One thing is obvious: roasting begins with heat.
Heat is far from obvious. We spend our professional lives thinking long and hard about the best ways – the healthiest and most efficient – to deliver heat to beans, beginning from the component that ignites it all: the burner.
Till they figure out a way to roast nuclear, burners come in three guises: electrical, gas, and wood. To avoid writing too much of a good thing in one single post, I broke down the topic into four separate articles. The first opens with an intro on heat transfer, thermal behavior and description of free aspirated burners (sometimes referred as atmospheric burners). The second delves into detailed description of the new generation of gas burners types: Pre-mix burners. The debate over gas versus electricity draws much heat in itself, which naturally takes us to electrical burners as the subject of my third article. However there is a fourth exciting option, namely, wood, to which I dedicate my final article.
This coming series of articles on heating methods in roasting was long in the making, at least till I find time to sit down and write my magnum opus on the subject (I am only partly kidding). There are surely technical issues to consider. However, at the end of the day, it may all be about your health as operator and consumer. Optimal thermal dialogue should by efficient. This can and should mean bringing down emissions. Just to be perfectly clear, these are the fumes that you, the operator, inhale.
Heat can be transferred in many ways: conduction, convection, radiation / reflection, or the natural escalation of thermosyphon. Each method differs in its mode of heat transfer, depending on the surface area through which heat is exchanged. For instance, all forms of heat transfer, except for conduction, have a fairly large area of thermal dialogue with the coffee beans. Conduction, on the other hand, relies on direct contact. Therefore, it has the disadvantage of a relatively small surface for heat exchange: a flat hot surface coming in contact with the tiny round bean.
The thermal conductivity of the beans and the metals involved also play an important role. Imagine, for instance, that we were to heat up one edge of several metallic rods made of different metals but equal in length and diameter. We would then measure the temperature on the opposite end. Such an experiment would determine which metal is the better thermal conductor, according to which opposite end heats up faster. We would then find that copper and aluminum conduct heat faster than ‘sluggish’ metals such as iron and stainless steel.
All these factors are important in understanding how materials transfer heat onto coffee beans – think instead of wet raw wood. A coffee bean has its own thermal absorption and conduction characteristics, which any good designer of coffee roasting machines must consider. Woody substances, such as beans, tend to burn and char when too much heat is applied all at once.
An important factor to consider when selecting materials for coffee roasting machines is thermal generation and continuity. Old-school thinking tended towards thermal generation, ergo, cast iron machines. Contemporary engineering leans towards responsive materials that offer heating on/off and cooling on command.
Designers choose a heat source for their coffee roaster based on multiple considerations. Heat sources range from a simple gas burner (a handcrafted, drilled tube with brass candles) to a highly sophisticated burner, the result of extensive research. Designers may follow necessity – cost reduction or shelf availability – or they may be after superb performance combined with great health and safety specs. It is not uncommon, in fact, to find burners installed in roasting machines that were initially intended for kebab machines or pizza ovens. Only, they still function as such, meaning, as part of a machine intended for that original purpose.
Generally speaking, gas burner performance depends on infrastructure as well as on environmental conditions. When using gas, a good deal of practice is required in order to guarantee consistent, reliable gas burner performance. Numerous parameters influence its caloric value: the surrounding temperature, gas type and purity, consumption rate (which can lead to gas freezing), gas tank capacity (especially when insufficient), pipeline diameter, the gas pressure reducer (type and location), the form of gas-projecting nozzles, jet volumes, adjustments made to air or fuel in the mixture, that is, to name but a few. In larger roasters, a gas vaporizer must be utilized when using liquefied propane gas (LPG).
Once all factors are taken into account, the burner is ready to be calibrated to min-max state using a control valve installed on the roaster’s control panel. The control valve must, in some cases, also be set to fit the scale working range, using the master delivery valve on the burner gas ramp. Compared to an electrical heating source, gas heating provides a broader energy range, much broader, in fact, than is actually needed. From a user’s standpoint, what’s crucial is to find the range of thermal values the burner can produce within its given limits.
Gas Burner Types
Gas burner types can be divided into two groups
Self aspirated burners (also called atmospheric burners):
- Self-aspirated candle burners
- Self-aspirated inline rail burners
- Self-aspirated, metal wool, projecting screen burners
- First-generation burners with air pump
- Pre-mix single nozzle burners
- Pre-mix single nozzle, multi-stage, or fully modulated burners
Self-aspirated candle burners
Self-aspirated candle burners are the most basic type of burners. Their main limitation is their localized thermal distribution and their diminished radiation capacity. Nevertheless, they are widely used by a broad range of manufacturers. Needless to say, no Coffee-Tech product is equipped with such a burner.
Self-aspirated inline rail burners
Self-aspirated inline rail burners are used ubiquitously in one form or another. They suffer from all the same problems as self-aspirated burners. One of the critical, yet neglected, issues involving inline rail burners relates to the linear calibration of air levels together with changes to the gas-regulating valve. It is pretty easy to sustain a dynamic mechanism that controls gas quantity, but linear calibration of preliminary air is a whole new ball game. Modulation during operation is not enough. These burners must be adjusted in terms of pressure at the point of entry, the volumetric flow rate through the nozzle, and the air opening that introduces air into the mixing chamber. The air opening is usually restricted in a pretty primitive way, using a slide ring that blocks part of it in order to set the amount. The default for static calibration is usually set to mid-range gas capacity.
This type of burner is supposed to operate as its name suggests: it should be self-aspirating. However, when it comes down to it, this is actually not the case, especially where low or high flame flow rates are concerned. There are different types of self-aspirated inline rail burners on the market. The difference in quality can be like night and day – from welded pipes to cast or stamped metal units with an efficient Venturi structure that allows for self-suction across the entire spectrum. The difference between the two extremes will be expressed in gas burning efficiency. Regardless, gas burning efficiency is still relatively poor for this technology. Proper combustion depends on gas quality. Gas, by itself, contains perishable compounds with no more than sufficient caloric value, not to mention the harmful byproducts of an incomplete combustion process that relies on secondary air alone. Mixing combustion fuel together with primary air in just the right ratio is the only way to ensure proper, efficient combustion.
Secondary air is certainly abundant. However, it is basically impossible for secondary air to mix with byproducts and burn completely under open chamber conditions. Some gas particles travel far from the combustion core. Eventually, they end up in the roasting drum, the flue, and from there on into the ambient environment. And the expected result? External drum panels get coated in soot. The drum panels inside the drum were supposed to look the exact same way, if it was not for the beans, yes, your precious beans, that absorbed the soot. The beans scrub the inner drum panels clean in the process of roasting. As you would expect, you will not find this type of burner in Coffee-Tech’s product line.
Self-aspirated, metal wool, projecting screen burners
What self-aspirated, metal wool, projecting screen plate burners have to offer is somewhat different. The setup includes a sealed compartment with a supply tube and a nozzle at the opening. One side of that compartment is usually covered by a two-stage dispersion system. During the first stage of dispersion, preliminary gas is distributed over a mesh screen. During the second stage, gas is redistributed more evenly over a second, denser mesh screen. Both screen are made of woven high-heat metal alloy that is durable to oxidation and high temperatures over long periods of time. The metallurgical composition of the woven metal alloy, of which there are many, determines its quality and durability over time.
Heat distribution using woven metal screens is fairly efficient. It has a definite extra plus: heated metal screens generate high frequency infrared radiation. When radiation is combined with a perforated drum – as is the case with Coffee-Tech’s infrared drum – it can be applied directly to the beans. This effect fixes a good level of sugars and coffee ingredients, generating caramel without scorching the woody content of the beans. It helps set off their encapsulation by caramelized sugars. This produces that familiar unctuous sheen and makes the beans look smooth and firm. Maturing time and shelf life after roasting are both extended due to sugar capsulation. Combining this type of burner with a non-perforated drum does not make any difference in terms of coffee quality compared to a regular burner. However, it does increase energy efficiency and reduce pollution, both good in their own right.
Another added bonus of woven metal screens is the extraction of fuel ingredients as gas passes through. The screen operates as an oxidizing agent, which helps complete the burning process of some of the more ‘obstinate’ gas particles, though not all. Coffee-Tech does not use this type of burner either.
(Ram A. Evgi)
Tune in for our next blog post in which we will continue with gas burners, this time we will dwell into the second burners group. On the next post I’ll describe these modern burners and relate their relevance for Coffee-Tech’s roasters.