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Kilowatts/BTU's Needed per CFM
oldgearhead
The best explanation I found is at:

https://farnam-cu...ifference/

No oil on my beans...
oldgearhead
I roasted the 3 pounds this afternoon, I must wait for a new meter to arrive in order to make a good delta T measurement. I was wrong, the delta T increases throughout the roast and its the same with beans as without. Think the delta T after six minutes is going to be around 180?F-210?F. However, first crack is repeatable 9:30 to 10:15... I had a typo in the above post. It should be SCFM and CFM.
No oil on my beans...
oldgearhead
I have proven the HVAC formula posted by the OP to be correct

CFM x delta T/3193 = kw

What I found with the 'Brewer to Roaster' today was:
1) The mass decrease, the buoyancy increase, and the temperature, pressure, volume make up a fraction of the efficiency. The mistake I kept making was guessing at the delta T. As you can see from the data set below the measured delta T is 80?F to 100?F.
2) Ambient temperature = 50?F
__T = 3 min__deltaT= 100?F__Bean temp = 208?F
__T = 6 min__delta T= 81?F__Bean Temp = 331?F
__T = 9 min__delta T = 92?F__Bean temp = 381?F
__T= 12 min__delta T = 98?F__Bean temp = 420?F

So if I plug 93?F delta T into equation:

50 CFM x 92?F/3193 = 1.46 kw (I had it set to 1395w)

So I know know at least two things:
1) The roaster is very efficient.
2) Don't guess, measure it....
Edited by oldgearhead on 02/13/2017 2:23 PM
No oil on my beans...
oldgearhead

Quote

oldgearhead wrote:

Well I figured it out, I think. The formulas posted by the OP are input CFMs, and I have been reading heater output CFMs. I believe there is a about a 40% difference. My delta T is 180?F and my output of 50 CFM must have an Input CFM of 20. So:

20 x 180/3193 = 1.2 Kw.

Will somebody please tell me if I'm right or wrong...
This wrong!
No oil on my beans...
allenb

Quote

As you can see from the data set below the measured delta T is 80?F to 100?F.
2) Ambient temperature = 50?F
__T = 3 min__deltaT= 100?F__Bean temp = 208?F
__T = 6 min__delta T= 81?F__Bean Temp = 331?F
__T = 9 min__delta T = 92?F__Bean temp = 381?F
__T= 12 min__delta T = 98?F__Bean temp = 420?F


Shoot some more numbers to us. Temperature entering your heating element and temperature entering the roast chamber which will give us the delta T rise portion of the formula.

Except for excruciatingly efficient designs as with the bubblebed roaster, one needs at least 480 F entering the roast chamber in the later stages to approach 1C in a reasonable amount of time. I've not been able to pull off a typical 10-12 minute medium roast without achieving at least 500 F entering the RC during that portion of a roast. If you're sending around 150 F to the inlet of your heating element, then your delta is higher than 100 degrees.

One of the problems with most of our designs is the inability to accurately measure the temperature leaving the element as it is quasi laminar. Moving the probe 1/4" can show 100 degrees different than the previous spot. It's close to impossible to get a completely mixed air stream without adding mixing baffles and subsequent additional length to the roaster.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
oldgearhead
Ill start with your equation. CFM x delta T / 3193 = kw.
This is probably a HVAC rule of thumb based on the physics equation SCFM x delta T / 3 = watts.
The key here is SCFM. The temperature should be taken at the inlet and outlet of the blower to know SCFM as well as the pressure (we ignore for now).
To correct this we need to convert the input and output temperatures to K and factor the answer.
Outlet temp = 500?F = 530k
Inlet temp = 160?F = 345k so the factor is 345/510 or 0.68 multiplier.
The must critical measurement is SCFM at the outlet.
apparently I have no way to measure that on my roaster without a 'hot wire' meter.
All this can be explained by the Master Appliance heat gun specs of 23 CFM at 1000?F. This is output CFM. To know what the input CF is we first convert to k (degrees): 1000?F = 811k and 68?F = 293k.
Therefore, 23 CFM at output is about 8 CFM at the inlet. Thus 8 x 800/3 = 2100 watts, starting to get close to their hype..at any rate my measurements are off.
But what a do know is my beans 'crack' between 9 and 10 minutes and the roast chamber is about 495?F..I just don't know why???
No oil on my beans...
oldgearhead

Quote

allenb wrote:
[quote] >snip<
Except for excruciatingly efficient designs as with the bubblebed roaster, one needs at least 480 F entering the roast chamber in the later stages to approach 1C in a reasonable amount of time. I've not been able to pull off a typical 10-12 minute medium roast without achieving at least 500 F entering the RC during that portion of a roast.


What about this and the 200?C, 110 pound load, 3 minute, 1954 Areothrem?

?Coffee roasting is essentially a process of exposing the green coffee beans to a warming process that is sufficiently fast to drive off the free and bound moisture of the bean and the dried bean residue is heated to more than 400 F. At about this temperature, pyrolysis, or thermal decomposition and chemical change, occurs within the bean.? -Sivetz, Coffee Technology

No oil on my beans...
allenb

Quote

?Coffee roasting is essentially a process of exposing the green coffee beans to a warming process that is sufficiently fast to drive off the free and bound moisture of the bean and the dried bean residue is heated to more than 400 F. At about this temperature, pyrolysis, or thermal decomposition and chemical change, occurs within the bean.? -Sivetz, Coffee Technology


Mr Sivetz is correct in his statement "and the dried bean residue is heated to more than 400 F. At about this temperature, pyrolysis, or thermal decomposition and chemical change, occurs within the bean"

Quote

What about this and the 200?C, 110 pound load, 3 minute, 1954 Areothrem?


Any printed media that states the Aerotherm was able to roast a batch of coffee to anywhere beyond the beginning of first crack while presenting an air stream to the beans of no hotter than 200C is factually incorrect but probably made good advertising hype.

The 1 1/2 bag Sivetz I roasted on had to deliver 570 F during the latter stages of the roast to pull off a 10 minute roast.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
oldgearhead
"Any printed media that states the Aerotherm was able to roast a batch of coffee to anywhere beyond the beginning of first crack while presenting an air stream to the beans of no hotter than 200C is factually incorrect but probably made good advertising hype."

Not my words, or Aerothrem but M.Stivets. He goes on the explain, cost, levitation, how the entire 110 pounds is processed with 27500 Btu/ batch, (250 Btu/pound).

"Aerotherm ? A roaster that levitates the beans in 392?F (200?C) hot air?..(Michael Sivetz 1963 ?Coffee Processing Technology?.

Edited by oldgearhead on 02/24/2017 9:05 AM
No oil on my beans...
oldgearhead
Test 2/24/2017
I fixed an incorrect t-couple and ran tests today.
Weather is 70?F today and it will be snowing tomorrow. I set the % on time of the temperature controller on the ?Brewer to Roaster? at 75% or 7.5 seconds on and 2.5 seconds off. This should be about 1175 watts (1565 x .75). The formula I use for 430 gram loads is (195 ? ?C) ambient temperature in garage. T2 is the t-couple in the air being returned to the blower inlet, Ta is the t-couple in the air below the perforation plate, and T1 is the roast chamber exhaust air.

Time/ T2/ Ta/ T1
+03 min/ 131?F/ 265?F/ 212?F
+06 min/ 155?F/ 325?F/ 277?F
+09 min/ 167?F/ 369?F/ 302?F
+9:28 m First Crack
+11 min/ 178?F/ 396?F/ 336?F
+11:30/ 180?F/ 409?F/ 339?F Finish (FC+)

Notes:
1) It was warm today so the temperature drop across the RC was only 50?F.
2) I don?t have a good enough air-flow (SCFM) reading, but I did order the stuff to update my Omega HHF616. It does SCFM. Temperature to 250?F, and pressure to 5?wc. I eagerly await the mailman..
Edited by oldgearhead on 02/25/2017 9:14 AM
No oil on my beans...
oldgearhead
...and the heat energy used for 2/24/2017 roast of 430 grams of beans was:
1565 watts = Kill-O-Watt measurement of heating element
1174 watts = .75% of 1565
4000 = BTU/hr (watts x 3.412)
800 = BTU/batch (4000/5) 12 minutes is 1/5 of an hour

Dos this look like the correct calculation?

No oil on my beans...
allenb
Your calculations look correct to me.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
oldgearhead
It was much colder 3/01/2017, 13?C. So using the 430 gram equation (95 - Ambient in garage ?C), I arrive at 82% on time or 1283 watts:

1283 x 3.414 = 4381 BTU/hr
4381/5 = 876 BTU per 12 minute batch.

That's almost 10% more energy than the previous days results. First crack still starts before 10 minutes and finishes before 11 minutes.

No oil on my beans...
CK

Quote

allenb wrote:

Quote

I'm basing my statement on two identical Heartware Precision fan/heater units I was experimenting with. Both had 8 ohm heating elements. One had #22 Kanthal A1 one had #19 Kanthal A1. With both blowers running off separate 120 outlets, there is a big difference in the discharge air temps. Yes, one was a dull red, the other was a very bright yellow almost, which told me that, yes they were both making the same heat, but one was not transferring that heat into the air stream and was just wasted heat.


If I had seen the same outcome as you did I would have come to the same conclusion as you. Hard to say why the different outcomes. The only thing that I can guess that would have caused the lower output temperature is losses from radiant heat through the enclosure walls. Again, the only choice a watt has is to be absorbed by the enclosure or take a ride in the airstream. No place to hide!

Allen


From post #9

Although an old thread, I've perhaps stumbled upon a simple explanation that tells why some people have found seeming discrepancies in the heat transfer of electrical heating setups of our roasters. I use exclusively electric elements on my builds and each heater is plugged into independent circuits. These circuits have different lengths of feed wire from the panel to the outlet location, so naturally there will be a difference in resistance the for power to reach each outlet. When one turns on each heater, whether different or identical, each wire type or element type produces what is possible from its supply, never altering physics and the math involved in heat transfer.

On a new build of mine that uses 2 heaters, 2 blowers, 2 supply paths, each with their own identical thermocouple, my temperature readings are greatly different for each pipe... everything in the system is identical... except the outlet location and extension cord length to the outlet. When I swap the cords around, the heat output changes for both heaters again...

The basic lesson shown here is that electric heaters will produce different heat outputs unless all the wiring is identical right back to the panel.

(This is also seen by adding an extension cord to a hot air popper to extend roast times... longer length and different gauge wire from the supply will equal less heat.)
renatoa
Assuming you have 10 amps through one heater, according to awg ampacity tables, someone could choose AWG 20 for the wiring, because is rated for 11 Amps.
But... this wire has 33 mO resistance for one meter, thus 66 mO for a pair, 660 mO for ten meters to the panel.
Voltage drop on 660 mOhms and 10 Amps is 6.6V, thus about 2.8% voltage drop for a 230V panel, which means about 8% power drop. Each % of power variation led to about 3-4 C degrees of ET variation, thus 8% => 24-32 degrees, less or more.
So, why wonder the wiring matters ? Grin
And also mains voltage stability, 1% voltage variations are something common, and can be seen in RoR shape very clear for machines with low thermal inertia.

To minimize the wiring influence, you should choose at least double sized wire than you read in ampacity tables.
This means 3AWG units lower, like AWG17 instead AWG20.
Remember this rule, every 3 AWG units the wire resistance doubles/halves, and ampacity the same.
Edited by renatoa on 12/28/2020 2:00 AM
CK
Thanks for the information, suggestions, and math Renatoa. Your work always puts equations to the problems to clarify them.
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