HomeRoast Digest


Topic: re hottop fan control (50 lines)
1) From: Jeffrey Pawlan
I did very extensive testing of the fan speed issue. I even made a special
version of my CCR program with a manual fan speed control in addition to the
rest of the program. I found that neither I nor any of the people who tasted the
resulting coffee roasts liked it as well as when the fan was on high all the
time. So it was not an oversight that the CCR program does not control the fan.
The roasts with the fan off or at low speed part of the time causes tipping at
the least and charring at the worst. The home roasters and the professional
cuppers who helped in the experiments did not like the taste.
The reason you and Mike are experiencing faster roasts with the fan off part of
the time is fully consistent which the thermal physics of the roaster which I
previously described. The heater is almost under-powered for the batch capacity
and the heat loss. The fan on DETRACTS from the heat rather than adds to the
heat. Open your hottop and remove the drum. You will see that the only place the
air can communicate with the space between the roasting cylinder and the outer
shell is through slits that are above the heating element. The fan is pulling
air out. Heat from the heater rises and is pulled out through those slits.
Also, you can easily see that there are no openings for the air to come in or
out through the wall at the back of the drum. The beans are roasted by a
combination of radiation from the close heater and by conduction to the drum.
More airflow adds convection and makes the environment more even. The drawback
is that the airflow unfortunately reduces the temperature and makes the roast
longer.
A gas fired roaster is just the opposite:
  In a professional gas fired roaster, the air is first pulled through the
firebox then it is directly sent through the open or perforated back of the
entire drum. Then it is sucked out through a vent in the top front of the drum
where the beans are entered. So in the professional roaster, the flames heat the
air and thus there is a great deal of convection heating of the beans. The
person doing the roasts decides how much air flow to use so he or she can get a
balance of convective heat from the air and conductive heat from the drum.
I had considered all the possible modifications of the airflow of the hottop but
decided that given the heater size and placement and the possible danger of
overheating of the cabinet, it was not worth it.
I hope that people will not mistake my comments as a criticism of the hottop. It
is a remarkably well made workhorse for homeroasting. The temperature monitoring
system aside, I would recommend buying a hottop.
Jeffrey Pawlan
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HomeRoast Digest