Air Flow

[kenhendarto]

Hey everyone

I am trying to understand the impact of airflow towards our roast curve.
I realize that if we increase the airflow during "high gas" it increase the convection and made the ROR plummet or go up. However, during "low gas" (1 min before first crack towards the end) increasing the airflow will bring down ET and made the ROR go down. Could you guys confirm this and I am also curious what is the theoretical explanation of airflow usage apart from increasing convection (does it necessary to increase airflow when conduction heat is enough), exhausting the chafs and smoke?

[SusanJoM]

It's a good question, and I don't think anyone has addressed it with any rigor.  First of all the air flow is going to be different if you have a solid drum than if you have a perforated drum, so there isn't a 'one size fits all' answer to your question.

I think to be addressed properly, the issues of smoke and chaff removal should be taken out of the equation.  They do not have anything to do with the roast itself, and in the case of the chaff, it can easily be removed post roast simply by putting the seive on top of a fan that is blowing rather than drawing.  Trying to incorporate chaff removal into the roast itself seems to me to be more likely to compromise the roast than to improve it.

[hankua]

Here's my unscientific opinion......

Not sure if the question is about airflow/convection during the RD (roast development) phase or the entire roast.

In general during the roast low air reduces convection, medium air increase convection and high air decreases convection. The cold air is drawn over the burners thru the back of the drum then out the top front. Too fast the air has not enough time to absorb heat. Too slow the air is hot enough, volume and flow insuffient.

Guessing about the RD question; most of the beans moisture have been driven out, most chemical reactions have occcured, and the cell struction has changed/expanded. As a result the beans take less heat to develop and change; less gas/heat is required and more airflow reduces ROR and evacuates smoke.

[SusanJoM]

[quote author=hankua link=topic=1325.msg13689#msg13689
Guessing about the RD question; most of the beans moisture have been driven out, most chemical reactions have occcured, and the cell struction has changed/expanded. As a result the beans take less heat to develop and change
[/quote]

This is the clearest explanation I have seen to date for that blip in ROR after 1C that happens with no change at all in the application of gas, or even in spite of fairly rigorous decreases in the gas. 

[flat_white]

Hi,

during my last roast I tried to stretch the drying phase acc. to sweet marias explanation.
I observed that there is only a little change in the length of drying phase with to different airflows.
First roast was a Guatemala 333gr, 220°C charge temp, 50% airflow, no gas upon charge, and 3.5kpa power reaching TP. Length of Drying phase was 4:25min (41,9%, 15.8°C/min).
Second roast was a Guatemala, 220°C charge temp, 25% airflow, no gas upon charge, and 3.0kpa power + 50% airflow reaching TP. Length of Drying Phase was 4:19min (42,7%, 16.9°C/min).

So how are you stretch the drying phase?
In my opinion, the airflow up to 50% will increase the RoR ( I tried different power settings 2.0-4.0kpa). So that makes it a bit difficult, to get a continous decreasing RoR, when I start to turn up the airflow (from 0 or 25% to 50%) only after End of Drying Phase. Otherwise I will get a significant RoR increase after end of DP, when I turn up the airflow stepwise.
I use only 4 airflow settings (0, 25, 50, 100%). How do you handle this problem?

[edtbjon]

Hi,

during my last roast I tried to stretch the drying phase acc. to sweet marias explanation.
I observed that there is only a little change in the length of drying phase with to different airflows.
First roast was a Guatemala 333gr, 220°C charge temp, 50% airflow, no gas upon charge, and 3.5kpa power reaching TP. Length of Drying phase was 4:25min (41,9%, 15.8°C/min).
Second roast was a Guatemala, 220°C charge temp, 25% airflow, no gas upon charge, and 3.0kpa power + 50% airflow reaching TP. Length of Drying Phase was 4:19min (42,7%, 16.9°C/min).

So how are you stretch the drying phase?
In my opinion, the airflow up to 50% will increase the RoR ( I tried different power settings 2.0-4.0kpa). So that makes it a bit difficult, to get a continous decreasing RoR, when I start to turn up the airflow (from 0 or 25% to 50%) only after End of Drying Phase. Otherwise I will get a significant RoR increase after end of DP, when I turn up the airflow stepwise.
I use only 4 airflow settings (0, 25, 50, 100%). How do you handle this problem?

First, it seems like the "general advice" is to use low (or no) airflow in the drying phase of the roast. Most of the videos on e.g MillCityRoasters are done with this approach. The idea is to let the water in the bean slowly steam off, distributing the heat in the bean mass and to prepare the roast for the later stages.
Now, it's tricky to speak "only" about the drying phase, as each and every part of the roast is a result from what you've previously done.
In short, the way I really started to get more consistent good quality was to lower my charge temp and hit the gas hard after a minute. Numbers? 400g charge at 160C, no heat for the first 70 sec, then 3.5-4.0kPa and about 10% air (TP at or below 80C). DP at around 6.00min, depending on how I want to enter the next phase(s).
The reason for using any air at all in the drying phase is to create some airflow for the heater. Just a little airflow makes the burner work much more effective. In my opinion, too much airflow takes away the moisture from the beans too quick, affecting the up and coming processes in the coffee.

[hankua]

Wow, 220c charge.

That sounds like something from Mr. Li's instructions, which my guess is for perforated drums. Could be a strategy for a Nortic roast, haven't ever tried it though.

Have you tried charging at 180c?

Here's a profile to try with 454g charge: (I call this a 4-3-2-1)
For 330g I would change it to (3-2-1-1 and possible delay 1st reduction)

Charge at 160-180c gas and air low.
At the turning point gas 4kPa 25% air
150c-3kPa 25% air
160c-2kPa 25% air
170c-1kPa 25% air
180c-1 kPa (use appropriate air till drop)
190c-.5-1kPa depending (appropriate air flow)
(From this point, it depends on the finish; city roast or full city)
ROR of @ 6-8c works for a medium roast

[flat_white]

@edtbjon:
Thanks for your feedback.
You are absolutely right, low airflow during the drying phase makes sense. Thats how I did all the roasts before. I just wanted to experiment in the early roast stage with airflow, to stretch the roast and to get rid of the RoR increase due to airflow turn up from 25% to 50%.
I think (what I can see from my different roast profiles) one of the main points to stretch the drying phase is the charge temperature.

@hankua:
Thank you for your profile.
I did already some roasts with 180C and less. Interesting is your airflow setup, I'll try that tomorrow on my next roast. Maybe I play to much with (too high) airflow settings until reaching FC.

As you can see below, that was a roast with 50% airflow. I was very happy with the RoR, but my goal was to hit a finish which is perfect for my espresso (something around full city plus).



That roast was good for a filter coffee, but not for espresso, because it is still too acidic. Thats the reason why I started to use higher charge temperatures.

On the next roast, I tried a higher charge temperature 50% of airflow at the beginning, and an airflow of approx. 75% 2 minutes after TP. I'm happy with the finish, only light acidic tastes, and nice sweetness, but the fruity aromas are gone 



Nevertheless - to stay on topic  - I think low to medium airflow with high gas settings implements an increase in RoR, medium to high airflow will decrease RoR independent of gas setting.

Tomorrow I will test hankuas way, constant ariflow until 170c.

 

[hankua]

Actually I don't use fixed airflow, with a Variac there are many choices. I was using your settings of 25% and 50%.

You need to come into 1C with more energy, try using the projected LCD function to estimate where the roast is going to land.

One basic principal at least I believe is that BT will always try to reach ET, so in your case the ET needs to be higher, more toward your projected.

Try using maximum gas, like I said and start reducing early; that's the idea behind that profile.

Another method would to be using fixed gas pressure and air, sort of "low and slow", with a somewhat higher ROR from 1C to finish. Don't be against boosting or bumping the gas pre 1C. Our machines are lightweight compared to the heavy pro roasters Rao uses.

GregR uses this method for espresso with success (pre 1C gas gas bump)

[kenhendarto]

Thank you everyone for the answer, it makes a lot of sense now.

Just a question for Huky users that have 220V electricity in their country, what is your maximum (100%) air flow in variac? Me myself never use more than 140V.

[edtbjon]

Thank you everyone for the answer, it makes a lot of sense now.

Just a question for Huky users that have 220V electricity in their country, what is your maximum (100%) air flow in variac? Me myself never use more than 140V.

I made some tests with an anemometer (wind meter) and will make some more to complete the test set. My tests accord with many other published here and it seems like the fan (actually the exhaust system from the roaster through the exhaust bowl etc...) flats out at 70-80% of the max setting on the Variac (maybe 170V). That is not far from the 140V that you suggest.