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Kilowatts/BTU's Needed per CFM
allenb
To help our members with understanding needed kilowatts/BTU's for a given cubic feet per minute of air flow, I'm re-posting some formulas from a previous thread. It seems that this area of engineering is something a lot of us are unfamiliar with and caused some recent heated discussions.

For calculating needed Kilowatts: (CFM X temperature rise F) divided by 3193 = Kilowatts

If you know the kilowatts of your heat source and want to know how many CFM you're limited to for being able to reach the delta rise in degrees F then:

KW X 3193 divided by delta rise = CFM

My first fluidbed which was able to roast 1/4 lb flowed around 10 cfm and could hit 500F leaving the heating element with a 75 degree inlet temp (425 degree delta). This roaster had a 1.2 kw element.

The formula proves this to be very close:

(10 x 425) / 3193 = 1.3 KW

Another example:

If we're flowing 40 cfm through an element at a delta T of 350 F then we have the following:

(40 x 350) / 3193 = 4.38 KW

If you're using a gas burner instead of an electric element then you'll need to convert your burner's BTU's to KW which uses this formula:

BTU's divided by 3412 = KW

FYI, some heatgun manufacturers routinely claim impossibly high CFM flow rates for a given discharge temperature. One that seems to be honest in their specs:

Steinel 34890-48047 Features:
Product Type: Heat Gun
Air Flow Rate: 3.6 to 17.6cfm
Nozzle Size: 1.3in
Cord Type: 6 ft Industrial Grade Rubber Wire
Power Rating: 1600W
Voltage Rating: 120V ac
Current Rating: 12.5A
Minimum Operating Temperature: +120F
Maximum Operating Temperature: +1200F
Dimensions: 10.2 x 3.5 x 8in
Length: 10.2in
Width: 3.5in
Height: 8in
Cord Length: 6ft
Standards Met: cULus Listed, RoHS Complaint, ISO9001:2008

Notice that it shows 3.6 CFM for minimum air flow. The CFM at 1200 degrees is probably not much higher than this.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
BenKeith
I make my own and can also tell you, not all heating elements are equal, even if they are the same wattage.

You can have a heavier wire that has way fewer turns designed to fit in a smaller space or you can have a smaller wire requiring a much larger space, and both be the same wattage. Because of the
difference in surface area between the two heating elements the amount of heat they can transfer to the air stream is very different.
 
renatoa
How can I ascertain the BTU of a burner without knowing anything about ?

For example a gas stove burner... can I place a pot with one liter of water and calculate BTU from boiling time ? Or a similar method...
 
allenb
You can have a heavier wire that has way fewer turns designed to fit in a smaller space or you can have a smaller wire requiring a much larger space, and both be the same wattage. Because of the
difference in surface area between the two heating elements the amount of heat they can transfer to the air stream is very different.


The physics of heat transfer between an electric resistive heating element and an air stream is such that if an element is flowing X amps then there is X watts, the resulting heat from that element must transfer to the air regardless of whether the element is wound from hair size wire or the size of a stovetop tubular element. The only difference will be the surface temperature of the element. The resulting heat has only two choices, 1. transfer all heat to the air only or 2.transfer some to the air and some to the enclosure or ducting via radiant heat transfer. A so-called "inefficient" element will have a much higher surface temperature than an "efficient" element due to it's reduced heat transfer rate.

I had an argument with one of our members a few years ago on this subject and he won the argument by showing the physics behind it. His statement was "Watts in = Heat Watts out". This isn't theory but is fact. I finally did an experiment using a tubular element versus a very well engineered wound nichrome element and with both being the same wattage, they both gave the same discharge temperature although one was bright yellow (tubular) and the other was a dull red (wound nichrome).

The only real difference is which will give you the most lifespan. The element with the cooler surface temp will last a lot longer!
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
BenKeith
Yes, that is correct, watts in=watts out, there is no way around that.
I looks like your experiment had a little different results that the ones I did several years ago.
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.

I guess that's why I don't calculate mine. I just measure how much air I need, and then using that, I make my heater to efficiently give me the temperature I want for that amount of air.
 
allenb
renatoa wrote:

How can I ascertain the BTU of a burner without knowing anything about ?

For example a gas stove burner... can I place a pot with one liter of water and calculate BTU from boiling time ? Or a similar method...


An average BTU rating for a stovetop burner is 7-8 kbtu's.

Calculating BTU's from boiling time etc. is too imprecise due to many variables that would change the outcome.

To find out what a burner can do, you need to know what the gas flow rate is
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
renatoa
What I know is the gas presure, the stove has a 20 mbar pressure regulator, and the nozzle diameter, 0.95 mm.
No idea if the burner diameter matters, or the flame size...

Are the 7-8 kBTU enough for 1 kg roasting ?
 
allenb
In looking over a chart I put together a while back which was based on what I saw advertised from various manufacturers, I would go a minimum of 12,000 BTU's and to have some headroom make it 14,000. So, two stovetop gas burners should do it.

Fluidbed--------------------Drum

lbs--- BTU's---KW---------lbs---BTU's---KW
1-----10200--3kw---------1----6800----2kw
2-----17000--5kw---------2----10200---3 kw
5-----25500--7.5kw-------5----17000---5kw
8-----34000--10kw--------8
10----40800--12kw-------10---35700---10.5kw
12----47600--14kw-------3k

Running the calcs with the orifice and pressure you've posted comes up to 9800 BTU's.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
allenb
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
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
coffeeroastersclub
Allen,

As a point to consider, I would state that reclaimed heat in applicable fluid bed roasters has BTU's that would reduce the needed kilowatts you state in your formulas.

Len
"If this is coffee, please bring me some tea but if this is tea, please bring me some coffee." ~Abraham Lincoln
 
http://www.coffeeroastersclub.com
allenb
Hi Len, the formulas work for any type of convection heating arrangement. The formulas are using delta rise and cfm. So, the only difference in a fluidbed using straight ambient temperature air versus a reclaim design is the reclaim has less delta rise to contend with but the math works for both designs.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
coffeeroastersclub
Thanks Allen. I am interested in seeing how OGH's design works with that formula. OGH care to chime in?

Len
"If this is coffee, please bring me some tea but if this is tea, please bring me some coffee." ~Abraham Lincoln
 
http://www.coffeeroastersclub.com
oldgearhead
"If we are flowing 40 cfm at a delta T of 350F, then we have:

(40x350) / 3193 = 4.38kw"


Since the variables, cfm at delta T are very close to what I have measured. I do not understand why my result (1400 watts) is so much less than yours?

Therefore, next Wednesday:
1) Load the roaster with 500 grams of green Kenya AA.
2) Set the temperature controller to 90% (1375 watts).
3) Install the 'short' pipe on the roaster so I can use the Florite (Right one in the pic) anemometer at the top of the RC.
4) I'll install a thermocouple in the Florite.
5) I will calibrate the Florite with the modern plastic anemometer (Left one), it will melt at a much lower temp than the Florite one.
6) I 'll measure both the flow and the air temperature at the top of the RC during the roast.

I will publish the results, plus the time to FC here.
oldgearhead attached the following image:
dsc_7664.jpg

No oil on my beans...
 
oldgearhead
02/06/2017 Airspeed tests Coffee Roaster
CFM per 400 and 500 gram loads

Calibration for both meters is 6 meters/second for the little plastic one and 1300 feet/minute on large cast metal one. Both are pretty close at 50 cubic feet/minutes (diameter is fixed 2.75 round), the ambient temperature was 53F, and the voltage on the Variac was 50 volts.

Does flow increase with temperature? The following test started with 53F air temperature and ended at 300F air temperature and the Variac controlling the blower was not changed from 50 volts:
_Air temperature = 53F and the air speed was 1300 feet/minute.
_Air temperature= 100F and the air speed was 1300 feet/minute.
_Air temperature= 200F and the air speed was 1300 feet/minute.
_Air temperature= 300F and the air speed was 1300 feet/minute.

How much air does it take to fluidize? All measurements were made with the Florite meter and 53F ambient air temperature:
400g roasted = 50 CF/M (1300 feet/minute, 58 Volts on the Variac).
400g green = 72 CF/M (1700 feet/minute, 70 volts on the Variac).
500g green = 74 CF/M (1800 feet/minute, 80 volts on the Variac).

It appears buoyancy has a big influence on flow rate. Archimedes proposed that when a sphere doubles in diameter, the surface area will be 4x the original. I know a coffee bean is not a sphere. However, we do know surface area does not affect buoyancy but volume does. Is this volume contained air or displaced volume? At any rate, its the ever-increasing buoyancy of the beans that make my roaster, roast.

I will do the flow and temperature measurements, while roasting, tomorrowoh I contacted Master Appliance about delta T of their model HG751B. Let's see if they respond..
Edited by oldgearhead on 02/07/2017 12:47
No oil on my beans...
 
oldgearhead
...another factor is the ever-decreasing delta T in the "brewer to Roaster". This has never been measured, accurately, on the roaster because of physical and equipment short comings. I may need to obtain additional testing tools. I will try to get a good delta T reading next.
No oil on my beans...
 
oldgearhead
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 180F 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...
No oil on my beans...
 
coffeeroastersclub
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 180F 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...


OGH, what accounts for the increase of CFM? Heat causing the air to expand?

Len
"If this is coffee, please bring me some tea but if this is tea, please bring me some coffee." ~Abraham Lincoln
 
http://www.coffeeroastersclub.com
oldgearhead
Well, Stevea said that the reason Master Appliance heat gun spec was 23 CFM at 1000F was they were stating output CFM vs. input CFM, and he mentioned the conversion 293/811 and I very quickly measured 58 CFM out and 39 CFM in, with no beans...btw - I also measured 180F delta T ..Does anyone concur?
No oil on my beans...
 
allenb
I had never thought about the difference between input and output cfm with the hotter output having been expanded but that makes perfect sense.

That is most likely the reason for the higher numbers you've been seeing.

That brings up another puzzle to solve. Does an anemometer have to have a correction factor added to a reading for temperature? You would think the lower air density of air in the 500 degree F range would cause the anemometer to read less than with room temperature air and would cancel out the higher velocity factor of the hotter air flow. Probably not the case but would be good to know the answer.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
oldgearhead
It depends if you want to go the trouble of SCPM vs. CPM. From high school, I know a balloon expands a lot when heated...This afternoon I will roast three batches of coffee, and measuring delta T with beans. I know it is smaller with beans than without..I do not have a good way to measure the input CFM, because the manifold is relieved by 1.5 square inches to drive the RPM of the blower higher to prevent blower self-heating...

SCFM = CFM * (Pactual/14.7psi)*(528R/Tactual)
No oil on my beans...
 
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