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Old 03-07-2013, 09:14 PM
Master Builder
 
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Location: South Australia
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Default Flue assistance.

Most would agree that a properly sized and used flue/chimney needs no assistance to keep the smoke out of your eyes.
However, there are always enquiries about what can be done to improve the draw of an already installed oven.
I was looking through the Ventilation Manual of the American Conference of Governmental Industrial Hygienists for some guidance on reading fan curves today.
I came across a reference to air ejectors as a means of moving hot air without exposing the fan to the direct heat.
See the attached pick, type B is the go. Essentially, if you inject some air into the flue in the direction you want the flue gases to travel, you will cause the air already in the flue to move along with it, creating a draw.
I reckon I'll hook up a pipe to my electric air matress pump, and try it next time I light my oven. It isn't actually a problem for me, but I'll see if I can do a poor job of laying the fire, generate excess smoke and see if squirting some fresh air up a one inch pipe up the flue accelerates the gases.
If it works, people who do have trouble should be able to insert the pipe from behind the flue, hide the air blower lot inside their enclosure and no-one would be any the wiser.
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Old 03-07-2013, 09:46 PM
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Thumbs up Re: Flue assistance.

Hi Mick,
I can't see why it would not work, sort of using a venturie effect!
I seriously believe that a good sized transition is the best solution to ensure a good draft BUT it also needs a suitably sized flue to handle the expected load, Once the smoke is directed up the transition, it will continue in the desired direction without further interference. I keep seeing many of the oven pictures with smoke stains on the front arches which indicate to me that their transitions and/or flues are inadequate.
Cheers.

Neill
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Old 03-07-2013, 10:04 PM
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Default Re: Flue assistance.

Mick,
this is an interesting idea. i'll be interested to see how well it works.
Dave
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Old 03-08-2013, 01:08 PM
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Default Re: Flue assistance.

Quote:
Originally Posted by nissanneill View Post
Hi Mick,
I can't see why it would not work, sort of using a venturie effect!
I seriously believe that a good sized transition is the best solution to ensure a good draft BUT it also needs a suitably sized flue to handle the expected load, Once the smoke is directed up the transition, it will continue in the desired direction without further interference. I keep seeing many of the oven pictures with smoke stains on the front arches which indicate to me that their transitions and/or flues are inadequate.
Cheers.

Neill
Neill I believe the same. However, I will try this for interest's sake, since my oven draughts OK.
I believe it will be a simpler, cheaper and more effective improvement to draughting than many other solutions I've seen proposed, like extending the chimney, inline fans, etc. I have the stuff I need to try it, and since I'm happy ('ish) with the way my oven draughts, I'll only be doing a removable install. We'll see how it goes.

The trouble I see with the transition, and I have yet to see a photo of an oven that addresses this, is that every single one is wider than it is long. We expect the smoke to make a sharp 90 degree turn up into the vent transition, and even in an oven with a 8 inch flue, the maximum distance generally available for the smoke to make this sharp turn is 8 inches. I believe this causes many draughting issues that are subsequently blamed on inadequate flue diameter/height.
By the time the smoke has made the turn, it is already halfway across the vent on its way to the front of the oven.
I don't believe I've seen an oven yet where the front edge of the top bricks of the inner arch are chamfered/bevelled to facilitate turning the smoke.
Flies in the face of all I've learned over the years about getting hot dirty air to go where you want it to go. Trouble is, to engineer a smooth turn into the transition, when you generally have only half a brick to play with, is not easy.
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Old 03-08-2013, 01:55 PM
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Default Re: Flue assistance.

I'm not sure smoothing the outside edge would have much effect on the airflow. In an air moving elbow this location is called the throat. Regardless of the shape a low pressure area tends to form in the throat. The backside of the elbow, called the heel, is where shape plays a key role in air flow. A radius heel leads to much smoother airflow and much lower friction loses that square. Obviously in my line of work there isn't always room for a large radius elbow, so instead we use square elbows(like our ovens) with turning vanes in the air stream. A turning vane is a 90 degree airfoil that bends helps bend the air around the corner, and in practice a square elbow with vanes is nearly as efficient as a radius elbow, just much more expensive.

Instead of a beveled edge, you could build a "door" that places a couple turning vanes in the upper section of the opening where the exhaust will be.
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Old 03-08-2013, 02:25 PM
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Default Re: Flue assistance.

In my line of work we avoid turning vanes like the plague. Fine for clean air, like big air conditioning systems where you have to build ducts square to fit the building, but horrible when the air is carrying your product to your baghouse for collection.
They catch solids and block up, especially where you lose entrainment velocity on a sharp right angle turn, or the air is moist, then you've got to take the plant down and blast them clean. At who knows how many thousands of dollars per hour of downtime. Shudder.
The radius on the inside of the turn reduces turbulence, meaning you product is more likely to stay entrained.

Never heard it called the throat. We normally call the constriction in a venturi the throat.

Of course we have strange archaic terms for everything. Bet you don't call your lunch "crib", your lunch break "crib break" and the lunch room the "crib room", either.

Last edited by wotavidone; 03-08-2013 at 02:28 PM.
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Old 03-08-2013, 02:27 PM
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Default Re: Flue assistance.

Quote:
Originally Posted by shuboyje View Post
I'm not sure smoothing the outside edge would have much effect on the airflow. In an air moving elbow this location is called the throat. Regardless of the shape a low pressure area tends to form in the throat. The backside of the elbow, called the heel, is where shape plays a key role in air flow. A radius heel leads to much smoother airflow and much lower friction loses that square. Obviously in my line of work there isn't always room for a large radius elbow, so instead we use square elbows(like our ovens) with turning vanes in the air stream. A turning vane is a 90 degree airfoil that bends helps bend the air around the corner, and in practice a square elbow with vanes is nearly as efficient as a radius elbow, just much more expensive.

Instead of a beveled edge, you could build a "door" that places a couple turning vanes in the upper section of the opening where the exhaust will be.


This is exactly the solution I came too as well. Something like a blast door with a curved wedge at the top.i didn't get around to making one because I believe that a blast door takes the temperature up too quickly to be safe for the refractory and is a nuisance to have to remove it every time you want to tend the fire.
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