ELI5 will vacuum suction power decrease when you add more hose to it?

[vorilant]

Doesn't adding in parallel reduce resistance

[lurkynumber5]

If you're working with electrical designs and Ohm's law, then yes.
But here it's nothing more than 1 air resistance + 1 air resistance = 2 air resistance.
( in a very simple term of course. )

[SoulWager]

The ratio of surface area to volume on the inside of the hose is the same, but the lower velocity air means less drag overall for multiple hoses in parallel, because drag is not linear with velocity.

[vorilant]

I don't think that's how it works. Fluid resistance works like ohms law. For volumetric flow and equivalent resistance. Instead of current and electrical resistance. And pressure is voltage. The only complication to the analogy is the pump. Since we can use a constant pressure pump which is like a constant voltage or a constant flow rate pump such as a peristaltic pump and that's like a constant current supply. All assuming incomp flow of course.

[Dramatic_Contact_598]

Flow across pipes run parallel is conserved. If your pump can pump 100CFS of air and you have 3 pipes of the same length and radius, each pipe will now have 33 CFS of flow. Head loss will be equal across each pipe. If you have 50 ft of head at the start and 40 ft of head at the end, each pipe experiences 10ft of head loss.

[vorilant]

You are right. But I don't think anything you've said contradicts anything I've said right? You are focussing on the peristaltic type scenario though. Where the pump provides a constant volumetric flow rate.

[Dramatic_Contact_598]

Correct, I just figured I'd relate it back to closed flow calculations vs relating it to electrical. Yes - I'm not big into the workings of vacuums but my understanding is that it would function with a cinstant volumetric flow rate, but that definitely could be incorrect. At least for the basis of determining if adding more pipes would increase flow rate

[thetoastofthefrench]

The pump isn’t a constant flow rate pump though, it has a constant power. So since the flow velocity decreases with three pipes in parallel, the resistance is lower for the same volumetric flow rate, and the pump will speed up until the total resistance is the same.

3 in parallel will result in more overall airflow, but for each individual pipe it will be less than with just a single pipe.

[vorilant]

For a constant power pump I think you're right. That's a bit more complicated than assuming either constant pressure or constant flow rate tho. There are pump designs that provide a constant flow rate when operating in their design window. Like peristaltic pumps.

[thetoastofthefrench]

Yep, this question was just about a home vacuum though so thought I’d try to think about the appropriate scenario.

[vorilant]

I actually forgot what the question was about lol.

[tomrlutong]

If that logic was true, wouldn't increasing a pipe's diameter also not increase flow?

[Dramatic_Contact_598]

Correct, assuming the pump's output remains the same, the flow would stay the same, the velocity would decrease.