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Thread: Batch sizes for a drum roaster

  1. #1
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    Batch sizes for a drum roaster

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Good afternoon all
    I apologise if through my pouring through the forum, I have missed this as a previously asked question.

    I am about to enter the world of Barbeque drum roasting. I will document the build in another thread to share the journey, but what I am interested in talking about and learning froim the brains trust today is optimal batch sizes for the drum I am looking at building.

    The roaster will be approximately 340mm long with a diameter of a little over 200mm

    I know how to calculate volume etc, but in all the reading and research I have done, I do not think that will really help because obviously I am not going to fill it to the brim and expect to roast 10kg properly and evenly!!

    Is there a calculation based on the size (volume) of the roaster to determine what is the ideal batch size of greens? I am happy to go the trial and error route, but I would much prefer not to if there is a method out there.

    Thanks all. Shannon.

  2. #2
    Senior Member greenman's Avatar
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    Hi Shannon
    I measured the drum of my 2kg Torrefattore roaster and it is 240x230 perforated drum.
    The maximum amount I roast is 1.8kg, adding more than this did not allow enough air circulation and the batches were quite roasty, the 1800g is ideal and produces consistent roasts.
    Can't help with the calculation but hopefully this will help.
    cheers
    Greenman
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  3. #3
    Mal Dimal's Avatar
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    G'day Shannon...

    Would also help matters if you installed lifters inside the drum, staggered to ensure thorough agitation; something like the included sketch to ensure thorough mixing.

    Drum Lifter Orientation.JPG

    Probably start at 500g and work your way up from there but you will also need to consider the peripheral speed of the drum, maybe aim for around 45 rpm +/- using a rotisserie motor or some such setup...

    You'll find the limit and optimum size of batches by gradually increasing the size of the batches until you observe inadequate agitation of the green bean mass or maybe that the drive system struggles to lift the bean mass effectively.

    Just a couple of ideas...

    Mal.
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  4. #4
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    Thanks for the replies Greenman and Mal, both very helpful.
    Greenman, your drum sounds reasonably similar in size to mine so that gives me a good idea of batch size capability without a specific calculation.
    1.8kg is way bigger batches than I was originally thinking of so this is really good news for me, gives me wiggle room for expansion if the need arises.

    Mal, your advice as always is very valuable. Most of what you have said I have covered -
    My BIL, sourced a motor is out of a golf buggy with car battery and vairiable speed controller unit. Can have it rotate anywhere up to about 75 rpm. Glad you mentioned RPM though because for some reason I thought I needed to be at around 60 - 70 RPM not 45ish so that is great to know.
    Fairly sure the drive system won't be the limiting factor for batch size, but at the moment I was only thinking about max 1kg batch size anyway as initially I will only be roasting for myself and my sisters family and between us we would only use 300 - 400g per week.
    Will definately start at probably even less than 500g though until I get the hang of the whole setup and slowly increase the batch size as I get more confident.

    I had planned on running 2 pieces of stainless angle (on opposite sides) down the length of the barrel for agitation. Is there merit in cutting the 2 pieces in half and staggering them as you have suggested Mal, or will the 2 longer pieces be sufficient? As I have just now collected all the pieces for the build but not yet begun assembly it will be easy to change the design now and if having 4 staggered pieces in the barrel is better overall than the 2.

    Before we begin assembly I will start a thread for it with our design so that people can have a look and maybe offer some suggestions that will make the roaster better/more efficient etc. If I have a chance today or tomorrow I'll get it up here.

    Thanks again all.
    Shannon.
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  5. #5
    Senior Member chokkidog's Avatar
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    What are you doing re airflow? Some sort of extraction is needed to prevent smokiness in the finished coffee (tho' this type of roaster is outside my expertise)
    Proper bean agitation, air flow through the bean mass and a suitable mix of conduction and convection heat transfer are all factors to consider in establishing
    the right charge weight, both upper and lower limits.

    Most commercial drum roasters have a sweet spot of about 80% of the rated capacity where optimum performance and productivity meet. In my 15 kg roaster the lowest batch size is 15% of capacity (decaf) or 20% for 'normal' beans and the largest charge weight is either 12 or 12.75kgs.

    Good on Mal for mentioning drum speed! Too fast and some of the beans may adhere to the drum wall through centrifugal force and roast unevenly or scorch, too slow and an uneven or unresolved roast will result.
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  6. #6
    Senior Member Lyrebird's Avatar
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    Quote Originally Posted by chokkidog View Post
    Good on Mal for mentioning drum speed! Too fast and some of the beans may adhere to the drum wall through centrifugal force and roast unevenly or scorch, too slow and an uneven or unresolved roast will result.
    It is surprising how slow a speed can cause this: for a 250mm diameter drum the critical speed is about 85 RPM.

    BTW that also means that you don't need much power to rotate the drum: in that drum at that speed the maximal power required to rotate a 1 kg bean mass is 11 watts.
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  7. #7
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    Thanks again for the replies. I'm really glad now I have the variable speed controller as I can find the sweet speed spot.

    Hmmmm airflow - not factored in as yet. In all the BBQ roasters I have seen thus far I hadn't noticed extraction in their set up. More research is about to happen.

    I have a 2 burner BBQ with a hood. Quick first thoughts are that I am also building a bean cooler using a thermo fan from my old hot rod. I might try to integrate this by running some ducting from the roaster to the thermo for extraction while roasting and then switching it to the bean cooler when I dump the beans.

    I'll have to make sure I don't have so much extraction that it dissapates the heat from the roaster though.
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  8. #8
    Mal Dimal's Avatar
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    G'day again Shannon...

    Re: considerations for airflow, it isn't just the flow volume but also direction. There is no point, for example, just having the airflow in an axial direction through the drum. To achieve better and more useful results, the airflow should be encouraged to flow in a generally vertical direction, bottom to top... even better if you can then incorporate a chaff collection/exhaust system rather than having it circulate inside the BBQ and catch fire...

    I really don't enjoy smoky flavoured coffee, ruins it completely for me.

    Mal.

  9. #9
    Mal Dimal's Avatar
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    Behmor Coffee Roaster
    Quote Originally Posted by Shannon View Post
    I had planned on running 2 pieces of stainless angle (on opposite sides) down the length of the barrel for agitation. Is there merit in cutting the 2 pieces in half and staggering them as you have suggested Mal, or will the 2 longer pieces be sufficient? As I have just now collected all the pieces for the build but not yet begun assembly it will be easy to change the design now and if having 4 staggered pieces in the barrel is better overall than the 2.
    No need to have a gap between the pieces, I just drew it like that for clarity.
    In my view, it is better to have the lifters angled to the axis, at the very least, to ensure that the beans are transported through the entire length of the drum so that no dead-spots are created that might lead to beans being over/under roasted or simply not being consistently roasted with the rest of the bean mass...

    Mal.



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