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Homeroasters - Home Roasting Coffee Community » BUILDING A ROASTER » Drum Roasters

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How drum mass affects the roast curve
oldgrumpus	#1 Posted on 09/03/2021 10:08 PM
1/4 Pounder Posts: 177 Joined: July 25, 2012	I'm a ponderer. This topic is something I've been giving a lot of thought to lately, especially when I calculated the weight of my drum. I should have weighed it before putting the roaster together, but by taking some measurements and doing some math, I discovered that my drum, including the vanes, the shaft and rear end components, I discovered it's about 35 lbs (16kg). Let me take a moment to talk about some experimenting I did a while back. I came to the realization that once preheated, I could experiment with different heat settings at the very beginning of the roast and make ONE HEAT SETTING, that if correct, might just work to get to first crack and possibly the right drop time and temperature also. So, the preheat temperature + initial heat setting (hopefully) = "hands-off" roasting Maybe this is impossible or worse yet, heresy. This graph shows what happened with one heat setting. 50% on my roaster.... oldgrumpus attached the following image: Clever Coffee Dripper Grinder: Macap M4 Roaster: Completed drum roaster project photos shown here: Photos https://goo.gl/ph...Da6K4wfqw5 Videos https://www.youtu...Bd1NrdpSUH


renatoa	#2 Posted on 09/04/2021 3:53 AM
Administrator Posts: 2664 Joined: September 30, 2016	Is exactly the reason I asked you to perform the step response test, it brings a lot of data with a simple move. Actually, even the graph above can be used, if adding the info about the power/gas level changes, and post as log file instead image.


oldgrumpus	#3 Posted on 09/04/2021 10:41 PM
1/4 Pounder Posts: 177 Joined: July 25, 2012	Yes, at some point I will do that. I'm just thinking through the physics of the drum mass and how it affects the roast curve. I think that when the drum is preheated and has absorbed as much heat as it is capable of storing, the effect is that it accelerates the roast at the beginning and as the curve moves upward the acceleration of the roast decreases because the drum temperature vs bean temp gets closer to matching each other. At some point the bean temp increases beyond the drum temp. Then the drum actually becomes a restraint to temperature increase because it is competing with the beans and absorbing heat itself. I think this is why I find myself with a slightly longer time after first crack unless I add a bit of heat at first crack.... Thoughts? My musing about this is really a separate discussion from my thread about automating the drum roaster.... Clever Coffee Dripper Grinder: Macap M4 Roaster: Completed drum roaster project photos shown here: Photos https://goo.gl/ph...Da6K4wfqw5 Videos https://www.youtu...Bd1NrdpSUH


renatoa	#4 Posted on 09/05/2021 1:51 AM
Administrator Posts: 2664 Joined: September 30, 2016	The drum temp is always much bigger than the beans, maybe you mean "bean temp increases beyond the air temp". Is the air the main factor that drives the roast, borrowing heat from the drum. To have a heat transfer enough to perform a roast in 10 minutes, the air should be about 40C hotter than beans at dry end, and about 25C more at FC. Else the RoR would drop in the "baked" values range. It's an heat energy balance. You could be right though. at some point the drum could be cooled by the air, and direction of heat transfer reverse. To prevent this, watch the air temperature, preferably in drum, not exhaust. When the air temp reach a maximum, and shows signs of regression, be prepared with the flame, to keep it at that maximum. This should be somewhere in the 232-240C ballpark.


allenb	#5 Posted on 09/05/2021 5:33 PM
Administrator Posts: 3725 Joined: February 23, 2010	Here's my understanding from taking measurements and lots of reading over the years. When room temp beans are added to the preheated drum, the drum gives off a lot of it's stored heat within the first 30 seconds giving the beans the needed rapid initial rise in temp. At that point, the drum temperature is maintained by the play between heat source on the outside and the temperature of the heat sink (beans) on the inside. The inner drum surface is always higher than the beans throughout the roast. Obviously it's much less of a spread during the final couple of minutes of the roast. Articles I've read but will most likely not be able to post a link to, advise against adjusting the heat at the onset of first crack in order to lessen the drop in rate of rise during the beans vapor release. Instead, they advise one to have a sufficient heat setting that will allow the beans, while dropping in rate of rise for a brief time, to complete the roast. 1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana


oldgrumpus	#6 Posted on 09/05/2021 8:53 PM
1/4 Pounder Posts: 177 Joined: July 25, 2012	I should know this, but "ideal" percent past first crack would be? 25%? or percentage range? Clever Coffee Dripper Grinder: Macap M4 Roaster: Completed drum roaster project photos shown here: Photos https://goo.gl/ph...Da6K4wfqw5 Videos https://www.youtu...Bd1NrdpSUH


allenb	#7 Posted on 09/08/2021 11:12 AM
Administrator Posts: 3725 Joined: February 23, 2010	That seems to be what I've heard as well. But is only a guideline. From what I've gleaned from hearing various experts is by stretching the start of first crack to end of roast phase to the 20 or 25% of total roast time, it is claimed by some to impart a sweeter, increased body cup versus letting the roast progress with same heat input as the browning phase. While it appears to not be difficult to achieve on larger shop roasters, trying to pull this off on our smaller roasters is another thing. The problem reported by many and experienced by me is the difficulty finding a heat input setting that when reduced as we near 1st crack, is sufficient to take us through 1st crack at a low enough rate of rise to not end up with a french roast at the end of the 2 to 3 minute time span. If we're too low even by a small margin, we don't get enough energy into the start of first crack which results in a poor outcome and, to compound this, with the temperature measurement error during the vapor release at the start of first crack, it's very difficult to use rate of rise as an indicator of how this phase of the roast is progressing. When I've been successful in doing a roast with the 25% post 1st crack phase, I haven't noticed much of a difference in sweetness or body so I don't see much value in stretching it versus letting it follow the normally declining curve which on most of my light roasts, ends up being closer to 1 to 1.5 minutes. 1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana


Ringo	#8 Posted on 09/09/2021 12:59 PM
Pounder Posts: 474 Joined: January 14, 2010	What I do one my drum. I have a temp probe on the air coming into the drum, and a probe in the beans. Both are fast reacting to temperature changes. When I hit first crack I lower the heat input till I see the incoming air start to drop slightly but make sure the beans keep going up. This gets me to maybe 30 seconds into 1st crack them I have to slowly add more heat and get the incoming air moving back up so i will not stall the load. I add as much heat as needed to finish the load in 2 min. At least that's my goal but i never hit it exactly. All you need in life is ignorance and confidence, and then success is sure. Mark Twain


oldgrumpus	#9 Posted on 09/10/2021 8:06 PM
1/4 Pounder Posts: 177 Joined: July 25, 2012	So Allenb and Ringo (and anyone else...) What are your thoughts on drum mass? Does it hinder the response time of control inputs? What if the burner WASN'T underneath the drum but hot air still was routed in through the back of the drum? Then wouldn't that make it closer to a fluid-bed roaster in theory? Also allowing for quicker responses? I know several of the commercial brands are designed with that concept, but has anyone here done that? Maybe then drum mass becomes less of a factor as well as concerns for even heating of the drum, and...? Clever Coffee Dripper Grinder: Macap M4 Roaster: Completed drum roaster project photos shown here: Photos https://goo.gl/ph...Da6K4wfqw5 Videos https://www.youtu...Bd1NrdpSUH


renatoa	#10 Posted on 09/11/2021 11:38 AM
Administrator Posts: 2664 Joined: September 30, 2016	Depends on your airflow capabilities. The heat capacity of two grams of iron equates to the heat capacity of one litre of air. I.e. one kg of iron = 500 litres = 0.5 cubic meters of air. iron = 0.5J/gram/Kelvin degree, and air is about 1J/gram = one litre. Other words, when the iron cease one degree of heat to the air, the temperature of 2 litres of air in the drum increase by same degree. However, this degree is ceased to the coffee in the next step... and the ratio is 1.4 litre s of air for one gram of coffee. Thus 1400 litres of air = 1.4 m3 for one kg of greens. Putting all together, a typical airflow of 120 m3/hour = about 70 cfm = 2 m3 per minute, should be enough for a 10 kg iron drum, and 10 minutes roast of 1.4 kg of greens. Of course, in an ideal lossless closed system...


Ringo	#11 Posted on 09/12/2021 5:08 PM
Pounder Posts: 474 Joined: January 14, 2010	I built a drum with a lot off mass but I do not think 30 pounds. I believe at the start of the roast with a heavy drum you do not preheat as hot. I try for a turnaround of 180 so with my drum I will preheat to 300 degs. If my drum was thinner I would preheat to a hotter charge temp. For the first minutes I like the heat to be radiant from the drum. After 300 deg bean temp I think all the steel slows the reaction to heat changes. I like this because I think it smooths out heat inputs but too much steel and you will not be able change the profile direction. All you need in life is ignorance and confidence, and then success is sure. Mark Twain

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