Heat transfer during a roast depends on the movement of air within the roaster.
Airflow can not only help regulate convection, but is important in keeping the oils volatile in the beans, ensuring they retain all of their best characteristics.

What is airflow?
Coffee is very sensitive to its environment. Minute changes in a variety of variables, from load temperature to ROR (Rate of Rise), can drastically affect the outcome of a roast.
Airflow is one of the most important of variables.
Defined as the way air moves in a roaster, it must be carefully controlled to ensure successful roasting.
The importance is largely due to heat transfer and the three ways in which energy (in the form of heat) is transferred: conduction, convection, and radiation.

Conduction
Conductive heat refers to the process by which heat is transferred from one object
to another through direct contact.
In coffee roasting, conduction occurs when the coffee beans come into contact
with the hot surface of the drum or when the beans come into contact with each other in a pile.
In “The Coffee Roaster’s Companion,” Scott Rao explains that heat transfer in the first few minutes of roasting in a classic drum roaster occurs predominantly by conduction from the hot surface of the drum. After the tipping point (the point at which bean and drum temperatures begin to rise in equilibrium), roasting is dominated by convective heat transfer.

Convection
Convective heat is the transfer of energy through a fluid medium.
In particular, there are two types of convection: natural and forced.
Natural convection is caused by the buoyancy effect of density difference (warmer, less dense air rising and denser, cooler air falling).

Forced convection, on the other hand, is when airflow is forced through an external source like a fan or pump. Convection energy is the most important aspect of heat transfer in roasting.

“Understanding how that heat transfer occurs through the moving hot air in a coffee roaster will give you insight into how to approach any given roast”

Radiation
Radiation is the transfer of heat through electromagnetic waves.
It is highly complex and therefore difficult to measure or control during a roast.
As such, it is better to focus on the other types of heat transfer, which are easier to measure and control.

 How can airflow affect the outcome of a roast?
For specialty coffee roasters, it is important to understand how changes in airflow can affect coffee characteristics.
The deeper their knowledge, the better control they will have over the outcome of a roast and the less risk of ruining the coffee batch.
Too little or too much airflow can cause a cooling effect on the roast resulting in dry flavors in the coffee.
When you are in a precise and continuous passage of air flow, a very clean coffee can be highlighted.
“Considering the processing methods of the beans, generally even with a naturally processed coffee, you’ll get a clean, or cleaner, cup compared to ones with too little or too much airflow.”

Airflow adjustment: How to find the optimal setting?
Airflow control is a fundamental part of coffee roasting. But how can specialty roasters find the optimal settings? One of the metrics we look at when adjusting airflow is the ROR (Rate Of Rise).
Specifically, you’ll look at how airflow affects RoR and roast duration. “Ambient temperature (or air temperature) works in correlation with grain temperature.” “If the ambient temperature increases at a higher rate, the temperature of the grain drags along with it. “When the t. If the grain is on top, if the air temperature is not right and it is dropping, increase the air speed or increase the convection energy through the roaster so that there is enough heat transfer to carry with it the temperature of the grain…

This is where an experienced roaster looks
“Finding a happy median between how those two variables work… 
Gives optimal airflow; that’s the method I generally “use” when adjusting the airflow.”
I point out that the optimal airflow setting can change from day to day, as well as between seasons, since the ambient temperature will affect the air pressure in the chimney. For example, one can get higher airflow in the toaster even on a low airflow setting during winter. Therefore, to get the same air pressure in the drum, roasters may need to reduce the air speed on cooler days or keep it at a higher setting during the summer.
As a general rule of thumb, keep the airflow setting constant once you have found an optimal airflow for a precise bump size/density in your roaster. “Vary the air flow configuration a lot”, “It’s time to change, the burner configuration too because it will be affected. So having two variables come into play makes it very difficult to understand which one is making a significant difference at a particular point in the roast. “Keep the same airflow from start to finish, you’ll have much more control over your burner.

It is not a generality for all; That’s how I see it.”

In the Fluid Bed toaster, pure convection burner control does not exist.
That’s why getting the perfect espresso is more difficult, despite everything, make a medium or medium high roast with the same fluid bed and you will get a very clean coffee, so a crystal clear cup.