BBQ Design

[leenowell]

Hi All,

I am in the middle of designing/ building a brick built BBQ and am struggling to understand the dynamics of chimney design. The overall design looks like this...

The red areas are all brick/ block/ concrete. The silver areas are stainless steel and the yellow area is the gap in the chimney where the smoke comes out.

What I am trying to do is to optimize the draw to minimize the amount of smoke hitting the cook (i.e. me!) and also to provide good flow of air across the coals. I guess the variables I have with this design are...

1. Changing cross sectional area of the chimney section
2. Varying the width v's depth of the chimney
3. Varying the height of the chimney (although I have a max height based on the surrounding bushes etc.)
4. Changing the angle of the various slopes Any others?

Does anyone have any idea how these factors (or others) impact the draw? E.g. if I reduce the cross sectional area of the chimney and keep everything else the same will this increase or decrease (or even have no effect!) the draw?

Any help gratefully appreciated.

thanks in advance

Lee.

[Muddymike]

Hi All,

All I see is a meaningless red shape!

Mike

[Capitol]

In any fireplace, the draw is most influenced by the aperture. ie keep the chimney entrance as close to the coals as you can. That's why you use a sheet of newspaper across the front of the fire when trying to light it. Fireplace design is not an exact science, there's a lot of experience required. Copying/scaling an existing design which works can be a good start. A Google.com search will give some US results.

[Tim Watts]

Try the "View 3D" button...

[Michael Chare]

You should have watched last night's programme about eating beef in Argentina. 9pm BBC2 IIRC

[Tim Watts]

A solid fuel stove might have a 5-6" diameter liner 7m long which is what we have (roughly)...

That will draw with very little effort from cold.

So perhaps working on a similar cross section area might be a start?

[Graham.]

click "3D view" and drag it round.

[leenowell]

Thanks very much all. In response to the various questions/ comments..

When you click on the link, if you then click the 3D button you can then sp in the design around by holding the left button and moving the mouse.

@Capitol - by putting the newspaper in front when lighting, I guess this ef fectively closes the front of the fire rather than having the effect of mov ing the chimney nearer the fire. Is this the case? If so, I assume this me ans that increasing the chimney area would increase the draw?

In terms of measurements, to give you an idea...

Front opening is 1770mm x 770mm Chimney cross section at minimum (i.e. once the front and side slopes have reduced it) is 1026mm x 246mm Angles of slopes is 45 degrees Height of chimney from where side slopes finish to top is around 1200mm

The only other thing I can think of which may help is that I have knocked i t up in wood to test it out. Lighting some paper to create the smoke resul ts in a bit of draw and smoke comes out of the chimney. After a little whi le (say 10 mins) it now seems to come out of the front as well as the top. Given it is paper (and not much of it), there was little real heat.

thanks

Lee.

[The Natural Philosopher]

Basically to avoid smoke coming out the front the aperture should be no more than 2.5 times the area of the chimney. A taller chimney can taper in from the throat too.

[Davidm]

When I opened out an old fireplace in my victorian house I created a throat at the top of the opening where the fireplace becomes the chimney, such that there was a smaller opening into the chimney before it then opened out into the full cross section inside the chimney.

I think that the idea is, that once some heat builds up in the fire, the hot air/smoke rising up past the throat creates a slightly lower pressure above it, thus "drawing" the smoke upwards.

More by luck than design it seems to work well once a bit of heat has been generated in the fire itself.

[leenowell]

So with the 2.5 rule, that would imply that I need to double the size of my chimney (if my calcs are correct). This seems very big?

@Davidm - was your throat flat or did it taper up the chimney reducing the hole size and then stopping after a bit so it then went into the full size chimney (if you see what I mean)?

[Chris French]

In message , [e-mail] writes

Is this worth it and going to work as it's for a BBQ?

At the point of using the BBQ to cook, there isn't much smoke - except from a bit of fat burning off maybe. and it's isn't really burning like a fire does, I doubt it would have enough draft to carry up the smoke that is produced

And to have enough space to cook, ISTM there will be such a large opening so far above the BBQ that it's not going to work that effectively

[leenowell]

@Chris, you raise an interesting point. You are right, in essence the main use of the chimney is at the beginning when you fire it up but after that is limited. In my case I need something to ensure any flames etc. don't se t fire to the surrounding bushes. I guess the other aspect is ... there is not really much of an issue with smoke hitting the cook now and again eith er.

@Michael - on your recommendation, I saw the Argentinian program last night on iPlayer. It gave me an idea... I could bring the cooking surface up hi gher and reduce the distance to the bottom of the chimney. The important t hing for me was the head height so I don't keep hitting my head on the chim ney. I may try this at the weekend.

I also have found some interesting information

This website seems to say that the rule is 1:8 for chimney area to fire are a

Based on this, I would need to reduce the width by about 40% (to 246mm x 66

7mm).

Also, building regs seems to suggest 15% as the optimal but it does say tha t for larger fireplaces, you need to consult and expert (or clearly this gr oup :) )

[Davidm]

See here for sketch of the cross section of the fireplace:

the throat goes the full width of the fireplace, and is only across the back. If I was doing it again I'd do something similar on each side as well. It has to go up into the chimney opening a short way, so that the "low pressure" area forms inside the chimney, to draw the smoke up into it.

It's just fashioned out of brick, smoothed over with cement render.

[leenowell]

Ah... I see - thanks for the sketch it was very good. In my current wooden mock-up, this would be easy to replicate to see how much of an impact it has...

thanks

Lee.