Creating a Vortex

[stuartthesmith2]

An Iforgeiron visitor to my shop remarked that he didn't smell smoke in my shop. I am posting this topic to explain the lack of smoke in my shop. If you look at the forge pipes going up the side of my barn/shop, you will see at the top of both smokestacks turbines like you see on the top of exhaust hoods at restaurants, which act like sails,catching wind and creating a vortex inside the pipes which suck the smoke right up the pipes.

[Wesley Chambers]

I know what your saying but I cant see what your talking about in those pics, and I have fairly young eyes hehehe

But they are very helpful in low wind situations!

[stuartthesmith2]

Thank you for the closeup pic...............this is EXACTLY what I use!!!!!!!!!!!! Those turbines could suck the hairs off a peach!

[Don A]

How tall are your pipes?

Also, do the turbines keep the rain out?

Thanks for showing your set-up.

Don

[stuartthesmith2]

each exhaust pipe is 45 feet high. If you use similar exhaust pipes, make sure they rise at least four feet above the peak of the roof, to allow the turbines to catch wind.

[stuartthesmith2]

at the bottom of each exhaust pipe, I have a t-connector with a cleanout. The cleanout consists of a cap, which I remove to clean out the bottom of the exhaust pipe. No rain water accumulates inside the pipe because they are always hot, evaporating any rainwater. Excellent question, though!

[Ramsberg]

I have to ask. Have you ever tried the stacks without the turbines?

Those are some very straight, large in diameter and tall stacks, perfect for a really good chimney effect with few losses.

I am just curious to know if you ever used just the stacks without the turbines and if so how big was the difference.

Thanks for indulging my inquisition.

Caleb Ramsby

[ThomasPowers]

You notice any LARGE industrial smoke stacks use turbines? If the stack is large enough the chimney effect, (hot gasses rise) is effective enough on their own.

Turbines help keep birds and rain out of the stack though but with 45' of rise they probably *slow* the gasses down than increase their speed!

[Nakedanvil - Grant Sarver]

You notice any LARGE industrial smoke stacks use turbines? If the stack is large enough the chimney effect, (hot gasses rise) is effective enough on their own.

Turbines help keep birds and rain out of the stack though but with 45' of rise they probably *slow* the gasses down than increase their speed!

I'm with you 100%! I don't see how they could add to the draft. If the draft is making them spin then it's slowing the draft. If the wind makes it spin then it's "catching" the wind in the vanes and working against the draft. Does keep birds and rain out and looks really, really cool. Now there are the "L" shaped ones that "weather cock" and they look like they would add draw in a wind.

[petere76]

Gents,

Remember that the rules of physics still apply. Force=Mass x Acceleration (f=MA). The mass is the smoke and air mixture and the acceleration, in this case increases, with the temp of the mass because the mass can expand (PVT rules apply here). The 45 foot tall stack serves as a very good conduit for the hot gas mixture. Anything directly on top of the stack is considered a design loss not a gain for the stacks efficency. Additionall, any bends, constrictions or irregular (rough) interior surfaces on the stack will decrease the mass acceleration and thus also decrease the systems efficency. The tech aspect of gas flow that is uniform and most efficent is called laminar (flow) and the term for disorganized or inefficent gas flow is called turbulant (flow). Anything uniform (gas or liquid) utilizing a pipe like structure for movement comes under the guise of a mass flow device. Jet engies and antiquated water systems are both governed by the same physical constraints.

As a practical example, ask yourself the question "does a duct exhuaster (fan) simply created a vacuum (low pressure) or increase the acceleration of the existing gas flow?" Conventional application in this case, increases the mass flow of the gas on the discharge side thus increasing the available CFM on on the suction side. We end of seeing the increase CFM as a increase of the (stacks) negative pressure. thus the suction is improved at the opening of the forge to the stack.

Peter

[Sweany]

Link removed at the request of Anvilfire.

[ThomasPowers]

You are making the assumption that the turbine effect adds into the chimney effect when both are present---it could subtract from the chimney effect due to flow interruption; or there could be a cut-over where the turbine helps for low flow and hinders when the flow increases.

By a poor analogy: take a straw; as it sits it does not pull water up it.
If you place your mouth over it and suck water goes up it. However if you attach a hose to it with water under pressure you may get less water going up it with your mouth in the way than if it wasn't there.

The effect of your mouth did not add to the effect of the hose pressure.

[MattBower]

As long as we're analogizing to vehicles....

Imagine a boat or aircraft with two props. The column of smoke in the chimney is the craft that we're trying to move through the water or air. One propeller is composed of the natural tendency of hot smoke to rise. This alone will cause the smoke column to want to move up through and out the chimney, absent some opposing force. The second prop consists of the suction caused by the spinning turbine, which is caused by outside wind. If the wind driven prop is moving too slowly relative to the other one -- that is, if there's no external wind, or not enough -- it begins to act as a drag on the craft. That's why, if one engine goes out on a twin engine aircraft, the pilot generally will want to feather that prop if he can. That way it provides less resistance to the efforts of the remaining prop.

In a short stack the first, heat-driven "prop" is weak. In a tall stack it's much stronger. So it's not hard to see how the turbine may end up often being helpful on a short chimney, but may do more harm than good (under most conditions) on a tall chimney. Make sense?

[Fosterob]

Stuart, thank you for clearing this up. Very good explanation of what was getting a little carried away.
Rob

[HWooldridge]

Following the same thought process (albeit independently) I added one of these turbines to a 18 foot stack on my first forge almost 30 years ago. Although it worked well enough in the beginning, the bearings went out fairly quickly and bound up - within a year as I recall. Maybe I got a lemon from the hardware store or maybe the heat and gases caused it to fail prematurely.

In any case, I got this design off of the Anvilfire site some years ago and have used it on a couple of stacks. We tried it here at work to accelerate a steam plume from a boiler simply by adding a sleeve to the existing stack - worked like a champ. I also have one on my forge at home and it pulls better than a straight stack - whether cold or hot.

stack.pdf

[windancer]

Preciate the PDF- will add this to my straight stack this weekend!

[DanBrassaw]

The sleeve is what I use. I haven't flashed the hole in my roof yet, so its hard to tell how well it keeps the rain out. I have found, however, that snow will fall right down it while I'm away. Which isn't a huge problem, I just take the few handfuls and throw it out the door, and what remains melts off quickly once I get started. For those of you who use it, have you had any "leakage" problems like that?

[Pat Roy]

HWooldridge

You essentially made you stack taller which will automatically increase the draft. I don't believe the velocity of the smoke is fast enough to actually draw in much ambient air through that annular space. In any case mixing ambient air with your hot smoke would be counter-productive because you would be lowering the temperature of the mixed air. That would work against you. Just make your stack taller, it's simpler anyway.

[ThomasPowers]

Dan you speak as if water actually falls from the sky---better get a CO detector! Since January we have had nearly a 1/4" of rain and that was severely localized. I haven't done anything to deal with precipitation down the stack---I haven't even put a ridge cover over the center of the shop roof or enclosed the truss ends. Ventilation *good* and we're wearing short sleeves and shorts out here now.

[DanBrassaw]

Dan you speak as if water actually falls from the sky---better get a CO detector! Since January we have had nearly a 1/4" of rain and that was severely localized. I haven't done anything to deal with precipitation down the stack---I haven't even put a ridge cover over the center of the shop roof or enclosed the truss ends. Ventilation *good* and we're wearing short sleeves and shorts out here now.

I'm pretty jealous of your location right now, it snowed here again the other day. Luckily, none stuck. I've got amazing ventilation in my shop, with no door, loose plastic over the broken windows I removed, and enough cracks in the walls to keep the place well lit on a sunny day. Until the weather gets nicer though, I find myself unmotivated to do anything about those issues. I'd rather stand next to a nice warm coal fire and swing a hammer...

[quick60]

I will put on my firefighter helmet here and offer some thoughts on the subject some of which has been mentioned in preveious posts from an physics perspective.

A taller stack when cold would not be not be very efficient without the turbine. The reason is as the smoke travels up the stack it will eventually cool and require more effort from below to push the cold smoke out. As the flue heats up and turbine spins faster this will become a more efficient process. In winter it will take much longer and require more btus and time to heat up the flue (espcially one 45 feet tall) and you may never get it hot enough to naturally take advantage of the flow of hot gases to travel up the flue. Hence a turbine would help this process especially when first starting your fire on a cold winter morning.

In certain weather conditions it will be difficult for smoke to travel up the flue due to differences in pressure and temp outside v/s inside. We call this stack effect in the FD and in certain conditions smoke actually can travel down to lower floors of a building and travel down stairwells instead of up.

One thing we have found in the fire service is positive pressure in the room or fire building is many more times efficient than negative pressure. If you pressureize your shop it would have a similar (and my bet is more efficient) effect on keeping smoke out of your shop and promoting flow up the flue. In a hot area it may improve temperatures in you shop as well as promote smoke travel up the flue. In a negative pressure situation (flue with no turbine) you must have the ability to replace air in the shop to improve the flow up the flue. Opening windows or installing grills in the shop to allow air flow into the shop will improve flow up the flue also. Adding power fans blowing into the shop will drastically improve flow. Think of putting that straw in your drink and plugging the top with your finger. Lift the straw out of the drink fluid stays in the straw. Take finger off and the fluid flows out. Blow on the straw and stand back cause you are going to make a mess.

A turbine may be effective in your area or it may be a hinderance. In a valley in winter with little natural wind it may restrict the flue if the flue is so tall it cannot develop any heat. Depending on wind in your shops location and length of flue the turbine may be a very handy thing to have or it may have little to no real effect. Try one and find out. It obviously works for stewartthesmith but may not work as well in your circumstance.

[Illi]

HWooldridge

You essentially made you stack taller which will automatically increase the draft. I don't believe the velocity of the smoke is fast enough to actually draw in much ambient air through that annular space. In any case mixing ambient air with your hot smoke would be counter-productive because you would be lowering the temperature of the mixed air. That would work against you. Just make your stack taller, it's simpler anyway.

I'm not sure, but I think the effect may be due to wind blowing across the top of the collar creating a negative pressure; increasing the cross-sectional area of the top of the stack would(?) result in more force pulling the smoke up the pipe (PSI times a larger area is a larger number of pounds). As with the turbine, I think this effect would likely be most helpful when the stack is cold and you're trying to start a draft. Once a good draft is going, I think that the draft would be the dominant effect.