Centrifugal fan DIY ...

[dkemppai]

Neat project!

That brings up a very good point, altering the back-pressure.

Pump design takes into account the operating point or load point of the fan. In your case you want a lot of volume and probably low pressure. A leaf blower is the opposite, less volume but higher pressure. Designing a fan for high pressure rise if it's not needed will most like just use more power than necessary.

My guess is looking at something like a furnace blower will get you closer to where you want to be. Big furnace blowers are both large in diameter and long axially. They can move a lot of air.

FYI. I use four 140mm PC fans to control ventilation in one of my shops. Bought a 5 pack off amazon. Didn't need much pressure rise, so these work well. It's for a parasitic system that derives heating/cooling from the house HVAC. An Arduino controlled damper and fans control air movement, to try to keep temperature to within a couple of degrees.

 

[tq60]

We have a 10 inch diameter computer type fan, does 700 to 900 cfm if I recall correctly.

It was a harvest from some communications equipment.

Top floor of shop has a full length ride vent but it does not do much, it can get to 140 in the summer.

Got a 25 ft length of accordion ducting, fan blows out a window, ducting on fan routed to high spot so it can remove the volume of air multiple times an hour.

It now stays just above outdoor temp.

Look for broken HVAC equipment, maybe ask around to see if you can get a removed unit.

The room air fan move a great amount of air and motor not really that big.

Sent from my SM-G781V using Tapatalk

 

[Flyinfool]

I salvage the blowers from my last 2 furnaces before the installer hauled them out. They work great for ventilating the shop quickly and have also done duty as carpet driers after a flood.

As far as the fan you are designing. you want the gap between the fan and the housing to be as tight as you can make it. Any gap is a significant air leak and the air will happily just flow over the edge of the blade as opposed to getting pushed on out.

The fans with few big reverse inclined blades move less air but will make a lot more pressure. That is why they are sometimes used in hvac where there is a lot of pressure drop through all of the ducting in the house.
The ones with a lot of forward inclined blades move lots of air but do not like any back pressure. It is rare to find a fan with flat blades, they are almost always curved.

The most airflow will be from a bladed fan like what is in the basic 20" box fan. It move a very large volume of air but can not handle any back pressure at all. Making a tight fitting ring around the blades will increase efficiency by eliminating tip vortices.

 

[projectnut]

I did the same thing. Repurposing a couple old furnace fans. The larger one (brown) has 5 speeds ranging from 500 cfm to 2200 cfm. The smaller one (green) 3 speeds ranging from 500 cfm to 1600 cfm. I use them to move air in the shop and the garage. The brown one has to be anchored in place when used at the 2 highest speeds or it will move itself across the room.

Both are mounted to cheap HF furniture dollies. I have about half a dozen each of the 500 lb. and 1,000 lb. dollies.

 

[MrCrankyface]

I have looked around the "used market" and found a few that could work but I'm way too deep into this experiment now.
Tried some more blade counts and definitely seems to be a sweet spot around 80 blades with the current setup.



So I figured I'd try altering the 80 blade wheel by making them twice as long(20mm instead of 10) and angling the original length blades 15 degrees forward.
Making them longer reduced air flow whilst angling them forwards increased it like expected.
An interesting aspect is an angle change of just 15 degrees increased the airflow by ~5% whilst still using the same amount of power(198W).


I really want to upsize this experiment but to save time and material I will first try a 30* wheel, if it has better flow I will also try 45*.
After that I might possibly try a wheel of the "best angle" which has a smooth angle rather than just a rotated flat blade.