Flue sizing

[mahesket]

Right then i'm thinking about making a patio heater from a small gas bottle. lots of threads seem to suggest that the correct flue size is around 4" my question is, if i were to use 4 x 2" flues located close together would i still recieve adequate draw from my flue? as the combined area of the flue would be the same.

Or would the individual diameters choke the flow?

Reasons are, I have 2" id pipe and i think it would make for a more interesting design.

The flue could be located as below please excuse my improvided drawings

OOOO

or

OO
OO (but closer together)

Cheers, Matt

 

[KemppiFrog]

Hi, curiously enough, the 4 will have the same x-section area as a single 4". Bit more drag from the increased surface area, but not a big prob. Best put them in your last 'box' style. Keep the temp up better than in line.
Post a pic when finished.
KF

Some stoves on boats have quite small flues, but seem to work, even if not very tall.

 

[mahesket]

Hi, curiously enough, the 4 will have the same x-section area as a single 4". Bit more drag from the increased surface area, but not a big prob. Best put them in your last 'box' style. Keep the temp up better than in line.
Post a pic when finished.
KF

Some stoves on boats have quite small flues, but seem to work, even if not very tall.

Excellent, thats what i was thinking, same cross sectional area (well pretty much) Box style it will be, just need to borrow the Jumbo transit to get the tube home now.

 

[normspanners]

as the combined area of the flue would be the same.

no it isnt, as has been said 4 is , the formula is 3.142 x (radius x radius)
1---- 4" 12.56
2-----2" 6.28
4-----2" 12.56

Some stoves on boats have quite small flues, but seem to work, even if not very tall.

by keeping the first few feet hot, me thinks

 

[KemppiFrog]

Hi Norm,
I don't think the OP was considering 2x 2" flues. Just the 4 x 2", which is the same X-section as 4", as in your calcs.

My first gas bottle job had a 2mtr 100mm (4") flue and roared right off. I lengthened it when istalled to about 3+mtrs, and this worked at all heats.

 

[mahesket]

Hi, Yeah i was meaning 4 x 2" flues.

As this will only be for outdoors what length of flue do people think will be adequate? I have seen all sorts from about 300mm up!

Also is it possible to make the flue removable to aid transport? maybe sealed with Fire rope/stove rope (i forget the name)?

 

[Shedendman]

Imho i always think one flue is better than a couple of smaller ones,aids better draw
If you want to join a few lenths simply weld a coller to one end and slip the non collered end in,perhaps with some locateing pins and cutouts to suit,in twist and locked,quick seal with this stuff
http://www.ebay.co.uk/itm/Siroflex-...ne-Sealant-Red-310ml-/271119823892#vi-content
goes off pretty quick to,as for height tbh above head high otherwise smoke gets in your eyes(i know a song like that)

 

[KemppiFrog]

Hi, Yeah i was meaning 4 x 2" flues.

As this will only be for outdoors what length of flue do people think will be adequate? I have seen all sorts from about 300mm up!

Also is it possible to make the flue removable to aid transport? maybe sealed with Fire rope/stove rope (i forget the name)?

Like the man says, a bit above head height. As for joints, as it is outside, sleeved tubes, forget the sealing rope if the sleeves are a half decent fit. (not difficult)