The Roasters Realm
Keeping Cool: Troubleshooting Cooling Problems in Drum Roasters
By Terry Davis
Photo by Kenneth R. Olson

Before I get to this month's inaugural Roaster's Realm topic, I'd like to take a few lines to pitch the Roaster's Guild. I firmly believe that no other organization in our industry today is doing more to promote quality in coffee than the Guild. It is helping to expand training, dealing not just with technical roasting and blending issues but also with cupping, trips to origin, fair trade, organics, brewing and grinding, and consumer education. Yet the Guild's greatest value lies in the membership itself. The Roaster's Guild acts as a forum, facilitating the open exchange of ideas and opinions amongst roasting professionals at all levels of the coffee industry. It is this very exchange that is advancing the operation of roasting equipment to where it can truly be considered a trade and a craft, and the roaster truly a coffee professional.
   Poor airflow is the most common problem in drum roasters. Although airflow problems affect all aspects of the roasting process, cooling is the most susceptible. If left unchecked, poor cooling can lengthen production times, reduce worker productivity, dampen sales, and alter the taste of the final product. In darker roasts, poor cooling can be fatal. In fact, many dark roasts are destroyed when coolers are unable to stop the roasting process, or cool in the proper time, while many lighter roasts are taken to another roast level. This can happen to anyone, regardless of experience, but it's usually the fate of beginning personnel or those who have only roasted on a single roaster in a single installation, and have inherited the problem. In a perfect world, cooler performance could be enhanced by simply placing a large fan near the roaster and "forcing" air down the cooling tray-but life is never that simple. So, how do you identify a problem with your cooling system?
   Your first clue is taste. Cupping should give the first indication that something is occurring in the roasting process to adversely affect the taste of your coffee. All serious roasters should be cupping anyway, for quality control. Cupping forms can be obtained from the Specialty Coffee Association of America (SCAA), the Coffee Quality Institute, Kenneth David's Home Coffee Roasting, or from other roasting professionals. If you're not cupping regularly, you'll need to listen even more closely to your customers. They will be the first to notice a change in the taste of your coffee, whether you're exclusively a retail roaster, a wholesale roaster or a little of both. If your once-dynamic French, Italian and espresso roasts have taken on a flat, burnt or baked aftertaste, you probably have a problem with airflow in the cooler.
   The second most likely indication that you have a problem with cooling is an increase in your cooling times. Because degradation of cooler airflow often occurs over many weeks or even months, it's important to keep accurate roasting logs that also track cooling times.
   Smoke should vanish downwards through your cooler tray. So if you see smoke rising from the tray, you have a cooler problem. Although poor airflow through the cooler can affect the taste of all coffees at all roast levels, it is more damaging to dark roasts, due to the higher amount of energy they contain.
   By now, you know you have an airflow problem. Now you should learn why, and what you can do about it.
   There are two major categories of causes for poor cooling airflow: poor installation and dirty equipment. Both factors can affect all drum roasters in all configurations, including those with separate cooling blowers or pollution control devices.
   Troubleshooting should begin with a simple question: "Has the problem always been there?" If the answer is yes, then your problem is installation. If the issue has only cropped up recently, then you need to do a little spring cleaning.
   Let's begin with dirty equipment. If your roaster is a single blower model, with a cooler that works off a damper system from the roaster blower, then the most likely culprit is the blower itself. Although this can also be seen in roasters with dedicated cooling blowers, this is less common. Both problems usually plague operators that do a lot of darker roasts. The darker roasts increase the amount of oily residue in the exhaust smoke and increase chaff, causing the blower to become dirty sooner. The fix for this is easy, but often painful: You must clean the blower.
   On most drum roasters this can only be accomplished by pulling the blower wheel (and usually the blower motor) and scraping the blades with a steel brush or paint scraper, depending on the blower type and blade configuration. If you are unsure of the best way to do this, call the manufacturer or distributor. You may also want to consult the Roaster's Guild (www.roastersguild.org) or track down someone with the same roaster-and good cleaning habits. Make no mistake, this is a dirty and time-consuming job, so you may want to combine it with other involved maintenance, like cleaning your stack, inspecting and changing belts and bearings, and the like. You can make the cleaning job easier by dousing the blower (not the blower motor) with a commercial coffee cleanser such as Urnex, Purocaf or Clean That Pot beforehand. We have found these products to be cheaper and easier to use in powder or pill form.
   The next most likely place to look if your cooling times are slowing is in your cooling tray itself. If your tray is even partially clogged it can drastically increase your cooling times. Clean it. And when cleaning your cooling screen be sure to clean the underside. This is also a good time to check the functioning of your cooling damper and the internal ducting (if any) between the cooler and the blower.
   The final place to check for problems is your exhaust ducting, especially the cap at the end of your ducting run. Any obstructions in the duct or cap will retard cooling times. Remember, airflow cleanliness issues have a way of compounding one another, so if one part needs cleaning, the others may also require attention. At any rate they should be inspected on a regular schedule.
   If your equipment is clean and well maintained, or your cooling times have always been too slow, then installation is the most probable culprit.
   Poor ducting installation is the usual cause for slow initial cooling. Both single and dual blower roasters can suffer from bad exhaust installation. Single blower roasters are more likely to experience slower cooling times, while dual blower units are more prone to poor roaster airflow and smoke. The list of subpar exhaust installations is long, varied and painful. It encompasses runs that are too long or have too many angles (each 90-degree angle is roughly equivalent to 10 feet of a straight run), horizontal runs, multiple pieces of equipment on one run, and restrictions. In technical terms, a restriction can be the wrong type of exhaust cap (screened caps are notorious cloggers), an inappropriate duct diameter, an undersized afterburner, or even wind patterns (wind can be especially detrimental if an exhaust is vented horizontally). Dual blower roasters are susceptible to restrictions caused by running both exhausts together at an overly sharp angle (a "T" instead of a "Y"), thus creating a cross current dam. If you don't increase the diameter of the duct after the intersection, you'll cause one exhaust to overpower the other. Keep in mind that it is usually the roasting system itself, and not the cooling system, that is affected by this particular problem.
   Another installation-related issue is competing equipment, especially hood systems, powerful exhaust fans and large A/C returns near the roaster. Drum roasting equipment creates negative pressure-it sucks air across the burners, through the drum and through the cooling tray. Other air-pulling systems can compete with the roaster. The effects of this conflict are usually noticed in the cooling system first. Remember those strong fans blowing across the cooler? Besides blowing smoke around your roasting space, they can actually have a negative effect on cooling. If you suspect that one of the above may be extending your cooling times, contact the manufacturer.
   What if you're unable to change your ducting installation enough to get your cooling times down, or are unable to roast and cool simultaneously? You're in luck. You can add a separate or additional blower to a single blower roaster, thereby separating your roasting and cooling systems. This is a fairly simple modification that can be done for under $250. It consists of a blower with a motor of adequate CFMs (cubic feet per minute) to handle the batch size, a little flexible ducting and an electrical cord set. Grainger sells a great blower (stock # 4YJ33) that can handle cooling up to 15 kilos fairly easily and is manufactured to handle the higher temperatures associated with dark roasts. On some drum roasters it can be mounted internally, on some externally, but is easiest to set up in a stand-alone "box" near the existing exhaust outlet.
   Part of the fun and challenge of being a coffee roasting professional is learning to understand and maintain your equipment. If you are patient and diligent, you'll soon find that cleanliness is next to godliness-and it makes your coffee taste better, too!

Terry Davis is President of Ambex Inc., a coffee roaster manufacturer, and New Harmony Coffee and Tea, a retail roastery located in Clearwater, Florida.