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Drum- Size and capacity
I've tried to find a magic formula for roast capacity and so far nothing concrete. Mostly advice about "as much as the drum will roast evenly..." I think I have seen a third of the drums volume mentioned but not fully advocated. I discussed it with a local commercial roaster, and on his Diedrich IR-12 the capacity was a lot less than one third of the drums volume.

Another question, do perforated drums have bigger roast capacities than solid drums?
I'm guessing surface area on solid drums takes on greater importance ie the surface imparts most of the heat whereas perf drums allow hot air to permeate through the bean mass and do the work.
This is Dan, checking in as ordered!

For drums that are closed on the end, you can fill them fuller. A rule of thumb I use is:

Roast size (ounces) = Drum Volume (cubic inches)/10

Sample roasters will be less than this, because they have an open end and beans spill out. For them, the formula is closer to:

Roast size (ounces) = Drum Volume (cubic inches)/15

Although I can roast 500g in mine. I usually do 454g (one pound) so I am not left with green remnants from a 5 pound bag. For beans I want a brighter, hot-air-like roast, I use 300g.

Thats near enough the formula i use Dan. I tend to roast 400g.

thanks for helping out here; very nice to see you.

So for us metricated mob 1kg = 5.8 litres give or take ;)

Anyone got any firm thoughts as to diameter to length ratios?
My name is Tim and I have a coffee equipment addiction problem :)

Two Hottops - modded
TJ 067 Electric 1kg 5+years old
Insert new 5-8kg Roaster here urgently BBQ grill
I'll give my 1.5 cents on the drum ratio.

Most of the commercial drums I've had the opportunity to measure have had close to an equal diameter to length ratio. I don't think it had as much to do with heat transfer qualities as it did with needing to stay with a given footprint and width / length proportion of the overall roaster design.

The only problem I can see with a disproportionately long drum to diameter ratio would be if at the same time having an inconsistent heat output down the length of the drum and poor bean recirculation back to the rear of the drum. On the other hand, even with this seeming design shortcoming it might not end up affecting cup quality at all.

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

that saves me having to go measure a few myself :) I was considering about 1 1/2 long to width but the even then the airflow and possible heat variation at the front of the drum was worrying me with drawing air at the rear. Back to tweaking my plans.
My name is Tim and I have a coffee equipment addiction problem :)

Two Hottops - modded
TJ 067 Electric 1kg 5+years old
Insert new 5-8kg Roaster here urgently BBQ grill
Hi all,

Just a quick question about the formula: Is this a capacity formula or average/ideal?

One of the reasons I want to build a drum roaster is that I usually drink coffee at home around once a week (have way too many roasted beans sitting around), so would like to roast an 1/8th to a 1/4 of a lb at a time.

Please correct me if I'm wrong: 4" diameter drum, 4" drum length.

volume=pi * radius squared * length = 50.26 inches cubed, divided by 10=~5 oz = 142 grams, which is above a 1/4 lb. Does this sound right in terms of calculations?

One issue might be welding vanes inside a 4" diameter tube. Sounds close to the limit of my torch set up (need a stubby I guess).

Thinking solid drum out of 1/8" SS 304 vs powder coated mild steel.

Thanks for any insights.

Hi Matt, welcome to HRO!

Your calculations are good.

From all of my experimentation with drum roasters I would use Dan's formula as a not to exceed weight.
One of my drum roasters had a 4.75" dia x 4.75" length and the max I was comfortable with was between a 6 and 7 oz batch. Using his formula on this drum we get a little over 8 oz which would have been stretching it a tad.

You should have no problem with a 4 oz batch and maybe 5 oz if you don't push the rate of rise too much.

On the vanes, there's always the option of using angle brackets and machine screws for mounting instead of welding. Cut each vane into two, half length pieces and stagger them which gives you really nice mixing.

FYI, I tried powder coating on some of the metal parts in one of my roasters and it bubbled up at around 400 F. Of course, you may be looking at a different powder coat capable of higher sustained temperatures than what I used.

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

4-5oz is more than enough, perfect. In terms of powder-coating, I believe there are 1000F-ish high temp coats, but seems like SS 304 would be easier and cheaper (i can get the tube for probably $10-15, services usually have a hefty minimum, although i should shop around).

Any thoughts on solid drum? Plan to use gas stove as heater for now. have lots to learn about design still!

Resurecting this old thread instead opening a new one, better keep all related info in one place...

Having two drums, none of them having ideal D/L ratio, one is 14Dx23L (3.5l) and the other 22Dx15L(5.7l), all centimetres (volumes in litre), which one would you consider having the potential for the biggest capacity even roasting using same heating source?

LE: none of these will be used for pure drum roasting, they are made from expanded tin mesh, and will be used in a hot air roaster, rather as bean agitators actually...
Edited by renatoa on 04/14/2017 7:15 AM
Roasters: DIY: TO based IR 200-640g
Moded commercial: Skywalker, Dieckmann RoestMeister, Nesco, popper(s).
Electrics: TC4ESP, MS6514, USB/Artisan/Apps, PID controllers
Grinders: Arco, Xeoleo ghost burrs, Krinder, vintage PeDe Dienes, MBK Feldgrind
Brew/presso: ALM pour over, Arin lever, Staresso Mirage, Hario Cafeor dripper, Flair, Kompresso, Rota Aeropress, Gaggia Mini
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