Welcome to the 2nd instalment in our series on our custom Probat 45kg coffee roaster that we finished designing and building this past summer (after 3 years of work!). This week, I want to talk about our burner design, which is really the heart of the roaster, as it produces the heat that actually roasts the coffee!
When selecting our burner, my two key considerations were combustion quality and dynamic range. First combustion quality: the vast majority of 45kg and smaller roasters have what are called atmospheric burners, which rely on an effect called Venturi. Although a brilliant concept, it only functions well at one fixed pressure. And here’s the rub: every atmospheric burner roaster I’ve seen varies pressure to achieve variable power and thus control the roast—this includes our old roaster, by the way! The problem here is moving away from the designed pressure causes non-optimal combustion which in-term produces negative combustion byproducts, one of the most common being carbon monoxide or CO. CO is basically soot, so what I’m saying is that it's coating the coffee with a thin layer of soot. This can certainly impart a negative taste. Not cool, right! For this reason, on our new roaster, I went with a forced combustion burner with a ratio gas valve. This burner varies the gas and air in tandem to achieve the stoichiometric ratio of fuel and oxygen, thus avoiding any negative combustion byproducts (aka soot)—ya!
But that was only half the battle, my other concern was dynamic range, called “turn-down ratio” in the burner world. The roasters I’ve seen are designed to run best from 50-80% of their total capacity. This is very logical because, of course, roasting large batches maximizes efficiency. However, some of our most exciting coffees are part of our exotic series. These coffees are a lot more expensive and, as such, we roast much smaller batches of them. To make sure we could roast very small batches, our burner has a special high turn-down option. With it, we can roast all the way down to 5kg. In fact, to my surprise, we have better control at 5kg on our new 45kg roaster than our old 15kg roaster!
Photos courtesy of Marnie Burkhart, Jazhart Studios.