Convection Roasting

[Jim Schulman]

Most specialty roasters use drum roasters with limited airflow, although a few, like the Roasterie, use fluid bed air roasters. However, the large mass and institutional market roasters gave up on these technologies about 25 years ago; and have overwhelmingly switched to large convection oven style roasters with mechanical agitation that finish roasts in around eight to twelve minutes.

All of these hold the beans in large perforated bowls through which the hot air flows. Some rotate the bowls, others have vanes, to agitate the beans. The air is recirculated for energy savings.

Heat transfer to the beans speeds up at higher airspeeds. This means both the oven temperature and the bean temperature can be independently profiled (one can run lower oven temperatures by using higher airflow). The literature discusses various profiling strategies and the beans for which they are suitable, although these are quite unspecific, since particular profiles are proprietary. In general, the lowest possible oven temperatures seem to get the nod except for darker espresso roasts. Clarke & Vitzthum and Illy both have discussions; and there is a dissertation available free of charge that also has information.

The only specialty roaster I know of who uses a similar system, vastly scaled down, of course, is Oren Blustein. He thinks that if these roasters can make silk purses out of the sow's ear coffees used in this sector; they should do a better job than conventional drums in the specialty market.

Does he have a point?

[nick]

Reminds me of the Loring Smartroast and/or Scirocco roasters.

[Jim Schulman]

The Loring (new to me) seems like it uses the same ideas. The Scirroco is a great little air roaster with a terrific heater, but it uses airflow to move the beans.

[jepy]

Jim, I think Nick is referring to this one.

[Christopher Schooley]

From what I saw, the Portuguese Joper is also similar in concept. I heard both the Loring and the Joper refered to as Hybrid roasters. The Loring has a stationary drum with moving armiture, it appears to have very little drum to bean conduction, and I'm curious as to how much radiant (non-fluid) heat is roasting the beans. I wanted to spend more time at the Loring booth at the show in order to get the skinny. The Joper's cumbustion is under the drum but housed in a seperate unit, I imagine that the construction creates a little more radiant energy than the Loring's rear mounted cyclone. I found both machines extremely fascinating. A couple years ago I took my Pop with me to visit the Allegro plant in Colorado and after seeing their Drum roasters he asked why the heated air that was being pulled from the drum wasn't being recycled back through to save energy. I told him that the biggest issue was probably filtering out the particulate, moisture and smoke and that to be able to do that thoroughly would also take a great deal of the energy away. I'm curious as to how some of these roaster manufacturers address that (air filtration) issue. Jim, I wouldn't say most specialy roasting drum roasters have limited airflow, it's just not pure convection. I think this is a really interesting topic for our industry as we face skyrocketing energy costs and many of us are seeking ways to not just be responsible coffee buyers but also responsible and environmentally aware roasters.

[Peter G]

There are two issues here: air recirculation and high airflow (which increases convection).

I happen to be a huge airflow-fanatic. I did not truly get control over my roasts until I began to understand airflow and the influence it has on roasts. Jim, you are correct that most specialty roasters use drum roasters, but drum roasters are engineered to have very different airflows from each other, and many roasters modify their roasters (as I have) to maximize airflow control. This gives the roaster control over convective energy, the most powerful force in the roaster's arsenal.

One of my three roasters is a Roure, which has a perforated drum and a power burner, and baffles at both the air intake and output that allow me to tightly control airflow. I can therefore make the roaster behave like a Probat (relatively low airflow) or crank the airflow up to where it acts almost like a fluid bed (it is possible to get 7 minute full-batch roasts on this roaster). The nice thing about moderate-to-high airflow is the coffee is much cleaner and clearer; this roasting style is well suited to crisp, floral, clean coffees. With lower airflow I tend to get enhanced molasses-type flavors, which works for other coffees. Our Renegade has intentionally lower airflow, which helps develop sweetness and dusky intensity, useful for darker roasts and low notes in an espresso blend. On my Samiac, we use both baffles and an inverter-controlled blower motor to tightly control airflow. Again, high airflow=crisp and clean (and shorter roasts, frequently), lower airflow=sweet and complex (and longer roasts). Of course this is very oversimplified....however it is consistent with the observations you mention.

I do think that putting all drum roasters in the same category is painting with too wide a brush. Fan size, blower speed, and perforated vs. solid drum construction all drastically change the behavior of drum roasters. I would say that each roaster design (and every roasting machine modified by tinkerers like us) occupies a different place in the airflow continuum. I think of it like this: since all roasting machines balance radiation energy transfer, conduction tranfer, and convection transfer, each unique machine has its own fingerprint (frequently controllable) as a balance of these things. This gives each roasting machine its unique personality. The conscious roaster is keenly aware of these variables, and seeks to control (and sometimes profile) each variable. It gets complex, but once you master it you gain true control over your coffee.

Now then, recirculating is another thing entirely. It is ridiculous that we do not recirculate air in most of our roasters, it is a big waste of energy. Most roasters are using the equivalent of Model T technology to roast their coffees. There are real problems reusing smoky air to roast with, but I believe the Loring technology is sound and I taste no artifacts of smoke or combustion byproducts in the coffee. This is the wave of the future.

The high-airflow bowl roasters are interesting, but there is no reason a roaster cannot gain the same control by using a properly designed (or redesigned) perforated drum roaster, or even a solid drum roaster (given the right configuration of burner(s) and fans).

Peter G

[trish]

I'm curious as to how some of these roaster manufacturers address that (air filtration) issue. Jim, I wouldn't say most specialy roasting drum roasters have limited airflow, it's just not pure convection. I think this is a really interesting topic for our industry as we face skyrocketing energy costs and many of us are seeking ways to not just be responsible coffee buyers but also responsible and environmentally aware roasters.

I roasted on the Loring, or the beta model of it, while I was at Taylor Maid Farms. Inman consulted on its design, so we were the first guinea pigs.
The Loring burns the exhaust with the same flame that heats the air- cleaning it as it goes. Very efficient. The engineer/inventor, Mark Ludwig, says the Loring is nearly smokeless. It has two stacks. the second is for the cooling tray, and that one still gives a puff of smoke off of dark roasts.
Yes..the design of the future for sure...just think about all of those afterburners we will not need someday!! Afterburners are such an environmental irony.

Flavor profile was not a big deal problem from my perspective. Coffees tasted a trace bit cleaner, but the jury is still out on that....a matter of opinon. Interesting note: we had a lot of roasters come through TMF to look at the Loring Smartroast. A lot of them were fluid bed roasters who wanted a better solution. Many of them didn't decide on the Loring. They couldn't seem to adjust to the idea of a drum roaster. When I got to TMF after years of drum roasting (Probats), I found it no trouble to switch over to the Loring. I set the times and temps as if it were a Probat....worked it just like a drum roaster. Times were the same...heat felt a lil different, but again, I hardly had to mess with the probat's profiles.

...and it's totally spongeable because it's stainless steel- clean as a whistle in that cooling tray.
LOVED IT

[Jim Schulman]

Thanks Peter & Trish. I had no clue how variable shop roasters are becoming in terms of profilability. I guess adding recirculation is a large capital expense, and may not have enough ROI for smaller roasterie until gas prices get even higher.

I use a nano-airroaster with variable speed fan, various insulation thicknesses (a rube goldberg style of control), and a PID controlled heater to get some independent control over roasting speed and maximum chamber (as opposed to bean) temperatures. The slow-sweet-smoky versus fast-bright-clean is part of my experience too, although the smokey part depends on having high environmental temperatures along with slower roasts (removing all the insulation in my case). I'm guessing if you try a high airspeed roast with lowered temperatures for slow speed and low envioronmental temperatures, you'll probably get the same mellow sweet roasts but with fewer low notes (useful for finicky brazil and other low grown espresso bases).

I'm curious about drum speed in low ventilation roasters. Does increasing the drum speed in this case increase the airflow around the bean enough to speed the roast significantly, or is it just a minor tweak? It would perhaps be a simpler modification than adding airflow.

[Peter G]

I would think that using drum speed to try to control bean velocity in the drum would be a little tricky. Usually, drum speed is set to control conduction; i.e. the amount of time a bean spends in contact with the drum. At high drum speeds, centrifigal force keeps beans plastered to the drum, increasing scorching. Low drum speed allows coffee to collect at the bottom of the drum and scorches too.

Most people think of drum roasters the wrong way: a drum is primarily an agitating container for the coffee. Although there is certainly some contact-transfer from the walls of the drum itself, that is not the primary purpose of it. It is there to tumble the coffee in the flow of hot air, which typically flows from one side of the drum to the other. Much of the contact-transfer happens at the thick metal (frequently iron) front plate of the roaster, which the coffee pushes up against given the design of the agitators. In this way, the front plate acts like a metal pan the coffee is roasting on, and the airflow moves over the coffee bed. It is a simple matter to use baffles and fan control to modify the airspeed, and that is a much more effective and controllable way to control airflow than adjusting drum speed. It is just as easy to control the fan motor as it is to control the drum motor.

Peter

[Brent]

In respect to the drum speed, the agitators affect how fast the drum can spin surely.

So my next question would be what is there to stop you putting non fixed vanes in the drum rotating at a different speed to the drum?

Or even the opposite direction...

[phaelon56]

Spinners inside the drum?

http://en.wikipedia.org/wiki/Spinner_(wheel)

How cool is that? Someone needs to tell Jay Caragay so he can trick out a roaster.

As a guy who's locked into using a fluid bed roaster at my present job (although I have a Diedrich of my own that hasn't been deployed yet...) I remain incredibly frustrated both by lack of control over airflow/temp and even more so by the amount of wasted heat.

The results - once I learned enough to put a gate valve in the gas line and correct some issues with make-up air - are remarkably consistent. The profile is still a bit brighter sometimes than I'd like but for the needs of the shop where I roast - Royal type generic co-op varietals that are blended before being ground and brewed - it's actually a very workable solution.

That said - it's now what I'll be using for myself down the road.

[barry]

I remain incredibly frustrated both by lack of control over airflow/temp and even more so by the amount of wasted heat.

run the exhaust through a heat exchanger, and then use that heat to melt the snow (or heat the building).

[phaelon56]

I remain incredibly frustrated both by lack of control over airflow/temp and even more so by the amount of wasted heat.

run the exhaust through a heat exchanger, and then use that heat to melt the snow (or heat the building).

Hmmmm... sounds "green" enough. But with average annual snowfall of about 125" to 150" and a few dozen zero degree F days... might not be so effective

[barry]

Hmmmm... sounds "green" enough. But with average annual snowfall of about 125" to 150" and a few dozen zero degree F days... might not be so effective

Supplemental heat... not primary heat. You're burning the fuel anyway, you might as well use some of the waste heat to offset your furnace bill.

[Jimmy Oneschuk]

Sorry to revive a dead post. I've become interested by Peter's post on convection, in particular, using airflow adjustments during roasting to take total control of the process.

Most of my science books talk about theoretics and chemical constituents in coffee, but do not develop the science into praxis. From what I could find on airflow/convection, I've seen proponents of using 100% open airflow for brighter coffees, and results in a faster roast time. I've also read using open airflow is "gentler" on coffees, which seems to contradict the faster roast time (increased airflow=increased convective action=shorter roast cycle)

I've also read proponents of adjusting airflow based on different stages of roasting, for example: closed (or 90% shut) during drying (to 300F), open momentarily (chaff/moisture release), then closed to first crack, then open again after first. This sort of technique has me most curious, especially closing airflow before 300F: is it true you would be trapping moisture in the drum and reducing drying before 300F? And obviously this will have an impact on your sugar development stage between 300 to 370? Would there be tactical situations where you would adjust airflow between 300-370 in order to develop specific flavours/sugars?

I'm about to do some profile cupping using specifically different airflow adjustments at varying stages, and figured I'd best ask ahead to better make sense of my recorded profiles later.

So far my results have been delicious with what I have been roasting - but I have a burning desire to actually understand what is actually happening!

I understand if there is reluctance in the roasting community to discuss - PM me if you wish?

[Jimmy Oneschuk]

Wow, I sure managed to use a lot of words to say nothing!

A more simplified question that might be easier to answer is, what impact does closing airflow versus having open airflow during the first phase of roasting to 300F? I am assuming less water is evacuated and there is more steam within the drum if airflow is fully closed: does this mean more moisture is retained at this phase or there is more active water in the bean when you hit 300F - and if so, what impact does this have on sugars and other reactions later on?

Thanks in advance!

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