Secondary Air Heated to a Higher Temperature

[Huntindog1]

Do some stoves have a longer pathways to heat the secondary air before it gets to the tubes in the upper part of the stove?

I wonder if making mods to increase the time the secondary air travels thru heated channels would make the stoves more efficient?

I wonder if air thats heated to a higher temps would ignite the smoke better and have a more complete burn.

 

[WellSeasoned]

From my experience, I think if there is the perfect mix of air the smoke will ignite before getting to the tubes. This happened to me just tonight when I made a channel from front to back in the coals. I think the perfect amount of air went through the tubes causing the wood gases to ignite before getting sucked through the tubes. That was at a low temp.then as stove temp increased, the gases ignited in/thru the tubes. I swear, there is something new I learn with this stove every week. I think the manufactures know what there doing, but I'm sure there is room for improvement with certain stoves. I would only think of modifying if I knew alot more than I do now.

 

[ozzy73]

I think the air is plenty hot once you get the stove up to temp. Every firebox I have seen has the channel that carries the air to the 2ndary tubes exposed pre heating the air before it gets to the jets. When the jets are really blasting I can actually hear a vacum as the air is sucked into the inlet for the 2ndary's.

 

[stejus]

I don't see how the air can be pre-heated before it exits the secondaries ports. The air is coming from the exterior of the stove somewhere, travels up and is being sucked out of the tubes due to the draft. My theory is the tubes are so hot (1000-1200 degrees?) that the air being drawn through heats up very fast as it travels through the burn tubes and once it hits the gases from the wood, it ignites.

 

[WellSeasoned]

I have seen different types of secondaries going off. One its the powerful jets of flames shooting from within the tubes and the second just a constant plume fireballish show. That happens in all sorts of ways. Like if you turn the air down too much too soon, you get the ghostlike flame show. Thats just the gases igniting before it gets to the tubes, like a back draft sort of.

 

[corey21]

My stove has a channel or square pipe in the main back to heat the air up and then a rather big baffle system tubes in side it. It seem to function well if the stove is up to 400 and above.

 

[Huntindog1]

Just saw this post that has a picture of the Progress Hybrid air flows , boy that design, the secondary air doesnt travel far.

https://www.hearth.com/econtent/index.php/forums/viewthread/89512/

 

[precaud]

Do some stoves have a longer pathways to heat the secondary air before it gets to the tubes in the upper part of the stove?

Absolutely. My Quad and Nestor Martin both have pretty long secondary channels.

I wonder if making mods to increase the time the secondary air travels thru heated channels would make the stoves more efficient?

Higher combustion efficiency, yes. More efficient overall, not necessarily. There are tradeoffs involved.

I wonder if air thats heated to a higher temps would ignite the smoke better and have a more complete burn.

Yes, sure it does. But let's look at the tradeoffs. The problem is the air channels themselves. They add mass to the stove, which slows down the temperature rise of the stove from the start of each load cycle. And they have insulating value, which does the same. Let's say you have a stove that has secondary air channels that run up the sides of the stove. The area under and near those channels will be at lower temperature than the areas where the channels aren't. The channels are cooled by the air passing through them.

And that's the main tradeoff. The surface area around the channels are at lower temperature and hence radiate less heat, essentially gaining combustion efficiency by lowering radiation efficiency. Where the channels are placed in the stove can make a big difference.

 

[wkpoor]

I have seen different types of secondaries going off. One its the powerful jets of flames shooting from within the tubes and the second just a constant plume fireballish show. That happens in all sorts of ways. Like if you turn the air down too much too soon, you get the ghostlike flame show. Thats just the gases igniting before it gets to the tubes, like a back draft sort of.

Thats is exactly what I see. Both what looks like gas pilots and the plume at times. Either way the secondary inlets are cool enough to put you finger in even with the stove top at 800 degrees. Its augmented air and it must heat up real fast as it enters the burn tubes.

 

[precaud]

The flame around the secondary air has a more well-defined shape when the air coming out of the tubes is cooler.

 

[Dune]

Sticking my neck out a little, but it is my understanding that cold air, being denser than hot air would aid combustion better.

 

[precaud]

Dune, it may be true, but I don't think that's a primary consideration compared to the other factors.

 

[Adios Pantalones]

From my experience, I think if there is the perfect mix of air the smoke will ignite before getting to the tubes. This happened to me just tonight when I made a channel from front to back in the coals. I think the perfect amount of air went through the tubes causing the wood gases to ignite before getting sucked through the tubes. That was at a low temp.then as stove temp increased, the gases ignited in/thru the tubes. I swear, there is something new I learn with this stove every week. I think the manufactures know what there doing, but I'm sure there is room for improvement with certain stoves. I would only think of modifying if I knew alot more than I do now.

Smoke sucked through tubes? Huh?

 

[Dune]

Dune, it may be true, but I don't think that's a primary consideration compared to the other factors.

Thanks.

 

[Stump_Branch]

Sticking my neck out a little, but it is my understanding that cold air, being denser than hot air would aid combustion better.

Physical limitations. Cooler air has oxygen etc. more per volume. But your thinking of an internal combustion engine, which operates in controlled volumes. Besides once compressed in a cyinder the air is heated.

AP, hilarious.

 

[Dune]

Sticking my neck out a little, but it is my understanding that cold air, being denser than hot air would aid combustion better.

Physical limitations. Cooler air has oxygen etc. more per volume. But your thinking of an internal combustion engine, which operates in controlled volumes. Besides once compressed in a cyinder the air is heated.

AP, hilarious.

Yes it is heated by compression but if it is cooler on entry, more oxygen will be present.
As related to a woodstove though, maybe it is not an issue since more than enough air is always readily available.
If so this would simplify construction of an Ericson cycle turbine. In other words you could run an open cycle and not have to cool the working fluid before boosting combustion with it.

 

[LLigetfa]

My stove has a double-wall construction between the intake and the secondaries as well as the door airwash. Since my stove is OAK fed bitter cold air, at higher settings, it cannot preheat it as well taking the stove out of the sweet spot.

 

[wkpoor]

I think a wood stove is closer to a turbine in volumetric efficiency and therefore temperature of secondary air may not be to important because it can have all the air it wants and needs.

 

[precaud]

I don't know about comparisons to a turbo, but I can say, excess air results in lower efficiency, especially if it's preheated air. Valuable "radiation real estate" was given up in order to preheat that air, if it's not used, its wasted right up the chimney...

 

[HotCoals]

I think the amount of air is more concerning then the temp of it...it's the oxygen imo.

 

[BillsWS]

Local wood stove dealer said the air is atomized.

 

[pen]

Local wood stove dealer said the air is atomized.

LOL

pen

 

[Stump_Branch]

Turbo, turbine, othe mechanical devices in which combustion occurs is different then what we are talking about. For example all of those mechanically compress the air. The only connection can be velocity and or volume of air.

The secondaries function is to provide heated air to the temps that cause smoke to ignite. The primary air is for combustion. Generally speaking of course. I doubt that adding more oxygen via a denser/cooler charge would do anything for the stove.
That being said im not sure that going above the needed temps to light off the smoke is needed. Smoke burns at a temp, 1100deg. Or so and thats that. Getting it hotter doesnt burn anymore as theres only so much smoke available. Only thing i am aware of to alter that is the chemical reaction of a catalyst.

Ive often wondered why no tubes along the top sides of the stove. Seems theres smoke there too coming out of the ends of splits loaded east west.

This is probably why i dont design woodstoves.

 

[mellow]

Ive often wondered why no tubes along the top sides of the stove. Seems theres smoke there too coming out of the ends of splits loaded east west.

In theory the baffle should be sealing the top and sides, gas on all sides should be routed over the tubes at some point, unless the wood is at the very front of the stove, then I think the air wash air would act as a secondary.