Increasing Conviction Heat Transfer


With the as-designed airflow path of the bullet, if you block the exhaust port, the exhaust fan impeller will no longer function in a stalled condition, and you will NOT create any additional airflow in the drum, as you think you will. You have 2 aerospace engineers (@bab and myself) and one of the Bullet’s designers telling you this, so I think that’s pretty safe information to heed.

The only way you could make something like this work as desired would to be to add an additional fan, somewhere inside the flow path of the roaster. I would suggest the best place to do that would be the path at the top of the roaster, that goes all the way front to back, behind the bean chute, while blocking off the exhaust port.

That said, blocking off the exhaust port is not suggested, lest you may damage the electronics.


I had a notice of a reply here, but looks like it was deleted.

To answer the question, the fan will “stall” out because it can’t pull enough air. It will continue spinning (the motor isn’t what stalls), but it can’t “suck” hard enough to move any air in an enclosed space, where the differential pressure is negligible. In an enclosed space like this, you would need a different type of fan, like a biscuit fan (with blades).

How about if just block the exhaust port 1/3 or 1/2 way? assume doing this will let some air out while raising the drum air temp, possible?

exactly, the forward curved fan of the bullet won’t work. You would need a backward curved fan (like we have in the AiO).

It would be like running F 0.5 - less than F1 on your airflow.

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@jacob so the F setting controls the squirrel cage / impeller fan? I asked earlier but got no answer…

Yes the fan settings controls the RPM on the squirrel cage fan.


Hi Jacob, the Aillio Air Flow video shows that the Aillio takes in Air from the front as well as from the back of the machine. The air sucked in from the front passes through the hot drum and is heated in the process while the air sucked in from the back of the machine is just cold air and acts to cool down or to moderate the hot air sucked in from the front of the machine.
So question:- if the aim is to increase convective heat transfer presumably by increasing the temperature of the air inside the unit relative to the temperature of the drum, would restricting the air from the back of the machine help in this endeavour??
Would greatly appreciate your thoughts on this and thanks in advance…

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I think you are a little bit confused. Where are you seeing that the air pulled through the front is going through the drum? It goes around the drum first. The air coming in through the rear goes through the opening for the drum shaft, so you won’t be able to seal that off completely.

That said… In reality, the rear opening is actually where most of the air enters the roaster (path of least resistance). Since the drum is not fully sealed at the front (there is still a small gap), a small amount of the air coming in through the front IS actually going into the front of the drum. However, since it then gets immediately sucked out the air tube at the top, it never really goes through the drum, it mostly just goes across the front panel, from the bottom up to the top.

My suspicion is that if you sealed off the rear air inlet, the majority of the air you sucked through the roaster would then follow the flow path I just mentioned. I think this would only serve to REDUCE the convection heating inside of the drum. Here’s a diagram of what I think would happen (my flow path in purple):

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Try running drum at F6, at Royal Coffee they test their coffees with different Roasters Bullet being one of them. They are running 500g batches. I turned my preheat up a little higher than they do and run 800. They show their profiles and data points for the 22 lb boxes on their website and roast world. That lower fan speed work real well for me, will copy and paste your basic profile it’s at the start of every analysis. >"Aillio Bullet R1 IBTS Analysis by Evan Gilman

Unless otherwise noted, we use both the site and Artisan software to document our roasts on the Bullet. You can find our roast documentation below, by searching on, or by clicking on the Artisan links below.

Generally, we have good results starting our 500g roasts with 428F preheating, P6 power, F2 fan, and d6 drum speed. Take a look at our roast profiles below, as they are constantly changing!"

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You raise a good point that I haven’t considered. I am still curious to see what the folks at Aillio think. Thanks for your feedback my friend. :blush:

Bless you Philip and thank you for your feedback. I’ll look at the roast profiles you mentioned and will try their settings to see if they improve my roasts. Kind regards :blush:

In theory yes, but the airflow from the back is also designed to cool down the shaft / pulleys, so you might overheat things. Also, the airflow area from under the drum is too small compared to the fan intake / suction capabilities. From all I have heard, limiting airflow will result in smoked flavors in your roasts.

Bless you Jacob and thank you for your valuable feedback. As a second question then:- what D, F, P settings in your opinion would maximise convective heat transfer in the Bullet?

Should Fan speed be kept as low as possible to not cool down the interior chamber temp or is there a mid way Fan setting that would optimise convective heat transfer?

Similarly for Drum speed?

Any feedback would be helpful.
Kind regards from the land down under :blush:

I don’t have clear facts, but an educated guess would be D9 so they spend as much time in the air, only heated with convection, then a low medium fan speed - F3-F4…but I am just guessing here, and I would not know how to test the theory.

God bless you for your honesty Jacob. I will try F3 and F4. Till now I have been using F2 mainly, thinking that anything above that may start to add more cooling. Thanks for your feedback…

I think I beg to differ on this though. I know several of us on this forums use F4 or F5 towards the end of the roast to kick out the chaff and I think some other folks had noted that a too high F at the end of roast seems to rob the roast of brightness. And since the F setting controls the speed of the impeller/squirrel cage fan which draws air out the exhaust, I would think an F4 during the roasting time may not give desired convection in the drum unless you also have the P cranked up - but I don’t know at which point the balance tips the “wrong” way. I would think closer to F2 or F3 might give you that convection condition. That said, most (if not all recently) of my roasts start with a F1 setting at charge for 1 min, then I bump it up to F2 and let it sit there until at least yellow point (around 335-340F) before changing to F3.