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Does a Fan On a Wood/Coal Stove Produce More Heat or Just Dispurse It?

by tony17112acst
Tags: dispurse, heat, produce, stove, wood or coal
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tony17112acst
#1
Jan12-14, 01:13 PM
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I posted this question in a wood/coal stove forum and after 100 responses, we do not have a convincing answer:

Will MORE heat be extracted from a heated metal object if you put a fan on it?

If we have a simple electric resistance heater (a glorified toaster) making 10,000 BTU's, it seems intuitive that we cannot make more than 10,000 BTU's by blowing (cooler) air across it with a fan. However, it ALSO seems intuitive with a wood or coal stove, that blowing a fan on it will give you more heat in the room (which is the opposite).

The electric heater seems simple enough but with a wood stove, we have heat passing through the stove and through exhaust piping to the chimney, and there may be a totally different principle at work here. I'm guessing the SAME principle applies when a fan blows across your radiator in your car to extract more heat from the liquid coolant which is passing through the radiator, much like heat is passing through the wood/coal stove up into the chimney.

But don't metals have a property that heat can only transfer through at a certain rate? Why would blowing (cooler) air change the rate of that transfer ...or doesn't it?

Thanks in advance! -Tony
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SteamKing
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Jan12-14, 03:55 PM
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You are confused on several points about heat transfer.

You pass air thru a car radiator to transfer the heat absorbed by the engine coolant to the air. If you didn't do this, the engine coolant would keep getting hotter, damaging the engine.

The heat produced by an electric heater come from the electric current passing thru a resistance element in the heater. If you blow air across the electric heater, you don't generate any additional electric current, so the heater will not output any additional heat.

As for turning a fan on a coal/wood stove, it depends on where the air from the fan is directed. If you blow air over the outside of the stove, all the stove will do is heat this air. If you blow air into the firebox of the stove, it is possible to generate additional heat from the burning wood/coal because the oxygen in this air can lead to more complete combustion of the fuel. This is why blacksmiths have a bellows attached to their forges: this additional air from the bellows leads to more complete combustion of the coals inside the forge, which raises the temperature.
Borek
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Jan12-14, 04:14 PM
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Raising the temperature doesn't mean producing more heat - it means producing it faster.

Buckleymanor
#4
Jan12-14, 05:13 PM
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Does a Fan On a Wood/Coal Stove Produce More Heat or Just Dispurse It?

Quote Quote by SteamKing View Post
You are confused on several points about heat transfer.

You pass air thru a car radiator to transfer the heat absorbed by the engine coolant to the air. If you didn't do this, the engine coolant would keep getting hotter, damaging the engine.

The heat produced by an electric heater come from the electric current passing thru a resistance element in the heater. If you blow air across the electric heater, you don't generate any additional electric current, so the heater will not output any additional heat.

As for turning a fan on a coal/wood stove, it depends on where the air from the fan is directed. If you blow air over the outside of the stove, all the stove will do is heat this air. If you blow air into the firebox of the stove, it is possible to generate additional heat from the burning wood/coal because the oxygen in this air can lead to more complete combustion of the fuel. This is why blacksmiths have a bellows attached to their forges: this additional air from the bellows leads to more complete combustion of the coals inside the forge, which raises the temperature.
The stove will heat the air if you blow air over the outside of the box and if a fan is used this warm air will be replaced more quickly with colder air to be heated which in turn will extract more heat from the stove which causes more rapid combustion.
You would be surprised at how much more fuel is consumed by a stove with a backboiler conected to radiators and a pump which is turned on, than one that is not.
A fan blowing air over a stove is pretty much the same effect but less efficeint.
tony17112acst
#5
Jan12-14, 05:18 PM
P: 10
I'm not sure why you say I'm confused, I agree with everything you mentioned. I have read my post and cannot see how you ascertained that I have any disagreement with your statements.

Of course air is passed thru radiator coils to cool the liquid in a car.
Of course an engine will get hotter if you don't do it.
Of course the heat produced by an electric heater is all that's going to be produced (It's even explicitly stated!).
Of course we all know blowing air/oxygen directly on coals will produce hotter temps, but that's far from my question.

My question: Is more heat is generated/extracted from a hollow metal object with high temps inside by blowing cooler air over it ...or is the same amount of heat transferred through the metal object because the property of that metal only allows a certain amount of heat to pass through?

So take ANY hollow metal object with heated air/liquid passing through it. Pretend you have no idea what's inside generating the heat (a fire, electric resistance, nuclear rods, a magic box, WHATEVER). We are NOT concerned with how this heat is generated, just that hot air is passing through a metal object (like a cylinder). Will blowing cooler air over this cylinder or hot metal box extract more heat from that system into the ambient room temperature?
tony17112acst
#6
Jan12-14, 05:21 PM
P: 10
Buckleymanor: Are you saying that when heat is radiated through a metal, that the rate the heat passes through gets faster as the temperature of the outside of it becomes cooler (which is what the fan is doing - it's moving the hot air away form the outside of the metal box/cylinder)?
SteamKing
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Jan12-14, 05:41 PM
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Look at it this way: if the stove has the same temperature as its surroundings, no heat transfer can take place.
sophiecentaur
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Jan12-14, 05:41 PM
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It's not surprising that you got no real sense from a wood burner forum. You clearly want the 'Physics' answer.
When the system is in equilibrium, the rate at which heat is generated will equal the rate that heat is dissipated into the room. It's the details that determine the subjective ('warming') effect of the heater. A fan will produce more warm air at a lower temperature and perhaps move it to parts of the room where it will be appreciated. If it's all near the ceiling then you have wasted a lot of your fuel.
The prime source of the heat (as stated earlier) is not very relevant (watt for watt) - except that the surface temperature of the heater will be affected by its surface area.
A wood burner with a good air flow may make you feel better - reverse convection with a fan may get your feet warm quicker than with natural convection.
tony17112acst
#9
Jan12-14, 05:58 PM
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Quote Quote by sophiecentaur View Post
It's not surprising that you got no real sense from a wood burner forum. You clearly want the 'Physics' answer.
When the system is in equilibrium, the rate at which heat is generated will equal the rate that heat is dissipated into the room. It's the details that determine the subjective ('warming') effect of the heater. A fan will produce more warm air at a lower temperature and perhaps move it to parts of the room where it will be appreciated. If it's all near the ceiling then you have wasted a lot of your fuel.
The prime source of the heat (as stated earlier) is not very relevant (watt for watt) - except that the surface temperature of the heater will be affected by its surface area.
A wood burner with a good air flow may make you feel better - reverse convection with a fan may get your feet warm quicker than with natural convection.
Regretfully, I cannot tell how that answers my question. Again, I ask: will blowing cooler air onto a hollow metal object with heat passing through it, extract MORE heat from that metal object, or just spread it around more quickly?
tony17112acst
#10
Jan12-14, 06:00 PM
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Quote Quote by SteamKing View Post
Look at it this way: if the stove has the same temperature as its surroundings, no heat transfer can take place.
I know.

Again, I ask: will blowing cooler air onto a hollow metal object with heat passing through it, extract MORE heat from that metal object, or just spread it around more quickly?
tony17112acst
#11
Jan12-14, 06:02 PM
P: 10
We all know a fan will spread the warmed air around to comfort the room better. But is the fan actually extracting more BTU's from the hollow metal object with heat passing through it?
sophiecentaur
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Jan12-14, 06:03 PM
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Quote Quote by tony17112acst View Post
We all know a fan will spread the warmed air around to comfort the room better. But is the fan actually extracting more BTU's from the hollow metal object with heat passing through it?
How could it, once equilibrium has been reached?
tony17112acst
#13
Jan12-14, 06:11 PM
P: 10
Quote Quote by sophiecentaur View Post
How could it, once equilibrium has been reached?
So are you answering "No?" If you are, then why are there fans mounted on car radiators? DOn't the fans extract more heat from the liquid passing through?

But equilibrium is never reached in my examples. Take a coal stove: Coal burns at say 800 and heats the stove surface to 400 and the rest goes up the chimney. The stove can never reach 800 because some of the BTU's are going up the chimney. This is why I use "heat passing through a hollow metal object" in my example/question. Some of the heat is lost, but we assume the surface area of the stove is a constant temperature (like 400 for example).

Whatever BTU's I get from that 400 stove, will I get MORE BTU's by blowing cooler (ambient) air directly onto the surface area of the stove?

So, I still do not know what the answer is.
CWatters
#14
Jan12-14, 06:12 PM
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Does a Fan On a Wood/Coal Stove Produce More Heat or Just Dispurse It?
The law on Conservation of energy says that you can't get more energy out than you put in. So if you put a fixed mount of energy in (in the form of logs and oxygen) then you can only ever get that much energy out in the form of heat.

However there are two ways for energy to leave the stove - into the room or up the stove pipe. If you cool the stove and stove pipe (eg with a fan) then more energy will go into the room and less energy will go up the stove pipe.

Consider doing the opposite...wrap the stove in insulation! Where is the energy going to go? It can only go up the stove pipe.
tony17112acst
#15
Jan12-14, 06:15 PM
P: 10
Quote Quote by CWatters View Post
The law on Conservation of energy says that you can't get more energy out than you put in. So if you put a fixed mount of energy in (in the form of logs and oxygen) then you can only ever get that much energy out in the form of heat.

However there are two ways for energy to leave the stove - into the room or up the stove pipe. If you cool the stove and stove pipe (eg with a fan) then more energy will go into the room and less energy will go up the stove pipe.
OMG, finally an answer!

Why will more heat be sent into the room by cooling the stove/pipe? Is there a principle that heat transfers through a medium (metal) more quickly if the inside and outside temperatures (relatively speaking) are further apart?
Drakkith
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Jan12-14, 06:25 PM
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Quote Quote by tony17112acst View Post
So take ANY hollow metal object with heated air/liquid passing through it. Pretend you have no idea what's inside generating the heat (a fire, electric resistance, nuclear rods, a magic box, WHATEVER). We are NOT concerned with how this heat is generated, just that hot air is passing through a metal object (like a cylinder). Will blowing cooler air over this cylinder or hot metal box extract more heat from that system into the ambient room temperature?
Let's do away with the "more" or "less" heat here. It's only confusing the situation.

Let's assume the heat source generates a steady source of heat like a toaster or electric heater. Now the key here is to realize that because you have a steady source of heat (aka thermal energy), the cylinder MUST heat up until it is hot enough that the rate of heat transfer away from the cylinder to the environment is equal to the rate of heat transfer into the cylinder from the heat source. The two ways for the cylinder to get rid of heat is through thermal radiation and thermal conduction.

Thermal conduction is when the air is heated by direct contact with the cylinder, while thermal radiation is the production of electromagnetic radiation solely due to the cylinder's temperature. Let's say our cylinder is at 1,000 kelvin so that it appears dull red and radiates mostly in the infrared range.

I'm sure you're aware that when you blow air over something hot it cools down quicker than if you don't. This is because the air near the object, in this case our cylinder, is hotter than the surrounding air. The rate of heat transfer between two objects depends strongly on their relative temperatures. Two objects at nearly the same temperature will only have a small rate of heat transfer between them, while two objects that are hundreds or thousands of degrees apart will have a great amount of heat transfer from the hot object to the cold object. The air near the cylinder is very nearly the same temperature as the cylinder, so the rate of heat transfer is much lower than if the air were cold. In effect, the air is acting as insulation. Blowing air over the cylinder will move the hot air out of the way and expose the cylinder to much colder air, greatly increasing the rate of heat transfer from the cylinder to the air.

We also have to consider the contribution of thermal radiation. If we put our cylinder out in space so that we have zero thermal conduction, it will simply heat up until the temperature becomes high enough so that the thermal radiation carries away heat at the same rate as it is generated by our heat source. However, back on Earth, the air carries away heat and keeps the temperature from getting so high.

So basically, not blowing air over your cylinder will cause it to reach a higher temperature and a greater proportion of heat will be radiated away as thermal radiation instead of being transferred to the air. (Though at least some of that will be transferred back to the air when the walls of the room are heated up by the thermal radiation)

Of course the real world isn't so simple. As others such as Sophiecentaur pointed out, there are plenty of other things to consider. Air flow can warm you up faster simply by getting the warm air to you faster.
Drakkith
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Jan12-14, 06:28 PM
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Quote Quote by tony17112acst View Post
Why will more heat be sent into the room by cooling the stove/pipe? Is there a principle that heat transfers through a medium (metal) more quickly if the inside and outside temperatures (relatively speaking) are further apart?
Inside the stove pipe, hot air is rising and escaping into the external environment. Blowing air over the pipe causes the air inside to absorb more of this heat than it does when static, keeping more of the heat inside the room.
Spinnor
#18
Jan12-14, 06:29 PM
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Quote Quote by tony17112acst View Post
OMG, finally an answer!

Why will more heat be sent into the room by cooling the stove/pipe? Is there a principle that heat transfers through a medium (metal) more quickly if the inside and outside temperatures (relatively speaking) are further apart?
Yes, if the inside and outside of the stove were at the same temperature no heat would flow. If you remove heat from the outside surface of the stove there would be a greater heat gradient and heat would flow faster form inside to outside. If this were done and the fire produced heat at the same rate the interior of the stove would be a little cooler, this in turn would tend to slow down combustion. I know that when my wood stove gets very hot it consumes wood very quickly if the intake air is not nearly closed.


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