Green Coffee Buying Club
Coffee Discussion boards => Hardware & Equipment => Topic started by: Tex on April 16, 2010, 11:43:49 AM
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I'm just at the start gate to designing an infra-red fluid bed roaster in the 2 - 4 lb batch range. Because of the BTU's needed I'll be using gas heaters - but I'll be using an infra-red heat chamber, rather than routing the heat directly to the beans. The fan will be directed over the heat chamber and then into the roast chamber, hopefully giving more precise heat control. I'm visualizing a heat chamber with a t/c mounted on the external surface to keep the chamber at a constant temp.
I'm wondering about bean heat management? It seems that the fan would be the best bean mass temp control - slowing it or speeding it up as needed. Can anyone see a flaw to this process? Boy, does a milowidget to control the fan seem perfect or what?
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If you have not already spend some time at Homeroasters.org in the Building a Roaster section. If you are going to build always lots of help there. I get a lot of help here and there as I am working through my build. Its over my head but some of the builders over there built a speed controler for a vac motor and control there roast with it. Lots of people on both places.
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After 3 passes I'm still not sure what you have in mind... :-X
You are going to blow a fan past IR elements from the top of a roast chamber down into a bean mass?
And the gas burners will be supplemental heat?
:icon_scratch:
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A friend dropped his new BBQ from his truck and smashed it. Since it had one of these heaters (http://www.tecinfrared.com/the_tec_advantage.php), we thought we'd see how it would work for a roaster.
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I'll take my steak Medium Rare.
Rob ::)
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Get the adjustment knobs off the grill also. Let the pid controll the heat with fan speed you could make small adjustments to the gas at times, maybe at 1c you would cut gas in half.
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A friend dropped his new BBQ from his truck and smashed it. Since it had one of these heaters ([url]http://www.tecinfrared.com/the_tec_advantage.php[/url]), we thought we'd see how it would work for a roaster.
After reading and rereading the material on this particular infrared heater I think we'll shoot for a drum roaster. Seems sorta pointless to use heat that's 100% radiant energy and then push air over it to roast beans.
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I've studied this problem in some depth. From the still confusing description is *sounds* like you are going to use the gas/IR heater to heat the air, of a fluid bed. There is nothing wrong with this at all, but hot air is hot air. There is a question of how effectively the IR heater will heat air vs being absorbed and by the chamber. You *may* need to create a baffle coated with lampblack(or candle soot) to transfer the energy from IR heating element to air efficiently.
The problem with fluid bed is that the same hot air you are use to transfer heat is also used to "jostle" the beans. If the air velocity is very much below the terminal velocity of the beans, then they just sit on the lower grid. If the velocity is much above the terminal velocity they get pegged on the upper grid (or go out the flue). One common design approach here is to create a roasting chamber that widens toward the exit side (funnel shaped) or else only has a small/partial inlet and this causes a decrease in air velocity as the chamber cross-section increases. Without that sort of approach you have almost no adjustment range for the air velocity.
Incidentally the terminal velocity of a green coffee bean is about 3000 ft/min (34mph), and *maybe* a bit higher for peaberries. The Vt drops by about 25% as the beans increase in size and lose mass. So to calculate your fan req you need to provide ~3000ft/min times the cross section of the lower aperature (determines the cu.ft/min, cfm).
So aside from the constrained airflow rates fluid bed has a problem in that the hot air passes over the beans and is then lost. That's a very expensive loss of energy. According to "Coffee Technology" (Sivetz & Browne) you need only to apply 250 BTU / lb of greens for roasting. Large commercial units actually use ~460BTU/lb about 55% efficient. I've calculated that a stir-crazy is ~50% efficient and a popper (fluid bed) ~20% efficient. To increase efficiency you need to increase the bean/air contact time. You can't make the bean mass height very large or else you have too much pressure for the fan to operate and jostle the beans. Note the top diagram here and how it increases the bean-air contact time.
http://www.sivetzcoffee.com/Fluid%20bed.htm (http://www.sivetzcoffee.com/Fluid%20bed.htm)
A big advantage of fluid bed is that you aren't recirculating the smokey air over the beans, which improves flavor, but OTOH you lose a lot of heat. One of the german companies produced a fluid bed where most of the exit air is re-routed to the combustion chamber where the combustion removes a lot of the smokey resin-y gunk. There used to be some better diagrams but ...
http://www.nepro-vortex.com/nepro.htm (http://www.nepro-vortex.com/nepro.htm)
FWIW a 22000BTU/hr grill produces 367 BTU/minute so you'd probably like to get the system efficiency up around 40% so you can roast 4 lbs in under 7 minutes. If the efficiency was as low as a popcorn popper you're 4lb target would take almost 14 minutes - too long.
[silly units error corrected in re-edit - see posts below]
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Thanks for the detailed information!
The original plan was to place the IR unit inside a heat chamber and then pass air over the chamber. After thinking about how an IR works, we've decided this didn't make the best use of the heat source. We're now considering using the IR unit for a drum roaster instead.
BTW: The TEC IR unit (http://www.tecinfrared.com/the_tec_advantage.php) we have uses a glass plate over the IR unit so almost all of the heat is radiant energy. That's one of the reasons we wanted to use this IR unit - no hydro carbon emissions passing over the beans should give us a cleaner roasted bean.
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3000 ft/sec ???
That's not a bean thats a bullet!!
30 MPH would be about 44 ft/sec
Aaron
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3000 ft/sec ???
That's not a bean thats a bullet!!
30 MPH would be about 44 ft/sec
Aaron
Should have been 3000 ft/minute (not per sec) . An odd but convenient unit since fans are rated in cfm un the US.
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Thanks for the detailed information!
The original plan was to place the IR unit inside a heat chamber and then pass air over the chamber. After thinking about how an IR works, we've decided this didn't make the best use of the heat source. We're now considering using the IR unit for a drum roaster instead.
Agreed. You'll get better efficiency by directly applying the IR to the bean surface. Most metals continue to reflect well light down into the infrared, so you can mirror the energy back at the beans from say aluminium sheet.
BTW: The TEC IR unit ([url]http://www.tecinfrared.com/the_tec_advantage.php[/url]) we have uses a glass plate over the IR unit so almost all of the heat is radiant energy. That's one of the reasons we wanted to use this IR unit - no hydro carbon emissions passing over the beans should give us a cleaner roasted bean.
You need the glass to protect the heating elements too I suspect. It sounds like the right course, tho' FWIW the smoke from burning chaff and bean oxidation issurely the greater flavor negative than any NG by-products.. If you are heating bean surface by IR then you don't need/want much airflow, just enough to carry off the smoke so mechanical stirring (like a drum) and a modest airflow is what you'll need.