Brazing hearth - Extraction

I am about to embark on my first boiler and I am setting up a brazing hearth. My workshop is a double garage, integral to the house. I have obtained a used Flamefast hearth which will be sited against an outside wall of the workshop. There is already a 6" dia hole through the wall above the hearth. In order to ensure maximum safety I plan to have some sort of hood over the hearth with an extract fan exhausting through the wall.
I am seeking advice on how to construct this and what kind of fan to use, bearing in mind the possible temperatures involved a few feet above the brazing operation and the need to ensure safety from fire. I want to keep costs sensible too!
How have others tackled this issue.
Thanks,
John

Couple of ideas for fume extraction below in sketch. Just food for thought. Standing by for the usual arguments disagreements etc from the armchair engineer fraternity. I've used both methods with inexpensive "found" motors / fans / blowers, they do work.

Re my sketch in previous post - needless to say a tee or rain cap should be used on the outlet pipe to keep rain out and away from fan or blower pipe. For the fan in tube method, a coupling and extension shaft from motor to fan will be needed. Nothing fancy needed though, mild steel shaft and ordinary ball brgs w metal seals will last quite a while in the heat. The blower method has no rotating parts in the heat, just a simple to fit metal tube. If a shopvac is used for blowing, it can be disconnected from the metal tube outdoors and brought indoors for other tasks when hearth is not in use.

You could also use a compressed air jet in place of the fan (or shop vac) in Jeff's second method. Use small drillings in the nozzle to reduce the volume of air used, if you three or four small jets appropriately angled (like a loco blower) you can get maximum effect for minimum compressed air volume.

Paul.

By the time the hot air reaches the fan it will be well diluted, I'd just use a 6" shower extractor fan and see how it goes.

Edited By duncan webster on 16/01/2019 23:35:55

Hmm, Spark arrestor - you wouldn't like any sparks going op into the soffits of the roof.

Just buy a cooker hood you may get one second hand on the net. Mine has stainless steel body and metal filter panels 

David

Edited By David George 1 on 17/01/2019 07:36:17

From what I remember of using a flamefast brazing hearth the extractor was very powerful, if you let go of a sheet of paper it would lift it into the canopy only stopped by the wire mesh. Probaly looking in terms of cubic meters min not per hour which bathroom and kitchen extractors are measured in.

Also don't forget to let air in otherwise your fan will just be pulling a vacuum

EDIT, just looked on Flamefast's site and their extractor is rated at 650cfm or 18 cubic meters per min

Edited By JasonB on 17/01/2019 08:01:43

John, a source of fans with metal impellers are those used on central heating boilers.

Chris Gunn

From the comfort of my well-appointed armchair I see Jeff's fan requires outdoor electrics. In the UK (240VAC) that means Building Regulations, at least for a permanent installation. I suppose it would be OK to power the fan with a temporary extension when needed, and not use the hearth in the wet?

Dave

In sketch 2, move the fan inside the building, mount it above the hood, and inject the air into the first bend , you could even take the pipe round the second bend if you are one of those picky perfectionists like me! Funnily enough, this is exactly what I was working on today, I also have an old flamefast hearth, the round type with the revolving table. I am making the flue ducting from sheet galv steel, and the flue will accomodate a woodburning space heater, a forge, the brazing hearth and a welding fume extractor, each will have a seperate shut off to maintain flue vacuum, and I have a selection of centrifugal fans of varying powers. When it is all finsihed (couple of weeks depending on time off for snow!) I will post some pics up if anyone is interested.

Phil

I expect a barrage of condemnation but as someone who silver solders almost every day exclusively on model steam boilers here are my thoughts for you. Unless you are going to be doing a lot of soldering an extraction canopy isnt cost effective, and may well be far more dangerous than you think. Fume extraction for welding is good because we weld well away from our body and face and have a full face mask on (ex asme coded welder) but with silver soldering we mostly work very close to the job, often needing to be looking down on it. An overhead extraction unit draws the fumes upwards and often straight into your face, trust me, it isnt pleasant. A very effective solution is to make sure the area is well ventilated and have a small office type fan blowing very gently at or just above the work and more importantly away from you.

My thanks to everyone with the ideas so far, much food for thought. Just to clarify, the flamefast I have acquired is just the basic brazing unit, i,e. a stand with a fireproof work surface and surround - no gas connections and no overhead extraction unit. The hole in the wall vents to the side of the garage over a path to a side door at the front of the house, thus I wouldn't want any external ducting or blower there.
I was thinking of erecting some sort of overhead extraction both the deal with the fumes, and to protect the garage ceiling, above which is a bedroom. However although it looks like a risk, am I overestimating the amount of heat rising up from brazing a boiler (in this case a 1 1/2" Allchin, about 13" long with a 3 3/4" barrel) ?
Fizzy's comments sound sensible regarding the fumes, but what about the fire risk to the building. Getting that much copper up to temperature needs a pretty large torch with a big output. This will be my first attempt at brazing on this scale, although I have done many smaller jobs, and some welding, hence my cautious approach (and the need to satisfy wife's concerns !)

Further thoughts welcome.

Regarding heat, you will likely end up with just a warm room. My brazing area has a lowish false ceiling of chipboard and after years of use there is absolutely no indication of heat affecting it. Just be absolutely sure that when you are brazing everything which might be the slighest bit flamable is removed from the surrounding area, especially things like spray cans and oils. Good luck and if you need any specific help just pm me.