Thread: Ducting update.

Originally Posted by jmk89

I would have thought that all three holes would have the same centres in terms of the angles of rotation around the axis.

You are correct they should have the same centres.
What you are seeing is the limitations and vagaries of drawing the plans in a less than appropriate application.

The important thing is to NOT mark out and cut the big ducting holes out until the back and front and the rocker are all assembled.
Then mark out where the centre of the duct hole should go on one of the sides.
Take care to make sure there is enough "meat" of the rocker left at the top of the hole as shown below and dont forget that the hole in the rocker should be the ID of the ducting (compared to the holes in the sides which should be the OD of the ducting)
Planxz.jpg

With the gate fully assembled, swing the rocker to one side and Drill a small pilot hole through the marked centre of the side and through the rocker and side underneath that.
Then use the pilot hole as centre to mark out the ducting hole in the rocker and other side.

That way all the ducting holes will be accurately aligned.

Thanks, Bob.

I thought that was the likely answer and, surprisingly, I had intuited my way to the same process- it was just that I was concerned that the process was going to lead to a result different from the plans.
Since I rarely have such good looking plans to work from, I was prepared to think I was missing something important.
Thanks for clarifying things and for making these drawings available to us

The plans were drawn up to (hopefully) minimise use of material but of course the sides and top of the gate don't have to be that shape and can be fully rectangular which might make them a bit easier to make.
I have noticed Fletty's rocker gate has a straight top.
One day I might make a rectangular sided gate to see if it is easier to make.

I wasn't sure where to put this but "ducting update" seems the best place.
There is precious little in the way of 125mm or 5" pvc fittings etc available and pvc pipe is expensive over on the east side.
On some of my units, I wanted to use 125mm not 150mm due to room and flexibility.
5" flexy is readily available but it's the fittings that are difficult to find.
On searching, I found a range of metal fittings by Defleco for heaters, stove ducting etc. which I have used to connect both 125mm and 150mm pipe to machinery.

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I was after a 5" 90degree elbow or a 45degree and found some sheet metal ones that where around $50.
I contacted Deflecto and found they supply an elbow unit which is completely adjustable from a 90 degree through to straight. Absolutely perfect for what I needed and reasonably cheap at $25.

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Local plumbing shop shop got it in for me and Bunnies supply Deflecto bits so maybe they could order it in?
Part number is ELB059030

Lappa your going to hate me for saying but those elbows only cost about $5 or less each here. Even an 8" dimeter one is less than $10. Benefits of residing in a forced air heating country I suppose. The pipe isn't much good if the DC is a powerful one. If the gates are all shut the pipes can collapse.

Pete

As you say, it's supply and demand and over here, that's the market price. $25 to me is a lot less that $50 for the alternative and it's adjustable and it's a less than an airfare to buy it for $5.

Mine is mounted to an adaptor plate and flange fitting so I can fit it to multiple machines.

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Just blocked the system, static pressure 6.75" WC and no problems apart from a little leakage at the joins but they can be taped when the final position is decided.

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Maybe it will collapse on a really big system but it's fine on mine and that's all that matters - to me

Yup much cheaper than flying out to buy. Not as much fun though. It was the long straight pipe that suck down not the elbows. Most of the elbows are the multi angle ones. Can't recall any fixed ones. I'll have to look next time I'm at the Borg. Seal the joints of the elbow when you're happy with the angle. Helps a little.

Pete

I asked my local HVAC and Plumbing outlet re 45 and 90 degree elbows and it was all too hard. Local hardware had the $50 elbow. A call to Deflecto and I had the part number. The thing that annoys me is the HVAC and Plumbing outlet is a Deflecto distributor.

I would love to visit Canada one day - sister lives in the States, son in England so might make it a round trip

Looks like a neat solution even if it is a bit on the exxy side.

It would be interesting to compare the flow reduction from the galv elbow with an equivalent length of smooth walled flexy.

I originally had flexy connected straight to the adaptor but it had to take a sharp right angle turn when I had the table back against the wall and was starting to crush the pipe. Hence my need for this elbow which allows a sharper bend. Works a treat and actually swivels as you move the router table so the hose is always in the best position and the angle is approx 45 degrees when using the router table.

Was looking through the Loren Cook Engineers cookbook and found this which may be of some interest.
The numbers in the char refer to equivalent length of straight smooth duct for each bend/
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Thus a 1.5R smooth bore 90º bend (1a in the picture) reduces the flow equal to 17ft of straight smooth pipe.
A 1.5R , 5 piece segmented Segmented 90º bend (1b) reduced the flow equal to 27ft of flow.
What this says is, if you can, stay clear of segmented bends.
This may also explain why 2 x 45º bends are not as good as 1 x 90º bend.

Good info
I'm sure that this has been discussed before but I can't find it
What is the correlation in area between a round opening and a rectangular one? Is it 1:1 or is there more loss through a rectangular opening so it has to be bigger? For example, a 125mm pipe has an area of 0.01277m^2 while a rectangular opening 20mm X 650mm has an area of 0.013m^2. Will they flow the same?

Cheers

Originally Posted by Lappa

Good info
I'm sure that this has been discussed before but I can't find it
What is the correlation in area between a round opening and a rectangular one? Is it 1:1 or is there more loss through a rectangular opening so it has to be bigger? For example, a 125mm pipe has an area of 0.01277m^2 while a rectangular opening 20mm X 650mm has an area of 0.013m^2. Will they flow the same?

The Loren Cook Engineering cookbook has a "duct equivalence diagram" and formula (page 71) that will give you an indication of the relationship.

A rectangular duct with side "a" and "b" will have a round duct diameter "d" = 1.265 x 5th root of ( (a*b)^3/(A + B))

For a 20 x 650 mm slot this translates to a "d" of 101 mm.

However this applies to ducts, for slots/openings all hell breaks loose because its flow dependent since turbulence from sideways air entry really upsets things especially long narrow slots.

I have tried to measure this effect and the narrower the slot the hard it was to get consistent results, probably because I could not make the duct edge profile identical enough
The edge profile even down to less than 1mm (i.e. a small burr) seems to make a significant difference.

What you can be sure off is the round duct diameter will be less than that described by the duct formula but more than a round duct of the smallest slot dimension.
A fully bell mouthed entry will get you closer to the Cookbook formula.

Here's a an interesting one from the engineering cookbook

This one is for expanding flow which is often used in 100 to 150 mm transitions such as on Bandsaws.
On the table thats and A₁/A₂ ratio of 2
The numbers in the table are the length of equivalent duct each

It clearly shows that a long tapered transition is the way to go.

It suggests an abrupt transition is better than a 45º transition angle 45 which what standard PVC level inverts use.
Might have to test these next



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