Background
Back in June 2024 I decided it was finally time to add wired ethernet throughout my townhouse, as part of a larger series of renovations.
As part of this I put a small 6 RU networking cabinet into the garage to terminate the 24 additional ethernet runs I put in. This networking rack contains the following:
- Router [Ubiquiti EdgeRouter X]
- 24 Port Gigabit Switch [TP-link TL-SG1024DE]
- 8 Port PoE Gigabit Switch [TL-SG108PE]
- PSU
- 2 x 24 Port Patch Panels
The garage remains reasonably cool, peaking at about 32 degrees celsius in the height of summer, however since I cheaped out on the Gigabit switch, it was only rated for 40 degrees operating. That didn’t leave much cooling margin so I knew I would need to add active cooling.
Of course, being just one of many things I was doing at the time I decided to take a bit of a shortcut on cooling – I left the sides off for ‘natural’ cooling and jerry rigged together an old 120 mm fan for the top of the rack. Job done? No way! But I decided I’d come back to it soon…
Well fast forward almost a year later and I figured it was time I should finally get around to building a fan controller and adding the fans.
Requirements
I had a number of specific requirements:
- Speed controlled fans to minimise noise and maximise fan life.
- Multiple sensors:
- Temperature sensors – including at least 2 sensors to ensure valid readings (1 primary, 1 backup).
- Humidity sensors – as there is a dryer in the garage (along with the rest of the laundry) and when that is running it gets steamy.
- Multiple fans:
- 1 x 120 mm exhaust fan (top of rack).
- 4 x 80 mm intake fans (mounted in 2 RU blanking panel, bottom of rack).
- Embedded, microcontroller based solution. Cheap to make and customise.
Fan Selection
For the fan selections I am looking for the following things:
- Available from www.mouser.com – the primary distributor I use.
- 12 V compatible (single power supply solution).
- PWM speed control.
- Tachometer for speed readout.
Exhaust fan – 120 mm
For the 120 mm fan I picked the CFM-A225BF-158-597-22 as an exhaust fan. This fan uses ~2.4 A max, and can move an impressive 185.5 CFM (cubic feet per minute in stupid units, ~ 5 m^3/minute in real units). That is running at a max speed of 5800 RPM and a sound level of 59.7 dBA – the sound level of a normal conversation. I suspect I won’t have to run this fan that hard.
This means for the selected enclosure (360 x 530 x 450 mm), we can exhaust the entire volume in just over 1 second. Not bad.
Overkill? Maybe. Lets do the maths.
According to those nerds at Stack Exchange, the required airflow (m^3/s) = (P*t)/(ΔT * D * SHC) where:
- P = Power [watts]
- t = Time [seconds]
- ΔT = Difference in Temperature [°C or K]
- D = Density of Air [kg/m^3]
- SHC = Specific Heat of Air [J/(kg*K)]
Lets say we need to cool switches, their power supplies etc (including allowing 100W for this project) ~ 350 W, have 8 degrees delta worst case (32-40), density of air is 1.16 kg/m^3 at 32 degrees and the specific heat is 1000. This results in 0.0378 m^3/s.
This gives us an airflow of 2.26 m^3/minute. So we have overrated by approx 50% volume. I am happy with that margin – I will be able to run my fans slow and silently.
Intake fans – 4 x 80 mm
For similar reasons as above I selected 4 x CFM8020BF-155-444-22 fans. These run at 5,500 RPM at 44 dBA max and move an impressive 53 CFM (~1.5 m^3/min). With all four running I can easily intake the maximum volume the exhaust fan can put out.
The only downside to these fans is that the datasheet lists the wire gauge at 28 AWG – which outside of the defacto standard sizes (12 – 26 AWG), so will have to consider that in their termination.
So that’s the hard part done right? Picked the fans! Will be moving on to power supply and cabling selection next time.
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