pipe diameter

[Eclip]

Hey guys i have been designing my own scrubber based loosly on SM's as the length is what i like about it as it is going on top of my DT so all the pods can drop into the DT. one thing is that the SM uses a 3/4" pipe and is flowing quiet alot of water (approx 900gal)which would be better suited to a 1" ID pipe. i know the discharge area from the slot(aprox 19cm^2) is much greater then the 3/4 pipe(3.32cm^2) but wouldn't a 3/4 pipe restrict flow by a large amount compared to a 1" pipe (5.4cm^2) and thus reduce the total flowrate of the pump?

[sklywag]

Smaller diameter would create more pressure forcing the water to squirt out faster. But depending on slot size that may not be a factor.

Like when you put your thumb on the end of a hose. Same size amount of water just at different pressures. One slow and thick. The other harder but thinner.

[rygh]

Surface friction is the square of the velocity.
But of course, pipes are not nearly that simple.

I did research pipes at one point for my main tank. Unfortunately, a bit high for this argument, but still might be helpful.
PIPE FLOW RATES:
1.5" @ 600 gph = 1.6 ft/s
1.5" @ 1800 gph = 4.84 ft/s
2" @ 600 gph = 0.9 ft/s
2" @ 1800 gph = 2.92 ft/s
2" @ 3600 gph = 5.84 ft/s
2.5" @ 600 gph = 0.7 ft/s
2.5" @ 1800 gph = 2.05 ft/s
2.5" @ 3600 gph = 2.05 ft/s

You really want to target less than 3 ft/s or you start to waste a fair bit of energy on the pipe.

So for 800 GPH, extrapolating a bit, 3/4" is a bit too small. Better would be 1".
For my 900 GPH, I ended up using 1.25", but not because of the pipe.

IMPORTANT: Look at the inside of your pipe union fitting.
A 1.25" union has a 1" hole.
This can be a real issue, and that is why I bumped the pipe up a size.


Note:
Smaller pipe really just means wasted energy. The water will go through, given the right pressure.
With a smaller pipe, you simply need a bigger pump.
So depends on if you want to optimize for power or ease of build.

[Eclip]

much appriciated rygh i also did some flow calcs and found that the 3/4 pipe was a little too small for the rated flow. I wanted to minimise the loss and maximise flow but couldnt be sure i wasnt missing something obvious. Since aquarium pumps work best at low pressure (as do most pumps) i guess its 1" ID pipe for me. I didnt know that about the unions i will haveto check it out when i get my stuff in the next few days. I should get the rest o my acrylic (the shops stufed up the cutting) and i'll get some pictures up and hopefully get my tank wet shortly :P

Damn work and its time consumingness

[SantaMonica]

One thing to consider is that the pressure can help keep growth out of the slot.

[Eclip]

I did have a quick thought of that but didnt think much of it due to the large difference in slot vs pipe diameter difference. it only pressure if the slot isnt big enough to flow the water and being that it is very much bigger. anyways enough theory crafting i will let you know when my pump arrives as i have a 1" ID pipe and a 3/4" ID pipe aroudn the place

[rygh]

One thing to consider is that the pressure can help keep growth out of the slot.
Yes, but that is based on the size of the slot, not the pipe.
You want all the restriction right at the slot, to maximize pressure there.

On that note: I actually put a hole in the TOP of the pipe, as an overflow when the slot gets full.
The theory being that it becomes very obvious when you need to clean the slot.
Now I wonder if I made a mistake, and in fact end up cleaning the slot more.
Hmm.

[Floyd R Turbo]

A 1.25" union has a 1" hole.
This can be a real issue, and that is why I bumped the pipe up a size.

What kind of union are you talking about here? The type of union I use is called a "PVC Union" but like any solvent union, the inner diameter matches the outer diameter of the pipe it is being welded to, so in the case of a 1" pipe it has a 1-5/16" inner diameter, not 3/4". I'm wondering if you're talking about a joiner, and not a disconnect-type union?

[rygh]

No, I am talking about a normal disconnect type PVC union.
Go measure it really carefully.
I just double checked.
A SC40 1.25" pipe has an ID of a bit less than 1+3/8.
The SC40 union for that pipe has an ID of a bit more than 1+1/8. So almost 1/4" difference.
From memory, the SC80 was a slight bit smaller.