Ever have the feeling that you’re about to have an aircraft take off from your house? If so, you’re in good company (and making money) but the FAA hasn’t found you yet and last thing you need is the HOA bitching about your plane not fitting in your garage (and you left your garbage cans out.)
So I had been running a Bitmain L7 aircraft, or miner, for about a week and couldn’t get the fans under ~5300RPM, which btw creates that nice high pitched whine you can hear throughout the house. My setup was an open inlet, an 8″ dual fan shroud (https://ebay.us/13zn51) on the exhaust to an 8″ duct (amzn.to/3DgKYy4) , then ducted pretty much straight out a window. I have a 6″ duct in the window as well to keep fresh air vented in. With this setup I saw the following on the L7:
Output Fan RPM – 5280 – 5500 RPM
Output Temps (highest board 74C/72C)
81 dB @ 1 foot (measured)
Good, not great, but good. How can we get these temps down I thought and if I get them down enough will the fans slow down?
Output Fan RPM – 5180 – 5280 RPM
Output Temps (highest board 73C/71C)
80 dB @ 1 foot (measured)
Duct fan power usage – 55W (measured)
Regardless of the dB level, the fans were still running high enough that the fan whine just resonated through the house, there’s got to be a better way, STOP THE MADNESS! Well, if some airflow brought it down a little, a lot of airflow may bring it down a lot?
If you read my previous bloguverse post on What’s my fans CFM and how do I measure it then you know that a 420 CFM vent fan might not be enough to create negative pressure with the Bitmain L7 fans running that high. A big reason for an exhaust duct fan is to create the negative pressure to help cool and vent the unit. A tell tale sign your duct fan is working in this fashion is the miner fans themselves slowing down while temps stay the same or drop. Another way to test this is to cut a small opening in the duct between the miner exhaust and duct fan. If air is blowing out then your duct fan isn’t creating negative pressure, it’s actually adding resistance. If air is drawn into the opening, then your duct fan is functioning as designed (fancy term for OK.)
So next step, I bought an adjustable 735 CFM 8″ duct fan (https://amzn.to/3IVnXC5) and cranked it up around 80%/600 CFM (measured) and got an interesting result. Slowly the fans on the L7 started slowing down, all the way down to about 3200 RPM, which is where they’re sitting now after a few days. They bounce between 3200-3400 RPM, but most important the fan whine is gone and the duct fan isn’t any louder than the lower RPM Bitmain fans. Another quick point, the first duct fan I used was all metal, and the second was a polymer. I did several temp measurements at the exhaust, just before the duct fan (32C/90F @ 1 foot), and found that they were low enough to use a polymer fan (rated at 140F) without the fear of it melting or having any deformation.
Here’s the data from the L7 and it’s hashing around 9300 MH/s:
Output Fan RPM – 3180 – 3380 RPM
Output Temps (highest board 74C/72C)
73 dB @ 1 foot (measured)
Duct fan power usage – 136W (measured)
So a significant drop in dB, but most importantly once the fans got under ~4000 RPM the fan whine was all gone. If you know much about the dB scale, or even if you don’t, it’s logarithmic. What that means is that if something, like having a conversation with your cat Fluffy (if you’re into that), is measured at 60 dB, then turning on a vacuum will scare the cat. Why? Maybe old Fluffs hates vacuums, but maybe it’s the fact that a vacuum would be considered a factor of 10 times louder, scaring your feline friend. Now what about carrying your little Fluffer-nutter outside near a busy intersection? Well you’ll get the crap scratched out of you, probably because traffic is roughly 20 dB louder than your friendly conversation with Fluff-daddy, which equates to a noise 100 times greater than the secrets only you and your cat share.
So when I go from 81 dB to start, down to 73 dB, I’ve cut my basement hobby down from Grand Central Station to a white noise coming from below, maybe even fooling my wife into thinking I’m vacuuming! Trust me, mama will never get mad at you if she thinks you’re cleaning.
In summary, and what’s most important other than making $$$, happy wife happy life!
No cats were harmed in the making of this bloguverse post.