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Homeroasters.org » BUILDING A ROASTER » Drum Roasters
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Exhaust Airflow
BobbyS
Mill City has a lot of info on their site about their roasters. I was surprised at the low max exhaust airflow. On their 2k roaster the max was about 41 cfm.
 
dmccallum
That's interesting to know and thanks for pointing this out.

If they're referring air draw from the roasting chamber, then I've been wondering what a typical commercial roasters metric would be and relevant to my build.
 
BobbyS
this brings into question how much air intake open area is needed. too much and the flow would be very low
 
dmccallum
Have wondered about that also, but assumed you'd be more concerned about the orifice being too small and wind up sucking bean out owing to the increased air velocity ?
 
allenb
BobbyS wrote:

Mill City has a lot of info on their site about their roasters. I was surprised at the low max exhaust airflow. On their 2k roaster the max was about 41 cfm.


Optimum exhaust air flow on a drum roaster depends on the desired ratio of the two primary heat transfer sources; air and radiant heat from the drum wall. On most Probat current models with dual drum wall construction, they've gone for a higher convection heat transfer versus drum radiant to bean which necessarily requires more air flow than is necessary to just get the chaff and smoke pulled from the drum. If one will be using a single wall design then there's no need to pull more exhaust than necessary to effectively remove the chaff and smoke which is less CFM than most would imagine.

For my 1lb gas drum, I'm guessing I'm not pulling more than around 15 to 20 CFM and there's never a problem with removal of chaff and smoke.

For a 2kg drum with single wall construction, 40 should be more than adequate from my experience.

On air intake area exiting the top front of the drum for 40 CFM I would not go less than 2 1/2" diameter to ensure velocity was low enough to not pull the beans out of the drum. Unfortunately, it's very difficult to measure CFM without setting up proper test conditions and without lab grade tools. It's probably best to go a little over on the design and include an adjustable damper to back it off if velocity is too high.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
dmccallum

On air intake area exiting the top front of the drum for 40 CFM I would not go less than 2 1/2" diameter to ensure velocity was low enough to not pull the beans out of the drum.

Allen


You had me scared for a moment. For my 2kg drum I'm running with a 1 3/4" exhaust pipe to the cyclone, but I'd increased the roast chambers front plate orifice behind the hopper to what looks like 2 3/4" and I do plan to implement a damper.

Quite a tricky topic this one as it's not easy to measure airflow. Might be a useful guideline for "Links and Formulas Drum Roaster Building" ?
dmccallum attached the following images:
roaster_exhaust_area.jpg roaster_exhaust_area_1.jpg

Edited by dmccallum on 07/03/2017 04:16
 
BobbyS
Good info Allen.

Nice drawings dm

Brings in the question of how large of openings for air in the burner area?
 
allenb
Always leave ample area around the burners for air to flow up, alongside them at very low velocity to ensure you don't cause irregular combustion. There's no known formula that I know of to determine this. You'll know if you have insufficient surface area for the opening if the flame is tossed back and forth and has trouble staying lit.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
BobbyS
ThumbsUp
 
renatoa
I don't see the OP 41cfm figure in the 2kg roaster page... :confused:

https://millcityroasters.com/shop/coffee-roasters/2kg-gas-coffee-roaster/
DIY: TurboOven, Popcorn
Moded commercial: Dieckmann RoestMeister, Nesco
PID/ramp/soak controllers
Grinder: mod'ed Porlex to 47 conical burrs
 
eschlukebir
I think I saw that mill city's roasters now come with double wall drums also.
 
BobbyS
renatoa wrote:

I don't see the OP 41cfm figure in the 2kg roaster page... :confused:

https://millcityroasters.com/shop/coffee-roasters/2kg-gas-coffee-roaster/


It is shown as 70 m3/h and converted/ 41.2cfm
 
BobbyS
What type of tubing are folks using on exhaust air from the drum on out to the chaff collector? Seems most thin-wall stuff I see is zinc coated or such and I don't know if that would be toxic or if it matters on exhaust.
 
allenb
Doesn't matter at all for that location. For there to be any potential health risk in that location, you would have to be hitting near melting point of zinc which is way over any roasting temperatures you'll ever expose it to.

On the other hand, health guidelines from most agencies do not allow zinc coated metal surfaces to be in contact with food regardless of temperature but with your use of it in the exhaust stream, you should be good to go.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
BobbyS
Thanks Allen! I'm thinking of using EMT conduit since it is easy to find in my area. where I live here in NM we are the end of the pipeline when it comes to supplies..
 
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