Q: I notice that I am getting a lot of chaff in the coffee
when I drop it into the cooling bin when I finish a roast?
A: The simple answer to this problem is Airflow, Airflow,
Airflow. The complication, however, is
determining just where the airflow problem is originating: technique, coffee,
weather, drum, blower, internal ducting, or external ducting.
Technique
In roasters that use a single blower for both roasting and
cooling- failure to move the damper from the cooling to the roasting positions
can cause a reduction in airflow through the drum and may cause chaff to remain
in the drum. Although
Coffee
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 Ehtiopian
Harrars come quickly to mind. And while
the amount of chaff thrown off by any one coffee should not be an issue in and
of itself- 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.
Weather
Weather is a problem most experienced by 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 all 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. And of course, if you determine that your
problem was caused by highly unusual weather, you can always not roast when
that type of weather reoccurs. Typical permanent fixes include: extending the
height if your exhaust, eliminating horizontal exhaust terminations and
changing the termination head of your exhaust.
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
the types of coffees mentioned above.
Additionally, darker roasts, because of the increase in bean size can
cause an over-packing situation, even if the roaster operates fine with that
load size at lighter roasts. However, if this problem is appearing after
roasting fine for some time, then over-packing is not the cause, although
reducing load size can affect 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 dominate 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 and eventually can be reduced to a point
where there is no longer 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 5 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 the chaff
collector, if the chaff collector is internal.
External chaff collectors are considered to be part of the exhaust,
except where the blower is mounted atop an external chaff collector, many table
top 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/clean this ducting. The
frequency of cleaning internal ducting can be quite long, many roasters do this
every year or more, but is very dependent upon the cleanliness of 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 back breaking task.
If you are unsure of how this duct runs, the usual frequency and 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 of 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 start-up of a
new, recently moved, or reducted 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 allow the air to not be pushed back down toward the open
duct. If at all possible all vertical
ducting terminations should use a no-loss or low loss stack head. (see Ask the Expert Roast Oct/Nov 09). Horizontal terminations should be avoided, if
at all 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 try and
understand the flow of air; the flow
through your roaster and through your stack. If not you may find yourself fixing a
problem, only to see it reoccur very quickly.
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