# Does a chimney work in this situation?

Hi friends,

We all know that chimneys are used to suck up smoke from a house's fireplace, in order for the smoke not to spread all over the house. But I wonder if I can use a pipe with a wide open end to create a similar effect outside the house? The following picture demonstrates what I mean (sorry for the bad drawing):

For example, when cooking outdoor, I don't want the smoke to spread out which can be annoying to my neighbours, can I just use a pipe like the one in the above picture to 'suck up' the smoke? And if the answer is yes, please explain to me which physics laws/rules can be applied here.

Thank you very much.

Riscilla.

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rcgldr
Homework Helper
If there's a wind, the chimney doesn't do much good.

I have seen some pre-fab firepits like this that have a chimney above the fire, but it does not capture all of the smoke, and with a wind it is just a large metal pipe in the way. Grilling bothers your neighbors? There is a special technique I like to use when people complain about grilling,
1)make a fist
2)extend the finger between index and ring finger
3)hold hand and said finger parallel to complaining neighbor
4)go back to grilling delicious meats.

I also guess that if there is a wind then the chimney might not work effectively. But in case there is no wind, how well does the chimney perform? As far as I know, if the chimney is high enough, there will be a difference in pressure between its 2 ends: At the high end, the air pressure should be smaller than at the low end (why?). As a result, the chimney can suck up the smoke. The questions are: how high the chimney should be? What is the maximum possible distance from the chimney's low end to the firewood in order for the smoke to be still sucked effectively?

Regards,

Riscilla

Q_Goest
Homework Helper
Gold Member
Hi Riscilla. As you say, there will be a difference in air pressure due to the difference in density between the hot air going up the chimney and the relatively cool air surrounding it. There's no easy way of calculating how hot it will be, there are too many unknowns such as how much air from the surroundings gets drawn up into the chimney, and how much heat transfer there is cooling the hot air in the chimney. Any calculation won't be very accurate I suspect. In still air, it will work, but how well isn't easy to determine with any accuracy.

sophiecentaur
Gold Member
Let's not use the word "suck". Excess pressure is what makes a gas move.
Why does a flue work?
It works better than just letting the smoke drift upwards because the flue itself gets hot and keeps the hot air from mixing / dispersing and cooling down as it passes through it. As the gas in the flue is less dense than the surrounding air, the cold (more dense) air outside pushes it upwards from the bottom. A chimney in a house always 'draws' better once it has heated up but even a simple flue as you describe, will help. A higher temperature difference between the inside gases and outer air is maintained so the 'heat engine' ('cos that's what it is) works better.
The system will cope with low speed air drift. Making the hood wider will help, of course, but why not make a partially enclosed brick oven? That would work much better AND cook more effectively.

I think that outdoor grilling is so common, along with the neighbor complaints, that a simple cost effective solution would have been discovered by now. With a very high flue, the smoke cools and sinks making the flue ineffective. Sizing a flue is a matter of trial and error. Commercial made grills that can be moved around the yard lets you stand upwind and place the grill as far away from the neighbors as possible. The smoke will be more dispersed and less annoying. Happy grillin'.
RS

sophiecentaur
Gold Member
There speaks someone with a large garden!

use a Van de graff generator to attract the charged smokes particles and pull them out of the air .

sophiecentaur
Gold Member
Or a "scrubber". That term always made us laugh at School. Can't think why.

Thank you all. Now the problem already becomes more clear to me. Well, I intend to install a small fan inside the chimney to increase the drawing force. How do you think about that?

Cleonis
Gold Member
Thank you all. Now the problem already becomes more clear to me. Well, I intend to install a small fan inside the chimney to increase the drawing force. How do you think about that?
To me that sounds quite over-engineered.
You've got moving parts then, and power requirement; not good.

Maybe a setup with two chimney pipes, one inside the other, so that there is a insulating layer of air. Possibly that will allow you to work with a longer chimney pipe.

More generally, moving outside physics realm: if you feel that grilling in the garden is unpleasant for your neighbours then maybe grilling in the garden is outside the boundaries of good neighbourly conduct. And while it's good fun to think about a technical solution, I doubt it's realistic.

Chimneys get extremely hot, and if you were to put a fan in there, if it were plastic, it would melt, if it were metal, the electronics would most likely malfunction. The reason for the use of a triple wall chimney is for the air to be an insulator from the inside hot pipe, to the outer pipe that is touching the house.

like I said before, if your neighbors dont like grilling tell them to p*ss off. It's a free country (assuming you are in the US)

Thank you for your helpful suggestions. I want the chimney to be as tall as possible. If I were to use that doubled-layer chimney, how long could the chimney be? It might be difficult to give an exact answer to this question, therefore I think an approximate one will help.

Thank you for your helpful suggestions. I want the chimney to be as tall as possible. If I were to use that doubled-layer chimney, how long could the chimney be? It might be difficult to give an exact answer to this question, therefore I think an approximate one will help.
Riscilla,
I don't think that double wall will help all that much. With no fan, I think 10 feet high and diameter to fit the size of the fire pit is no problem. The fan idea would most likely help increase the height possible. I don't know where you live but there are fireplace companies who sell inserts and that sort of thing and may be able to assist, and undoubtedly have experience in this area. Is cosmetic effect part of what you are shooting for? Your drawing looks more like a campfire than a cooking fire. You might keep in mind that no matter what you decide on, on a windy day the smoke issue will be back. A movable windscreen may help your odds of having a good grilling day. How bad do you want perfection?

RS