Roaster Dynamics

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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#1: Post by arriflex »

Ed "Farmroast" Bourgeois suggested we start a new thread on the topic of roaster dynamics with regards to a question I posed on the topic of agitation.

Specifically, I am designing and acquiring parts for a roaster more similar to his "Dreamroast" rather than the Stir Crazy Turbo Oven design. However, the mechanical agitation poses so very many opportunities for experimentation that I could use some direction from those with experience. In my case, I have acquired an 1800 watt convective element and fan for heat. I anticipate controlling the heat output using an SSR connected to tc4 hardware by way of thermocouples and an Arduino with whatever I hack together for an interface.

The question for this topic is to consider the most flexible combination of drum shape, diameter and mass along with best means of agitating the beans and how on earth to integrate some form of a tryer! Mass or volume of the beans is not so important as my needs are modest in that department and I'm confident that 1800 watts should allow me some headroom for everything I want to do. My goal is to have a system that I can use to repeatably control and share with others.


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#2: Post by farmroast »

My thought was more in a generic sense. Better understanding, heat transfer, batch size, bean mass, agitation, air flow, ET/MET, conduction/convection/radiant application, power demands, roast chamber size, etc. And how these effect each other. You must know what the beans need from a roaster and then how your roaster can accomplish it.
LMWDP #167 "with coffee we create with wine we celebrate"

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arriflex (original poster)

#3: Post by arriflex (original poster) »

With the more generic aspect in mind, my real questions stem from a basic curiosity regarding the means by which energy is consumed in the bean. It's my understanding that roasting and baking are very different things, but that there are those who flirt with the latter if unintentionally.

If I am able to maintain a given Rate of Rise that best drives the roast, could the same RoR be applied to a different batch size (within reason) of the same beans and accomplish the same result?

One of the big questions I have is with regard to air flow in a system that is basically closed. IIRC, the only exhaust on the Dreamroast is the chaff collector. Where does the air get made up? Should one consider a controllable make-up air system a possible advantage?

There have been plenty of conversations regarding conductive vs convective heating. My inexperienced opinion is that radiant heating is more responsive than conductive, but otherwise similar in results with radiant having the ability to affect irregular surfaces more substantially.

My goal is to understand what the beans react most positively to, decide which of the numerous variables I would like to control vs. have remain static, and put together a machine that offers me the ability to replicate and engage in the discussions here.

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#4: Post by DJR »

You might check my thread Making Your Own 57 Ounce Roaster Updated Again regarding using a milk can and a stainless tube as a tryer.

I use propane, however, and no forced air, so these observations might not apply:
1. It definitely takes longer to roast two pounds than one. About 25% longer. However, I didn't measure the energy consumed, so I might be adjusting my burner to compensate and it really probably takes double the time with the same amount of energy.

2. There is a point after 1C where energy input can be reduced because, I think the beans are contributing their own energy to the roast. What that means is that with 2 pounds it is easy for the roast to "get away" from you because there is twice the mass and the heat from the beans builds up quickly. For the first time I "ruined" a roast by letting it get a bit oily on the surface. The time between a good roast C+ and the oily one was only about 45 seconds.

3. I am still a bit unclear what the forced air does. I don't pretend to understand that aspect of it, and what I have learned has been on this forum and through trial and error.

4. I did try three short vanes rather than two large ones and noticed no difference. I did try without holes in the side and no holes, and there is maybe a difference, but I'm not sure. The holes in the side, however, with propane basically burn off 90% of the chaff, which is very convenient.

5. I put a heavy hood on the drum. It uses less energy that way. But it interferes with the dumping of the beans, so I just made it instantly removable, and I'll try it tomorrow.

6. If I could locate a servo controlled propane valve cheap, I would hook an arduino up with controls, but so far I haven't found anything and I don't want to build my own.


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arriflex (original poster)

#5: Post by arriflex (original poster) »

it really probably takes double the time with the same amount of energy.
I'm more curious about whether or not one could (assuming all other things being equal) drive the roast with RoR alone for different batch sizes with predictable results understanding it would utilize more energy.

The idea of both desiring to maintain energy in the vessel as well as exhaust it at the right moment is something it appears drum roaster are more capable of than most other home-designs. However, I think that a design like Ed's could approximate it with make up air if appropriately valved. What I haven't seen is the concept of external cooling of the vessel like the fins on a Hottop. I wonder (In the context of Ed's roaster) if a double boiler design where the gap was static air during high RoR periods and forced air during periods when trying to dump energy out of the vessel might be worth exploring.

My concern is controlling ET in a mostly unvented container when hitting the peak of BT.

BTW, if you are savvy with an Arduino, and you have the propane valve that the burner kit you linked up came with, I think you'd find it trivial to attach a little $15 servo to it and use the driver code available on Arduino's playground. IIRC, it doesn't take much torque to turn those valves, but that would be the only remaining question. Cool roaster!

germantown rob

#6: Post by germantown rob »

After 5 years of roasting I am just now starting to pay more attention to RoR in hopes of fine tuning my roasts. It maybe an overly complicated way to control a roaster since each bean variety will have different RoR to get a bean to the same roast degree. I break my roasts into 3 phases, drop in-drying phase, drying phase-1st crack, and 1st crack- end of roast. Depending on bean and how the roast will be brewed will effect how much time I spend in each phase.

My roaster weights 125+lbs, is IR heat, and airflow travels over heat syncs to warm it before entering the drum. In theory it is considered a lab roaster to design profiles on that can be translated to larger IR roasters. I do not use double the heat when going from 1lb roasts to 2.2lb roasts but that may be due to it's large thermal mass compared to say my Hottop which required far less max heat for 150g then it did for 300g.

I use airflow at different times, towards the end of the drying phase I use it to get rid of chaff, after drying I use less air for removing moisture and pressure as well as convection, then no air for the rest of the approach to 1st crack, once 1st crack has started I use it for smoke removal, chaff, and convection again.

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arriflex (original poster)

#7: Post by arriflex (original poster) »

Interesting! When, if ever, do you find the considerable thermal mass to be a problem? Or are you able to offset that issue with airflow?

BTW, I hadn't considered using airflow to help facilitate chaff collection (duh!) with all that mass, I suppose you aren't to concerned about it. Wouldn't the end of the drying phase usually be a time when dumping energy is discouraged?


germantown rob

#8: Post by germantown rob »

The only time I find thermal mass is a problem is when I want to do a 1lb roast and I have pre-heated to 415f, because of all the thermal mass and heat syncs when I cool the roaster to 350f I will be at the end of the drying phase with in 4 min. Even though my BMP is saying 415 it is the furthest from the mass of heat and the rest of the materials are much hotter so it is better if I pre-heat to 400f for a 1lb charge if I want a longer dry then 4min. BTW when I drop in I cut all gas flow to 0 until the turning point and then I turn the gas on and start to add heat to achieve the desired time of drying I want.

Airflow in my case is not eliminating heat until much later in the roast since the path the air travels it is being preheated (within roasting times, if I run x gas pressure and full airflow eventually I would cool the heat syncs), I would say somewhere just before 1st crack is when the roaster and heat that it has absorbed is starting to stabilize, before that airflow is going to give added heat and raise RoR. Once 1st crack has started or just before air flow can help bring a fast RoR down but if I control the gas flow proper it is not needed.

Edit: I have never run a roaster with anything but manual control so I have no idea how automated roasting is controlled.