Balancing airflow and heat in the FZ94

My Coffee-Tech FZ94 roaster has a perforated drum which makes the airflow very effective, enabling me to blow out any excessive heat while ensuring that enough heat remains inside the drum area to navigate the planned roast profile.

Airflow control
Until recently I kept gradually increasing the airflow during the roast, mostly following a routine developed over many roasts. I did lower the heat of the elements somewhat as FC approached, preventing an escalation of heat when at FC the beans exhale much steam when the cells expand under the great internal pressure that also develops to create the delicious taste and aroma of the coffee beans.

In the Artisan profile above one can see how airflow (the blue graph) is gradually stepped up from its lowest (corresponding to about 30% of max) up to 75% of max airflow capacity. At the lowest airflow, enough air is pushed through the drum to prevent smoke from interfering with the taste of the beans and at the high end it pulls out any excess heat, without cooling down the process too much. The heater is dimmed slightly in two steps well before FC occurs.

Heat control
In a discussion with Marko Luther from the Artisan team, he suggested I try to keep a moderate airflow and instead lower the heat energy from the heating elements.  Ideally, this would create a situation where the burst of energy coming from the beans around FC is absorbed by the mass of the drum and roast chamber and one could aim to avoid both the bump in the Rate of Rise  and the slump of this RoR which can occur when the excess heat is pulled away too agressively.

It did take me a few weeks of figuring out the best timing and proportion of the new values of airflow and heat energy but in the above Artisan profile you can see it's working out nicely. There is still a slight bump & slump between 8:00 -- 10:00 and this can be smoothed over more.

Airflow is stepped up to a little over 50% of capacity and kept there mostly. In that situation, there is a lot of margin to momentarily increase airflow to help excess heat escape while there is also a good margin to suddenly lower airflow if for any reason the RoR tends to drop. The lower airflow will then conserve more heat radiating from the elements while keeping enough airflow to prevent the beans from rolling around in their own smoke.

Shortly after 2 minutes into the roast, the heat is already lowered a bit and well before FC it is gradually lowered further to about 80% of capacity in mid development of the beans. Changing the energy radiating from the elements has a long response time compared to gas heaters where the flames don't linger, so one has to plan well ahead.

Here, the 'Alarms' functionality of Artisan comes in very handy again. I have locked the steps not to the time of the roast but to the BT so if a new bean responds much quicker and temperatures rise faster, the correspoding changes in airflow and heater energy also occur sooner.

screen snapshot of Alarms plan


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