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How far into second crack, if at all?
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Discussions > Coffee > Home Roast > How far into...  
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Frost
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Frost
Joined: 26 Jul 2007
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Posted Tue Feb 4, 2014, 8:42pm
Subject: Re: How far into second crack, if at all?
 

Sorry to the OP for the thread drift.    

The 'throttle' controls the heater power. The heater power, the BTU's pumped into the roaster, ultimately determines the ET.

The 'heat transfer efficiency' of the roaster is the roaster specific complex relationship between heat given off by the element or burner, and how it makes it's way through the roaster, drum, air, losses through the housing, etc... and ultimately raises the temperature of the bean mass.
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boar_d_laze
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Joined: 21 Nov 2006
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Posted Wed Feb 5, 2014, 8:36am
Subject: Re: How far into second crack, if at all?
 

Frost Said:

Sorry to the OP for the thread drift.

Posted February 4, 2014 link

Me too.

The 'throttle' controls the heater power. The heater power, the BTU's pumped into the roaster, ultimately determines the ET.

If all you meant was that the heat source drives ET, you're (a) right; but (b) didn't mean very much.

The 'heat transfer efficiency' of the roaster is the roaster specific complex relationship between heat given off by the element or burner, and how it makes it's way through the roaster, drum, air, losses through the housing, etc... and ultimately raises the temperature of the bean mass.

"Heat transference" and "efficiency" each have meanings.  "Heat transfer efficiency" as used here is a made-up term which could mean everything or nothing. It may sound "technical," but it's not; it's just word salad.

A picture is worth a thousand words.

The attached profile of a very successful roast disproves your assertion that ET leads the way in a professional type drum roaster.  FWIW, almost all my profiles are similar in that BT exceeds ET for at least some part of the roast; and that's the way it is for most roasters using drums.  

Then there's the very obvious fact that drum roasters heat the outside of the drum, but (most) do not heat the air going into it.

With my drum roaster (and Henry's, AND Henry' hypothetical Giesen), as with most drums which have independent gas, and fan (and sometimes tray/roaster damper) controls, the more airflow you push through the roaster, the more you slow the RoR.    

You can see in my plot that I'm using increased airflow to slow RoR (red ET line and BT line cross, ET falls), get rid of smoke and chaff, and increase the first part of Drying and Development times.  While I can use the damper and fan to control (to some extent) the differential in ET and BT RoR, there's nothing I can do to raise air temp independently; heat enters system comes ONLY as a direct result of the burner flame heating the drum.  

(Note also, that if I wanted, I could make BT exceed ET very early in the roast, and keep it well ahead all the way through -- simply by running with the gas and fan cranked, and using the damper to maximize airflow through the roast chamber.  It may be worth it to illustrate the point, if doing so would make the disagreement go away.  If you or anyone else is interested, I'll run a load of sweepings I have around with max gas and max air and we'll see what it looks like from Turn on.)

In my roaster, as in most drums, air temp in the chamber is an artifact of heat applied to the drum and not the other way around.    

BDL

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Frost
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Frost
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Posted Wed Feb 5, 2014, 10:24am
Subject: Re: How far into second crack, if at all?
 

Nothing I have said contradicts what you are showing here;

  •  'Heat Transfer Efficiency' is a made up term, to lump all the complexity of the roaster thermal behavior, thermal mass, phase lag, heat loss.  All that occurs between energy applied to the heater and finally the resulting rise in Bean Temperature.

  • Burners directly heating the drum and the ratio of convection to conduction heat are related but not nearly as much as you might think. Bean agitation by the drum is an important component in the heat transfer too. Many of the smaller drum roasters could use improvement here.

  • The big roasters do have much better control of convection heating. They have to as it is the only way to effectively heat the much larger bean mass without baking it.

  • Increasing the air flow will reduce air temperature. Consider also where this air comes from, how it is heated, and where it goes.

  • Per your ET BT data; I would look for a better ET probe location as this one does not represent the roaster temperature; You want the ET to be at least a reasonable PROXY for the heat energy of the roaster available to the beans.   If the bean mass was actually exposed to that ET during finish the roast reactions would be stalled where this happens.
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boar_d_laze
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Posted Wed Feb 5, 2014, 11:46am
Subject: Re: How far into second crack, if at all?
 

Frost Said:

3 min google search:  http://probatburns.com/resources/articles/spotlight-drum-roasting.php "Only about 85% convection in solid drum roasters...." I did research many years ago and the old  links are now dead.

Posted February 4, 2014 link

The Probat material, either refers to open throat, sample roasters only; or is pervasively ignorant of the ordinary physics of cooking.  U pick'm.

Here's the second paragraph of the article, in full:

There are two types of drum roasters: solid drum and perforated drum roasters. The perforated design allows for more convective heat transfer — or heat transfer through air — than the solid drum.

While there may be a bit of truth to the statement that the perforated design allows for more convective heat transfer, it is very misleading.  

Before we analyze it, let's look at something very basic.  There are three primary forms of energy transference (aka ("conduction").  Contact, convective, and radiant.  (That brings up a separate issue; the erroneous conflation of contact with conduction -- but it's so widespread the error is moot).  In cooking we often see hybrid forms such as immersion contact (frying, poaching, etc.), immersion/convective (steaming), etc.  

With coffee, the energy transfer during skillet roasting is almost purely contact; while the energy transfer in a popcorn roaster is almost purely convective.  As far as I know, there are no roasters which are almost purely based on radiance.

On to the analysis.  With a perforated drum in which the beans can "see" the heat source, surely you can see that at least some energy will be transferred by radiance; but radiance is ignored in the article.  

On the other hand, if perforated drums are more "efficient" because they allow more efficient energy transfer because of convection... where's the source for the extra air which wasn't already working in a solid drum?  If anything, convective conduction is more efficient in a solid drum, because the air is moving through the smaller volume of the drum only, as opposed to moving through the large volume of the roaster's entire body.      

...that’s because drum roasting uses less convective heat transfer (only about 85 percent) than other types of roasters. The remaining 15 percent of the heat transfer in drum roasting happens through conduction, or transfer of heat from bean to bean. The beans stay in constant contact as the drum turns, allowing flavor compounds, aroma and acidity to transfer between the beans for flavor development...

This is the statement which you seem to find dispositive, but -- not to put to fine a point on it -- it's almost pure horse$#!t.

Here's the part with the pony:  Yes, drum roasting uses less convection than air roasters.  

However, to say that contact conduction is limited to heat transfer from bean to bean without considering the role that contact with the drum plays itself is a way of saying the beans heat themselves and denies the laws of thermodynamics.  After this twaddle, maybe you can take the rest of the remark as good authority; but I can't.  I'm too busy scraping the bottom of my shoes.  

Also the Behmor is a unique case as it uses a radiant element; I have no idea where it operates.

Not just the Behmor, but the HotTop, Quest, Dalian Amazon, and quite a few others as well.  I've owned and used three of those four.  Their respective radiant elements "operate" inside the roaster between drum and shell.  The location may be to the side (Behmor), above (Dalian Amazon), above and to the sides (HT), above and below (Quest), etc., etc.  

Finally:
This has become as tedious for me as I'm sure it has for everyone else.  I have no interest in teaching Jr. High physics for any amount of money, and less than no interest in doing it for free for the benefit of someone who's already convinced by some other truth.    

It seems rather obvious to me that a typical, professional style drum roaster in which the drum is exposed to the heat, is helluva lot more like a skillet bent into a cylinder than it is like a popcorn machine which has hot air pumped through it.  There's just not much to say after that.

BDL
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boar_d_laze
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Posted Wed Feb 5, 2014, 12:49pm
Subject: Re: How far into second crack, if at all?
 

That didn't take long. So much for good intentions. Why do I let myself get roped in?

Frost Said:

'Heat Transfer Efficiency' is a made up term, to lump all the complexity of the roaster thermal behavior, thermal mass, phase lag, heat loss.  All that occurs between energy applied to the heater and finally the resulting rise in Bean Temperature.

Posted February 5, 2014 link

OK.  It's a made up term which you used without defining in hopes of improving communication.  Whatever you meant by it then and now (still not entirely clear) it has little do with efficiency, or transferring heat.  

While I'm familiar with the term "phase lag" in a few contexts, I'm not sure what you mean by it here.  

Maybe I'm wrong about this -- I hope I am -- but technical terms strewn about like confetti without their "term of art" meanings, is often indicia that the author is trying to sound more knowledgeable than (s)he actually is.  

Burners directly heating the drum and the ratio of convection to conduction heat are related but not nearly as much as you might think.

How could you possibly know what I "might think" about ratios?  I haven't said a word except about skillets and popcorn roasters.  You're probably thinking of the guy who ghost writes for Karl Schmidt, President, Probat Inc.

Bean agitation by the drum is an important component in the heat transfer too. Many of the smaller drum roasters could use improvement here.

I'm not sure whether or not "component" is the right word insofar as agitation relates to transfer.  I think it's more about how evenly the heat is transferred throughout the mass.  But it's close enough for me to at least understand what you mean by it.  But there's something I don't get... What's your point?  Just musing?  Or, does "agitation" relate to the usefulness of ET's importance as an indicator of what's happening to the beans through every phase of the roast in some non-obvious way.    

The big roasters do have much better control of convection heating.

Better roasters do offer better control than worser roasters; one thing which makes them better.  

Bigger machines are not necessarily better.  And -- just speaking generally -- at a certain level of capacity you lose a great deal control to the thermal momentum of the bean mass, the roaster mass, limitations on how much heat you can actually pump into a roaster (no matter how it's pumped) without damaging the roast, and surface area-volume considerations.  

That's partly why even high volume artisanal roasters add mid-sized roasters to meet supply, rather than investing in a big boy.      

They have to as it is the only way to effectively heat the much larger bean mass without baking it.

If you're saying roasting a large charge requires a roaster large enough to handle it; you're not adding much to the conversation but you're right.    

Increasing the air flow will reduce air temperature.

Yes.

Consider also where this air comes from, how it is heated, and where it goes.

I not only have, but have discussed it in some detail in several posts within this thread.

Per your ET BT data; I would look for a better ET probe location as this one does not represent the roaster temperature; You want the ET to be at least a reasonable PROXY for the heat energy of the roaster available to the beans.

That takes nerve.  

You don't have even a shred of an idea where my ET probe is located.  The only problem with the its location and the accuracy of the information provided is that it doesn't fit your idea of good information.  It not only works for me, but works for the people doing the roasting in the world-class shop where I trained under one of the best roasters in the country.

If the bean mass was actually exposed to that ET during finish the roast reactions would be stalled where this happens.

Or -- second possibility here -- you're wrong and the beans take energy from another source besides hot air.  

BDL
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Frost
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Frost
Joined: 26 Jul 2007
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Location: Sierra
Expertise: I love coffee

Espresso: Isomac Venus
Grinder: Lelit PL53
Roaster: Poppery I w/variac, MET, BT
Posted Wed Feb 5, 2014, 12:51pm
Subject: Re: How far into second crack, if at all?
 

The irony that your reader would give credibility to the rant of a lay person over a brief statement on the subject by Probat Burns.  Anyone really interested in researching the subject can do better by Google search such terms as 'Drum Roaster Convection, Conduction'  etc. along with the normal (high level of) signal to noise filtering required of any web based research. First rule: consider the source.

What is the approximate ratio of convection to conduction heating in your roaster? How would you go about measuring this? How much control do you have over this ratio?
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rmongiovi
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Posted Wed Feb 5, 2014, 3:14pm
Subject: Re: How far into second crack, if at all?
 

Frost Said:

The ideal ET probe placement is one that accurately reflects the temperature of the air the bean mass is immersed in or tumbling through. There will be a temperature gradient across the roaster from the heater element or burner itself to the bean mass. Something like the Hottop has significant temperature gradients in the roast chamber as the fan does not assist much in convecting heat through the roaster.

Posted February 4, 2014 link

Yep, and that's exactly why I ask.  If I wanted to add an ET probe to my hottop I'd need a theory on which to base my choice of location.  As near to the bean mass as possible without being in it?  On the side of the roaster nearer to the heating element or the side further away?  I'd expect a huge variance outside the drum, so almost certainly not there.  But where?  (and apologies to OP for hijacking his topic).
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Frost
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Frost
Joined: 26 Jul 2007
Posts: 2,093
Location: Sierra
Expertise: I love coffee

Espresso: Isomac Venus
Grinder: Lelit PL53
Roaster: Poppery I w/variac, MET, BT
Posted Wed Feb 5, 2014, 4:46pm
Subject: Re: How far into second crack, if at all?
 

Fortunately for you, the Hottop may be the most probed home roaster out there. Spend some time here and mostly on HB looking at the data and probe placement others have collected. The main stratification of heat you will see (due to the lack of forced convection) is that the top of the roast chamber will be hotter than the bottom...
Study the various Hottop ET/BT roast profile data and pick the location most favorable to your needs.
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rmongiovi
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Roaster: Hottop B
Posted Wed Feb 5, 2014, 11:54pm
Subject: Re: How far into second crack, if at all?
 

I've seen articles about installing a BT probe through the bean chute, but never a discussion about proper ET probe placement.  You realize, of course, that "Study the various Hottop ET/BT roast profile data and pick the location most favorable to your needs" contains absolutely no information.  My needs are, obviously, to get a better handle on what's happening inside the roaster.  If I knew what was most favorable to that I assure you I wouldn't be asking this question.

If you don't know the answer, it's OK to say so.
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Frost
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Frost
Joined: 26 Jul 2007
Posts: 2,093
Location: Sierra
Expertise: I love coffee

Espresso: Isomac Venus
Grinder: Lelit PL53
Roaster: Poppery I w/variac, MET, BT
Posted Thu Feb 6, 2014, 9:37am
Subject: Re: How far into second crack, if at all?
 

Before you start drilling holes in the back plate, spend a few hours researching and reading about those who have already done it so you can avoid repeating the same mistakes. I don't use a Hottop.

Here is a starter:

Click Here (www.home-barista.com)

Most of the value in these sites is not in the daily chatter, but to ferret out the important relevant information from the archives. It's not always easy and it does take time. Good Luck with your project. I'm sure it will help you better understand and control your roaster.
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