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fan to extract air from the drum.
I am manufacturing a 6 kg drum coffee roaster, and I would like to know how to calculate the amount of air and what specifications I need for the fan to extract air from the drum.
Thanks for any help you can give me.

don't know how to calculate it, but mill city gives 212 cfm for their 6kg roaster
There isn't a standard formula that I know of since there are so many variables that can affect what a drum roaster requires. Some drum roasters utilize one fan for both roasting exhaust and cooling tray, some have two separate fans. Some designs using double wall drums look to minimize drum to bean heat transfer and have convective air be the dominate transfer medium. With these, there will be larger burners or electric heaters to allow the higher air flow.

I would go with the cfm krampe posted used in the Mill City roaster and utilize an air flow damper to allow dialing in an appropriate flow. With typical classic drum roasters, you want just enough exhaust to allow chaff and smoke removal but not so much as to require your heat source to be at maximum fire throughout the roast. This is of course assuming an appropriately sized heat source.
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
Aicardo also didn't mention whether it's solely to draw out exhaust air or will there be a cyclone separator in the mix.
I have a cyclone separator in my system, and although it will reduce airflow to some extent, I believe for the purposes of a coffee roaster the loss should be minimal, especially if the duct size is large enough.
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