pylon:
One Mega2560 should be enough to control at least 8 fans.If I understood your description correctly you need to control only 3 PWM signals, an UNO should be able to handle that. If you take I2C LCD moduls the rest should be no problem too.
If the value you supply a fan is a relative figure rather than an exact figure this would be the case. From what I understand, when controlling an PWM fan, you supply it with a figure of 0-255 (off to full speed)? If this is the case, then I could send the same value (eg 122) to both the in and out fans of one section (even though they are different fans capable of different top speeds) and this would set them both to 50%?
A Mega does sound an attractive option if it will be capable of supplying 25mHz PWM to all my fans from a single device without it struggling, especially if it can also cope with the rest of my requirements!
wvmarle:
In that case you can also get a PWM port expander. Much easier to handle than multiple Arduinos. I haven't checked the specs of that one, just given as example.
That one doesn't seem to be capable of 25mHz but I'm guessing others are out there. Are these easy to work with in terms of programming multiple PWM devices through one of these?