Electric Recirculating BIAB rig, Temp overshoot problems

[klangers]

PID tuning won't solve process design issues. PIDs are certainly not a silver bullet and in my time doing control systems work there's been plenty of instances where they're more harm than good. Over/undershoot and oscillations are typical behaviour.

PIDs work by apply gain to the error, the integral of the error (error accumulated over time) and derivative of the error (rate of change). PIDs are more applicable to mechanical systems (eg cruise control) where there are actual proportional (speed), integral (distance) and derivative (acceleration) terms that are measurable and controllable by one output (eg throttle) but can change extremely rapidly and to a certain degree influenced by random external factors (eg when you start rolling down a hill).


I agree with MHB - 4500W is WAY too high for direct-contact mash heating. Your wort flow is insufficient to absorb this amount of energy, as mentioned above. You have an unstable system that is prone to huge oscillations as you've massively increased your rate of change of temperature and this means your derivative term may actually need some gain, and this will impose further instability on the system which requires further tuning.... ad infinitum.

 

[MastersBrewery]

As with Klangers above, some times too much power is too much ( my motor cycle days too much power was barely enough) note in the previously link to takai's thread; he has a 4500w element but he has reduce it's output with an SVR and POT(similar to PWM) so in effect when his element fires it's only producing 2400-3000w (or what ever he has dialed in) when mashing his already ultra low watt density element has an even lower watt density, this in turn controlled by PID. The advantage is he can punch up the power when ramping a double batch to the boil. Note both the grainfather and the Braumeister 50L use 2 elements; one low watt, specifically for the mash.

ED: clarity + spulling

 

[QldKev]

I disagree that 4800w is too much power for direct contact mash heating. I think it is more a combination of watt-density, total volumes and wort speed past the element. I know of 1 setup with 10800w RIMS style heating in the main vessel using just an on-off controller with no main issues.

I think the outcome is a bigger pump with a second return to get that sump area moving.

 

[klangers]

I disagree that 4800w is too much power for direct contact mash heating. I think it is more a combination of watt-density, total volumes and wort speed past the element. I know of 1 setup with 10800w RIMS style heating in the main vessel using just an on-off controller with no main issues.

I think the outcome is a bigger pump with a second return to get that sump area moving.

It's a combination of everything. My point is that having huge power inputs in combination with PID control causes issues as you're trying to control an inherently unstable system (due to the flow issues and probe placement)..


Remember our aim - to have accurate, precise and steady control of the mash. We're not trying to heat it as fast as possible.
I still think dialling back your 4800 W will be the cheaper and most effective option. You can buy bigger pumps and fiddle with PID settings as well - there are always multiple solutions.

There are existing overshoot issues and varying the flow has non-linear effects on heat transfer that will further complicate the situation and make it more difficult to predict. Also, most elements have internal resistance that increases with element temperature - increased flow could well allow a greater heat transfer rate which takes away the heat faster, and causes it to draw more current and input more power in order to maintain the same temperature. We just don't know these details.

As for on/off control - there is nothing wrong with this as it rarely causes an unstable system and the worst you'll get is regular oscillations which bracket your setpoint. Plenty of large breweries control temperatures with on/off control. Good control system design starts with fixing the process.

 

[Hpal]

Thankyou guys for all your detailed responses. After talking with a control guy at work and reading the 'Getting the most out of PID Control' threads I am more and more confident about getting this thing working well! I will try putting a limit of say 80% on the element output so it wont whack 4500w in at one time. Try Kev's method of dual wort return, and play around with different probe positions and loop tuning. Maybe even on/off control will work for me?

 

[hairydog]

Hpal,
Some photos of the Malleys old wash tub,i wrapped the outside with rubber sheet to improve eficency.I have received the 4-20ma input,240vac 25a output SSR
in the mail so i will put together next week and test,i have some lag issues as you have but will be happy with keeping temp stable within 0.5 deg.

 

[Adr_0]

You need to put your temp probe within an inch or two of your element.

I'm pretty sure you have this, but you should have a gap of a few inches of clear wort where your element can sit.

If your probe is close to your element it should regulate its own power - that's how the PID controller will work.

The catch then is you need to keep bringing 'cold' wort across the element. Flowrate will govern your overall ramp rate, and will also limit the peak temperature you see in the boundary layer around the element.

Low flow = hot and slow, with limited power from the element
High flow = lower surface temperature, fast ramp rate, maximise available element power.

A 2000w, 4500 or 20,000W element will only heat to the point where the probe picks up that temperature has been met. So if it's a large distance, you add power to the wort but you get overshoot, lessened at very high flowrates.

If the probe is close, you will still get very high temps at no flow or low flow, but you have much more room to move. You should still run at the highest possible flow you can though, for the purpose of ramp times and getting every bit of that 4500W being used.

 

[Adr_0]

Oh yeah, and as you've probably learned... Stay the hell always from integral congrol.

 

[Adr_0]

Oh yeah, and as you've probably learned... Stay the hell always from integral congrol.

Stay the hell away...

 

[Mardoo]

What was that? Stay the course with integral control?

 

[Adr_0]

What was that? Stay the course with integral control?

The only thing within reach is a plastic spoon and a roll of paper towel.

Consider both thrown in your general direction