Fermentation BTUs

[Eric Tyira]

All,

I'm working on sizing a chiller and piping system for a cidery. For the piping, they need flow rate and velocity of glycol to determine the header size. For the chiller, they need the peak BTU/h of heat generation.

I mapped out a rough timeline of cider temperature across the process from initial fermentation to packaging over time. The cold crash is the single dominant feature since you're trying to bring the temperature of a lot of cider down quickly.

The calculation for flow of glycol got me thinking about fermentation BTU in a container. So I did some calculations and made some assumptions about behavior. I want to bounce this off of you to see if it makes sense.

The first part was to calculate the total BTUs given off from fermentation. The key input is the 13 kcal generated during fermentation per 100g of sugar. I got this (13 kcal...) from a blog Q&A I found.

Then I wanted to graph this over time. Without having actual data, my best guess was a bell curve (normal distribution). Yeasties have to build up their population, reach some peak rate and then as sugar becomes less available, the rate of fermentation slows. So I used the bell curve equation of this:

To generate this curve showing BTU/h, I used a standard deviation of 100 and x of 0-504 (504 hours in 21 days). Since the area under the graph of the above equation equals 1, we just multiply each hourly result of the equation by the total BTU of the container size. This would give us the BTU/h at any moment during fermentation. That is, the area under the curve is 66,965 BTU.

For a 275 gallon container fermenting happily, this curve indicates a max of 267 BTU/h.

Does this at all seem reasonable?

Eric

[Dave Mitchell]

I would map your curve to your density/sugar drop.  I think you'll have your best correlation to heat generation over time there.

It sounds like you're making your own chiller?  If not, then the manufacturer will be be able to do it all for you very quickly.

For maintaining fermentation temperatures you're only dropping the temperature of the fermentor by whatever you have the tolerance set to.  So if you're fermenting at 60ºF and you have your variance set to +/- 2º.  Cider heats up to 62º and then it gets cooled to 58º.  So the load on the system is only that small drop of 4º.  So that multiple that by maximum fermentors that you would have fermenting at one time.  The real draw in cooling systems is crashing.  Going from fermentation temperature to your crash temp take a lot of energy.  Generally it's calculated to take 18 hours, at least in the brewing world it is.  So you figure out your BTU/hr for that then you multiple that by how many vessels you think you would crash at one time.  Then you should have how much cooling capacity required figured out.

[AW]

Interesting, from my understanding that peak heat generation is enough to raise the temperature of the IBC by 1/4 degree C per hour...assuming no heat dissipation. 

Which reminds me of a problem from this winter:  How long does it take for a frozen IBC of juice to defrost...give some know temperature differential?  Any help is appreciated - It would really save some trips to the cellar.  

[Eric Tyira]

Thanks Dave.

We may be getting a used chiller, so I'm not sure how willing the manufacturer will be with pipe sizing.

I agree that the cold crashing is the dominant feature. We don't go so cold so quickly, but it's still dominant.

What I like to do is break things down to their elemental particles and build them back up. This way I have a thorough understanding. So here I'm trying to model just fermentation to wrap my head around the heat generated and how that would go over time. The normal distribution may not be the correct model, but it may be close to reality. Or at least as close as necessary.

What really got me started on this was the piping supplier asking for the required flow rate of the glycol. This way they can determine pipe size to keep velocity to a reasonable level. To determine flow rate, we need to do the calculations of heat transfer, so therefore we need to know the heat generated. One source uses 1.94 BTU/gallon/hr for fermentation. I don't know if that's peak, average or where it came from. If we use it, then:

1.94 x 275 = 533.5 BTU/hr.

In my graph I had it reaching a peak of 267 BTU/hr. Only a difference of almost exactly 2:1. That's a 100% error depending on which one is correct.

The heat generated starts at zero and ramps up. It reaches some peak for some duration. Then it ramps back down as the sugar density decreases. I would bet on the 267 being more accurate than the 534 number.

For cold crashing, we have:

BTU/h = (gal x density x ΔT x 1 BTU/lb/F)

275 x 8.5 x (60-40) x 1 = 46,750 btu in 1 hr.

46.750 / 24 = 1,948 btu/24 hr (average value but with a peak of 2,610 BTU/h)

1,948 x 1.34 = 2,610 btu/hr (to handle the first hour of heat transfer)

So cold crashing requires nearly 10x the energy of fermentation. But if they happen simultaneously in multiple tanks, we have to include them in the glycol flow rate calculation.

So I created a cold crashing calculator to determine the GPM flow of a 30/70 glycol mix at 20 F. Because of the logarithmic nature of cooling, I calculated it by the hour. For every hour that passes, the tank liquid temperature drops so the delta T gets smaller (temperature difference between the liquid and glycol), so the heat transfer slows down as the tank approaches setpoint given a constant flow rate of glycol.

Below is an example for 1,000 gallons of liquid at 60 F to be cooled to 32 F with a 30,000 BTU/h chiller (assuming no losses). This says it can be done in 13 hours assuming no other tanks are using glycol. The heat transfer through the stainless is never a bottleneck, so I ignore that in the calculations. In this particular scenario, the glycol flow ends up at about 1.55 gpm which is super low. The chiller in question can move glycol at 40 gpm and provide 32,717 BTU/h at 20 F. So in the end, it's the chiller performance that is the bottleneck if you want to crash faster.

All of this ignores the action inside the tank where cold liquid has to drop and natural currents are created to expose warmer liquid to the cold tank wall, etc. Way beyond my skill set.

Eric

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[Eric Tyira]

I ended up doing some pretty extensive calculations for fermentation, cold crashing, BTU requirements, etc. I mapped out the process in daily detail so I could get a full picture of when and how much the chiller would be used. If you avoid temperature extremes, you'd be surprised how small of a chiller one can get away with.

On Tue, May 23, 2023 at 7:08 PM Dave Mitchell <da...@junctionvictoria.com> wrote:

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