The benefit of the stirring is you can pretty much guarantee the wort is the same temp throughout, so you only need the thermometer a few inches into the top of the wort.

Also, keep in mind that the element & hotter wort below the false bottom will throw a bit more heat once you turn it off (if you didn't realise this already). So that means if you switch it off when the stirred wort hits 65°C, it'll probably keep rising to ~66°C (depends of the system, of course) over the next few minutes before stabilising.
So for me, if i'm targeting 66°C, i turn my urn off once the thermometer reads 64.5°C (mine gains ~1.5°C).

 

[CyberAle]

So really for best results here and for automated circulation I shoud get a pump and pump the wort from the bottom and pump it into the top? I would prefer not to have to stir it every time the element turns on, I mean it is controlled by an STC and depending on how accurate I have the STC set will depend how often I have to stir. I'll have a go with stirring it this weekend and then determine if I will need to install a pump.

Thanks guys

 

[MastersBrewery]

Tips for recirculating.

Mash in and let rest no recircing for a few minutes

To start no more than 1/3 full flow for 10 minutes to 'set the grain bed'

Gradually increase to full flow.

Note flow is controlled
with a ball valve, the ball valve should be located on the outlet of the pump in all cases.

 

[sstacey]

The stockpot is 40 mm in diameter

Mmmm... I'm no good at thermodynamics, but I think this is your problem...