The DD4021 series comes standard with tach output, so I have to assume that that's what they are. I'll have to see how it would run at different voltages. The "yellow" cable, which would normally control the fan, is not an option, but there's a reference to an optional "thermal speed control" and "programmable speed control".
These "motorized impellers" suck air from the center: the rotation throws the air around, by centripedal force. Typically, the blades are straight, but the more advanced designs have "backward" blades, like this one, which I believe provides even more pressure, at the cost of a bit of flow.
I'll have to confirm the typo. I can't believe that the same size wheel would only draw ".25 amps". The RPM would have to be substantially lower, possibly under 500 rpm, and I just can't imagine that as being realistic.
So if it's good, I could pick up two of these, since they're only 4" in diameter, and ~2" thick. The trick would be to duct an air inlet to both... More thought required...
Any idea about the unit used in the pressure chart?
I really need to figure that one out!