Increasing Conviction Heat Transfer

Hello everyone,

Couple of days ago, I replied to @gabyritaseek.qiAO, suggesting to run the aillio on max drum speed and minimize the batch, which already had been done and there was no more room for any thing to do.

But in the same topic “ What would you suggest to improve Aillio Bullet design!?”, one of the features I suggested was to add a cap on the exhaust.

I used some aluminum foil layers, squeezed them, tried as much as possible to fill the hole.

It wasn’t fancy :sweat_smile:, and this is how it looked :slight_smile:

I know, it will not block the exhaust 100%, but I was hoping to trap as much heat as possible while the fan is on a high speed.

And this is the result of 2 roasts for the same beans, one entered the milard phase with F3 and ahead of the FC, turned to F5 to kick the smoke.

The other roast, I used the handmade cap i shown to you earlier entered the milard phase with F8 until ahead of the FC turned to F5 and removed the cap.

I managed to roast on F8 during the milard without using the momentum. I assume i was able to expose the beans to a higher rate of hot airflow thanks to the exhaust cap.

Did it work?! I still don’t know as the roast is still fresh, I didn’t do cupping yet.

Does it make any difference?! As a roast chart it did, but how this higher airflow will impact the cup, thats what I will find after the cupping.

The experiment is still new, I don’t know if anyone else done it. if did, hope they can share their experience.

Last thing, normally it takes somewhere between 23-25 minutes for my roaster to be ready and the pop message appears. This time, it took less time, 18 minutes (%33) less preheating time.

This is a mere single experiment, need more trials and observation.

Thank you

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Just done with the cupping, Compared to the original roast, the new roast (with F8 milard phase), is less in body, less bitterness and need more development time.

I will pull a similar experiment on an Ethiopian Natural Process and get back to you with the updates.

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God bless you alghamdi.majid1b1s, great suggestion and certainly worth a try. I will follow your experiment closely and learn from your findings. If this approach ends up working we may need to find a way of standardising the modifications so all the modified machines are the same and we can compare roasts and results across different machines better. We cross that bridge when we come to it. Again, thanks for sharing…

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Hello Gaby,

I did roast an Ethiopian beans, with F6 fan speed during the milard phase, compared to the previous profile with F3 during the milard phase for the same beans, the latest roast with F6 had had a faster milard phase (20 seconds shorter).

Managed to increase the airflow rate and and maintain a high temperature heat transfer led to shorten the milard phase.

Though, its early to judge the experiment, still needs more trials, and I hope someone else can step up and try it, so we have more than one individual’s experiment. Yet, its promising.

To standardize it, there is already an adapter for the bullet’s exhaust. And it’s available on Aillio’s official website. It’s normally used with a 3D printer. We can use it to get measurements to creat a stainless steel or an aluminum alloy cap using a CNC machine. It will fir perfectly.

Hi [alghamdi.majid1b1s],
Lovely to hear from you so fast. What time zone are you in? I am in Sydney Australia for what it is worth. I am new to the Bullet and still learning how to get the most out of it for the sort of taste I like. I tend to roast light to medium for espresso type extraction with a little bit of milk and I prefer Ethiopian beans for their distinctive bright/aromatic taste. That’s what I am trying to learn how to achieve on my bullet and why I think more convection will help.
I will check the adapter on the Aillio website and may try a few roasts with your method and see how I go.
In the meantime I was also planning on trying some roasts with less fan speed, to reduce cold air coming in and lowering the the inside environment temp in the hope that this may also improve convective heat transfer. Anyhow, thanks again for your great suggestions and sharing your findings. :slight_smile:

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Hello Gaby,

I am from Saudi Arabia, our timezone is +03:00 GMT, we roast the coffee as same as you do in Australia. So basically, I understand the coffee you like the most.

My advise, ahead of trying any cnc caps, i prefer you do as you suggested, and as same as I did, and observe the outcome.

And let us both share our tests results, of course more participants are welcome.


I don’t understand the purpose of the exhaust cap. If you only want to block air why not just use F1 during roasting?


Salam to you [alghamdi.majid1b1s] and belated blessed Eid to you. I will try a few different roasts over the coming days and will do my best to share results. Kind regards…


I think the aim was to try and cause more air turbulence inside the bullet by using the higher fan speeds while adding the air flow resistance. We will try lower fan speeds too to see if this gives similar results to the above or not. :slight_smile:

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Thank you for your kind congratulation,

Sure I will be awaiting for your feedback :star_struck:.

Happy Roasts.

Hello Jacob,

Gaby already has explained it, but I will explain the idea.

Roasting on F1, will maintain the the heat inside the roaster, with a low airflow. While higher fan speed F8 for example, has a higher airflow, but it will blow the heat out of the roaster.

The exhaust cap, will allow to have a higher airflow, but it will prevent the heat from sneaking out of the roaster, it will go back to the toasting chamber. Allowing for more convection heating transfer while roasting.

If you block the exhaust port, air doesn’t move inside the drum except for that air movement provided by the muffin fan (which is a fan with “blades”) behind the drum moving some air thru the chaff exit below the door. The exhaust fan is a centrifugal fan which “stalls” (stops moving air) when it is blocked. It is unable to pull air from the rear entry holes at the back of the drum because it has effectively stopped working… the pressure at the exhaust exit goes to zero when the path is blocked. Air movement with a centrifugal fan must have an unimpeded path for exhaust.


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Hello Bruce,

I will try to simplify my idea, When you increase the airflow at a higher fan speed, more you lose energy, because more hot air flows out of the exhaust port. When we block the exhaust, either we recycling the hot air or we are kicking back the hot air (reversing airflow). Thats my explanation to what happened.

You will not recycle the air by blocking the exhaust port. As Bruce pointed out, the impeller will not function when it is blocked. You will need a backwards curved fan to work in the environment.
I also don’t understand where the air is going to flow when you block the exhaust? If you can please draw this for me, I am interested.


Hello Jacob,

Normally, when the exhaust open, on a high fan speed, the hot air goes through the upper deck towards the the chaff collector. And I am losing the heat, due to the high fan speed. To prevent that normally we go on low fan speed.

As you see in the picture, sorry I am loser when ir comes to drawing. If we block the exhaust, we will reduce the hot air passive through the upper deck, because there is no more place to go, some of the hot air will find its way., but mostly the upper deck will become a dead area or low traffic are, when the air can’t find its way out, it will recycle inside the chamber.

Recently, I am using the exhaust cap, to maintain the high speed airflow inside the roasting chamber without losing the energy, to increase the conviction heat transfer. But ahead of the First crack, I remove the cap, to release the energy and the smoke out of the roaster.

I’ve been following this thread with interest. In my mind the best analogy is a oven that has convection feature. When you roast a chicken without the convection feature on you may not get a roast chicken with evenly browned and crispy skin, and even cooking of the chicken without some manual turning of the chicken often - which means opening the oven door often resulting in heat loss. Whereas with the convection fan on, the fan moves the air all around which would result in evenly browned and crispy skin on the roast chicken and a more evenly cooked chicken, all without opening the oven door.

I think what @alghamdi.majid1b1s is attempting to achieve is similar to the convection oven. Now the question is whether the existing fans in the Bullet can actually achieve this if that exhaust port at the top is sealed. Also, when we change the F setting, which fan are we changing? The squirrel cage / impeller fan or the muffin fan?

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Hello @blacklabs,

Yes, its almost the same oven concept :sweat_smile:. Yet its one of the roasting heat transfer method, and there are many roasters works with conviction heat.

The result as you seen, managed to pull almost the same profile curves, but with a higher fan speed. The result in the cup is quite noticeable, less bitterness and better uniformity.

And if i wasn’t mistakes, using the exhaust plug / cap will increase the pressure as well.

This is impossible to do. You cannot have high speed / airflow inside the drum without having any airflow at the fan. I am 120% sure what you are trying to do will not work. You can decrease airflow by blocking the fan yes, but you cannot have both; no airflow at exhaust and high airflow in the drum.

One thing I would like to point out is that the majority of failures we see are from users who forget to clean the chaff filter. A blocked filter restricts airflow and the humid air while roasting gets into the electronics and over time damages it. So I very strongly advice against doing this hack.

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Hello Jacob,

I didn’t say without airflow in the fan, the opposite, high speed fan while blocking the exhaust to creat a vortex inside the drum.

Again, I didn’t say block the fan, i said block the exhaust.

Ahead of the first crack, the cap will be removed, to let the energy and the smoke goes out of the roaster, its will not be blocked all the way from start to the end.

Ok, its an experiment I wanted to share, wether its worth trying or not, thats up to users themselves.