Thermal Gradients

[DrPaulsen]

Since I'm still in possession of my electric keggle, I decided to run an experiment on it regarding thermal gradients and any potential "dead space" below the element. From experience with my HLT, I know a dead space can exist below the heating element if the water is below boiling. However, I did not have any data to suggest one way or the other after boiling began. I've heard from others (e.g. Kal from The Electric Brewery) and have suspected the full volume of wort is properly boiled inside the keggle, due to agitation during the boil, but I'd never actually taken data on my setup.

Instead of using the bottom port for a drain, I inserted a thermowell to be able to measure temps below the heating element. The tri-clamp thermowell I had on hand extended to just under one inch below the element.

I filled the keggle with 6 gallons of tap water and let it equilibrate for 10 mins. At this point, I measured the temperature of the water, just below the surface, the temp of the thermowell interior, as far up as the probe would reach, and the temp of the exterior base of the keggle, behind the insulation. I used a thermapen for each measurement and did my best to apply pressure to the probe tip to reduce thermal resistance when measuring the thermowell inner wall and the keggle base. It obviously took longer for these two measurements to settle, but it seemed to work okay.

After taking a baseline measurement, I put on the keggle lid and turned the heating element up to max power (4500W). I took temperature data every 5 mins from each of the three sample locations, taking care to sample the data in the same order every time (Top, Thermowell, Bottom). Once the boil started (around 30 mins), I removed the lid. A plot of the data can be found, below.

I found the shape of the curves very interesting, but not all that surprising, given the currents/mixing I observed in the keggle. Below 200F, the water above the element was very still and there was almost no mixing taking place. Above 200F the water became continuously more agitated until a full rolling boil was in place (around 30 mins). I attribute the 5F offset in the "bottom" curve at a full boil due to heat spreading into the body of the thermowell and being cooled by the ambient air. With better thermal bonding of the probe to the base of keggle and better insulation I see no reason to believe it wouldn't be the same temp.

So, there you have it. For anyone that has ever wondered about thermal gradients inside an electric keggle, they seem to exist up to the point of a rolling boil and vanish shortly thereafter.

[bf514921]

DrPaulsen, so for electric brewers does this imply a roling boil is necessary for a proper hop utilization? or does the proper mixing occur at a simmer of a boil?

[DrPaulsen]

That seems like a reasonable conclusion. I'd be more concerned about achieving a "consistent" hop utilization than a "good" hop utilization and a vigorous boil looks like it will achieve that. I didn't check alternate cases, e.g. attempting to bring the wort just to the point of a boil then turning the power down to more of a simmer. I think my experiment would be simple enough to replicate and you're welcome to do so. Send me an IM if you have any setup questions.

My primary concern when thinking about dead space was with respect to making sure the entire volume was sufficiently boiled for protein denaturation and protein/polyphenol binding (that's what the trub floaties you see in the boil are mostly composed of).

[terpsichoreankid]

Interesting! I'm curious to know how a standard kettle with a propane burner shapes up against the electric setup now... With your new kettles and burner, do you think you'll be able to conduct a similar test? I would imagine that the gradient would be much more smooth, since it is bottom heating, and the overall container (kettle) would heat up much more uniformly than a single heating element...