THE HEAT IS ON

THE HEAT IS ON

God grant me the serenity
to accept the things I cannot change;
the courage to change the things I can;
and the wisdom to know the difference

THE SERENITY PRAYER seems tailor-made for the task of controlling and understanding the process of roasting coffee, especially when the discussion turns to heat and heat transfer. Conduction, convection and radiation--the three forms of heat transfer - are deceptively simple scientific concepts that underscore some of the more fractious debates within the coffee roasting industry. In our current age, when scientific terms are often used and misused, to prove or disprove often diametrically opposed points of view, these three terms are at the heart of two of modern roasting's most fundamental debates, one practical and the other very much philosophical.

The practical debate centers on how to control the rate of heat transfer in the roasting process or, more simply, how to control the roast itself. This debate is at the base of the more specific drum vs. air question, as well as any number of smaller arguments about what tools are the most necessary and return the best value for the investment, which manufacturer's equipment has the perfect balance between differing types of heat, and other niggling controversies. Many of these debates do nothing more than serve to distract us from the practical task at hand--controlling the roast.

The philosophical debate is simply whether the act of coffee roasting is an art or a science. And like many modern philosophical debates, both sides tend to use science, specifically the science of heat transfer, when making their case. Not unlike the debate over evolution and creation science, this dispute often reflects more about the user than the principles used to bolster either argument. And like the seemingly never-ending discussion about evolution, the absolutists on both sides seem to be the only ones with an opinion. The fallacy that is the art vs. science debate, and the absolutists on both sides, make it difficult for the rest of us to find common ground, to gain a better understanding of what we need to know and, more importantly, how that knowledge pertains to our own distinctive roasting operations.

Since most of us already have our roasting equipment, let's set both debates aside and assume that what we're really attempting to do is to gain better control of the roasting process within our own operations. Better control will help us create a better and more consistent product in a much more repeatable, and hopefully in a more efficient, manner. Better control requires a basic working understanding of the three types of heat transfer--conduction, convection and radiation--and how they work within a drum roaster. In short, as roasters we are interested in the application of science, not necessarily in the science itself.

Conduction

Conduction is the transfer of heat from direct contact between the molecules of a hotter substance to a cooler one. If you were to accidently touch the end of the trier to your nose while attempting to smell your coffee, and burn your nose, this would be the result of conducted heat: the hotter molecules of your trier directly transferring heat to the cooler molecules of your skin. In drum roasters, we have three potential sources of conducted heat: the drum, the faceplate and the beans. There are those who would argue that the metal of a hot cooling tray is also a potential conductor, but for this to occur the sides and/or bottom of the tray would need to be hotter than the coffee itself. And while warm cooling trays can lengthen cooling times, they should never be hot enough to conduct heat directly to the coffee.

The rate and ratio of conduction in a drum roaster is initially affected by drum preheat temperature and load mass. In most modern drum roasters, approximately 80 percent of the heat transference is via forced convection. In air roasters, the percentage is significantly higher.

Convection

Convection is the transfer of heat through currents in a liquid or gas. In the case of coffee roasting, the transferring substance is air and the receiving substance is coffee. There are two major types of convection: natural convection and forced convection. Natural convection occurs as our air heats up, causing density changes; as air grows hotter, it gets lighter and rises, while the denser, cooler air falls. This flow then allows heat to transfer through the natural movement of buoyancy. Forced convection is heat transferred through currents that are moved by an outside force, such as a pump or fan. Forced convection is a quicker, more efficient method of heat transfer than natural convection.

In both drum and air roasters, forced convection is the major mode of heat transfer in the roasting process. Air roasters force air through the roasting chamber via positive pressure (blow), and drum roasters use negative pressure (suck). Either way, when roasting personnel discuss convection in the roasting process, it is forced convection to which they are referring.

The rate and ratio of convection in a drum roaster is directly affected by airflow and energy supplied by the burner. The higher the airflow and the higher the energy input from the burner, the faster the roast.

Radiation

 

As the roaster, it is your job to begin to gain a better understanding of each source of heat.

Radiated heat is thermal radiation that is defined as electromagnetic waves, and it occurs naturally between two bodies of differing temperatures. It needs no carrying medium, unlike conduction and convection, and travels at the speed of light. A substance's ability to accept and/or throw off radiated heat is affected by its color, temperature, density, surface area, finish and geographical orientation to other thermal-producing bodies. In short, radiated heat is the most complex type of heat transfer for laymen to understand, and in the case of coffee roasting, very difficult to measure or to control. For roaster operators, the important thing to remember about radiated heat is that it exists, period. You can neither measure it nor control it, so realize it's there and then focus on the types of heat you can both measure and control.

There is, however, some confusion over radiant heat or infrared burners in drum roaster applications. Even using infra-red burners, it is still the conduction of the drum and the beans, plus the forced convection of the air, that is of primary concern to the operator.

The rate and ratio of radiation in a roaster is an unknown.

It Is About Total Energy

Drum, Air, Bean
Air, Drum, Bean
Bean, Drum, Air

These are the three sources of heat over which you, as a roaster, have some level of control. The little mantra above represents the periods of the roast at which each type of heat is at its most influential. At the beginning of the roast, the amount of stored energy in the drum--represented by drum or preheat temperature--is at its most important and potentially most damaging to the bean. Air or convection is the dominant form of heat transfer throughout the roast, but air is also the allimportant driver for the body and flavor formation portion of the roast. Toward the end of the roast, the coffee beans themselves become an important source of energy and can actually become the dominant way that heat is transferred in some roasts and/or roasters. At the end of the day, however, all of the above forms of energy play their part in the process. As the roaster, it is your job to begin to gain a better understanding of each source of heat, and then exercise that knowledge of each within your own equipment, to better control your roasting. This is what the concept of total energy is about.

Three important things to remember regarding total energy

1. Coffee roasting is a dynamic process that changes throughout the course of the roast.
2. There is infinitely more energy later in the process than earlier.
3. None of the forms of heat transfer are independent of one another.

Strategies For Gaining Control of the Roast

Conduction (Drum/Faceplate to Bean)

Many roasters believe that they have little or no control over drumto - bean conduction. This simply is not true. What is true is that you only truly have control over this type of heat transfer at the beginning of the roast. Once a roast has begun, there is little you can do to affect this type of heat transfer. But, at the beginning of the roast, there is plenty that you can do. Controlling drum-tobean conduction is all about preheat temperatures.

Roasters should set and follow preheat temperatures. Preheat temperatures represent stored energy. The higher the preheat temperature, the hotter the roaster, the more energy is stored in the drum and the faceplate, and the more energy can be transferred via conduction. By being consistent in your preheating, you will be starting every roast with approximately the same amount of stored energy, allowing you to roast in a more consistent manner.

For partial batches, it is absolutely imperative that you lower your preheat temperatures if you wish to follow a similar profile as when you are roasting a full batch. Less coffee (mass) requires less preheat energy--pretty simple. Partial-batch preheat temperatures can easily be determined with a little experimentation; just take note of the lowest reading via your bean probe after the coffee continued on page 70 The HEAT IS ON: A Roaster's Guide to Heat Transfer (continued) is dropped in the drum. You want this point of equilibrium (sometimes called "turning point") to be the same, or nearly the same, regardless of batch size. If, when roasting partial batches, you see that the point of equilibrium is above that of a full batch, then lower your preheat temperature the next time you roast this size of batch. Eventually, you will be able to determine the correct preheat temperatures for the varying load size of your roaster. Remember this when roasting partial batches in drum roasters: It is always easier to add energy than to take it away once your roast has begun.

Symptoms of too much conductive heat (drum to bean)

• Tipping
• Uneven roasting/too fast
• Mottled/scorched beans

Conduction (Bean to Bean)

Through the majority of the roast cycle, you have two sources of heat energy--the drum and the air. Just before you hear first crack, you have a third source of energy--the coffee beans themselves.

 

A well-functioning cooling system is critical for gaining control of roast profiles.

As the coffee approaches first crack, it begins to go exothermic and throws off heat, hence the sound associated with the cracking of the bean. Sound is a form of energy, and the cracking of the bean signifies that energy is being released. If this energy is not accounted for in the overall energy equation, then the roaster risks losing control of the profile, or the roast. There are three strategies for taking control of the roast at this point: adjust the burner down or off, increase airflow, or both. In essence, you are manipulating the rate of convection in order to control the total energy and hence the profile of the roast.

Symptoms of too much heat at first crack (bean to bean)

• Uneven roasting/too fast
• Unusual amounts of smoke
• Moving almost immediately from first to second crack

As you approach the end of the roast, you must be aware of the possible consequences of bean energy once again. The faster and harder you approach the termination of the roast, the more kinetic energy will need to be dissipated by the cooler. In other words, the more aggressive your profile curve is at this point, the harder it will be to stop the roast at your desired termination temperature. This can become especially critical if you are operating in a building that is not climatized, in an area where there are significant swings in temperatures throughout the year. Additionally, the darker the roast, the more energy is available to push the coffee past your desired stopping point. This potential problem can be handled in the same manner as you deal with gaining control at first crack: adjust the burner down or off, adjust the airflow, or both. By "slowing down" or reducing heat at the end of your roast, you will gain more control and use less energy as well. Once again, you are manipulating the rate of convection to lessen the impact of the energy of the beans themselves.

Note: A well-functioning cooling system is critical for gaining control of roast profiles. Many a darker-roast coffee has been, and still is, being ruined by inadequate or ill-functioning cooling systems. Research possible fixes.

Convection

Convection is the cornerstone of the roasting process for both drum and air roasters. It is the most dominant, the most easily understood, the most measurable and the most controllable. Once again, forced convection is heat carried by currents created by a fan or blower. You can change the rate of convection by changing the airflow, changing the energy output of the burner, or by a combination of the two. Unlike all forms of conduction in the roasting process, it is possible to make adjustments in the rate of convection that can have near-immediate effects on your roast profile. Although you cannot truly read convection, you can begin to get a handle on its effects by reading the drum environment temperature along with a bean probe, or through the use of a real-time datalogger.

Convection is truly a modern roaster's friend. A high rate of convection means coffee is roasted more evenly, more cleanly--as most smoke and chaff is pulled away from the coffee--and each roast is more controllable and repeatable. The trick is finding the technique that works best for you in your installation.

Here are 10 questions to consider or to obtain answers to as you seek to gain better control over convection, and hence the roast itself:

1. What style of burner do I have?
2. What does the style of burner I have mean to me as a roaster?
3. How does the manufacturer recommend that I control the burner?
4. How do I change airflow in my roaster?
5. Do I need to change the airflow of my roaster?
6. What does the manufacturer of my roaster recommend as the best way to control the rate of convection in the roasting process?
7. Is my roaster equipped with the necessary tools to allow me to do what I wish to do with my roasts?
8. What tools should I consider adding to my roaster, if any?
9. Is my installation causing me to lose some level of control of my roaster?
10. Is my roaster clean and well maintained?

Convection is the most dominant and the most controllable form of heat transfer and, as such, it is the place to start in gaining better control. Working through the questions at left can help you create a better, more flavorful product in a more efficient and more consistent manner--and that is truly the hallmark of a professional roaster.