The essential process in coffee roasting involves elevating the temperature of green coffees from low to high. The coffee roasting machine is designed to ensure even temperature increase throughout the coffees and supplies the required heat.

There are various brands and series models of coffee roasting machines available. The internal structure and heat supply principles may vary among different brands or models of roasting machines. Therefore, it is crucial for roasters to understand the structure and heat supply principles of the roasting machine they use to operate it properly and effectively raise the temperature of coffees inside.

This content focuses on the most commonly used types of roasting machines, which are the drum direct-heat roasting machine and the drum indirect-heat roasting machine.

In Korea and Japan, the term commonly used for the "drum direct-heat roasting machine" is "ban-yeolpung roasting machine." This type of roasting machine operates with a horizontal drum that rotates on top of heaters, such as burners, and the drum is directly heated by the heaters.<Figure 1> illustrates the basic structure of the drum direct-heat roasting machine and the direction of the hot air generated by the heaters for heating.

The air moves in the following direction: The ambient air around the heaters, which has increased in temperature, enters the drum chamber through perforations on one side of the drum. Inside the drum chamber, the hot air meets the coffee green beans, and the air that transfers heat to the green beans is then expelled through an exhaust pipe. After passing through a cyclone for dust and silver skin collection, the air is exhausted through a duct.

Now, let's introduce some representative models of the drum direct-heat roasting machine:

1. Giesen
2. Diedrich
3. Easyster
4. Proaster

As shown in <Figure 2>, the Diedrich machine has a different air flow direction compared to the one depicted in <Figure 1>. In the Diedrich machine, the hot air flows into the drum chamber from the coffee bean inlet and then exits through the exhaust pipe. Nevertheless, the underlying principle remains the same.

"Drum indirect-heat roasting machine," known as "yeolpung roasting machine" in Korea and Japan, operates differently from the drum direct-heat roasting machine. In this type of roasting machine, the heaters, such as burners, do not come into direct contact with the drum, and as a result, the drum is not heated directly. Instead, the hot air generated by the external heaters enters the drum chamber, indirectly heating the drum.

The drum indirect-heat roasting machine operates by injecting air into the heaters, causing intense combustion and generating high-temperature air that enters the drum chamber. Inside the drum chamber, the coffee green beans meet the hot air, and the air transfers heat to the coffee beans. The air that delivers heat to the coffee beans exits through an exhaust pipe after passing through a cyclone to collect aerosols and silver skins. Finally, the air is discharged through a duct.

In this process, some of the air that exits the drum and enters the exhaust pipe is recirculated back to the heaters. This recirculation structure allows the air to be reheated before reentering the drum. Many machines use this recirculation method to maintain a consistent and controlled roasting temperature.

Here are representative models of the drum indirect-heat coffee roasting machine.

Understanding the essential aspects of the structure and heat supply in both drum direct-heat and drum indirect-heat coffee roasting machines was the main point. The most crucial difference between these two types lies in the drum's direct or indirect heating, affecting the rate of temperature increase during roasting. However, the fundamental principle of heat transfer remains the same, relying on the movement of hot air and the temperature difference between the coffee beans and the air.

Even within the same type of roasting machine, there are variations in detailed specifications among brands, manufacturers, and series models. For instance, single drum versus double drum configurations, the structure of agitating paddles, mixing speed, the airflow capacity of heaters and exhausts, and more, can differ.

However, understanding how the hot air moves to raise the temperature of the coffee beans and the drum's temperature increase is crucial before customizing roast profiles to accommodate individual specifications. This knowledge enables better utilization of the differences in profile design and ultimately leads to improved roasting results.

Aside from the two types described, there are other types of roasting machines available. Regardless of the machine type, comprehending the structure and principles of the roasting machine used by the roaster is of utmost importance.

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