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Part 10 - Convections
10 Convection:

Linked integrally with the afterburner, a recirculating fan so heat is not being wasted to atmosphere and to provide convection in general. I have this proven out on my bench top currently. I would recommend a small metal squirrel cage blower. We should discuss a simple baffle (or two) for bringing in cool air and releasing hot air for additional control

That's from the Design thread. I know you have been wanting a little discussion of this. I found and David has picked up a small steel blower wheel 4.5" dia, 2 1/4" thick. This will do the dirty work of moving air around. If we can find a steel blower cage, great, otherwise we will make one out of sheet metal - no biggie for this scale. We are looking for a motor that can be kept outside the hot air path. Something that moves 1200 rpm or so, 110 AC, 5/16" shaft and pretty small (I hope only 2-3" I hope and cheap.

Drawings will follow, but the plan for the moment is that this blower will be mounted under the drum and chaff collector. We have the space already allocated due to the cooling tray up front.

The ductwork. I am surprised you (or anyone else) did comment about the flat walls on the inside. This was specifically to run ducting against. The planned path is the afterburner/hot air "intake" (where it leaves the roasting chamber" will be at the top rear of the roast chamber. It will go left and down the side. There will be an adjustable vent going back also (hence the reason to mount at the back). At that point it has to be left "open" so we can still lift the drum during EOR. It will mate (at an 11 degree angle) with a duct in the bottom, continue down and turn under the chaff tray to come to the top of the mounted blower. The blower will pull the hot air down (into the center of the squirrel cage) and out at 90 degrees to the front. I want a cool air inlet (this area will be cool due to insulation and probably forced venting - yes another little fan) at the top of the squirrel cage that we can adjust if we want cool air in to slow the roast. The duct that is coming to the front of the roaster will then turn left back to the front left edge of the roaster wall duct, turn up and then l turn right to meet up with the chaff collection area and roast chamber - full circle.
Alchemist attached the following image:
Actually, I held off buying this wheel because I have struck out so far on getting a small AC motor to run it. Right now it seems more cost effective to get the entire squirrel cage blower assembly. We could disassemble it, isolate the motor, and then use your ducting. Also, it looks like the smaller motors use a smaller shaft than in the wheel in your picture above.

I have an answer about speed. 3450 rpm is the limit on this one. Very easy to work with speed wise.

Give me a bit more info about what you are finding. "Isolate" the motor is an easy phrase to use, but most of what I have found is a very short shaft that will not allow us much space to flip the motor to the out side.

And, for the assemblies you have found, are the the wheels a suitable metal? This is just the kind of thing we need to go through from a design point of view.

Hit me with a few links if you have them.

I haven't had a chance to go out to look for the small squirrel cage fans yet.

However, looking across the room I got the hot idea of the day (so far).

Dual window fans! Shock

It just now occurred to me that the motors are just what we are looking for: small, light-weight, adjustable speeds. They are going on seasonal clearance at the big stores. I picked mine up over the past weekend. The next step would be to find the right metal wheels to move the air through the ductwork.

A couple of convection oven fans would work also. But I bet they'd be a lot harder to find.
Funny how great ideas occur together. I was at a thrift store yesterday and thought the same thing about a small fan. Can you post a picture to give me an idea of what you have? How about shaft size? Metal fan blades I can find. Heck, I have wondered about a P1 blade.

It takes up more room, but I thought of going for a traditional metal fan blade in a duct. Not as easy to get in there, but I think it will work, and with us doing the leg work of design, I think it would be very approachable for others.

The other thing I thought of was to go with an adapter on a smaller shafted motor. Most adapters are pretty cheap, and I bet we could get a 5/16" adapter.
Check out this. Nice little bath fan replacement motor.

1/4" shaft. Plenty long. We then add a $2.10 bushing for conversion to 5/16" from here and I think we are good to go with that wheel above.

I am seeing if there is a buy now price on the motor and combined shipping should we decide to go that direction.


BTW, I just thought of and checked the rotation of the motor and blower wheel - CCW BOTH !!!!!!Grin
Edited by Alchemist on 09/06/2007 5:09 PM
OK, here's the proposed duct work and basic arrangement. This is a rear of the roaster view. The space below the roaster ended up around 6". A little more than I wanted, but there we are.

Center square is the drum motor. Arrows indicate air flow direction. The yellow is insulation. At the bottom you can see the motor and wheel I have found. Air is drawn from the top left (as seen from the drawing) of the roast chamber, into the blower wheel, and out left and up back into the chamber. You will note two squares with holes. These are air regulation vents that we will control from levers (that I have all designed) i the front. One controls (the left one) cool air in, the other (the right) hot exhaust. In a perfect world, I want them both fully closed and we control fully by power input and our knowledge of how to roast.

I an only assume this will not be fully clear.

What questions do you have?
Alchemist attached the following image:
I think I follow what you have drawn.

I can see how the motor thickness will directly affect the height of the Zen 4.2.

Back when we we teasing each other with those pictures of the professional roasters, we were discussing whether to have the control module out to the side vs under the roaster.

Might there be enough room under the roaster for our variac (or triac) and could we use the inlet air to cool the controls? I don't seriously expect that they will need it, but it's a thought.

Or conversely, what if we put the fan motor "outside" the main housing, sill using it to cool the side-mounted controls?


I'm thinking that the interior ductwork that you drew would stay mostly the same.
There might indeed be enough room underneath for controls, but I don't think we can rely on the convection draw to keep the underneath cool. OTOH, cooling fans are cheap and easy to place. From a practical standpoint, don't forget we will have a cooling tray up front where controls would go, and that area will be crowded with the stirring motor and fan.

As for putting the fan motor outside, good idea. I have a couple rough sketches and we might well be able to put it "out back" and maybe even flip it to conserve room.

Oh, and I want to mention conserving space. Please catch me if I try and get this too small. I was noting I was fretting over the 6" and that the profile was going to be 18" tallShock. Upon going into my shop last night, I measured the Zen II and it is 16" cubic. Looking at what I would be comfortable with, up to 22" tall (including the top handle) doesn't cause me any grief. What do you think of that kind of height?
I am going to reiterate I really want this to be buildable by most anyone. The ductwork has really be causing my grief because whereas I can imagine what I want (always the first step), I don't think we can bend and shape a portion of the duct (where the lower and upper duct has to join tightly, but separate when we tilt the drum to dump) tightly and exactly enough.

What I do think will work will be to use rectangular aluminum tubing. That I am pretty sure we can make mate.

Just another piece of the puzzle.

Oh, and wait until you see what I have in mind for the housing of the blower wheel.
Have a look at these three items.

Grainger Item # 4C761
C09050393 Blower Wheel bore-1/4"ID ,od-3" x 1" $7.00

Split A3/16" B1/4" C9/16": $3.00each

C-Frame Motor,1/70 HP,3000rpm,120v,OPAO

This would be an alternative to the other grouping I gave. I little more $$ than I wanted for the motor, but that 2" shaft is nice.

What do you think? Do we keep looking?
Edited by Alchemist on 09/10/2007 4:40 PM
OK, I just can't leave a bone alone until I am done with it.

The blower wheel above btw is cw. Hence the reason for the new parts.

This motor (99080145 Motor CW - 162-B Price: $29.88) is the same as the one I originally spec'ed from Broan, but CW and less money. Plus we don't need the adapter sleeve. win-win

Any good hardware or appliance store should be able to get it, although I do note, many of the "new" prices are much more. Online might be the way to go.

I'll give this a rest until you answer.
Edited by Alchemist on 09/10/2007 6:09 PM
OK, one more thing. Our "final" option is money in lieu of home construction. What I mean by that is we could go with the
Grainger Item # 4C761
for $44.15 plus a 1/4" shaft coupler for $5.50 (plus a piece of 1/4" shaft).

That way we don't have to worry about the construction of the blower housing (that I do think we can do). With the coupler, we can extend the motor away from the heat source so it doesn't burn out (the whole point of these longer shaft motors). And we don't have to fret that the motor we have chosen as a replacement does not have sufficient strength.

So, convenience or a little more money? ( $50 vs $45 (30 + 8 + 4 + 3xs/h ) hrm - you know, after running those number we just might want to do the this option.
I was looking at the convection ducting and wondering if I was not understanding how it would work. As drawn, it seems to me that we would not get the full convection effect if the input and output ducts were so close together. Perhaps there is some separation that is not apparent from the drawing.

David attached the following image:

Edited by David on 09/16/2007 3:06 PM
David, (and Dan assuming you are reading), if you can give other suggestions, that would be great. There are multiple design features that have to be met and this was the only one I could find that met a majority of them.

First off, I want to point out that that the return air will be going effectively into a turbulent flow, i.e. the edge of rotating drum. Think in 3-d of Newton's 2nd law. An object in motion will remain in motion.... The air upon exiting the duct is going to want to go straight. It will be acted upon by the drum motion. That is my take. In a static roasting chamber (like a tradition convection oven) I would agree we would want them placed differently, but I think we are fine here.

As for design criteria.

1) As smoke "generally" rises, I want the afterburner and hence the exit duct at the top.

2) We need cool air intake. Correspondingly, we need a heat exit. Both need to be on an edge of the roaster so we can control them. Right on the edge (back) without extra ducting was the most simple.

3) The ducting needs to be a simple as possible since we need to be able to build it and have it seal well.

4) Any exit or entry on the top has to make it to the bottom, and that duct has to separate and join cleanly as we tip the drum. This is the tough point.

Please, sketch or describe something else that meets the above or rework what is above. I am all ears.
This is part lecture, part talking to myself, part information.

I did a lot of thinking about how badly we wanted this convection, and afterburner (they are integral) and how complicated it is going to make this. My hope is the "complicated" portion is on this side while we hash out and nail down ideas. And if someone doesn't want to bother, I suspect it (the roaster) will work just fine without it, albeit without a bit of the control. On that note, we need simply connections, attachments and tolerances. But also, I want the design fully thought out before we touch a piece of metal. If I (we) can't draw it, we can't build it. Part of that means working out construction order, attachment (shit, the head of that rivet ISN"T flat - it sticks up), alignment, etc. And even sheet metal, as thin as it is, is NOT insignificant in it's thickness (lesson from Zen I) and has to be accounted for. So, assuming we stay with the path I have planned, here is a bit more of the detail of the ducting that I have in mind.

A few notes of importance. The red angles are our mating surface between the roast chamber and the rest of the roaster. We have to keep this perfectly flat and smooth if we want anything close to air tight (we might talk gasket later). That means not duct fitting or attachments to it. I want to literally strap the duct tightly to the back wall of the roast chamber (the black straps). Those keep it in place horizontally. We also need a cap to the duct since it is open 1" x 2" rectangular stock. That bent sheet metal cap will keep the pressure horizontally to keep it in place against the red angle. There will be an opening cut in the angle, and aluminum tape against leak assurance. That cap is also going to provide us with the proper alignment since the duct is to the left of the angle and we need to maintain true and flush alignments.

And yes, I am being a bit picky at this stage, and I really expect you to have no trouble with this. By the end I expect the roaster will be a little out of true, but if we want any chance of actually having it fit together at the end, we get the frame and base alignments thought out and as true as possible.
Alchemist attached the following image:

Edited by David on 09/27/2007 7:23 PM
I've looked at some 1 1/2" drain pipe and I think that would be nice to work with. I am going to proceed accordingly. It has a nice flare at the end with should make attaching it pretty simple.

In addition, I think I have a design for the air control inlet/outlet dampers. I have thought of many designs and kept rejecting them for complexity.

See if this makes any sense. If that works, we/I have the after burner to flesh out and then the back bone of the design is done and we can start hitting the details and design drawings for you to build off of.
Alchemist attached the following image:

Edited by Alchemist on 09/24/2007 9:53 PM
Lemme see if I understand this drawing

There two ways for the air to flow: Path A and Path B.

The choice of path appears to be controlled by two damper which are "ganged" to work simultaneously.

If so, then Path B is easier for me to understand and it seems to be the recirculating, convection air flow through the convection pipes.

Path A is the alternative path. I'm not sure what its function is.
Does it bring fresh air into the chamber?
Does it exhaust heated air to the outside?
Is it the path though the afterburner which eliminates smoke?

David attached the following image:


David wrote:
Lemme see if I understand this drawing

There two ways for the air to flow: Path A and Path B.

The choice of path appears to be controlled by two damper which are "ganged" to work simultaneously.

If so, then Path B is easier for me to understand and it seems to be the recirculating, convection air flow through the convection pipes.

Path A is the alternative path. I'm not sure what its function is.
Does it bring fresh air into the chamber?
Does it exhaust heated air to the outside?
Is it the path though the afterburner which eliminates smoke?


First off, you mostly made good detective work of my drawing. They are not ganged though. They are independent. Maximum control. In the perfect roast, the both would be closed to outside air. You would be right on the money with power input and the roast would coast right along with variac input only.

But from my own experience, that just isn't the case. You sometimes need to cool the roast chamber. You get you ramp to steep and/or want to put the brakes on the roast. Hence the ability to have cool air come in. If we have cool air coming in that "extra" air has to go somewhere and creating a vent seems the way to go. I considered ganging them, but would rather have two levers. Technically when one opens, you want the other open also, but in some cases you may have moisture or after burner exhaust that needs/wants to be ejected, but no new cool air has to come in.

Path A is just what you say it is. Cool air in, hot air out. Oh, and these are meant to be infinitely variable. They are not meant to be all or nothing.
OK, I think I get it.
Here is how I am understanding it now.
David attached the following image:
BTW, I just realized something. Assuming we do gang these, we don't ever want both dampers fully in A/C mode. If we do, all cooling effect stops as all convection to and from the drum is cut off. 50% - 75% flaps may be all we want/need. That way we always have air moving through the system.
Alright, I have been thinking about this a lot and am confident that the blower fan from Grainer I speced out and piece of sink plumbing (the 90 degree bend drain pipe - $7-8) will let this work just fine.

Would you review the blower $$'s vs building our own from parts and let me know what you think. Are you good going with the off the shelf unit with modifications?
I went by a local Grainger outlet today and looked at some of their small centrifugal blowers. I checked out the one you mentioned above [4C761] as well as it's replacement, the 1TND4. I also compared the 1TDN3, which has a smaller wheel and motor. It has about the same cfm rating via faster RPMs. I chose it because I thought the lower profile would be better for our use.

Here's the link for this small blower. It was $41.95. I hated to spend that much, but we'd end up spending close to that if we did it piecemeal after counting shipping.

They had a high-temperature model for only twice the price!!
I asked about shaft extenders, but they did not have any in their catalog.
So, we'll need the ones that you sourced as while back.

I'll take some pictures as I disassemble it so you can design the ductwork.
You said in another thread that you found that a kilo filled your 11.5 inch bake pan to a depth of about 2 inches. How big a pan diameter would we need to reduce that to about 1 inch?

The cooler blower --
I am a bit spoiled in this regard as my two coolers are powered by 1) a leaf blower and 2) a blower assembly salvaged from an old clothes drier! Shock

Along those lines, I found this blower the other day and got one to try out. Actually I got mine via Amazon for a better price/shipping combo. If someone goes looking for this item on eBay, beware, as it is offered by at least three sellers at three different prices and three different shipping costs. YMMV. I wouldn't pay over $15 for it. I thought it looked lke a toy at first. But I saw that it had a place to insert new motor brushes, which is more than I can see for my full-sized leaf blower.

In any case it is a lot smaller than a regular leaf blower, but it puts out a substantial volume of air at a pretty good pressure. The problem with it for our purposes is that the motor sticks out quite a ways from the impeller blade. Actually the full depth [from base to the top of the volute] is a little under 6 inches, but I still think it's too tall.

And, you may not even want to consider something with a universal motor with all its noise.
David attached the following image:
I looked for sheet metal a few days ago and found that the Big home improvement stores only carried 24" x 30" stock. I think we need a sheet that is 48" to put a skin around the front of the cooler, right?

BTW, what thickness shall I look for next trip out? Please tell me in inches and/or gauge for aluminum and for galvanized steel. Tnx.
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