The calculation used for loads fitting a Mason jar (430-440 grams) has proven very good: (95-garage ambient in °C = % on time of the heater). Because I use % on time instead of PID temperature control the energy used per batch is very easy to calculate power converted to energy used:
2/24/2017 - Temperature in the garage was 70°F (95-21°C = 74%), 1147 watts x 3.412 = 3916 BTU/hr.
Divide by 5 to see how much was used in 12 minutes = 783 BTUs/batch.
3/01/2017 – It was 20°F colder today (95-13°C = 82%) on time or 1271 Watts equals 1271 x 3.414 = 4339 BTU/hr x 20% = 868 BTU/batch.
These, energy used, calculations are over two times the theoretical number of 350 btus/pound set forth by Michael Sivetz in his first book "Coffee Processing Technology". We know from further reading that recycling 204°C commercial fluid-bed roasters did not get much better than 1200 btus/pound.
First crack was a very repeatable nine minutes and thirty seconds and my delta T appears variable from 134°F to 229°F and the final temperature was 412°F at 11.5 minutes. I am sure the delta T is not that variable because the cool beans influence the reading on the temperature probe mounted only 1.5 inches below them. However, the temperature rate-of-rise is rather dependent on the blower speed adjustment, but it is ever increasing during each roast.
My roaster mixes ambient air directly with the hot air exiting the top of the roast chamber and introduces it to the blower inlet. Inlet temperatures range from 150°F to 180°F. The spa-used, two-stage, flow-through, vac blower has been used twice a week for more than five years.
OGH, here is something I always wanted to ask you. What percentage do you think is recycled air? Do you have the ambient air inlet located at the bottom of the mix box and the final outlet on the very top? This is where I get confused due to negative (?) air pressure at the final exit? Positive air pressure at the ambient inlet?
"If this is coffee, please bring me some tea but if this is tea, please bring me some coffee." ~Abraham Lincoln
The volume of air being pulled down through the inlet pipe is 16.5-17.5 SCFM and it's temperature is 150-180°F. At a standard ambient air temperature of 68°F that is about 100°F increase, or about 25%...The pipe marked with the red tape is the inlet air supply. Ambient air is available under no positive or negative pressure..I need to update my photos, the chaff collector was removed years ago..
Please you look at the image I posted above, you will notice the top of the unit hinges down, covering both the inlet and RC outlet pipes. The 'box' is open at the back, unless it is below freezing, then it is baffled. Any positive pressure inside the 'box' is exited out through the small, former. water door. If you are familiar with commercial pour-over brewers you will get the picture.
The temperature drop across the roast chamber is controlled by insulating it during 30-50F garage temperatures..It will not be fun today, roasting in 40F ambient with an un-insulated roast chamber. However, I will post the energy used BTUs/batch later. I will also record the temperature drop across the roast chamber. It is 17F this morning so I'll wait till afternoon...
3/16/2017- The garage temperature was a cool 8°C (46°F) and this was not good for the temperature drop across the un-insulated glass roast chamber with an average drop of 87°F. I like to keep it below 60°F. The % output setting was 95-8 = 87% or 1550 x .87 = 1384 watts.
Load was 454 grams of Rwanda Kiru Lake from Happy Bean.
Energy used = (1348 x 3.414)/5 = 920 btus/pound.
The shrinkage was 454-388 = 66 grams or 14%.
I would call the resulting roasted beans a City roast. I prefer City+. But the cracks around 10 minutes were very hard to hear, so I ended the roast at exactly 12 minutes and 407°F.