Flow restriction in pipes

[RSKY]

Any physics majors out there?

I am having to reroute my dryer vents. Is it better to have one 90 degree angle in the pipe or would it be less restrictive to have two 45 degree angles 18" apart?

Or would it be the same restriction?

Thanks for your help.

RSKY

 

[CobyRupert]

Only did a quick search and found info on PVC, but maybe this website also has info on metal duct. For 4" PVC this says that each 45 bend equals 5.1 feet of straight pipe whereas a 90 bend equals between 12 & 13.1 feet.
One would assume these principles would be similar for metal duct too. Maybe.
PVC - Equivalent Length Friction Loss in Fittings

 

[RSKY]

I am using PVC. Thanks for your help. That is the info I need.

I knew there would be somebody smarter than me on here.

RSKY

 

[CobyRupert]

Do people use PVC for dryer vents? Doesn't it get too hot?

 

[ozley]

Do people use PVC for dryer vents? Doesn't get too hot?

I wouldn't but PVC will burn but will not support a fire. A long time ago they were thinking of outlawing ABS because it would burn.

 

[RSKY]

Have used pipe for years. The instruction manual says using the flexible venting will void the warranty. This is a very large dryer. When it was new, before I read the manual, you could wash two loads before one would be dry. Went to pipe, took out a couple twists and turns, and it dries in half the time.

 

[kenmac]

Never seen or vented a dryer with PVC. Not saying it can't be done, I've just never read any manufacture's instructions or code that state you can use PVC to vent a dryer.. Back to your question.. 45's are preferred over 90's because of less restriction. I try to use hard pipe instead of flexible metal for dryer vents

 

[boggen]

a street 90 (quick sudden turn, can put a square to it, and be right at it)
then you have "sweeping 90s" check the electrical section / piping, you will find say 2" pvc 90's that are about 1.5' x 1.'5 in dimensions.

a street 90 = more friction = bad
a sweeping 90 / long 90 = less friction = good

more pipe = more friction = bad
less pipe = less friction = good

smaller diameter inside of pipe = more friction = bad
larger diameter inside of pipe = less friction = good

corrugated pipe (has kinda like threads of the nut inside the pipe = more friction = bad
smooth pipe, like various metal pipe, pvc, abs, etc.. type of pipe = less friction = good

more fittings, lots of pipe work = bad.
less fittings, straight run to the outside = good.

=============
2 45's act more like a long / sweeping 90, than a street 90.
it doesn't matter how far apart the 45's are, the primary goal. is to reduce "quick sudden change in direction"

i am not up to par for regs/codes for dryer piping. so will leave that to others. but rest of above, is quick and dirty run down that you should try your best and follow (goods) that is.

 

[PILOON]

I also recall seeing in a dryer manual that the first ? 5or 6 ft must be metal (due to heat), with balance OK to be plastics.
However I did use all expandable aluminum ducting in my installation and only dual 45's instead of single 90.

A mistake I did make in my bathroom vent was to use aluminum vent pipe. Under certain conditions water vapor condenses on the cooled (due to cool attic) alu pipe and drips onto the BR floor.
Kind of jolting when you step out of the hot shower. LOL.

 

[RSKY]

Got it done!!

Thanks to everybody for your input.

The first foot or so of my vent goes thru a metal pipe that slides into a 4" PVC pipe. Final total was 12" straight, a 45 angle, a 22.5 angle, 21" straight, a 45 angle, into the concrete slab where there is a 90 angle then 6' pipe. Pipe goes from the dryer thru two walls and a vanity before going into the slab. Believe it or not it still dries well.

The perils of remodeling a twenty year old addition.

The room was built with the washer and dryer as far away from the bedrooms as possible because I worked the midnight shift for more than twenty years. Since we are now retired we are trying to figure out how to move them closer to the bedrooms.

Thanks again for your input.

RSKY