I want a 6" bellmouth

[guy48065]

Where can I buy or how can I build a 6" ID bellmouth for my 2-stage impeller intake?  It really makes a lot of sense to me to ease the transition into the fan so I want to flare the intake--but not spend a bunch to do it.  I've considered mounting a section of 6" PVC on a lathe or drillpress & see if I can expand the mouth with a torch as it spins.  Also considered combing the aisles at Toys R Us for a plastic horn the right diameter.  Used baritones are still very expensive :) I've been eyeing kitchen departments when I'm with the wife...looking for a jello or cake mold with a large ID.  Etc.

Has anybody done this before or do I need to be the one?

[cts1085]

Do you mean something like this???
http://www.pennstateind.com/store/N-BELL06.html

[JeffQ]

Here's one neat trick shown on youtube for a DIY PVC version... I think if you look and use the right search terms (aeroport), you can find others.

http://www.youtube.com/watch?v=fzdnlBkG2mI

[guy48065]

Cool--I was hoping I wouldn't need to "re-invent the wheel".  Thanks to you both. 

[BernardNaish]

Hi, I have ordered one of these.

5" Metal Air Valve / Ceiling Grille in White for Ducting.
http://www.ebay.co.uk/itm/221197924324?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

They are cheap but give a bell mouth with a 5" adaptor at the back for connecting tubes. They are also available in other sizes.

Hope you are also able to find these in the U.S.A.

[retired2]

Hi, I have ordered one of these.

5" Metal Air Valve / Ceiling Grille in White for Ducting.
http://www.ebay.co.uk/itm/221197924324?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

They are cheap but give a bell mouth with a 5" adaptor at the back for connecting tubes. They are also available in other sizes.

Hope you are also able to find these in the U.S.A.

I'm sure it will be better than a straight pipe, but the radius on the mouth of this device seems much tighter than a true bellmouth.

[BernardNaish]

The central bit screws out and I do not know what that will reveal. I will report back when it arrives. The mounting plate on the left will not be needed. It seems a shame to buy it and not use two parts but then it is very cheap! I will store the unused parts as you never know when they will come in handy.

Regarding transitions from round pipe to rectangular I have now made two and it was very easy. I wanted a rectangle that was 1 1/8th" wide so I planed a block of hard wood to this size and then bent and hammered a section of ducting down to that size. I found that once it was close to what I wanted light hammer blows soon brought it to tight bends and flat surfaces. I was able to make the transition as long as I might need and chose 2 1/2 times the diameter of the pipe. This method ensures that the rectangular section has exactly the same sectional area as the pipe and I am hoping this will ensure there are no sudden changes in energy distribution that might cause more turbulence.

Like you I have a thirst to innovate; understand how things work and make them function better. When I have a moment to spare I am always sketching ideas even to the extent of a better clothes peg and an umbrella that does not turn inside out. Both these ideas I have since seen offered commercially and although slightly irksome it is also pleasant to think that I probably thought of it first.

Thanks for your persistent and carefully thought through comments.

[cts1085]

Bernard, I am curious has to your method for a round to rectangular transition.  If I were to scale this to a 5" pipe - you would take a 2.5*5" length of pipe (12.5") and if I wanted a 8" high opening on the other end make a small 8" x 2" x 2" block and use it as a form (left and right) to square off the end until I had a rectangle approx. 8" x 3.9" ? - How does the transition from round to rectangular work? just hammer away???  Thanks for any assistance on this...

[retired2]

...... This method ensures that the rectangular section has exactly the same sectional area as the pipe and I am hoping this will ensure there are no sudden changes in energy distribution that might cause more turbulence.

Rectangular duct is not as efficient as round duct for moving air, so the HVAC industry typcially adds 10% to the area of the rectangular end of a transition piece to compensate.  I followed this guidance when I had my transition piece fabricated because it made absolutely no difference to the cost.

So, how much difference does it make if the areas are the same rather than the rectangular end being 10% larger?  Probably little or none, since in this application there really isn't any "pipe" attached to the rectangular end.  Just food for thought.

[alan m]

as for the transitions.,
if you want a small diameter transition , hammering it out of a round pieces will do but for larger diameters it is easier to make the transition out of sheet metal and bend it your self
the first one is tricky while you work out how to do it. then you fly. i made the 8"transition for my next build in about 3 hours , going from no clue what the drawings where showing me to sealing it up
i made a template while i was at it so that i could remake that size again if i wanted


bell mouth fittings
im not sure them hvac air intake fittings are the right shape. they are probably better than a straight pipe but not as good as a properly engineered fitting
im planing on building mine out of a circle of sheet metal and a pipe roled into a pipe (not spiral because it would fall apart)
i will cut radial cuts in the circle center and cuts along the pipe. then align them so that when they are bent to the shape the gap in one piece is covered by the piece on the other. then weld hopefully
not tried it yet

[BernardNaish]

Alan. I have hammered a transition in 4" and 5" spiral wound pipe without a problem but I think it may not work for snap fit.

CTS1085 When doing this, the thickness of the hard wood former should be the same size as the smallest internal required but it cannot start out as full size in the other dimension as it would not then fit into the round tube. Yes you just hammer the bends at each edge of the former on one of the short side then the same on the other side then hammer the longer sides flat. Then adjust the round end as needed and fair out the steel sheet between the two. Piece of cake....well steel actually.

Alan. You might be better off silver soldering your bell mouth than welding such thin steel. You and Retired2 were right the thing I bought arrived this morning and is no good for this work. I am thinking of turning one from wood! What do you think? The best way would be to spin it from soft aluminum sheet on a heavy mechanical engineering lathe but I do not have one any more nor access. However I have just remembered that I had some success make aluminum leaves just as a silver smith would. I know you would be able to "raise" a silver one but that would be rather too fancy even for me. Perhaps I will try with soft brass as aluminum work hardens pretty fast and does not anneal so well.

Retired2 Thanks for the information on rectangular duct efficiency. I did not know that fact. Does it hold true for an air and wood debris mixture? You are probably right about the 10% difference not mattering one way or the other but will probably follow your lead and fabricate one with the rectangle increased by 10%. Just want to try and make sure I squeeze out (or is it in) as much efficiency

[alan m]

you could make it out of wood but i would be worried about it spliting  or deforming.
my back up plan is fiberglass around a mould

[retired2]

Is there no place "across the pond" that you can simply buy a commercial bellmouth?  Here in the states they cost less than $25 for any size.  I can't believe the trouble people are willing to go through to avoid that expenditure.  Any homemade version costs something, and that assumes your time is worth nothing.  And in the end what you have is a compromise.

[alan m]

Is there no place "across the pond" that you can simply buy a commercial bellmouth?  Here in the states they cost less than $25 for any size.  I can't believe the trouble people are willing to go through to avoid that expenditure.  Any homemade version costs something, and that assumes your time is worth nothing.  And in the end what you have is a compromise.

i know . its a total nightmare.
if i could buy one i would. there doesnt seem to be any real need for them
i got a quote of close to 90 euro to make one. thats a bit too much . i would rather save that for some other tools etc

[retired2]

Is there no place "across the pond" that you can simply buy a commercial bellmouth?  Here in the states they cost less than $25 for any size.  I can't believe the trouble people are willing to go through to avoid that expenditure.  Any homemade version costs something, and that assumes your time is worth nothing.  And in the end what you have is a compromise.

i know . its a total nightmare.
if i could buy one i would. there doesnt seem to be any real need for them
i got a quote of close to 90 euro to make one. thats a bit too much . i would rather save that for some other tools etc

90 euro is a bit over the top.  Penn State Industries does ship internationally,  but I have no idea what it would cost.