Which fan/pipe combination is the most efficient?

[Mads C]

I'm making a ventilation system with a fan that will blow air through a 1700mm long pipe that has a 45 degree bent in the middle. I should decide on a pipe diameter of 125mm or 160mm.

Also I need to choose between 3 different fans that has the following specs from the manufacturer:
120mm fan: 234 m³/h - uses 10W
125mm fan: 190 m³/h - uses 16W
255mm fan: 800 m³/h - uses 72W

I'm looking to find the most efficient solution with as much m³/h as possible.

My own guess is that thicker is better for the pipe. Would it make sense to use a 255mm fan on a 160mm pipe - maybe just blowing on 60-70% speed? Or could it make sense to use 2 of the small, energy-efficient 120mm-fans and somehow combine them with one 160mm pipe? Or is it best to choose a fan that precisly fits with the pipe (125mm fan with 125mm pipe)?

Anyone have an opinion of the most sensible solution? And perhaps someone even has a guesstimate of what m³/h I can hope to achieve? Any help is greatly appreciated! Many thanks in advance

All the best,
Mads

 

[anorlunda]

In engineering, we typically start with requiremens. How much flow? How much pressure drop? The word "best" is not a good way to start.

My suggestion is that you do a search for "air flow calculator" and spend some time playing with one such as this:
https://www.pipeflowcalculations.com/airflow/

That should give you some feel for the pressure/flow/dimiensions trade-offs.

 

[Mads C]

Thank you for your feedback, anorlunda :-)
Ideally, the airflow would reach around 400 m³/h. I'll look into the calculator- thanks!

If you already have some feel for the trade-offs, I'd love to hear which of these 2 solutions you'd opt for: the 255mm fan with a fitting to a 160mm pipe - or a dual set of the 120mm fans put next to each others, fitting to a 160mm pipe.Mads

 

[rbelli1]

a dual set of the 120mm fans put next to each others, fitting to a 160mm pipe.

A Drawback to this is that if one fan fails then you lose most of your airflow back through the failed unit.

BoB

 

[russ_watters]

Do you want the most cooling, least energy use or highest efficiency? They are three different things.

...You'd also need to know the pressure at the rated flow to be sure of which is most efficient.

 

[Tom.G]

Might be a little noisy. Using the 160mm pipe I get an exit speed of 20k/h. The exit speed with the125mm pipe would be 33kph.
(someone please check my math, it's getting late here)

 

[Mads C]

Do you want the most cooling, least energy use or highest efficiency? They are three different things.
...You'd also need to know the pressure at the rated flow to be sure of which is most efficient.

I'm looking for an airflow that approaches 400 m³/h (if possible) using a combination of the items I listed. Secondary, as little energy use as possible.

In danger of exposing a very flawed logic, I tried to calculate the airflow for the big fan combined with a 160mm pipe like this:
I put 250mm diameter and 800 m³/h airflow into the equation and find an airspeed of 4,53 m/s. If I then use that airspeed with a 160mm pipe diameter, I get an airflow of 328 m³/h. If that calculation is correct, I guess that airflow is the maximum I could hope to achieve (with some additional loss due to the pipe path and friction, I guess)

A Drawback to this is that if one fan fails then you lose most of your airflow back through the failed unit.
BoB

Perhaps it's better to place them "after each others, in the same flow", instead of "next to each others"? I wonder how well they will "add together"...

Might be a little noisy. Using the 160mm pipe I get an exit speed of 20k/h. The exit speed with the125mm pipe would be 33kph.
(someone please check my math, it's getting late here)

My own calculation of 4,53m/s which is 16,3 km/h is close to your calculation. I didn't think about noise. Some noise can be tolerated though, since it will not run all the time.

 

[anorlunda]

I'm looking to find the most efficient solution with as much m³/h as possible.

Some noise can be tolerated though,

You are talking with engineers here. Engineers don't work with hand waving requirements like that. "Efficient" (but I don't want to say what that means.) "Noise but not too much" I don't think you are ready to ask the specific question in your OP yet.

Let's go back to basics. What are you trying to accomplish?

 

[jrmichler]

Go back and reread the @anorlunda posts. You know what flow rate you want. The flow rate by itself is not enough to specify a fan. You need to calculate the pressure to get that flow rate. Then, and only then, can you select a fan.

 

[russ_watters]

I'm looking for an airflow that approaches 400 m³/h (if possible) using a combination of the items I listed. Secondary, as little energy use as possible.

Ok, in that case the largest fan - preferably on a speed controller is really your ownly option.

In danger of exposing a very flawed logic, I tried to calculate the airflow for the big fan combined with a 160mm pipe like this:
I put 250mm diameter and 800 m³/h airflow into the equation and find an airspeed of 4,53 m/s. If I then use that airspeed with a 160mm pipe diameter, I get an airflow of 328 m³/h. If that calculation is correct...

It's not. In a lossless condition, the velocity will go up proportional to the cross sectional area drop, like in a Venturi tube. In a real duct with loss, the smaller tube creates a loss and higher pressure, which reduces the flow rate somewhat.

Real fans don't just have a specified airflow, they have a curve relating pressure and airflow. As you add restriction, you increase the pressure and reduce the airflow. Thus the most efficient way to get a certain airflow is with the largest duct possible and a speed controller to reduce the fan speed until you get the airflow you want.

I didn't think about noise. Some noise can be tolerated though, since it will not run all the time.

Noise is mostly a function of velocity. So again, large duct, low fan speed = low noise.

 

[Mads C]

Thank you everyone for your replies!

All the best