Rocket stove for a coffee roaster?

Hey everyone,

This is my first post, but I've been lurking on and off for some time now. It's a GREAT resource, and I find that I lose hours at a time reading up on this stuff. Thanks for all the information.

I'm starting to build some prototype rocket stoves (not RMH yet), with the end goal of replacing the old tired propane grill body that supplies the heat to my coffee roaster. I'm currently playing around with a 20-gallon galvanized trash can, some 6" black stove pipe (it's the only size I can find readily around here, at least so far), and a bunch of perlite I grabbed from the local nursery. I've already measured temps > 1000F at the top of my very short stack (20" from the bend) and I can see that I need a taller stack already.

1000F seems like it's plenty enough for coffee roasting, IF I can translate into bean temp with reasonable efficiency. I need to end my roast around 470F, hopefully with up to 4 lbs of beans. I'm using a rotisserie-style drum with vanes to stir the beans, but I digress. I'm looking at a taller stack to try to make it burn cleaner. I figure I can always vent in some cooler air using a different duct and fan to get to the temp I need. But, I digress...

My main questions are:

1. I need to channel the flue gases into more of a rectangular shape to hit more of the spinning drum. Will it work for me to split the exhaust into, say, 9 1-inch pipes and create a sort of manifold that I could reshape into a line?

2. Looking over the setup above, am I taking any wrong turns, or missing something important? I'm still learning how all of this stuff works, and how to make it work optimally...

3. I'm looking around for something that will hold up better than stove pipe, assuming this thin-walled stuff may not have a long lifetime in such a harsh environment. I'm currently checking out refractory supplies for when I get the prototype stove working and I'm ready to create something more permanent...is there a really GOOD material that I should be checking out?

John Kc : no! this will not work ! when you divide your combined air flow below 4'' you suffer from a condition called laminar flow, it should be called laminar non-flow !

Basically, there is always a thin layer of still air air the near molecular level of a surface ,when surrounded with a stream of hot, thermally active gases this barrier is
vigorously scrubbed at, and minimized. To use a military term when you divide your forces, you increase your resistance !

Having said that I don't know what will happen if you tried actively pumping your gases past the container that holds your beans, not much I think.

I hope I do not sound to snarky, to say - the Rocket stove is capable of providing all the heat you will ever need, see what has already been invented
out there, no use reinventing the wheel, tho you might look at peanut roasters too.

For the good of the Craft, be safe, keep warm PYRO AL












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I know very little about roasting coffee, but this link might give you some ideas...

http://www.aprovecho.org/public/Publications/Still-The.Winiarski.Wood.Fired.Agricultural.Food.Dryer.pdf

allen, thanks for the warning...it's good that I asked before diving in, I guess.

Is there any way to overcome the laminar flow problem, maybe by increasing the number of pipes in the outflow well above the equivalent area of the 6" stack? Say, up to 12" or even 18" of total area? Or, is the division into separate channels going to cause problems too?

I've read that wood has a lot more BTUs to offer than, say, propane. My current roaster (propane) tops out each time I use it, and it's basically useless in a Kansas winter (never reaches 475F). I think I can get partway there using decent insulation, but was really hoping to find a way to use wood instead of propane. And the rocket stove seemed to offer the most in terms of quick warm up and limited wood use.

Well, even if it won't work, playing with these prototypes has definitely got the gears turning in my head!

Maybe the answer is to try to run the drum vertically with an auger inside...that's a much more complicated build though, in terms of bearings (that will be in the heat stream) and the auger design.

John Kc : as always, the B.T.U. that you save, is the cheapest B.T. U. that you will ever have, If your present coffee roaster is still in its original location,
then the H.V.A.C., or air plumbing system was designed in the days of cheap energy to pump the smell of Roasting Coffee outside the building to tempt
passer byes, and create walk in business.

If you have moved the Roaster, how much ductwork have you taken with you ! I would google info on your roaster, and talk with any one at any trade
organization, specifically about your machine, and Roaster Air Handling Equipment,

It sounds as though you are saying that you are venting the high temperature products of burning propane - carbon dioxide and water vapor directly
onto the coffee beans. and want to do the same thing with the products of combustion from a rocket stove.

While it is possible to visually check the propane flame and determine that you are burning the propane cleanly, efficiently and safely with out producing
any Carbon Monoxide, you should have a couple of CO detectors and check them daily the same as you check your propane flame to be blue on blue with
no yellow flames and no orange sparks !

If you have a heat exchanger so that you are heating air to pass over and roast your beans then basically I am sorry that I misunderstood your description.
It was your use of the term 'Flue gases' that lead me to believe you were trying to use the propane fired exhaust stream for roasting the beans, I don't know
enough about roasting beans but it would seem to me that the high moisture content of the propane exhaust gas stream would work against roasting beans

Every thing I have said about roasting beans with propane goes double for using the Rocket Stove to roast beans,without using a heat exchanger I would not
attempt to roast beans for sale to the permies. Big AL