Fire and Heat — Gas, Burners, Thermocouples and More
By Liz Bishop and Doug Graf
Now for the hot stuff. We cannot talk about roasters without talking about fire and heat, the components that help you turn your green beans brown. In this piece, we will focus on natural gas- and propane-powered roasters.
As a reminder, working on your roaster’s burner or any gas appliance is not only unsafe, but also illegal in many places. This article is meant only to provide information to help you get to know your roasting machine better. Please call your registered gas professional before touching anything related to the gas source or your roaster’s burner.
What is a BTU?
The British thermal unit (BTU) is a unit of measurement used to quantify heat energy. It is defined as the amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit. BTUs are commonly used to measure the energy content of fuels like natural gas and propane, and you will find the required BTU minimum to operate your roasting equipment on the rating tag (along with many other useful technical tidbits about your roaster).
BTUs are used to calculate fuel consumption rates for propane and natural gas on various types of equipment. By knowing the BTU rating of your equipment and the energy content of the fuel, it is theoretically possible to estimate how much fuel will be consumed. But when it comes to roasting, your energy consumption also depends on many other factors: batch size, type of roaster and burner, between batch protocols, type and processing of your green coffee, etc.
Every roaster can benefit from a regular burner service performed by a qualified gas fitter. Photo courtesy of Vintage Coffee
Natural Gas and Propane
Natural gas is generally the gas of choice in locations where it is readily available. The default choice for most manufacturers is a natural gas burner and valve setup. In some areas and specific situations—such as more rural areas or nonpermanent settings like a roasting tent—propane tanks are typically used. Propane comes with some extra regulatory requirements, so if propane is your gas of choice, make sure to speak with your gas professional to avoid problems and subsequent delays. These regulations involve strict guidelines for storage, handling, installation, and safety measures to prevent leaks and explosions and ensure proper ventilation.
Propane has a higher energy content than natural gas, two and a half times the calorific value, meaning it contains more energy per unit volume. However, just hooking your roaster up to propane won’t mean that you get more power out of your roaster. When a roaster is set up correctly, it will have burner components that are appropriate to the type of fuel used, which will give you the same BTU output regardless of gas type.
Most gas-powered roasting machines can use either natural gas or propane, and the choice can be made when the roaster is purchased new. If you buy a used roaster, check what type of gas it has been set up for; most roasters are easily modified, if necessary, by a qualified professional. Your roaster manufacturer may have a gas conversion kit available, but if you must source the items independently, make sure you get the appropriate parts for your equipment. Again, work with a qualified professional who can guide you to the correct choices.
Photo courtesy of Vintage Coffee
Different Types of Gas Burners
There are two main types of burners used for gas-fired coffee roasters: atmospheric, and fan-assist or power burners. Most traditional drum roasters use an atmospheric burner, and they come in many shapes and sizes. These are generally what we think of when we picture a burner in a coffee roaster. Their operation relies on air from the room, or atmosphere being drawn over and around the burner for combustion, like a barbecue. A burner manifold, tube, block, or flame distribution setup is present, usually under the drum, with several burner orifices and jets of some type. The actual design and setup of the burner is something that most manufacturers put a lot of thought and effort into. There will either be a pilot light or a spark setup of some kind for ignition and safety. Gas is pushed out the orifices, grabs the oxygen for combustion from the air around the burner, and lights via the pilot or spark. The mix of fuel and oxygen is based on what’s available and reasonably efficient around the burner. The roaster then adjusts the burner by some type of gas valve to modulate the heat and roast the coffee.
Atmospheric burners are almost always located underneath or directly beside the wall of the drum. This gives us a mix of conductive and convective heat, as the hot air is drawn over the burner, through the roast chamber and out the exhaust. The balance of the two types of heat and energy is dependent on the specific design, size, jet type and distribution, airflow, and position of the burner itself. This is often one of the biggest differences between roasting machine brands. The specific performance of your roaster, as well as the balance of heat—and therefore energy sources—can vary based on the burner design and airflow in the system.
These types of burners, because they rely largely on the atmospheric air conditions within your roasting environment, may vary in effectiveness seasonally due to temperature, humidity, and other weather-based variations. They may also vary a certain amount through the course of a day due to environmental swings. This can lead to having to vary profiles to respond to these changes—one more reason to make sure your quality control program is consistent.
The other main type of burner is known as either a power, fan-assist or pre-mix burner. This type of burner uses a combustion air blower and some type of mixing chamber to control the fuel and air mixture before it’s ignited. The mix of the two components is set by a gas professional using a variety of analytical methods to make sure the combustion is as efficient as possible. The burner is usually set up in a combustion chamber near (but typically not directly below or beside) the drum. The air is then circulated through the roaster by the roaster’s exhaust fan, which gives these roasters a mostly convective heat source, with much less or virtually no conductive heat.
Because power burners use a highly controlled gas-to-air mixture for combustion, they are usually more efficient at burning fuel and have more precise modulation when it comes to heat control. When you adjust the gas on a power burner, you are not generally adjusting a gas valve, you are adjusting the speed of the combustion air blower which brings the gas with it at the efficient ratio that you set up. This leads to a burner that has a much wider range of control than most atmospheric burners, with much finer control throughout its operating range. It’s like a barbecue where the burner knob says zero to 10, but we all know that, from a practical standpoint, it functions best between 3 and 7—that’s how an atmospheric burner operates. A power burner, on the other hand, provides a much wider range of control. All that said, both styles of burners perform well and will roast great coffee if you learn how to maximize their potential.
Something often discussed when roasters talk about getting the best from their roasting machine is burner tuning. There is no doubt that your burner needs to be set up properly, by a gas professional, to get the most efficient fire at the correct pressure and firing rate. If your roaster is rated at 125,000 BTU, it should be running at 125,000 BTU, and your burner should be running cleanly and efficiently. There are a variety of tools that can be used to adjust a power burner in a way that measurably improves performance. The exhaust can be tested to make sure that the mix of fuel and oxygen is optimal and the burner is set up according to the manufacturer’s specifications.
With an atmospheric burner, there is much less opportunity for adjustment. Because it gets oxygen from the environment, that can’t be controlled, and adjusting the gas really only adjusts pressure. Some adjustments can be made, but it is a much simpler system, so fine-tuning is limited. When your roaster is installed and set up, the installers should absolutely adjust your burner to get the best possible performance out of it. Like everything else in life, though, we must work with the limitations of the equipment provided. Understanding what’s possible with your specific system will help you to get the most from what you have and understand what may or may not be possible. This can help avoid a lot of frustration and unrealistic expectations.
How Hot is it? Thermocouples and Resistance Thermal Detectors
We receive a lot of questions about thermocouples and other ways to measure temperature and collect accurate data. The questions mostly seem to be about how many probes to use, how and where to install them, their diameter and accuracy, and their lifespan. For some reason, the use of thermocouples seems to have the most pseudoscience surrounding it. The myths, misconceptions and just plain silliness around thermocouples and how they work are vast and varied. With that in mind, perhaps a bit of basic information to start would be smart.
First, there are two main types of tools to measure what’s happening in your roaster related to temperature: thermocouples and resistance thermal detectors (RTDs). Thermocouples are voltage-based and RTDs are resistance-based. Although they both exist to perform the same function, they read the temperatures in somewhat different ways. For coffee roasters, thermocouples are by far the most common type of temperature probe.
Thermocouples operate according to the phenomenon in physics known as the Seebeck effect, where a temperature difference between two conductors produces a voltage difference that can be read by some type of sensor. In each thermocouple, there are two wires of different metals that are joined at the tip to form a measuring point. When the temperature changes at the junction where the wires of the thermocouple are joined together, a voltage is generated. This voltage is proportional to the temperature difference between the hot and cold junctions of the thermocouple. It is then sent back down the wires and into the data collection point, which could be a Phidget box, programmable logic controller, or temperature readout device. Different temperatures correspond to specific voltages for a given type of thermocouple. Standardized tables are available that list these corresponding voltages and temperatures. In a pinch, these tables can be used to test if a thermocouple is reading correctly. By double-checking the measured voltage with the expected voltage for the corresponding temperature using an electrical tester, you can verify the accuracy of the thermocouple’s readings.
When installing and wiring the thermocouples to the coffee roaster, it’s vital to use only components, wires, terminals and connectors that are made specifically for the exact type of thermocouple being used. The voltages being generated can be quite small, so any issues with connections can render the probes useless. Thermocouples can also have issues with being too close to higher voltage wires, as induced voltage from an adjacent wire can affect readings, in some cases quite dramatically. There’s quite a variety of thermocouple types available, and each has its optimal range of operation and measurement. For coffee roasting, we typically use J type or K type (see Table 1, page 85). The easiest way to identify which type of thermocouple you have is by wire colors, but see Table 1 for details of our two favorites for roasters.
One of the biggest misconceptions about thermocouples is that temperature can be read all along the thermocouple metal arm, and getting a thermocouple deeper into the coffee gives a more accurate reading. Thermocouples only measure temperature where the two wires join, at the tip of the probe. Insertion depth of your probe and the specific placement of the probe are vital to getting good data, but it is only where the tip of the probe sits that matters. Varying the depth or placement of a probe can lead to dramatic changes in readings. If a probe must be removed for any reason, such as to clean it, make sure to mark it so that it goes back in the exact same spot.
It’s also important to remember that your probe is not measuring the temperature of the coffee. It is measuring the air in between the coffee. That’s as close as we can currently get in a practical sense, but it is an important distinction to remember. This is especially important if you’re varying batch sizes in your roaster. Probe placement is essential to keep consistency in your readings. Choosing an appropriate size and diameter of probe is also important. Where the probe is going, and what specifically it is measuring, is an important part of this decision.
Photo by Juan José Sánchez Macías
Lots of people seem to think that getting the smallest probe possible is always the best choice, but that’s not always the case. People also talk about thermocouples as being fast or slow. This refers to the diameter of the probe, with a smaller diameter being seen as faster. Obviously, all probes measure at the same speed; a smaller probe is not going to give you a quicker reading. A probe with a smaller diameter will react a bit more quickly to changes in temperatures and will give a slightly more accurate reading at any given moment, however, larger probes can provide durability, longevity, consistency and compatibility benefits that may be more important depending on your specific needs and preferences. The key is consistency. If your profiles have been created with a 3-mm or a 6-mm probe, the key is to keep the same probe if you want to use the same profiles. If you do decide to switch to a smaller or larger probe, some work will have to be done to redo profiles with the new probes. It is possible for thermocouples to drift slightly over time, although it is a bit unusual as they will normally fail for mechanical reasons before they drift. Unfortunately, thermocouples cannot be recalibrated. If you are experiencing odd readings or encountering other issues, such as mechanical damage, corrosion or electrical problems, the thermocouples will need to be replaced.
RTDs operate on the principle that the flow of the electrical resistance changes depending on the temperature of the metal. As the temperature of the metal changes, so does the electrical flow of resistance, which is what RTDs measure and display.
RTDs can generally be more accurate than thermocouples, though they are slightly slower, perform better at lower temperatures, and are slightly more expensive as they use costlier components such as platinum, nickel or copper. RTDs are also much less susceptible to electrical interference than thermocouples, and don’t require as much specialized material and equipment to connect and run, making them simpler and more robust.
Thermocouples, however, are our industry standard at this point in all their complicated variety. We almost exclusively use them for our roasters and afterburners. They work very well for all our data collection needs if chosen and installed carefully in any coffee roaster.
It’s All About the Heat
Heat—and information about how much heat, when and exactly how it is applied, and its rate of change—is the backbone of coffee roasting. It has become one of the main topics of conversation almost any time roasters get together, and one of the main topics for roasting education in every form. Knowing more about the main sources of heat, how they work, and how they can be accurately measured will make most everyone a better roaster.
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Liz Bishop has been on her coffee journey since 2005. Walking down every path of the industry from barista to educator to roaster to Q Grader and green buyer, she is now on a deep dive into roasting machines and what makes them run smoothly. Based in Halifax, Canada, she can be found working alongside Doug Graf of Vintage Coffee at roasting operations around the globe.
Doug Graf has been doing all things roaster-related since 1986 and is now traveling the world as Vintage Coffee, working with coffee roasting companies to provide consulting and training. You can find his adventures on Instagram @doug_graf.
Bishop and Graf are Loring Smart Roaster certified technicians and sales representatives for Canada.
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