Thank you Commie for your advice!
Actually I have a big Mitsubishi 24,000 BTU Commercial AC so the fans I bought, the P12 PWM PST CO
https://www.arctic.de/media/05/64/2f/1583824874/spec_sheet_P12_PWM_PST_CO_190328_r6_EN.pdf
are good just to move enough air, as you can see the temperature of my GPUs before:
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and after
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Glad it works for you. However, these profits aren't going to last forever and using AC to cool your rig might send you into negative profit numbers
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Hopefully my electricity cost is 0,07$/kWh and my AC has an automatic energy-saving operation i.e. the room temperature is automatically adjusted in order to maintain a fixed effective temperature, cooling operation is performed a few degrees warmer than the set room temperature once the temperature is reached.
I have difficulty to envision negative profit numbers. Let's simply take for example three 480 cards, from the website whattomine.com, mining ETH, it generates $12.51 in revenue per day (in date of May 15th 2:22 P.M. Easter Time) and with my 0,07$/kWh, it costs 0.71$ of electricity per day. I calculated my AC cost me in average 4$ per day.
So in this hypothetical example, the total electricity cost from the mining rig + A.C. = $4.71/day or
37% of the revenu goes to pay the electricity in this example but in reality I have much more than 3 GPUs so my actual electricity cost percentage which I monitor and calculate with my Power Distribution Units is around 5% of my total revenue.
There will be EIP-1559 in July which could impact negatively ETH mining revenues (-30%) and then next year ETH 2.0 will make it impossible to mine ETH but even when I look at the second most profitable coin to mine with GPUs, my electricity cost percentage will still remain very low.
I agree there's actually a frenzy around crypto mining but it is a really profitable business!