Hard piping?

[tvman44]

Currently I use flex hose from HF DC to Thien type separator and from the 4" PVC trunk line to separator.  Thinking about changing the flex hose to hard pipe and wondering if it is even worth the effort, I know & understand there is supposedly less resistance but the line from DC to separator is about 4' long and the one from trunk line to separator is also abt. 4'.  Also thinking that if I do this I would also change the line from DC to separator from 4" to 5" using metal ducting.  Anyone have any experience doing this?
TIA,
Bob

[alan m]

it is definetly worth doing..

[retired2]

Currently I use flex hose from HF DC to Thien type separator and from the 4" PVC trunk line to separator.  Thinking about changing the flex hose to hard pipe and wondering if it is even worth the effort, I know & understand there is supposedly less resistance but the line from DC to separator is about 4' long and the one from trunk line to separator is also abt. 4'.  Also thinking that if I do this I would also change the line from DC to separator from 4" to 5" using metal ducting.  Anyone have any experience doing this?
TIA,
Bob

If I understand you correctly, you are considering replacing two 4ft. sections of 4" flex hose with one 4ft. section of 4" rigid pipe and one 4ft. section of 5" rigid pipe.  Ignoring bends for the moment, your current setup creates about 1.3" of SP loss while your modification would only cost you about .4" of SP.  Depending on the fan curve for you unit, that difference could amount to an additional 100 cfm or more.

Now, that may sound like it is worth the trouble, but the devil is in the details.  Flex pipe allows for long sweeping bends which have far less SP loss than tight bends, and when you replace that flex pipe with rigid pipe, my guess is you are going to need some 90 or 45 degree fittings.  Depending on how many and how tight the bend radius, you can quickly negate any benefit. 

I have seen many systems built with plastic fittings that are 1D bends.  These things should be avoided at all costs.  A 1D bend has twice the SP loss of a 1.5D bend.  To put that in perspective, your current 8ft. of 4" flex has losses equivalent to 24ft of straight, comparably sized rigid pipe.  However, the losses of one 4" 1D 90 bend is equivalent to a 12ft. section of straight pipe, and a 1.5D 90 bend is  equivalent to 6ft. of straight pipe.  So, you can see the adverse effect of having to add fittings. 

My guess is your change will provide some benefit, but the fittings required will keep the improvement modest.

[blackhorse16a]

What is "1 D" bend?

[WayTooLate]

"D" is diameter. 

A "1D" bend turns within its diameter. 
ie:  If you made a 180 degree turn, the two pipes would be next to each other.  Or, 4" pipes would be 4" OC apart. 

A 1.5D U-turn would put 4" pipes 2" between them (6" on center).   To make things worse, most of these small radius fitting shrink the circumference (pinching down the size) at the apex of the bend.  So you are penalized for a small radius, and penalized again for the smaller size... 

Keep the bend radii as large as possible.  That is why Retired2 was explaining why large radius flex hoses aren't always so bad... 

Hope this helps!

[retired2]

What is "1 D" bend?

Let me try to simplify WayTooLate's explanation even further.

A 1D bend is simply shorthand for a bend radius that is equal to 1 times the nominal diameter of the pipe.  So, a 4" 90 elbow with a 1D bend has a bend radius of 4".  If it is a 1.5D bend the radius is 6".  If it is a 2D bend, the radius is 8".

I purchased all my 5" elbows from Oneida air.  They offer 5 segment 1.75D 90 degree bends.  The five segments can be individually adjusted to create a bend anywhere from 0 degrees (straight pipe) to 90 degrees.  Any bend less than 90 degrees will have a bend radius larger than 1.75D.   Once they are set to the angle you want, it is a good idea to silicone caulk each segment to insure they are air tight.  These fittings are not cheap, but they are high quality and they will outperform 1D bends by a huge margin.