Not your ordinary application - silent 92mm fan needed

[cubic25]

Hey all,

I read the 'must read' thread regarding fans and the rules of thumb, so with that in mind, I have selected 92mm as the size I need.
I know this has been asked a few times before, but seeing as my application for this fan is a little different, I also need it to be ultra reliable, and none of the threads seem to address that.
My budget is $20.
Noctua fans was recommended in another thread on another forum (whirlpool), but on here, I read that they are noisy.
The CFM varies heaps between manufactures, with Noctua offering the highest between the ones I am looking at: https://www.pccasegear.com/category/9_5 ... -size-92mm
I will consider running it at lower voltage, but I am concerned that the fan won't move enough air to be effective, so chances are, I'll be running it at 12V.

It isn't really for a computer. I'm going to be using it as an exhaust fan on a kitty litter, much like this: http://www.instructables.com/id/Cat-Lit ... /?ALLSTEPS

I need it to be quit so that the cat doesn't freak out and ends up shitting in the sink rather than the litter box. Even though the fan won't be mounted on the box, (it will be at the end of the piping, rather than start), it still needs to be silent.

Any recommendations?

(I fully understand if this thread gets deleted, but I figured this was the best place to ask the question).
If it is relevant, I will go into more detail about how the contraption will be constructed if other users ask.

[cubic25]

After reading a few other posts, it seems position of the fan makes a pretty big difference to noise.

What I am planning on doing is sourcing some 90mm conduit. The conduit will have one 90degree bend in it and then a straight length of about 500mm. The fan will be at the end of this length. It will be venting to outside and I will use some mesh as a filter so that bugs don't make their way down the conduit into the kitty litter.

[cubic25]

Link to drawing:

Mod: deleted link as it was a download. Feel free to upload an image here.

[Zolishoru]

IMO, the noise level isn't the main problem for this particular application; more important would be how constant is the noise level, and how well protected the fan is(ideally both intake/exhaust protected), so almost any 92mm/25mm(thick)@1500rpm with good bearings and covered in fan grills on both sides should so the job.

[lodestar]

SPCR tested the current 92mm Noctua NF-A9 as part of the Noctua 92mm CPU coolers review early last year and noted that "...Measuring 29 dBA@1m off the heatsink(s) in our custom frame, the stock fan is fairly loud at their top speed of ~2,000 RPM. The smaller diameter blades, compared to 120mm and larger fans, sound higher pitched, but it's a fairly smooth sounding fan. The low noise adapter reduces speed to ~1,550 RPM, which makes the fan fairly quiet at 21~22 dBA@1m...". This was the PWM fan but it might suggest that for under $20 Australian you could consider the Noctua NF-B9 Redux 92mm 1600 rpm 3 pin fan.

[edh]

Just to expand on it: how much airflow do you need? This is an interesting question actually.

For a human public toilet you would want 6-8 air changes per hour. Looking at that kitty portaloo thing it may be a bit more than 100 litres in volume. This would mean you would want to move getting on for a cubic metre of air per hour. The slowest Noctua 92mm fan is still over 50m^3/h.

Why am I saying this? Well you really don't need much airflow at all. A basic 92mm fan running at absolute minimum voltage will be indistinguishable noise wise and still move enough air.

I'm guessing if you're in Aus that heat retention may not be the biggest concern but if you live in a house with forced ventilation and a heat exchanger just be aware that this may interfere with airflow in the rest of the house.

[cubic25]

Just to expand on it: how much airflow do you need?

Looking at that kitty portaloo thing it may be a bit more than 100 litres in volume. This would mean you would want to move getting on for a cubic metre of air per hour. The slowest Noctua 92mm fan is still over 50m^3/h.

Why am I saying this? Well you really don't need much airflow at all. A basic 92mm fan running at absolute minimum voltage will be indistinguishable noise wise and still move enough air.

The kittyloo in question is much smaller than the one in that pic. It would be about 50-60L.
So you think a 92mm fan on its slowest setting would still move enough air? That'l be awesome if possible, because it would also mean less bearing wear and power consumption.
Thanks for the advice so far.

[edh]

It would be about 50-60L.
So you think a 92mm fan on its slowest setting would still move enough air?

Let's assume you take a 50^3/h fan and run it directly at 5V. It will run at about half speed, be imperceivable and will push about 25m^3 of air. If it's 60 litres volume then that's 0.06m^3 and you will get hundreds of air changes an hour. 5V will be more than sufficient and the differences between fans at 5V will be much less than at 12V. If you can get a fan that runs even lower and use resistors in series to drop the voltage further then this could make it even quieter.

[cubic25]

Awesome! That means I could run the fan off an old 5V phone charger.

Anything highly recommended, yet cheaper than the Noctua? It came in at $30 after shipping. Some ebay shops have it at $28.
Anything under $20 that is as good?

[lodestar]

If you are planning to use a 5V supply then start voltages for these fans could be an issue. The Fractal Design 92mm for examples needs 6V. Noctua don't say but it is probably 6V or even 7V. The stated starting voltage for the Nanoxia Deep Silence 1400 rpm 92mm 3 pin is 5V and it only costs $9, so this might be the one to go for.

[cubic25]

If you are planning to use a 5V supply then start voltages for these fans could be an issue.

I realised that after I made my statement about the phone charger. I got really excited about the https://www.arctic.ac/worldwide_en/arct ... wm-co.html because, apparently, its ball bearings are made to run 24/7. The I read that its start up voltage is 9V. Doh!

I'll have a look at that 5V fan you mentioned.