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Heat Gun Driven Fluid Bed Roaster
I posted about my initial foray into roasting in the "First Roasters" forum and this is the follow-up. As I mentioned in that first post, I got tired of the small batch size, the individuality of the roasters and the lack of control. So, I decided to build my own. My going-in objectives were to be able to roast at least a cup of green beans per batch and in an 8-10 minute time frame with good control over air flow and temperature. I also wanted to build it inexpenively, but was willing to spend $200 or so. I had no interest in automation or computer control. I like to be very involved in the process, so I knew it would be a "manual" roaster.

I had read about roasting beans with a heat gun in a stainless steel dog bowl. I even tried it, but it was hard to do a good job and WAY too manual. However, I liked the idea of using heat guns as a heat source, so I found these Porter-Cable adjustable, two-speed heat guns for about $40. I then built a manifold out of black iron pipe and wrapped it in fiberglass manifold tape to insulate it. The base of the roaster is just 3/4" plywood covered by cement board for insulation. There are three heat gun ports in the manifold, though it turns out I really only needed two. There is also a cold air inlet that is supplied by a small Shop-vac fed through a split valve arrangement to help control air flow. There is a thermocouple in the roaster air inlet to monitor the incoming air temperature. The roasting chambers are old Uno-Vac stainless steel vacuum bottles with the bottom cut off and a coarse stainless steel screen in the neck to keep the beans in place. These work well because they are double-walled and they still have the handles on them. I also put a small 3" elbow in the top of the roaster to direct chaff and keep the beans from hopping out when they are fluidized.

The heat guns are just press-fit into the fittings in the manifold and there is a ring of gasket material on the barrel of the heat guns to prevent any blowback. There is a crude wooden fixture that holds the heat guns in place. It is functional, but really needs to be replaced with something better. The roasting chambers just sit in the reducer on the end of the manifold. There is no need to clamp them in place.

I had to experiment a bit to get airflows right. I turn two heat guns on to high blower speed and adjust temperature as needed. The air flow from the Shop-Vac is controlled with the split valve.

I can roast up to 500+ F, but I usually roast around 450 F. Once the roaster gets to steady state, it holds very constant within about 4-5 degrees which is perfectly fine. When a batch is finished, I just grab the handle on the Uno-Vac and pull the elbow out of the top and pour the beans onto a metal sheet pan to cool. I have roasted more than 50 batches so far and the results have been great. Batch times are about 8 minutes and I can easily roast 1 cup of green beans per batch. I could probably push the roast size a little more, but I am content to leave it at 1 cup for now. The only thing I would like to change is to get a slightly smaller mesh for the screen in the roast chamber to prevent any beans from falling through. I'd be happy to answer any questions you have about my roaster. Thanks for reading.
coffeeforblood attached the following images:
img_0070.jpg img_0071.jpg img_0072.jpg img_0073_1.jpg img_0074_1.jpg img_0075_1.jpg img_0076_1.jpg
That is a pretty ingenious setup. My only question is have you noticed any negative effects of the vacuums positive air pressure affecting the heat gun outputs? I would think it would reduce the heat gun air pressure output and possibly fry the heat gun elements.

"If this is coffee, please bring me some tea but if this is tea, please bring me some coffee." ~Abraham Lincoln
Len - you have hit on the main unanswered question for me - how long are the heat guns going to last? This is garage engineering at its finest, so let me assure you that no fluid dynamics calculations were done in the design phase! My thinking is that "the path of least resistance" for air flow is through the roast chamber. Is there some possible back-pressure in the system? Almost certainly, but I am hoping it is low. Also, most of the Shop-Vac output is split to waste. I am using only enough cold air to supplement the heat guns in order to fluidize the beans. I'd be much more concerned if I was directing the full flow into the system. So, your guess is as good as mine. So far, I have roasted more than 50 batches and the heat guns have not shown any signs of distress, but they could just fall off a cliff at some point. I have not checked, but I would imagine that the elements are replaceable. If I can get 200 batches out of a heating element, I will be ok with that.
If you have done 50 batches so far it would seem you do not have the backpressure I envisioned. So as the old saying goes, if it ain't broke don't fix it! ThumbsUp

"If this is coffee, please bring me some tea but if this is tea, please bring me some coffee." ~Abraham Lincoln
I am also depending on a Venturi effect wherein the cold air rushing past the heat gun outlets will "pull" the hot air out. Who knows if that is true, but that's my story and I'm sticking to it. Ha ha.
Edited by coffeeforblood on 03/07/2019 8:26 AM

I am also curious how much air flow you are getting through each heat gun compared to it's usual air flow.

Definitely makes it easier to replace a heatgun/element.
The ease of interchanging heat sources was actually a secondary design criterion. Burn up a heat gun? Yank it out and throw in another one and keep roasting!
Thanks so much for taking the time to post your narrative and all the great photos. What you did will be a big help to me in thinking about a design. It is always fascinating to me to see how different people approach the general objective in accord with different preferences. In my brief journey, I've learned how coffee is and area where personal preferences are prevalent. While you specifically wanted a manual system for hands-on operation, I've lately been seeking to automate so results will be as reproducible as possible and I can forget the roasting process and focus instead on single origin effects etc. But I can clearly see the issues you identified in the ways you decided to address them, and this will no doubt save me a lot of the error in the trial-and-error design process. Thanks again !

good post
So many beans; so little time....
John - I am even more into wine than coffee and your maxim of personal preference holds true there as well. I once met Michael Mondavi (son of the California wine icon Robert Mondavi) and he very explicitly said "drink what you like. Don't let anyone tell you what you "should" drink." Amen.
John - thought you would appreciate this as well:

"There are no standards of taste in wine, cigars, poetry, prose, etc. Each man's own taste is the standard and a majority vote cannot decide for him or in any slightest degree affect the supremacy of his own standard." - Mark Twain, 1895
Nice build! Heat guns all have a flow problem and the system will require a source of pressured air,.such as your shop vac. Heat guns,typically, have 1200-1500 watts elements and in my case are rated to deliver 1000F at 23 CFM. Since 23 CFM is NOT enough flow to fluidize my bed of coffee beans (500 grams). I, like most home-roaster builders before me start with a source of air that is much higher (100 CFM) than a heat gun and install the heat gun element heater between the air source and the roast chamber.
My primary concern with your roaster is understanding the capacity. If one cup is 100 grams then you must be wasting a lot of hot I have with this missive..
No oil on my beans...
You are probably right about wasting hot air. I know I could lift more beans easily, but the height of my column is the limitation. I would have beans reaching escape velocity if I put many more in and increased air flow at all. I could probably get another piece of straight 3" pipe to "extend" the column and roast more beans per batch, but that would undoubtedly increase back pressure in the system with unknown consequences. I might give it a try just to see how (or if) it works.
Out of curiosity, I weighed a cup of green coffee beans. Turns out that it weighed 180 grams. I am sure that the weight varies with bean identity, but it is a little more than we were thinking.
Since there is .035274 ounce per gram. I assume you are using a 6.35 ounce cup. I was using a 4 ounce cup. A typical coffee maker is marked in 5 ounce cups. A cup is not always the same..
No oil on my beans...
Actually, I used a standard kitchen measuring cup that would be used in cooking, since this is the way I dose beans into my roaster. So, I measured 8 ounces of beans by volume and then weighed them on a digital scale.
Eight ounces of greens, by volume, usually weighs in at 85 grams depending on %M. At least that's what I get when I fill my moisture tester..My 100 grams per cup statement, was very wrong and miss-leading, because there are four different 'cup' sizes used with coffee.
No oil on my beans...


oldgearhead wrote:

Eight ounces of greens, by volume, usually weighs in at 85 grams depending on %M. At least that's what I get when I fill my moisture tester..My 100 grams per cup statement, was very wrong and miss-leading, because there are four different 'cup' sizes used with coffee.

I just weighed a level measuring cup of yirgacheffe. It weighs 160 gm. I then filled the cup with water. That weighs 238 grams, so we know my cup holds 8 oz by volume.
So many beans; so little time....
oops! Chicago John is correct. My %M tester uses one-half cup (85 grams)..
No oil on my beans...
I have now roasted over 100 batches with this rig and performance continues to be very steady. The heat guns appear to be working fine and are currently showing no signs that they are having problems. I did figure out the hard way that one 15 amp circuit will not supply a heat gun plus a Shop-Vac. Oops! I figured that was probably too big of a load and I confirmed my suspicion. I am about to order some new SS screen material though because I am constantly having beans fall through or get stuck in the current mesh size. It's not a show stopper, just an annoyance, but it's easy to fix.
I replaced the #4 stainless steel mesh screen in the bottom of my roasting chambers with #5 screen and it worked just as I hoped it would. There was no discernible impact on roasting time or efficiency and I didn't have any beans get stuck in the mesh or have any beans fall through the screen. Happy, happy, joy, joy!

My next experiment will be extending the height of my column to see if my roasting rig can handle up to 2 cups of green beans (ca. 360 grams) per batch. This might be a little more of an issue because the back pressure will increase proportionally with bed depth and this could have unanticipated consequences. However, the only way to know for sure is to run the experiment! I just need to wait for a slightly warmer day.
I tried roasting two cups (about 360 g) of green beans today, but the experiment wasn't too successful. I, of course, had to increase airflow to lift the taller column of beans, but then, it was hard to keep the temperature high enough. It seemed as though the heat guns were not able to keep up. I was able to roast 270 g of beans pretty well, but that might be about the limit of this roaster. It was pretty cold and windy today, so maybe it wasn't the best day to try it. I will have to give it another shot on a warmer and calmer day.
It's been about a year since my original post, so I thought an update is in order. I have continued to use my roaster regularly and have roasted more than 200 batches (180 grams each). In fact, I just finished about 20 batches a couple of days ago. So far, results continues to be excellent. I am using the same two heat guns that I started with and have seen no noticeable loss of heating performance. I have confirmed that 1 cup of green beans per roast is the optimal size for this set-up. I have settled on a roast time of 9-10 minutes to reach Full City + for most of what I roast with an inlet air temp of 450F - 455F. If and when I have a heat gun failure, I will let you all know.
Time once again for an update. It is now a little over two years and many hundreds of batches of coffee and I can finally report that my heat guns are beginning to fail. I had completed a roast session in early November with no issues, but I went to roast beans today and uh-oh, I could only get to about 320 F. I pulled the third heat gun out of its port and tried using it and even though it had never been used for roasting, it was barely warm. I have to think that it was damaged by having hot air forced back through it for a couple of years. Fortunately, I was able to find a couple of the Porter-Cable heat guns that I use for $20 each (new) on EBay, so I will be back in business soon. Then I can worry about repairing the failing heat guns (or recycling them if repair is not an option).
I have a little more information to share. The third heat gun was indeed dead and the heating element is not replaceable (as per Porter-Cable factory rep), so it went to the recycler. It turns out though that my two workhorse heat guns that I?ve been using since the beginning of the project are fine! They are still going strong. I am roasting even as I write this. Apparently, it was just an extra-cold day and I was being impatient. I did cap the port for the third heat gun since it is not necessary and I FINALLY built my wooden screw hold-downs for the heat guns, so they are securely in their ports now with no wiggle and less leakage. I now have three spare heat guns in the shop, so I should be in business for a long time to come.
Edited by coffeeforblood on 02/27/2019 5:50 PM
I run heat guns cold (mine has a cold/hot switch) for a while after running. I find it prolongs the life of the heating element. Lets it cool down gradually. I have some heat guns that are 20 years old that I use for melting heat shrink tubing and other electronic repair tasks.

KKTO Roaster.
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