Does a Fan Produce More Heat or Just Blow the Air around?

Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: oliver power On: Tue Jan 14, 2014 7:36 pm

"THE END"
oliver power
 
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: tony17112acst On: Tue Jan 14, 2014 9:42 pm

THE END, CLOSED THREAD:

I am the OP; In case someone in the future wants to know the answer and not read thorough 100's of posts:

I have confirmed in a physics forum that it is absolutely true that heat will transfer faster through metal as the difference in temperature on either side of the metal is greater. So cooling the outside of the metal box (stove) with a fan WILL cause heat to transfer more quickly from inside the box (stove) to the outside.

So THE FINAL summary of this thread is:

Q: Does a Fan Produce More Heat or Just Blow the Air Around?
A: It produces more heat because of the INDISPUTABLE principle that heat transfers more quickly through a medium as the difference of temperatures on either side of that medium becomes greater.
tony17112acst
 

Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: lsayre On: Tue Jan 14, 2014 11:09 pm

The first law of thermodynamics states that the change in internal energy of a system (Delta U) equals the net heat transfer into the system (Q) minus the net work done by the system (W). In equation form, the first law of thermodynamics is:

Delta U = Q - W
(where Delta = the change in)

If for whatever reason (such as air blowing across it) the system does more work (if it is giving off more heat) then Delta U (the change in internal energy of the system) goes negative (the stove gets colder) unless Q (the heat transferred into the system by the burning coal) is increased (which means unless more coal is being burned).

The first law of thermodynamics states in a nutshell that you can't get something for nothing.
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: tony17112acst On: Wed Jan 15, 2014 1:14 am

Lee, Allow me...

But Larry, We all agree that you cannot get more heat from a certain (pre-determined) amount of coal. Your equation is totally valid, but it doesn't apply to my question. Here's why:

To apply your equation directly, the Change in U would apply to a system like a toaster, or if a pile of coal were burning in a room with no exhaust. 100% of the heat generated is transferred into the room. Putting a fan on it cannot make more heat and your equation is obeyed and observed. If you DO get more heat out either example, you've used more energy ...and your equation applies and we'd all agree and lock this thread down.

But our system is different. It has heat from that certain amount of coal (or energy source) moving through a tube (piping/chimney and the stove itself) and we want to move the heat from that tube to the room. Your equation applies ONLY to the heat/energy INSIDE the tube (the coal)! We are conversely focusing on MOVING THAT HEAT INTO THE ROOM from inside the tube or Thermal Resistance. These are a totally different set of equations.

Thermal Resistance is how a material resists heat flow. If we encased your coal stove with glass, and asbestos, all of the heat generated by the coal would simply go right up the chimney and almost none would be transferred into the room. Notice that your 1st law of thermodynamics is true and applied only to the coal INSIDE the stove. But it DOESN'T apply to whether we transferred the heat (from the coal) to the room yet, or Thermal Resistance. If our stove and pipe was made of aluminum foil, I'm sure you'd agree that we'd get more heat from it than the duplicate glass/asbestos stove with the exact same amount of energy/coal in both examples. Yet we are not disobeying the 1st law of thermodynamics!

On this site: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatloss.html#c1 They have:

Heat Loss Rate = Q/t = [(area) x (Temp Inside - Temp Outside)] / Thermal Resistance.

So Heat Loss (which is what we want) is greater as Temp Inside and Temp outside are further apart. Try some examples, like only 10 degrees apart will give a certain number, but 400 degrees apart will give a much larger number, or heat loss. Our fan makes the Temp Outside lower and gives us a bigger number in the numerator.

One more: http://en.wikipedia.org/wiki/Thermal_conduction: and scroll down to "Cylindrical Shells" (e.g. pipes).

look for the words: "the rate of heat transfer is" and you'll see another equation with T2-T1 in the numerator where T2 and T1 are the temperature on the outer and inner wall. You'll agree that as T2-T1 is greater, the larger the number, or heat loss!

These are the equations showing that the larger the difference in the temperature inside and outside of the stove, the greater the rate of heat transfer occurs. A fan makes the outside temp lower, so heat is transferred at a greater rate

If you still disagree with this: Ask yourself this fundamental question: Why do they put fans on car radiators? We say it's because the fan transfers (not create) the heat inside the material (radiator) faster by decreasing the outside temperature.

Please say you agree. We all love you Lar!
tony17112acst
 

Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: lsayre On: Wed Jan 15, 2014 4:12 am

If you feel that thermal heat transfer differential through a medium based on outer surface vs internal temperature differential of same medium violates the laws of thermodynamics, you are totally misunderstanding both the first law of thermodynamics and what you have read on your linked site.

I have never disputed that "if" the energy going out of the chimney in relation to the energy entering the room is shifted in favor of the room there is a net gain in heating efficiency, as witnessed in my very first post to this thread. Everyone should take the time to revisit it. A fan does however have the potential to shift (reposition) the energy (heat) to where the occupants may feel more comfortable with it, but that is altogether irrelevant to your initial subject line question. This is why I said earlier that we are merely talking in circles here (when everyone jumped in to your defense and questioned why I would say this, apparently thinking it was a chink in my armor).

What I do seriously dispute is your absolute 100% certainty (premise) that in all cases for coal fired appliances there will actually be such a net gain to the room vs. the chimney (without the consumption of additional coal) merely by blowing air across the coal fired appliances heated outside surface. You are not considering the effects to the chimney itself (and the draft it is providing) in your line of reasoning. You are apparently not fully grasping the implications of thermal equilibrium either. Be sure that you are considering the entire system and check your premises carefully.
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: kstills On: Wed Jan 15, 2014 8:25 am

tony17112acst wrote:THE END, CLOSED THREAD:

I am the OP; In case someone in the future wants to know the answer and not read thorough 100's of posts:

.


Ah, the science is settled.

And I thought we weren't discussing global warming. :P


From a wood burning site:

Does a fan increase stove efficiency?
Great site! Here's something I've been puzzling over. How much difference does having a fan blowing across the stove make with regard to it's heat delivery efficiency? I have a "high velocity" fan on it's lowest setting blowing across the stove pretty much all the time, and I've been wondering how much this increases the efficiency of the stove. Thanks, Mac.


--------------------------------------------------------------------------------

Hi Mac,

I don't use a fan of any kind and I don't promote them. There hasn't been much independent research on the efficiency of fan use, but many years ago I do recall one obscure study that found only a very slight difference in efficiency when the fan was used, and this was on a fireplace insert with small air circulation passages around the entire firebox.

I think the effect of stove fans is mostly psychological. If you could see air flow, you would see a huge plume of warm air flowing towards the ceiling from your stove. It would then flow down the far walls, across the floor and back to the stove. That convection flow of air, while it is gentle, is probably moving much more air than the fan, and it costs no electricity to run it. I suppose a fan might be useful to aggressively mix the air if a room had cold floors, but I can't think of another advantage.

Stove manufacturers like to sell accessory fans for their stoves because it runs the price up quite a bit. A retailer friend of mine likes to tell customers who ask about efficiency boosts from fans that 'you'll get the same effect if you let me tape your $200 to the back of the stove'.

John, Posted January, 2005
Last edited by kstills on Wed Jan 15, 2014 8:29 am, edited 1 time in total.
kstills
 
Stoker Coal Boiler: WL 110
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: kstills On: Wed Jan 15, 2014 8:28 am

You have a 2000 def f fire in your stove, encased in a chamber that is 3400 times more conductive to heat then the air that surrounds it.

You're already both radiating and convecting heat into your room at an enormous rate, based on the size of your fire, and the assumption being put forward is that passing a few cfm of a conductor that with heat conductivity of 0.023 is going to change the rate of burn INSIDE the chamber?

That's hard to believe....
kstills
 
Stoker Coal Boiler: WL 110
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: Sunny Boy On: Wed Jan 15, 2014 8:38 am

A very simple test would be to use an I R gun and take stove surface, stove pipe, wall, and floor readings. Then turn on the fan and about 10 minutes later take readings in the exact same places and compare.

That's science.

Someone's opinion from a wood burning site is not.

Paul
Sunny Boy
 
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: kstills On: Wed Jan 15, 2014 8:49 am

Sunny Boy wrote:A very simple test would be to use an I R gun and take stove surface, stove pipe, wall, and floor readings. Then turn on the fan and about 10 minutes later take readings in the exact same places and compare.

That's science.

Someone's opinion from a wood burning site is not.

Paul



No one, I don't think, is disputing that heat can be transferred (IE taken away from the stove and put somewhere else) with a fan.

And as Larry pointed out, if one closes off the exhaust, one can significantly increase the usable btu's captured by the dwelling.

However, what was being presented that because of an increase in delta between the outside of the box and the inside, the amount of energy produced in the fire chamber itself would increase.

I've read papers for optimizing coal fired electricity generators that restrict airflow inside the fire chamber to increase the efficiency with which the electricity is generated, however that's air being applied directly to the fuel source. In these situations, it appears that reduced internal temperature is desired.

I would suggest that the vast amount of heat being given off by the stove is radiant in nature, and is not dependent on the air for transport at all. The amount of air that comes in contact with the box and then begins to convect the heat off of it would have to be understood before any calculation about how efficient a fan would be wrt increasing the heat transfer could take place.
kstills
 
Stoker Coal Boiler: WL 110
Stove/Furnace Make: Leisure Line

Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: Sunny Boy On: Wed Jan 15, 2014 9:01 am

I think, like others have done, your missing the point by over complicating this.

He just wants to know if the fan will have an affect, or not.

After ten pages, this thread reminds me of the board of experts convened to investigate the space shuttle Challenger disaster. A long table of the best-of-the best going on about all their theories and calculations of how it blew up.

Meantime, while they were going on and on, one scientist took a sample of the o-ring material used to seal the booster rocket sections and dropped it in his glass of ice water. After a while he pulled it out and showed how it stiffened at cold temperatures. The morning of the launch it was freezing.

They later discovered pictures showing one of the seals failed and burned through to the large fuel tank, causing the explosion.

Sometimes the answer is so simple even the best engineers don't see it.

Paul
Sunny Boy
 
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: titleist1 On: Wed Jan 15, 2014 9:11 am

The trees have hidden the forest......

kstills wrote:However, what was being presented that because of an increase in delta between the outside of the box and the inside, the amount of energy produced in the fire chamber itself would increase.


Although this seems to be the root cause of all the discussion, this is not what was being presented, EVER.

The very first post in the thread the OP hoped the fan could make more heat TRANSFER through the metal and as demonstrated by the equations posted above it does. Never does it say that a fan blowing on the outside of the box could make the lb of coal inside the box produce more BTU's, only that the TRANSFER of those same BTU's through the metal might be improved.

"I am leaning towards that the heat output is the same whether there's a fan blowing on the metal or not. I'd think how much heat that transfers through metal is a function of the properties of that metal, if everything else is the same. I'm thinking that the fan cannot make more heat transfer through the metal frame of the stove or exhaust pipes. But I'm hoping it would!"
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: kstills On: Wed Jan 15, 2014 9:29 am

titleist1 wrote:The trees have hidden the forest......

kstills wrote:However, what was being presented that because of an increase in delta between the outside of the box and the inside, the amount of energy produced in the fire chamber itself would increase.


Although this seems to be the root cause of all the discussion, this is not what was being presented, EVER.

The very first post in the thread the OP hoped the fan could make more heat TRANSFER through the metal and as demonstrated by the equations posted above it does. Never does it say that a fan blowing on the outside of the box could make the lb of coal inside the box produce more BTU's, only that the TRANSFER of those same BTU's through the metal might be improved.

"I am leaning towards that the heat output is the same whether there's a fan blowing on the metal or not. I'd think how much heat that transfers through metal is a function of the properties of that metal, if everything else is the same. I'm thinking that the fan cannot make more heat transfer through the metal frame of the stove or exhaust pipes. But I'm hoping it would!"



Actually, here is the first post:

I probably should ask an actual physicist this, but here it goes:

Does blowing air onto a hot object (like a coal stove) give you MORE HEAT output from the object, or does the fan just spread that heat around the room?

No. The heat is a function of the rate of combustion of the coal. Unless this is increased, the rate of heat from the object remains constant.

I have been using a fan, but I hate to use more electricity if it doesn't produce MORE heat; I really don't need to spread it around the room in my specific situation. I am leaning towards that the heat output is the same whether there's a fan blowing on the metal or not.


Heat output does remain constant, with some potentially immeasurable amount of extra heat generated by increased air flow, however according to the requirements of the op this would not meet his needs.

I'd think how much heat that transfers through metal is a function of the properties of that metal, if everything else is the same. I'm thinking that the fan cannot make more heat transfer through the metal frame of the stove or exhaust pipes. But I'm hoping it would!


Correct, the heat transfer of the metal is constant, but heat transfer away from the black body can be accelerated by increasing the use of a fluid medium to conduct the heat from it.



So, he asks both if the object will produce more heat, and if heat transfer will improve.

However, he also clearly states that he doesn't care if heat transfer improves, because that's not what he's concerned with.

Hence, the confusion....
kstills
 
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Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: titleist1 On: Wed Jan 15, 2014 9:34 am

"I'm the OP and franco is correct. My sole concern was: if I point a fan at my stove, will I get more heat in the room than if I did not have a fan at all."

Seems to simplify it pretty well......
titleist1
 
Hot Air Coal Stoker Stove: Harman Mag Stoker (old style) one in basement, one in workshop
Hand Fed Coal Stove: Harman Mark III on standby for long power outages
Coal Size/Type: Rice/Anthracite; Nut/Anthracite

Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: kstills On: Wed Jan 15, 2014 10:07 am

titleist1 wrote:"I'm the OP and franco is correct. My sole concern was: if I point a fan at my stove, will I get more heat in the room than if I did not have a fan at all."

Seems to simplify it pretty well......



So THE FINAL summary of this thread is:

Q: Does a Fan Produce More Heat or Just Blow the Air Around?
A: It produces more heat because of the INDISPUTABLE principle that heat transfers more quickly through a medium as the difference of temperatures on either side of that medium becomes greater.


I'm also leaning towards 'simple'.... :lol:
kstills
 
Stoker Coal Boiler: WL 110
Stove/Furnace Make: Leisure Line

Re: Does a Fan Produce More Heat or Just Blow the Air around?

PostBy: Sunny Boy On: Wed Jan 15, 2014 10:40 am

Numbers and formulas are great, but as we techs say, "To prove it, ya gotta get to where the rubber meets the road".

Here's some quick test results. Make of them what you will.

I put a small table fan on the range's trivet pointing down over the firebox end of the stove.

I took I R gun readings at three location around the kitchen. Plus, a thermometer on a kitchen cabinet on an outside wall.

1. The gas stove top, nine feet away from the range. (a horizontal surface with a refrigerator blocking any radiation heat).
2. At chest height, on a kitchen cabinet door, 16 feet away at the opposite end of the kitchen (a vertical surface).
3. On the counter top below a window, 11 feet away ( A horizontal surface with cold air convection, from the window, moving across it).
4. The thermometer is at head height on the cabinet, above #3 and to the left of the window.

The day is cloudy and no wind.

I first took ten readings, a couple of seconds apart, at each point to get an average.

I then turned the fan on and after 15 minutes took more readings at these same points.

The pictures show the first readings on top with the fan off. The readings below are those with the fan on. Each time the IR gun was pointed at the area between the two arrows.

After 15 minutes the two left hand stove top plates dropped in temp about 25 degrees. The stack temp went down about 3 degrees.

The wall thermometer went from 72 to 75.

Paul
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Hand Fed Coal Boiler: Anthracite Industrial, domestic hot water heater
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