Hi guys,
This is my first post in the sub, and I’m excited to talk with professionals in the field. A bit about myself — I’m not an expert or a physics student. Actually, I always get terrible marks in physics when I was in school. Now I’m just an ordinary coffee roaster who has been working in this field for about three years. But because of this job — and having to do my own reading to earn a better salary — I’ve realized that I really love science.

Most of my understanding of the laws of physics in coffee roasting comes from trial and error with different approaches. Other times, I learn from books, blogs, YouTube videos, or research articles about coffee. I thought I was good at my job, but I realized I wasn’t — because lacking a proper understanding of physics fundamentals really holds me back. None of my hands-on experience could answer the question: “Why did this happen?”

So, hopefully after I share the “system” inside my brain, we can discuss the topic together. Personally, I love reading any kind of critique because it helps me refine my skills.

What I’d love to discuss is the mechanism of airflow in a closed environment (in this case, a coffee roasting machine) — what happens physically when we apply heat, and how complex it can become.

I’ll break this topic into the following sections to make it easier to follow:

1. The coffee roasting machine – how I understand it from the manual book.
2. Physics terms I’ve encountered while learning.
3. My understanding of airflow from using the machine.

1. The Machine

The machine I’m working with is a Probat, made in Germany, and the model dates back to around 2014, so we can expect some technology inside. I’ve provided two images: the first is a 3D mockup of what the machine looks like, and the second is a process diagram from the manual.

From the second image, when D1 turns on, it brings in the flame and heats the R1 environment (the red area). This area is called the drum, which spins thanks to the motor marked in blue on the left. The yellow area, F1, is called the cyclone, and it sucks air through the orange pipeline out of the drum (R1).

When coffee beans are added from the hopper (B1), they enter the drum (R1). With the heat from the burner (D1) and the suction from the cyclone (F1), the beans roast through:

• Conduction (contact with the drum surface, which is made of cast iron)
• Convection (from the moving hot air)
• Radiation (emitted from the beans themselves)

For this roaster, I can control the heat level (from 0% to 100%) to manage roast time and environment temperature.
In the middle of the orange area in the diagram, there’s a black line with a curved hook — that’s the damper. You can manually raise or lower it to adjust the airflow through the pipeline.

Compared with other brands that allow full control of the airflow (by changing cyclone fan speed) and drum rotation speed on-screen, this Probat model doesn’t have those features.

2. Physics Terms I’ve Gathered

During my time using the machine, I’ve come across some basic physics terms:

• Heat transfer: conduction, convection, radiation
• Airflow / ventilation

And some more advanced ones that I’ve recently encountered:

• Air pressure and velocity
• Fluid mechanics
• Thermodynamics
• Endothermic & exothermic reactions
• Entropy (no idea what this actually!)
• Bernoulli’s principle
• Venturi effect

Even though I can name these terms, that doesn’t mean I fully understand them — but I try to use them because my instinct tells me they’re important within the scope of my work.

3. What I Understand About Airflow from Using the Machine

When we heat the drum environment, we’re heating not only the drum material (cast iron) but also the air surrounding it. This Probat drum is designed with a double layer. The outer layer is in direct contact with the flame underneath, while the inner layer touches the coffee beans.

Since the drum keeps spinning, the beans are constantly being lifted and dropped, allowing enough contact time for conduction. The main driving force of heat transfer is the drum material itself (cast iron), which retains a lot of heat throughout the roast. Meanwhile, hot air flows from inside the drum, through the pipeline, and out through the cyclone to the chimney.

This machine only allows me to adjust heat level during production. I can’t change the drum speed or the cyclone fan speed, as both are fixed. So, I assume the airflow velocity remains steady throughout the process.

Then there’s the damper.
When I close the damper to around 15% open, the “air sound” around that pipeline area raises its pitch — it sounds like a hiss. When the heat reaches 90–100%, this hiss becomes much louder for a minute or two, then quiets down.

When I open the damper wider (around 45% open), the hiss disappears and is replaced by a deeper, bass-like sound. So by adjusting the damper, it somehow affects the air exhaust velocity — where I believe Bernoulli’s principle and the Venturi effect might come into play — and therefore the convection inside the drum.

The manufacturer recommends setting the damper to around 40–45% open throughout the roast, no less and no more. But being curious, I experimented by opening and closing it at different levels. Here’s what I observed and tried to make sense of:

• When the damper is too narrow, the heated air at the start of the roast (when the drum is preheated and the beans are at room temperature) produces that loud hiss — possibly due to increased pressure.
• When the damper is more open (around 30–45%), the sound shifts to a deeper tone, suggesting lower pressure.
• I’m not sure what exactly causes the loud hiss, but my theory is that narrowing the damper traps hot air in the drum, and when that air passes through the pipeline and hits the damper area, the pressure increases sharply. This might actually improve convection.
• Conversely, when I open the damper wider, the pressure seems to stabilize — but interestingly, the drum temperature appears hotter, which I can’t fully explain. Could it be that the airflow is moving slower? Shouldn’t it exhaust faster instead?

So, I expected that opening the damper wider would lose more convection heat, but instead, I observed that the rate of temperature rise increased. And when narrowing the damper, I thought it would trap more heat — but it didn’t behave that way. This Probat is often described as a heavy-conduction machine, rather than a more versatile conduction/convection type like some other brands. Because I haven’t been fully satisfied with the results I’ve been getting, I really want to understand more about hot air and how it works in the roasting process.

This is where I’m completely lost about how airflow really works — and I hope you guys can help me understand it better. Thank you so much for reading and make a reply to my thread.