Calculate BTU’s Produced By a Pellet Stove

[whitsett2014]

knock it off.

Do you disagree with this calculation?
If so please explain
Thanks!
Whitsett

 

[kofkorn]

The stove rating of 48K BTU is not the amount of BTU's that the stove puts into the room, it's the total amount that the stove consumes, as BDPVT says "Input Rating". So for a VERY simple calculation, the heat output into the room would be the Input Rating multiplied by the efficiency of the stove:

48000BTU/hr * .78 = 37440 BTU/hr

Since we are calculating this backwards, I am doing a quick sanity check to see if our calculated heat output matches the stated stove rating. Since we are working backwards from the heat output into the room, we divide by the efficiency rating.

Now, I (and I think many other people) don't agree with the way that the EPA comes up with an efficiency rating. It is grossly overstating the capabilities of nearly every stove. As stated previously, the efficiency is a combination of combustion efficiency (~98%), electrical efficiency (~99%), and heat exchanger efficiency (~60%). The EPA takes these numbers and averages them, which really doesn't make sense to me. However, this is a conversation for a whole different thread though.

 

[SmokeyTheBear]

The stove rating of 48K BTU is not the amount of BTU's that the stove puts into the room, it's the total amount that the stove consumes, as BDPVT says "Input Rating". So for a VERY simple calculation, the heat output into the room would be the Input Rating multiplied by the efficiency of the stove:

48000BTU/hr * .78 = 37440 BTU/hr

Since we are calculating this backwards, I am doing a quick sanity check to see if our calculated heat output matches the stated stove rating. Since we are working backwards from the heat output into the room, we divide by the efficiency rating.

Now, I (and I think many other people) don't agree with the way that the EPA comes up with an efficiency rating. It is grossly overstating the capabilities of nearly every stove. As stated previously, the efficiency is a combination of combustion efficiency (~98%), electrical efficiency (~99%), and heat exchanger efficiency (~60%). The EPA takes these numbers and averages them, which really doesn't make sense to me. However, this is a conversation for a whole different thread though.

And likely has been in one form or another.

 

[jtakeman]

As stated previously, the efficiency is a combination of combustion efficiency (~98%), electrical efficiency (~99%), and heat exchanger efficiency (~60%). The EPA takes these numbers and averages them, which really doesn't make sense to me. However, this is a conversation for a whole different thread though.

Yep, I do have to say though. The reasoning to include combustion efficiency (~98%), electrical efficiency (~99%) was a way to get the majority of the stoves approved. If they only used the heat exchanger efficiency a lot of stoves woud not be approved.

You guys are really getting into this ey?

 

[whitsett2014]

So,
I have presented the method that both kofkorn and I have been discussing to my Project Manager, and he agreed with most of the calculations. We concluded that because we are measuring output temperatures directly, that the overall efficiency does not need to be taken into account. (feel free to comment if you disagree)

I presented him with the thought that much of the heat that the stove produces is also transferred by radiation. I'm now going to try to come up with some kind of calculation for the amount of heat that is gained from this. I would like to thank whomever brought this point up earlier in the discussion.

Does anyone have any "ballpark" idea of how much heat the stove produces through radiation?

Thanks,
Whitsett

 

[kofkorn]

The efficiency calculation is simply a check to make sure the rest of the calculations made sense. You are correct that they are not needed.

The radiated heat output is a much tougher challenge without some direct measurements. Every stove design is different, with many heat exchanger designs and shapes. Some of the stoves have a significant amount of the combustion chamber as the outside of the stove. Many also use double wall design that greatly reduces the radiated heat.

I think in this case it is relatively simple to take some direct measurements on the exterior of the stove you intend to use using a thermocouple or non-contact temp probe, measure the area that is at each temperature and use the radiated energy calculations to get your answer. It should be a fairly accurate calculation.

 

[checkthisout]

knock it off.

Do you disagree with this calculation?

I do not understand the calculations enough to judge if they are correct or not.

You were just making my head hurt.

 

[checkthisout]

Now, I (and I think many other people) don't agree with the way that the EPA comes up with an efficiency rating. It is grossly overstating the capabilities of nearly every stove. As stated previously, the efficiency is a combination of combustion efficiency (~98%), electrical efficiency (~99%), and heat exchanger efficiency (~60%). The EPA takes these numbers and averages them, which really doesn't make sense to me. However, this is a conversation for a whole different thread though.

I think it's a nice sidebar while this thread gets underway.

It makes absolutely ZERO sense to anyone who at least knows how to open a pack of Starbursts.

Are furnaces rated this same way?

 

[tjnamtiw]

So,
I We concluded that because we are measuring output temperatures directly, that the overall efficiency does not need to be taken into account. (feel free to comment if you disagree)

If you are a renewable energy consuming company (or hope to be), why would you not want to consider how much energy you are consuming in order to get a given amount of output? That is efficiency. Even your renewable energy takes energy to manufacture. That's why your pellets aren't free. It's like the very plausible argument against using corn to produce ethanol for cars. If you are consuming more energy to make your fuel than you are getting out of it, then it is no longer viable.

 

[whitsett2014]

If you are a renewable energy consuming company (or hope to be), why would you not want to consider how much energy you are consuming in order to get a given amount of output? That is efficiency. Even your renewable energy takes energy to manufacture. That's why your pellets aren't free. It's like the very plausible argument against using corn to produce ethanol for cars. If you are consuming more energy to make your fuel than you are getting out of it, then it is no longer viable.

I can assure you we are. Our building is LEED platinum certified . The efficiency is important, however at the present time we are just looking for the actual offset that the stove is providing for us. The efficiency will undoubtedly be taken into account at a later time.

Thanks,
Whitsett

 

[whitsett2014]

I hit a small road block...
I don't know what type of material the stove surface which is emitting the radiation is made out of... Might be safe to assume iron? I need to know this to find the emissivity of the material which is used to calculate the thermal radiation.
It's a HarMan Advance.... the surfaces I measured the temperatures of appeared to all be made of the same material.
Surfaces I measured were: Metal above, below left and right of the glass window.
All of these surfaces showed a significant temperature increase from room temperature.
Upper:75 C
Right: 58 C
Left: 57 C
Room Temp: 16 C

 

[kofkorn]

Steel or cast iron depending on the stove. If its a relatively smooth surface, it steel. Rougher surface and it's cast iron. Did you measure the sides of the stove around the corners from the window? Heat is thrown off of the side of the stove as well.

 

[whitsett2014]

Well,
I used an infrared temperature gun to gather that rough temperature data. I'm going to get some temperature sensors/loggers to gather the information regarding the window itself. Obviously the IR temperature sensor was going through the glass and reading the fire temperature . The stove is magnetic, so I'm assuming that it is Iron (I know that steel can be made magnetic but there is really no other way of differentiating). However, the surface is VERY smooth. Which according to kofkorn would point towards steel. So, magnetized steel, or smooth iron?
Emissivity Values
Cast Iron, newly turned 0.44
Cast Iron, turned and heated 0.60 - 0.70

These are the only plausible types of metals that I have come across. I think that its a shot in the dark really... I could just average them, worked for the EPA efficiency, why not work for me?

Is this considered a black body? I honestly have no clue...

 

[kofkorn]

In general, unless you are talking about 300 series stainless steel or some special compound, all steels are magnetic. Additionally, you will very rarely find pure iron in a product sold on the mass market. It's too soft and difficult to work with. So you're dealing with a steel surface.

After a little research, it appears that the emissivity is more a function of the coating on the surface than the substrate under it.

http://www.molalla.net/members/leeper/coatbar.htm

Based on measurements in this site:

http://snap.fnal.gov/crshield/crs-mech/emissivity-eoi.html

The black paints average in the .85 - .9 emissivity range. I would use middle of the range @ .88 which seems to line up nicely with the steel numbers above.

A black body is one of those text book terms that never happens in reality. To simplify things though, you can use the black body assumption.

 

[whitsett2014]

Excellent! I'm going to run some numbers and I will post them shortly.
Thanks a lot kofkorn!

 

[whitsett2014]

So, some calculations here.
Using the emissivity of .88 for the unknown but well estimated metal.
e=.88
Three Areas to Evaluate.
A1=A3=.033m^2
A2=.0685m^2
s=sigma=Stfan-Boltzmann constant= 5.67x10^-8
Tk= Temperature in Kelvin
Power=P=A*e*s*(Tk)^4
for A1 (57 C)=19.563W
A2 (75 C)=50.214 W
A3 (58 C)= 19.8 W
Sum all of these and convert to BTU/hr comes out to 305.65
This is only for the three surfaces located at the foot of the stove. (shown in the picture they are directly below the glass in the following link)
http://www.harmanstoves.com/products/details.asp?f=ADVPSTV&nav=features
I'm not trying to advertise for harman, just wanted to let you guys see what I am working with.

The glass is going to propose a separate challenge, as of right now I do not have an accurate way of measuring the temperature. I'm working on it. I believe the emissivity of glass should be around .9 though (correct me if I'm wrong) and this changes with thickness.

 

[kofkorn]

Sum all of these and convert to BTU/hr comes out to 305.65

Wow, I thought it would be more than that, although those three surfaces are a small part of the radiated area. Don't forget the sides of the stove, the same surface as those vent holes. Probably getting some radiated heat off of those as well.

 

[whitsett2014]

Yes,
These are probably the "cooler" of the six surfaces I was intending on doing the calculation for. The side is where most of the control switches are so there is not much heat radiation there. I just measured the temperatures, and on average its less than a 10 degree C increase (not enough to justify a calculation).

 

[tjnamtiw]

The sides and the back are where most of the snap switches are on many brands so I would expect them to get rather hot.

 

[whitsett2014]

Infrared temperature gun says otherwise
nothing notable as far as surface temperature change is concerned about the top, back, or sides. Less than 15 degrees C, most cases less than 10.

 

[kofkorn]

The very reason I said that you need to measure. Every stove is different. You can rest your hand on the sides of some stoves, others would give you a 2nd degree burn in a second. My stove gives off significant radiant heat from the sides.

I'm glad the exercise is going well.

Good Luck!

 

[tjnamtiw]

Infrared temperature gun says otherwise
nothing notable as far as surface temperature change is concerned about the top, back, or sides. Less than 15 degrees C, most cases less than 10.

Now I'm talking about the side and back surfaces of the outside of the 'firebox', not the trim pieces. How could the convection blower snap switch ever turn the blower on? Some of these trip at 110F and others up to 130F. Something is rotten in Denmark, me thinks. Perhaps your IR gun?

 

[whitsett2014]

I'm still unsure of what surface you are referring to. I am only measuring surfaces which are in direct contact with the ambient air. All other surfaces are being neglected on account of I cant get to them without taking it apart... which sounds like a lot of work for a couple hundred BTU/hr.

 

[Delta-T]

neat thread...I have a bit of input....the Advance may not be the best choice for this test...the actual amount of pellets being fed is semi related to the feed adjuster knob and the temperature being read at the ESP probe. The stove will manipulate the actual on/off times of the auger in relationship to the exhaust temp. If I were to be trying to calculate this stuff, I might use a mass air flow meter on the intake ad take a temp reading and put another mass airflow meter on the exhaust and measure the temp and calculate the difference using the avg BTU/lb on the energy side. The amount of air exhausted should be greater than the intake (gasses expanding during heating and all). Your whole process might end up being simpler if you use a simpler stove. Of course, I could be way off base, and in that case/either way...you are doing an excellent job. I am intersted in what you end up with. Super LEEDS platinum ultra is cool./ do they have that yet? ;-)

 

[tjnamtiw]

I'm still unsure of what surface you are referring to. I am only measuring surfaces which are in direct contact with the ambient air. All other surfaces are being neglected on account of I cant get to them without taking it apart... which sounds like a lot of work for a couple hundred BTU/hr.

If you notice on the Advance, there are ventilation slots on the sides and back. Behind them is the exterior of the burn chamber (probably double wall) which will get quite hot, hence the slots. These surfaces give off a lot of heat.