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12 kg Roaster Design
Axel
Hi all,
I'm building a drum roaster to handle about 20-30lbs.
still trying to decide between a double wall drum made of two 14gauge carbon steel layers with 1/2" air space or one layer of 11gauge. the outside diameter of whichever one I decide on is planned to be 24"x30".
2"shaft on high heat bearings.
Approximately 140,000BTU's(to much,too little?)

anyways...

right now I am designing the exhaust system.
any blower/motor units that can be recommended for my application?
I've seen a lot on eBay but the ones that are "high temperature rated" have 450?F as the maximum.
should the blower be built to withstand a chimney fire(1,400?F)?
I am planning to use 18gauge welded pipe ~6"diameter to run the exhaust to the
chaff collection. probably regular wood stove pipe(maybe 8") after that for the vertical.
Edited by allenb on 07/30/2015 9:58 PM
 
allenb
Welcome to HRO!
This must be the largest drum roaster home-build we've seen to date!

In case you haven't seen examples of probat's double wall drum construction and for others benefit I'll go over what I know. The front 4" or so of the drum is a cast iron hub with integral spokes. The hub is made with a rabbeted ledge on the ID of its rear portion to allow affixing the inner sheet metal skin to the hub. The outer sheet metal skin wraps around the OD of the rear end of the hub making a sandwich of sorts. The rear plate of the drum is fabricated to hold the two skins in a similar fashion. From my understanding, the sheet metal is not very thick and doesn't need to be due to the air gap. My guess is they're using more like 24 gauge. To me, using anything thicker wouldn't be of any benefit and would be a pain to work with.

From memory, the L12 drum is somewhere around 22" diameter so going 24 should work for a 20 to 30 lb capacity especially if you're planning on a 30" length and should give a little extra headroom. This is all off the top of my head so if it's critical to hit the 30 lb batch size you may want to have someone take a real measurement of an existing shop roaster.

On shaft size, since there's no overhung load, a 1" and definitely no larger than a 1 1/4" will be more than adequate unless you'll be shipping the completed roaster via UPS and then I'd go for a 5" diameter shaft and insure it for double it's value.

My thumbnail calcs come up with a 120 kbtu burner but having an extra 20 for headroom can't hurt at all. I've seen a couple of 12 kilo probats and another brand sporting a 140 kbtu so you're not out of line.

On the blower on your cyclone, since it should always be pulling a portion of your cooling tray air allowing a mix of room ambient temperature air and roasting exhaust, the blower doesn't need to be a high temp blower at all. With my gas fired 1 lb drum roaster, by blending both cooling tray and roasting exhaust together, the blower isn't seeing hotter than around 130 F anytime during a roast. If you end up designing for a two fan roaster then allow a way to introduce ambient air into the mix to keep the temp entering the blower at a reasonable number. But, don't allow the temp to go too low to avoid hitting dew point which would allow volatiles to condense on the cyclone and fan parts.



Allen
allenb attached the following image:
7533_probatdrum.jpg

1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
allenb
I want to bring up something I've been puzzled about with the double wall drum construction specifically with Probat models. Regardless of thin or thick inner wall sheet metal, it appears to me they're brazing or welding the return vane struts to the sheet metal and I can't imagine how that would be very solid unless there was additional metal backing them up in between the skins.

I'd sure like to hear from any Probat experts for shedding light on this subject.

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

Your reply is much more than I was expecting.
there seems to be very little info out there on building a roaster of this size.

I'm still thinking about how best to make the hub,spokes,and ledge without having it cast.
Do you think making a double wall is worth the extra trouble?

A 2" shaft instead of 1.25" is mainly so that there is less vibration and noise.
I'd like to hear the crackling of the beans as best as possible...

since I'm planning to use a separate blower for the cooling tray, where would it be best to introduce outside air? closer to the roaster exit or closer to the blower?
My original plan was to use a sliding door to adjust vacuum as a way of draft control.
is 900CFM too much?
 
allenb
The question on whether double wall construction is worth the trouble is a tough one to answer. I can't see why Probat would have gone through so much extra engineering and expense if it didn't provide an increase in roast quality. The best tasting coffee I've ever cupped was roasted either in a Probat shop roaster with double walled drum or in a Gothot belly dump. Both of these roasters have something in common. Probat has the double walled drum limiting the amount of flame to drum transfer and the Gothot belly dump is a side firing arrangement which also limits flame to drum transfer and both depend more on convection versus radiant from the drum surface. If I were going through the time and effort to build a larger shop roaster from the ground up I would be inclined to go for the double walled drum.

On drum shaft diameter. I'm certain you will not have more vibration/noise using less than a 2" diameter shaft.

On where to introduce outside air? Some builders have incorporated an adjustable opening in the duct just after it leaves the fill funnel heading back to the cyclone. Since the blower is typically attached directly to the top of the cyclone it would be difficult to add an opening to the blower inlet unless you added a short section of duct between them.

I'm not sure how many cfm are typical for drum exhaust flow in a 12 kilo shop roaster but I wouldn't think they would need more than 200. More than that would most likely over dilute the heat from your burner and require a much larger btu output. You'll obviously want to incorporate a means of adjusting exhaust airflow regardless of blower capacity.

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

Quote

allenb wrote:
The best tasting coffee I've ever cupped was roasted either in a Probat shop roaster with double walled drum or in a Gothot belly dump. Both of these roasters have something in common. Probat has the double walled drum limiting the amount of flame to drum transfer and the Gothot belly dump is a side firing arrangement which also limits flame to drum transfer and both depend more on convection versus radiant from the drum surface.
Allen


is there a diagram somewhere of how a Gothot belly dump works?
I remember seeing a roasting video with Inteligentcia roasters and I think they were using one.
I was very intrigued and still am.

If I was to use flat stock for the spokes(cutting it on my 4'x8' CNC plasma table), what thickness sheet should I use? I'm thinking 3/8" with 3/8" flat stock welded on perpendicular would make them ridged.

Btw, my CNC table was a project kind of like this one, it's probably overbuilt...
I've got a Hypertherm45 hooked up for cutting(pierces up to 1/2") but recently also started using a small router (with feedback RPM control) to machine aluminum.

is a 1/2" gap between drum layers ideal?
is 24gauge the heaviest you would go with for each double wall layer? seems very thin to me.
If I remember correctly.. someone i spoke with told me 18gauge is probably what the L12 has for wall material.
 
Axel
How much horsepower should I have for the cooling tray paddles?
what RPM should the paddles spin at?
what diameter is the cooling tray on a Probat L12?
Edited by Axel on 07/26/2015 12:39 PM
 
Axel
skip to minute 3:17 of this video:

[video]www.youtube.com/watch?v=M2hOPWNVdN4[/video]

www.youtube.com/w...2hOPWNVdN4

I used a stop watch and got 21RPM on the cooling tray paddle rotation.
Edited by JackH on 07/26/2015 6:41 PM
 
allenb

Quote

is there a diagram somewhere of how a Gothot belly dump works?


Not that I'm aware of.The wheel at the front of the roaster has a threaded rod extending through the hollow drum shaft and as it's turned, slides a sheet metal door on the side of the drum open and closed. The larger models have a little more complicated mechanism that doesn't require you to spin the wheel throughout an open/close cycle.

Quote

If I was to use flat stock for the spokes(cutting it on my 4'x8' CNC plasma table), what thickness sheet should I use? I'm thinking 3/8" with 3/8" flat stock welded on perpendicular would make them ridged.


For a 24" drum I wouldn't go less than 3/8" x 1" if you'll be welding them. The rear hub can take care of handling forward/reverse thrust of the drum so the front spokes would only see a radial load. Another option is to use three 1/2" rods and thread the ends with lock washers and nuts. Drill and tap the drum shaft hub to accept the other ends. This allows centering the drum to the shaft.

Quote

is a 1/2" gap between drum layers ideal?
is 24gauge the heaviest you would go with for each double wall layer? seems very thin to me.
If I remember correctly.. someone i spoke with told me 18gauge is probably what the L12 has for wall material.


If you can roll 18 gauge and work with it then this would definitely give you a beefier drum and should work fine. Even with using the thicker gauge, I would mount the return vanes off of spokes coming off of the drum shaft rather than mounting the spokes to the inner drum wall. I'm not sure what the optimum spacing is between the two layers. I don't think there's much of a gap and if you allow too much, you may be reducing the drums ability to transfer enough heat to the beans and might rely too much on convection which would alter the balance of the two heat transfer modes. If it were me I wouldn't go more than 1/4".

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

Quote

How much horsepower should I have for the cooling tray paddles?
what RPM should the paddles spin at?
what diameter is the cooling tray on a Probat L12?


I can't imagine paddling 20 to 25 lbs of roasted coffee in a circle would need any more than a 1/10 hp gearmotor.

In watching the video I came up with 15 rpm (one rev every 4 seconds)

Not sure on L12 tray diameter. I would calculate a diameter that would hold 1 cubic foot of roasted coffee (1 cubic foot of roasted beans is around 28 lbs) at no more than a 2" bed depth.

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

Quote

allenb wrote:
I can't imagine paddling 20 to 25 lbs of roasted coffee in a circle would need any more than a 1/10 hp gearmotor.

In watching the video I came up with 15 rpm (one rev every 4 seconds)

Allen

great catch,
I was calculating 100sec in a minute haha!
but I counted 5 revolutions in 23 sec for more precision (so with 60sec in a minute this time) 13rpm
so with a 3/4hp motor 17rpm should work? that's the closest to 13rpm that the two gear reducers I'm looking at can get to together.

Quote

allenb wrote:

Another option is to use three 1/2" rods and thread the ends with lock washers and nuts. Drill and tap the drum shaft hub to accept the other ends. This allows centering the drum to the shaft.

Allen

planning to use a keyed shaft so that I don't have to drill into it.(don't like the idea of drilling into the shaft)
But I like your idea of centering, maybe a keyed sleeve with drilled/threaded holes..

for the cooling tray I'm thinking a bit more horsepower if the paddle arms are long and four of them, seems like there could be some torque with that.
I read that the faster the cooling the better. so I'm thinking a 3' wide cooling tray?

I think I read that one manufacturer is using 1/3hp for cooling tray paddles in their 12k roaster

ordered a 1hp motor for the drum, planning to use it with VFD for fine tuning the drum speed. same with the 1/5hp blower motor.

maybe I should make the drum bigger?
I read somewhere that it's best to go with the biggest roaster you can. you will want to upsize if you don't...
Edited by Axel on 07/27/2015 9:04 AM
 
Axel
what does the bean distribution look like during a roast?
are there any beans in the very back of the drum?
if looking at the side of the drum(with X-ray vision), where should the highest and lowest concentration of beans be?
 
Axel
would using a high pressure blower be ok for the exhaust?
would it effect the dew point in the blower?
 
allenb

Quote

planning to use a keyed shaft so that I don't have to drill into it.(don't like the idea of drilling into the shaft)
But I like your idea of centering, maybe a keyed sleeve with drilled/threaded holes..


I had stated Drill and tap the drum shaft hub to accept the other ends. This allows centering the drum to the shaft.

Drum shaft hub was probably not the best words to describe the central part of the front hub assembly that the shaft passes through. Yes, don't drill and tap the shaft as this would obviously weaken it severely if tapping 1/2" holes.

You mention possibly going bigger on the drum for future upsizing potential. How large of a drum to go with depends on what your smallest batch size will be. Too small of a batch in a given roaster prevents optimal bean contact with your bean temp sensor and causes you to fly blind.

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

Quote

Axel wrote:

what does the bean distribution look like during a roast?
are there any beans in the very back of the drum?
if looking at the side of the drum(with X-ray vision), where should the highest and lowest concentration of beans be?


I've had the opportunity as many others here have, to see bean distribution while testing different vane configurations using clear acrylic drums. Bean distribution can vary quite a bit depending on how steep or shallow your forward vanes are pitched as well as how large and how close to the forward vanes the reverse vanes are. From my experience, forward vanes pitched at 45 degrees can cause excessive stacking at the front of the drum while vanes closer to 30 degrees or a little less lessen the tendency while still able to empty the drum quickly. If the reverse vanes are sufficiently large compared to the forward vanes and located immediately above or next to the forward vanes, you'll find just about as much backward bean motion as forward and very little stacking at the front will occur.
This will obviously lessen the tendency for there to be uneven distribution on the bottom of the drum.

What does the bean distribution look like?

If properly executing the vanes it will look like a swarm of bees from the bottom to around 3/4 high with a fairly even small layer of higher density beans across the bottom.

Unfortunately, large reverse vanes in a small drum makes it difficult and in some cases impossible to incorporate a bean trier as there will be very little open area between the inner edge of the reverse vanes and the shaft. A solution to this is to use a large diameter, hollow drum shaft for the trier. The hollow shaft has openings in it to allow coffee to drop into the trier and the openings go far enough back to prevent the trier from hanging up on beans while reinserting it. This requires some engineering challenges and much larger diameter front bearing but might be worth the trouble.

Beans in the back of the drum?

If vanes are done correctly, drum diameter and length are equal and batch size is optimal for the drum size then you'll find the bean depth to be fairly even from front to rear but will always be a little deeper at the front.

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

Quote

Axel wrote:

would using a high pressure blower be ok for the exhaust?
would it effect the dew point in the blower?


Low, medium or high pressure shouldn't make any difference as long as you have a way to control flow. Obviously, if the duct leading to the fan is smaller than optimum then a higher pressure fan is preferred.

As far as I know, dewpoint/condensation shouldn't be affected by static pressure potential of your fan.

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

Quote

allenb wrote:

If vanes are done correctly, drum diameter and length are equal and batch size is optimal for the drum size then you'll find the bean depth to be fairly even from front to rear but will always be a little deeper at the front.

Allen


does probat use a 1 to 1 drum size ratio?

is there a video of the clear drum presentation?
Edited by Axel on 07/29/2015 2:25 PM
 
allenb
All of the Probats I've seen are 1:1 or at least very close to that from all the ones I've been able to take a peek at.

I've never seen a video of an acrylic drum showing a side view to see bean distribution but who knows, lots of things I haven't seen yet. You should toss one together and do some experimentation using various vane pitch angles and various reverse vanes to pick out best practice. When I did my experiments I found acrylic tube remnants at our local plastic supply and fabrication shop and got it for next to nothing. I used thin aluminum bent into an angle for vanes and mounted at various angles. It's actually a fun part of the build seeing the beans react to differing vane configurations.

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

Quote

allenb wrote:

Bean distribution can vary quite a bit depending on how steep or shallow your forward vanes are pitched as well as how large and how close to the forward vanes the reverse vanes are. From my experience, forward vanes pitched at 45 degrees can cause excessive stacking at the front of the drum while vanes closer to 30 degrees or a little less lessen the tendency while still able to empty the drum quickly. If the reverse vanes are sufficiently large compared to the forward vanes and located immediately above or next to the forward vanes, you'll find just about as much backward bean motion as forward and very little stacking at the front will occur.
This will obviously lessen the tendency for there to be uneven distribution on the bottom of the drum.

If properly executing the vanes it will look like a swarm of bees from the bottom to around 3/4 high with a fairly even small layer of higher density beans across the bottom.
Allen

how does this relate to the "Spiral Flight Development" formulas?
https://techboard...opment.pdf

Quote

allenb wrote:
large reverse vanes in a small drum makes it difficult and in some cases impossible to incorporate a bean trier as there will be very little open area between the inner edge of the reverse vanes and the shaft. A solution to this is to use a large diameter, hollow drum shaft for the trier. The hollow shaft has openings in it to allow coffee to drop into the trier and the openings go far enough back to prevent the trier from hanging up on beans while reinserting it. This requires some engineering challenges and much larger diameter front bearing but might be worth the trouble.

If vanes are done correctly, drum diameter and length are equal and batch size is optimal for the drum size then you'll find the bean depth to be fairly even from front to rear but will always be a little deeper at the front.

What do you mean(more specifically) by "small drum"?

Quote

allenb wrote:
All of the Probats I've seen are 1:1 or at least very close to that from all the ones I've been able to take a peek at.


do the Probat L12 and P25 drums also have 1:1 heigh:length ratios?

My original design was for a 24":30" drum because I found some info that the outside of the L12 is about that size.
I'm beginning to seriously rethink my drum size...

Would a 24"x24" drum be considered small(in terms of needing a hollow shaft)?
Edited by Axel on 07/29/2015 4:05 PM
 
Axel

Quote

Axel wrote:


My original design was for a 24":30" drum because I found some info that the outside of the L12 is about that size.


meant to say that I found info stating that the outside of the L12 drum is 24"x30"
 
allenb
If my mental calculations are correct, using Pi x diameter/4 in steps 1 and 2 instead of Pi x diameter will give you a vane pitch that's less than 45 degrees but I'm not sure how much less. The math needed to adjust the formulas to end up with x degree pitch is over my head. My guess is if we use Pi x diameter/5, we'd be pretty close to a 30 degree pitch but someone would need to give it a try to verify it.

By small drum I'm thinking of a drum less than around 14" diameter but to be sure, one needs to break out the graph paper and draw the drum with vanes and see if there's room between front bearing flange and reverse vane for a trier to comfortably fit in.

I've only seen the L12 in person and photos of other drums but I'll bet all models will be similar. A 24 x 24 would not be considered small in relation to needing another arrangement for the trier.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
Axel
I'm probably going to bolt the vanes to the drum wall and just wing the pitch at 40 degrees(I like the number 40). Is that for both the horizontal and vertical angle?
how do I determine the height of the vane from the wall?

after some more calculation and preliminary cut drawings for my CNC table, it looks like i'll be going with a 22"outside diameter drum. much more efficiency of material. still thinking of making it 24"long but not sure if that would be a benefit.

Also I haven't been able to find any conclusive info on wall thicknesses for double wall drums of this size. waiting to hear back from Probat...
Thinking about using 18 or 20Gauge with 1/4" air space.

the burners are 90,000BTU each
I already have one since someone was selling an extra for a great price
 
allenb

Quote

Axel wrote:

I'm probably going to bolt the vanes to the drum wall and just wing the pitch at 40 degrees(I like the number 40). Is that for both the horizontal and vertical angle?
how do I determine the height of the vane from the wall?

after some more calculation and preliminary cut drawings for my CNC table, it looks like i'll be going with a 22"outside diameter drum. much more efficiency of material. still thinking of making it 24"long but not sure if that would be a benefit.

Also I haven't been able to find any conclusive info on wall thicknesses for double wall drums of this size. waiting to hear back from Probat...
Thinking about using 18 or 20Gauge with 1/4" air space.

the burners are 90,000BTU each
I already have one since someone was selling an extra for a great price


40 degrees should be fine. The vane angle is based on the shaft line. So, shaft is 0 degrees, front plate of the roaster is 90 degrees.

From my calculations, a 22 x 22 will be sufficient for a 12 to 13.5 kilo batch.

From my experience, getting a response from Probat on questions related to design aspects of their roasters that's not already published is next to impossible unless you know someone on the inside. I think you'll be good with the 1/4" airspace gap.

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

Quote

how do I determine the height of the vane from the wall?


From memory, the forward vanes on the L12 were around 2 to 2 1/4" tall and many of the older drum roasters used wider reverse vanes compared to the forward which makes sense if you consider they are grabbing less coffee at the height they're at. If I were designing a 12 kilo drum roaster I would use around 2 1/4" for the forward and 2 1/2 or 2 3/4 for the reverse vanes. I would also place the reverse no more than 1/4" or so above the forward vanes for good mixing.

Different manufacturers differ widely on vane shape, size and placement.

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

Any last thoughts on drum wall thickness?
I'm about to order the sheet metal for the two layers.
aiming at 18Gauge..
there should be plenty of BTUs (180,000) so maybe even 16gauge would work, however i'm worried that the responsiveness when lowering the temperature would be delayed with the thicker metal.
The benefit of thicker layers seems to be that the temperature drop would be lessened when the beans go in.

in the continued design optimization of the sheet and plate metals I'm heading back towards a 24" outside diameter and 24" depth.
 
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