"Nonexistence of Heat Momentum" - Page 2

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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another_jim
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#11: Post by another_jim » Nov 03, 2019, 3:36 pm

Almico wrote: ... the same amount of heat pre-DE will cause the RoR to increase too much post-DE. At 1C when all the bound water is released, the coffee is now dry and much less heat is needed to continue to roast.
When I wrote my roast simulator, I had to use a faster heat transfer constant post first crack than pre-first crack to fit the roast curves. I always assumed that this was just a modelling fudge factor rather than anything real (my training in isn econometrics -- lots and lots of unreal or arguable fudge factors).

The thermal mass of each bean decreases as it releases water, so less heat is required to heat it. But I always thought the heat transfer rate would reduce, since dry cellulose is such a good insulator.
Jim Schulman

crunchybean

#12: Post by crunchybean » Nov 03, 2019, 8:59 pm

You people are mistaking your probes for beans. And coming to different conclusions...

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Almico
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#13: Post by Almico » Nov 03, 2019, 9:17 pm

another_jim wrote:The thermal mass of each bean decreases as it releases water, so less heat is required to heat it. But I always thought the heat transfer rate would reduce, since dry cellulose is such a good insulator.
I'm of the mind, and it's only in an effort to explain what I see in my roasting data, that the water in the bean, although a good conductor of heat, will retard heat increases (over 212*F?) similar to a car radiator. As the water dissipates, the rest of the bean is free to heat up unencumbered. Thus the need to begin reducing heat, for me, as early as 30s pre-DE.

So the water will aid heat transfer to the inside the bean, but keep that temperature lower until the water is gone.

HoldTheOnions

#14: Post by HoldTheOnions » Nov 03, 2019, 9:30 pm

All I know is when people say momentum, I understand what they are referring to. That's good enough for me.

EddyQ

#15: Post by EddyQ » Nov 04, 2019, 9:11 am

I'm not sure I follow what he is saying in part II. Heat momentum does exist in roasters. The drum and possibly other roaster parts hold all the heat and hence produce this momentum while roasting. With a fluid bed, there would be much less thermal mass in the roaster, but it still is some momentum (especially if electric).

Also, his part II plot shows no vapor leaving at around FC, which is simply incorrect. Maybe I am reading his plot incorrectly. I cannot tell if this is modeled or measured results.
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DaveC

#16: Post by DaveC » Nov 04, 2019, 9:13 am

another_jim wrote:Thanks for this antidote to ROR blather (the main reason I've stopped posting a lot on home roasting -- I gave a much simplified version of these equations some time ago, and was ignored. I hope people will pay more attention to these)

So the real question is what does happen in drum roasters to create a faster rise in temperature during the first crack. My KISS answer is that the first crack happens. This is a release of superheated steam. In an air roaster, where the volume of airflow per bean is very high, this steam heating is lost. In an drum roaster, where the volume of airflow per bean is low, the super heated steam affects the thermometry.
For almost a decade I have used the term "less endothermic" when describing 1st crack, whilst all around me claimed the beans became exothermic. When using a CBR1200 roaster, clearly I needed less heat but not "no" heat to continue at 1st crack. Fair enough but you are putting cold air in people say.

I have dumped beans into an insulated container a little into 1st crack, they stop roasting instantly

I've killed the heat to the roaster (drum roaster), a little before 1st, just enough to they just get into 1st, reduced the airflow to nothing....and 1st crack stops.

These things would be different, especially the last one if coffee was exothermic at 1st crack. In fact, if it was in most drum roasters when you turn the heat off at 1st, the beans would not only continue to rise with the residual energy being imparted to them by the heat left in the roaster, but would continue way beyond the normal 6-7C rise that you see. If they were indeed exothermic, they would give a lot more heat than that to the roaster.

I believe there are 4 things happening.

1. There is a phase change of water as it bursts out of the bean
2. There is a structural change as it cracks, reducing bean density
3. There is a size change as the beans expand
4. Airflow within the drum/roaster changes

All of these things make the beans absorb more heat and absorb it faster from the environment (the roaster)....hence we have to turn the heating elements off in a drum roaster (or burners off). The probes in the bean mass then start registering an increasing temperature. My external probe, the one just outside the perforated drum, but in the casing it rotates in, shows a much higher temperature...reflecting the energy still in the roaster, this one often stays static, or reduces slightly.

My roaster takes 25 minutes to warm up, the heating elements being off for a few minutes, isn't exactly going to cool it down much.
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Almico
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#17: Post by Almico » Nov 04, 2019, 1:15 pm

DaveC wrote:My roaster takes 25 minutes to warm up, the heating elements being off for a few minutes, isn't exactly going to cool it down much.
Exactly, but my query is why the RoR would increase (significantly) almost 2 minutes after the burner has been shut off.

Next time I do a darkish roast, I'll record the screen.

Marcelnl

#18: Post by Marcelnl » Nov 04, 2019, 2:56 pm

could (part of) those 2 minutes not be lag time due to probe data averaging? I know from Oxygen Saturation monitoring that you're always looking at old data, thinking you look at real time events due to averaging that is needed to get a stable reading on the screennwithout making the observer go crazy.
Or a combination between probe lag and heat conduction delay?
LMWDP #483

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another_jim
Team HB

#19: Post by another_jim » Nov 04, 2019, 3:40 pm

DaveC wrote:I believe there are 4 things happening.
1. There is a phase change of water as it bursts out of the bean
2. There is a structural change as it cracks, reducing bean density
3. There is a size change as the beans expand
4. Airflow within the drum/roaster changes
Almico wrote:Exactly, but my query is why the RoR would increase (significantly) almost 2 minutes after the burner has been shut off.

Next time I do a darkish roast, I'll record the screen.
It's good to get some real observations and real physics rather than "thermal momentum" and "exothermy." I'm glad Tom started this thread.
Jim Schulman

crunchybean

#20: Post by crunchybean » replying to another_jim » Nov 04, 2019, 4:33 pm

I dont understand this, real observations are still based on that one person's machine, setup, environment, etc... you and Almico both have conflicting data. Secondly, all those "physics" equations are chemistry equations...