Quadrafire Castile Experiment

[tjnamtiw]

For quite a while, I have been thinking of ways to upgrade the caveman technology that Quad uses to transfer heat using the smooth aluminum tubes in their Castile and Sante Fe stoves. The nice round, smooth tubes give the least amount of surface area for heat transfer both on the outside where they are exposed to the fire's heat and inside where they are supposed to transfer heat to the room air. Why they aren't made of corrugated or finned material is beyond me based on the asking price for the stoves. In any event, because of the mechanical wiper system, the outside didn't afford much opportunity for easy improvement. I, therefore, focused on the inside of the tubes looking for ways to break up the assumed laminar flow of air that promotes lousy heat exchange and at the same time increase the surface area exposed to the air flow for more heat transfer.
As an experiment, I purchased from McMasterCarr some 36" long 1 1/4" diameter spring material (9662K46) and cut it into 9" lengths. I then bent the one end so that I could grab it with pliers and push and rotate it clockwise into the 8 center tubes, leaving the first and last tube as is since little air flows through them. Here is hopefully a picture.

Next I let the stove reach steady state at Medium with a flame height just barely out of the fire pot. Then I measured the exhaust temperature just past the quick disconnect on the stove. Finally, starting from the left, I measured the room temperature air coming out of each tube. Then I inserted the springs in tubes #2-9 and waited 1/2 hour for stabilization and then measured everything again. Here are my results:

Exhaust temp before = 170F after = 160F
Tube 1 before = 150F after = 135F
Tube 2 before = 145F after = 152F
Tube 3 before = 140F after = 161F
Tube 4 before = 142F after = 162F
Tube 5 before = 160F after = 158F
Tube 6 before = 160F after = 172F
Tube 7 before = 141F after = 150F
Tube 8 before = 140F after = 150F
Tube 9 before = 160F after = 165F
Tube 10 before = 200F after = 165F

Conclusions > The exhaust temperature appears to show that less heat is going up the chimney, which logically means more is going into the room.
Tube 1's lower temperature is a bit of a mystery since it is the first tube to see the fire's heat. Very little airflow comes out of that tube so effect is minimal.
Tubes 2 through 9, except 5, show an increase in heat output due, I imagine, from the added surface area and the spiral effect of the air flowing through the tubes and breaking up the laminar flow. Tube 5 was really a wash for increase.
Tube 10 has very, very little airflow through it so it had plenty of time to pick up heat before the exhaust exits the stove. The fact that the temperature dropped dramatically could be and indication that the previous tubes have removed much more of the heat from the exhaust, leaving less to transfer to the tube #10.

Next, I will order enough springs to do my Sante Fe PLUS order a Dwyer Meter, which will supply one important bit of information, the air flow rate out of each tube. This will answer the question as to if the springs severally limit the airflow, which would explain the higher temps. However, the lowering of the exhaust temperature points toward a positive net gain in efficiency.

Sorry it's so long but some may find this interesting. Others may be asleep by now. %-P

 

[Countryboymo]

That is a great idea but I would be afraid of cutting the airflow. I am curious of the results.

 

[tjnamtiw]

Without an airflow meter, I can't really tell but, to me, it's still blowing like gang busters and the air is definitely hotter than before. The real telling piece of evidence is the exhaust temp going down. Twice today I overshot my thermostat setting by an extra degree also, meaning it was putting out more heat faster before the thermostat could shut it down. That never happened before.
No worries on cutting the airflow, IF THAT IS HAPPENING, because of the overheat snap disc, which hasn't tripped.
It's definitely working.

Thanks for reading the thesis. )

 

[flynfrfun]

Very interesting. Thanks for doing this experimentation. Like you I have wondered why something that would be so simple for a manufacturer to design is not done? I like how your spring idea is so easy to do for the rest of us if your final result is positive.
Flynfrfun

 

[tjnamtiw]

Very interesting. Thanks for doing this experimentation. Like you I have wondered why something that would be so simple for a manufacturer to design is not done? I like how your spring idea is so easy to do for the rest of us if your final result is positive.
Flynfrfun

Thanks flynfrfun (full scale or RC planes????). I wish I could figure a way to clamp some fins on the outside of the tubes and lose the scrapers. I'm ordering spring material tomorrow for my Sante Fe and I will redo the experiment with it. All in all, I think it's a positive step for very little money and completely reversible if questions of warranty arise.

 

[Wi Thundercat]

Very interesting :gulp: Please keep us posted! ;-)

 

[Countryboymo]

I am curious what could be done inside the stove in conjunction to this. The more I ponder the more I am intrigued. Great job!

 

[Nicholas440]

I have a Castile insert, and I've always thought they used the cast aluminum for the fastest heat absorption like the aluminum discs used in good cookware. I'm not sure what the idea is about the round tubes, I just assumed the engineers at Quadrafire researched this and came up with the round tube design. The hot air flows over the tubes and gets them very hot. Personally I think the idea of fins sounds fine however, have you seen how much ash gets on the topside of those tubes? This is I think why those tubes are round. Any excess ash build up will not stick to the round sides of the tubes it drops off onto that stainless steel baffle plate just below it. If you had fins in there those fins would be clogged with ash in no time and they would be a real pain to clean. You want something in that hot air flow that will be quick to absorb a lot of heat and not load up with fly ash the could eventually clog the air flow thus making the cleaning job a lot more work.


I dont think putting springs or anything inside the tubes is going to improve their efficiency. What it will do is cut the air flow down in the tubes and you have less air coming out. It might be hotter air because you have restricted it somewhat so its slower, and the air sits in the tubes a second or so longer therefore it is hotter. The trick is to push a lot of air through a heat exchanger and into the room thats carrying with it the greatest amount of heat.

My thinking long ago was they could have used a second heat exchanger , in other words pass the air over the main heat exchanger, then instead of having the hot air exit the stove right away pass it over a smaller secondary exchanger just as it exits up the exhaust vent pipe. A larger diameter vent pipe with hollow copper coil in it could capture some heat also and being hollow this tube could have air flowing through it back into the main exhanger or have its own vent into the room. Anything to pull more heat out of the exhaust air and into the room would be a good thing. You could have a long copper tube inside the vent pipe that goes up about 5 feet which would capture a lot of escaping heat thats in the exhausting air.

This is a good topic for discussion for sure. I think many of us, with whatever brand stove we have has often thought about how to get more heat out of them. It would be interesting to hear what ideas some of the others here in the forum might have to get more heat out of their stoves.

 

[tjnamtiw]

I have a Castile insert, and I've always thought they used the cast aluminum for the fastest heat absorption like the aluminum discs used in good cookware. I'm not sure what the idea is about the round tubes, I just assumed the engineers at Quadrafire researched this and came up with the round tube design. The hot air flows over the tubes and gets them very hot. Personally I think the idea of fins sounds fine however, have you seen how much ash gets on the topside of those tubes? This is I think why those tubes are round. Any excess ash build up will not stick to the round sides of the tubes it drops off onto that stainless steel baffle plate just below it. If you had fins in there those fins would be clogged with ash in no time and they would be a real pain to clean. You want something in that hot air flow that will be quick to absorb a lot of heat and not load up with fly ash the could eventually clog the air flow thus making the cleaning job a lot more work.


I dont think putting springs or anything inside the tubes is going to improve their efficiency. What it will do is cut the air flow down in the tubes and you have less air coming out. It might be hotter air because you have restricted it somewhat so its slower, and the air sits in the tubes a second or so longer therefore it is hotter. The trick is to push a lot of air through a heat exchanger and into the room thats carrying with it the greatest amount of heat.

My thinking long ago was they could have used a second heat exchanger , in other words pass the air over the main heat exchanger, then instead of having the hot air exit the stove right away pass it over a smaller secondary exchanger just as it exits up the exhaust vent pipe. A larger diameter vent pipe with hollow copper coil in it could capture some heat also and being hollow this tube could have air flowing through it back into the main exhanger or have its own vent into the room. Anything to pull more heat out of the exhaust air and into the room would be a good thing. You could have a long copper tube inside the vent pipe that goes up about 5 feet which would capture a lot of escaping heat thats in the exhausting air.

This is a good topic for discussion for sure. I think many of us, with whatever brand stove we have has often thought about how to get more heat out of them. It would be interesting to hear what ideas some of the others here in the forum might have to get more heat out of their stoves.

Enjoyed your input. Yes, the aluminum tubes (not sure they are cast though) are four times better conductors of heat than steel; however, copper would be almost twice as good as aluminum! Also, you're also right about the fins on top of the tubes collecting ash. If you put fins up there, an alternate means of cleaning would have to be available, such as access holes for a small brush or scraper.

40 Years ago I had a heat extraction mechanism in my fireplace that doubled as andirons. It was curved tubes that were open at the top and bottom and served to heat the air flowing through them. That's the SAME technology used today on these stoves! We now have the capability of extruding all shapes of aluminum that would present much more surface area on both the inner and out surfaces for better heat transfer. One glaring problem with using smooth walled tubing is that you get laminar flow. With laminar flow, you have a stagnant, slow-moving layer of air at the surface, which is not mixing with the central, faster-moving stream of air. Heat transfer is poor. Pushing even more air faster through the tubes, as you suggest, would only compound the problem. What you need is some means of breaking up that stagnant layer and mixing it with the rest of the air. You can use corrugations, a rough surface, fins or other irregularities to do that. At the same time, these irregularities present a greater surface area to the air for more heat transfer.

I can't argue with the assumption that POSSIBLY I am restricting air flow by putting the springs in the tubes since I have no data to refute it. That WOULD allow the air more time to pick up heat before exiting. However, I am NOT reducing the inside diameter of the tube by the thickness of the spring since it is a spiral and still allows air to flow along the spiral and out the exhaust. Certainly there is some restriction but, based on the fact that the exhaust temperature has gone down, the NET effect of heat transfer improvement is positive. I have broken up the laminar flow. Because the springs fit tightly against the tubing, I have also increased the surface area presented to the airflow significantly, thereby putting more heat into the air stream.

I also agree with you that a second heat exchange mechanism would be great to extract more heat from the exhaust. By my measurement, the exhaust is about 160F, so there is still some heat going up the stack. One more pass over another set of tubes would have been possible to design. Of course, if they would use more modern designs of heat exchangers such as in the AE, there would not be a need for this. I wonder what the measured exhaust temperature of the AE is....... Can someone measure that????? Caution: infrared temperature measuring devices act differently on dark and shiny surfaces. Check to see if you have a switch or setting for each type.

Finally, even if you wanted to stay with smooth tubing, Quad could have used two or three times as many smaller diameter copper tubes in a staggered pattern, which would have reduced the room airflow resistance and at the same time, presented much more surface area of tubing having twice the heat transfer coefficient of aluminum.

 

[tjnamtiw]

I am curious what could be done inside the stove in conjunction to this. The more I ponder the more I am intrigued. Great job!

me too! Maybe we need a Quad Testing Lab, but I don't want to go to China!

 

[Czech]

Excellent post TJ. I've own a Castile for around 6 years now, yes I've played with a bit because that is my nature. One question I've always had on this stove, why the baffle that covers the tubes and collects ash? The only big mod I've done is the door gasket, I like the cheap less than 10 dollar 3/4 stuff and not the $50+ tad pole! Keep us posted please.

 

[tjnamtiw]

Excellent post TJ. I've own a Castile for around 6 years now, yes I've played with a bit because that is my nature. One question I've always had on this stove, why the baffle that covers the tubes and collects ash? The only big mod I've done is the door gasket, I like the cheap less than 10 dollar 3/4 stuff and not the $50+ tad pole! Keep us posted please.

Thanks! I've tried to make sense out of that baffle too :ahhh: There's some really half-arsed technology in the whole heat transfer system IMHO.

Like the 'Czech' sig. Mine should be 'Slovak'.. Jasenov, Slovakia > grandpop's home

 

[flynfrfun]

Very interesting. Thanks for doing this experimentation. Like you I have wondered why something that would be so simple for a manufacturer to design is not done? I like how your spring idea is so easy to do for the rest of us if your final result is positive.
Flynfrfun

Thanks flynfrfun (full scale or RC planes????). I wish I could figure a way to clamp some fins on the outside of the tubes and lose the scrapers. I'm ordering spring material tomorrow for my Sante Fe and I will redo the experiment with it. All in all, I think it's a positive step for very little money and completely reversible if questions of warranty arise.

I fly small RC electric planes & helis from my driveway and a paramotor from local farmer's fields. Lots of fun! I like your idea of the spring (if it works out) because it is an easy retrofit to existing pellet stoves. I know we can all come up with different designed heat exchangers that would be more efficient, but unless you want to cut apart your stove and re-weld it all back together, very few people will be willing to do that. I would think that a corrugated or star shaped tubing could have just as easily been used on a lot of these stove heat exchangers and been a lot more efficient. It would still be easy to have a scraper with a star shape instead of a circle. I think it's about time pellet stove manufacturers put a lot more thought into the heat exchanger. I know Quad did the airfoil exchange on the Mt Vernon and I think Harman has a fluted type of setup. The airfoil is neat, but I haven't heard any discussions about how it compares side by side to plain round tube exhanger stoves. In other words, how much air flow at a certain temp with a measured rate of pellet usage for both stoves. I see manufacturers efficiency ratings, but have to wonder how they come up with them. For instance my Enviro M55 cast insert was advertised a week ago with 76.5% efficiency (round tube heat exchanger). I noticed this week they have updated their website and now they claim 83.5% efficiency. That's a big difference. I emailed them to find out just what changed? No response yet. Do these mfg just grab a number out of thin air similar to the other stove's specs to keep their stoves selling?
Flynfrfun

 

[Czech]

The name follows me from forum to forum on the 'net, thanks. Rather than springs, how about flat alum stock cut to fit the diameter? Say 1 foot (whatever the top tube length is) by 1 1/2" (whatever diameter tube are) by 1/8 thick? I know it would take some shop time, but what else do we have to do all winter except fill the stove and blow snow? And let's get some measurements without that darn baffle all other varibles controlled.

 

[skidozer]

I work for a large company that makes heat exchangers for power industry.

Sometimes we use twisted strips inside of the tubes like czec mentioned above then you gain surface area.
Its always a balance of heat transfer and pressure drop.

 

[tjnamtiw]

I work for a large company that makes heat exchangers for power industry.

Sometimes we use twisted strips inside of the tubes like czec mentioned above then you gain surface area.
Its always a balance of heat transfer and pressure drop.

That's interesting! Twisting the 'Czech' strips would spiral the airflow and break up the laminar effect! hmmmmmmmmmmmmmmm. We might actually come up with something effective for all the tube exchangers. The Quad tubes are 1.25" ID. I almost got thrown because I measured them with a mike but didn't go in far enough and measures 1.32". Here they have a little flare at the end for some reason.

Too bad there isn't a Quad factory rep on here to tell us what that baffle is for. Maybe just to slow down the exhaust speed? It almost looks like it's BLOCKING the flow!

 

[tjnamtiw]

I fly small RC electric planes & helis from my driveway and a paramotor from local farmer's fields.
Flynfrfun

I fly RC too. 1/3 scale 9' wingspan Sopwith Pup, .60 size YouCanDo, .40 size Kaos, .60 size Stuka. etc etc. Boy's toys.............

 

[B-Mod]

I think the plate is there to slow down the exhaust. I know a few have tried removing the plate on a Bixby, and all the heat went straight out the exhaust.........

 

[slls]

And to keep from warping the tubes, even though some have warped the tubes with baffle plates installed.

 

[tjnamtiw]

And to keep from warping the tubes, even though some have warped the tubes with baffle plates installed.

So people have removed the little plate that is welded to the upper stainless steel plate that covers to tubes? Just want to make sure we are talking about the same thing.

I ordered two of those adjustable snap discs, Bmod. Just timed my Castile. It took 7 minutes to start the blower after establishing a flame.

 

[Czech]

I'm talking the baffle that is removable, slot hooks on the edges and covers the tubes on the top inside of the firebox, side forward and down to remove. Reasons above make sense I guess.

 

[SmokeyTheBear]

That's a nice easily reversible little experiment you are running TJ.

Most of the baffles I've seen are used to direct the exhaust gases so they don't exit the stove before they have somewhat of a chance to heat the heat exchanger instead of making a beeline for the vent.

 

[tjnamtiw]

That's a nice easily reversible little experiment you are running TJ.

Most of the baffles I've seen are used to direct the exhaust gases so they don't exit the stove before they have somewhat of a chance to heat the heat exchanger instead of making a beeline for the vent.

Thanks, Smokey, I appreciate the compliment!

 

[Wachusett]

I do agree, is there a better way to extract/transfer heat out of the stove.
However is this really increasing the heat gained. The total net. difference in temperature output
only appears to be about 3 degrees. (before avg. temp =153.8 after avg. temp=157). I would think the
springs also would cause more friction vs. a smooth tube = less velocity and fan working harder.

Just my 2 pennies, interesting idea though.

 

[SmokeyTheBear]

It is quite possible that the convection fan hardly knows those springs are there.

How big is the fan outlet on the convection fan? Before the reduction the fan was seeing a 12.27 square inch outlet at the stove front.