You can control the airflow in most roasting machines in either of two ways: by means of a damper or by means of a variable speed drive (VSD), also known as a variable frequency drive (VFD). Most roasters feature one of these two controls; some include both.
A damper is a flap or valve that regulates the airflow through the roaster by partially blocking the exhaust flue. Opening the damper increases the amount of airflow through the machine, and closing the damper reduces it.
A damper mechanism can be as simple as a flap inserted in the exhaust flue of the roaster.
If your roasting machine uses a damper to control airflow, it’s best to set it to a position that allows reasonable airflow for roasting and then leave it on this setting. It is difficult to make precise airflow adjustments with a damper, and you’ll find it challenging to maintain consistency in your results if you adjust the damper setting between roasts or during a roast.
A VSD, on the other hand, enables the operator to control the speed of the fan directly. Increasing the fan speed increases the amount of airflow. A VSD allows a much bigger range of adjustments than a damper, and it enables more-precise control of the airflow through the roaster. By controlling airflow with a VSD, therefore, it’s possible for you to use different settings for different roasts and still maintain consistency between batches.
Even if you have a VSD installed, however, we advise you to use a single airflow setting per roast and avoid adjusting the setting during roasting. It might be tempting to experiment with changing the airflow settings when you’re developing a roast profile, but alterations in airflow aren’t necessary for achieving a successful roast. Alterations to the airflow result in a more complex roast profile — one that will be harder for you to follow consistently.
The only exception to this recommendation is a situation in which the roaster is underpowered or your batch is too big for the machine to roast effectively.