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The long and short of it is...
THE SIMPLE ANSWER to this problem is airflow, airflow, airflow. The complication, however, is determining just where the airflow problem is originating: coffee, technique, weather, drum, blower, internal ducting or external ducting.
Let's start with the coffee. Changes in coffee, or increases in the use of certain types of coffees, can significantly increase the amount of chaff. Brazils, Sumatras and Ethiopian Harrars come quickly to mind, especially if they are dry processed. The amount of chaff thrown off by any one coffee should not be an issue in and of itself, however this increases in the use of these softer-bean coffees necessitate an increase in the frequency of cleaning and general maintenance of the roaster. It is pretty simple, really: more chaff, more cleaning.
Furthermore, these coffees are the very ones where you will first notice this problem. They are not unlike the canaries in the coal mine. Because they produce more chaff, you are more likely to begin to notice airflow problems with these particular coffees. If you see this problem with these coffees and no others, you still have an airflow problem, and you will eventually see it with other coffees if you do not determine the cause and take the appropriate actions.
Technique
In roasters that use a single blower for both roasting and cooling, failure to move the damper from the cooling bin to the roasting bin positions can cause a reduction in airflow through the drum and may cause chaff to remain in the drum.
Weather
Weather-related problems primarily affect the least-experienced roasters, or those that have recently moved a roaster or changed an existing duct. Thermal inversions, abnormally strong or gusty winds, or winds from unusual directions can cause transient airflow problems with roasters. The best way to determine if you have an issue with weather is by keeping a roasting log and documenting your weather so that you detect any problem patterns. If you are experiencing transient weather issues, there are several things you may be able to do to lessen, or even eliminate, the problem depending on the particulars of that weather, your exhaust set-up and even the surrounding buildings. Permanent fixes include extending the height of your exhaust, eliminating horizontal exhaust terminations and changing the termination head of your exhaust. And, of course, if you determine that your problem was caused by highly unusual weather, you can always choose not to roast when that type of weather occurs.
Drum
Over-packing the drum can cause airflow restrictions that will not allow the chaff to be efficiently pulled off the coffee. Once again, you will see this most often with Brazils, Sumatras and Ethiopian Harrars. Additionally, the increase in bean size with darker roasts can cause an over-packing situation, even if the roaster operates well with that load size at lighter roasts. However, if this problem is appearing after roasting well for some time, then over-packing is not the cause, although reducing load size can produce a short-term solution.
Blower
The heart of drum and air roasters is the blower. It is the blower that provides circulation that allows for convective action — the dominant form of heat transfer in both air and drum roasters — and that removes smoke and chaff from the roasted coffee. For drum roasters, insufficient cleaning of the roaster blower is the single greatest cause of chaff remaining in coffee. This is true because drum roaster blowers move dirty air. This air contains coffee oils, chaff in varying degrees and green-coffee dust. If allowed to accumulate on the blades of the blower, these "dirty" elements will begin to reduce the amount of air that the blower can pull through the roaster. Eventually, there may not be enough airflow to pull the chaff out of the drum (this is also the greatest cause of smoke in the roastery). How often you clean the blower is mostly dependent upon five things:
- Type of blower (impeller or squirrel cage)
- Exhaust run (how much resistance your blower has to overcome to push out the exhaust)
- Type of coffees roasted (see above)
- Darkness of roast (the darker the roasts, the more blower cleaning that is necessary)
- Frequency of roasting (the more often you roast, the more often you need to clean)
Internal Ducting
Internal ducting is generally accepted to be the ducting between the drum and the blower, and may or may not include an internal chaff collector. External chaff collectors are considered to be part of the exhaust, except where the blower is mounted atop an external chaff collector. Many tabletop roasters are configured in this manner.
It is important that roasters understand how the air moves within their respective roasting system in order to know how, and how often, to inspect and clean this ducting. Many roasters clean internal ducting very infrequently — every year or so, depending upon the cleanliness of the blower. The dirtier you allow your blower to get, the likelier you are to experience a partial or complete clogging of your internal duct. Complete cleaning of this ducting can be a time-consuming and backbreaking task. If you are unsure of how this duct runs, the usual frequency of cleaning or how to clean it, go back to your manual or ask your manufacturer.
External Ducting
Improper ducting materials or poorly laid out external ducting runs are the second-greatest cause of poor airflow through restriction; ducting problems are, in fact, the greatest cause when roasters experience this phenomenon upon initial installation. Ducts that have too many angles, long horizontal runs, horizontal terminations, restrictive caps, or are the wrong size can all cause chaff to remain on the coffee by negatively affecting airflow through the drum. These types of ducting restrictions increase back pressure upon the blower, decreasing the amount of air that can be pulled through the drum. They can often be noted upon startup of a new, recently moved or re-ducted machine.
However, these types of restrictions are not always noted at start-up, but they will increase the likelihood of trouble in the future as well as increase the amount of maintenance that must be performed — especially cleaning of the roaster blower. Roaster ducts should be inspected, if not cleaned, at the same frequency as the main blower. When cleaned, they should be free of oils, chaff and dust. Mushroom caps, or any cap with screen, should be avoided at all costs. China caps should be modified to prevent the air from being pushed back down toward the open duct. If possible, all vertical ducting terminations should use a no-loss or low-loss stackhead (see Roast's September/October 2009 issue for more on this). Horizontal terminations should be avoided, if possible.
Most recurring problems with roasters come down to airflow issues. And chaff on coffee is no different. Perhaps the most important thing to remember about all airflow issues is that a problem in one area can cause connected problems in another. For this reason, it is important to understand the flow of air — both the flow through your roaster and through your stack. If not, you may find yourself fixing a problem only to see it recur very quickly.
One last thing
Make sure that the airflow in the roaster is cleanded out, this in itself can cause blockage and then the air cannot flow as it needs.
Thanks to Terry Davis
