Fermentation Metabolic Power

I'm trying to get a ballpark range for fermentation metabolic power as a function of temperature. Of course it will depend on many factors, such as nutrient levels, sugar levels, etc. I'm considering the maximum power, which I figure would be at high krausen or thereabouts.

The reason I'm asking is because I've just put together an upright freezer fermentation chamber for my brewhemoth conical. I've disabled the compressor delay circuitry, and am using a ranco-type controller. The probe will be taped to the conical and well insulated. This has given me good measurements in the past, although before I used a thinner RTD which probably would have made better contact with the fermenter shell. I want to make sure that when the compressor clicks off, it will stay off for long enough for the refrigerant to drain. I'm not sure how long this should be - I think I've heard ten minutes or so.

If anyone has theoretical parameters for, say, watts/mL at 70degF and high krausen, I'd be all ears. Alternatively, empirical observation would be useful as well. Figure that I will have 17gal of wort/beer in there.

Intuitively, it seems to me that 17gal could certainly increase its temperature faster than 1degF in 10min. That's why I'm worried.

Hopefully a 1degF hysteresis isn't too small for the system. Any takers?

Thanks a bunch!

Maintaining temperature doesn't require nearly as much power as crashing and chilling.

The most intense heat generation by yeast is in the first 72 hours. There is a logarithmic rise in heat as yeast goes from low to high krausen, but in most cases the available chilling power needed to maintain fermentation can be averaged. Heat generation after the first 72 hours plummets pretty quickly with normal gravities.

Yeast metabolism produces about 9 btu/gal/deg plato.

Assuming 17 gallons of 15 plato (~1.060) wort:
9*17*15 = 2295 total btu in the first 72 hours.

Power needed to maintain temperature:
2295/72 = 32 btu per hour.

This doesn't include power for chilling to fermentation temperature or any other losses.

If I use your numbers, 32btu/hr is about 10w. What I really need to know is what the peak could likely be.

17gal ~ 65L ~ 65kg

beer ~ water ~ 4kJ/kgK

1degF ~0.5K

So about 130kJ to bring 17gal beer up one degF

To do this in 600s would take ~200w.

Sound right? Any way that the peak rate could be 200w for highest krausen?

Cheers...

thebrewingnetwork.com/forum/viewtopic.php?f=19&t=20602

This one's saying about 3x the rate I used above, or about 1w/L.

StMarcos said:

This one's saying about 3x the rate I used above, or about 1w/L.

Click to expand...

What I've given you is the metabolic heat generated during fermentation. Once started, yeast works at a pretty consistent pace when given optimal controlled temperatures. So, "peak" is going to be relative to your conditions (including OG). The thing to do is look at the total btu created and divide that across your expected fermentation time.

This isn't accounting for surface area, specific heat, insulation (yeast itself can be a good insulator) or chilling below ambient temperature. Those items are going to add to your peak power needs.

What I'd do is consider a full volume lager fermentation in summer, with high gravity. This would represent "worst case" chilling needs.

I'm not trying to figure out my chilling needs. I'm trying to figure out the peak power density of the fermentation so I can figure out the shortest time the compressor will likely be off. Cheers.