Zalman's CNPS9900DF heatsink is a twin tower cooler that stands 154mm tall and is built around three 6mm diameter composite wick copper heatpipes. For a look inside a composite wick heatpipe, check this out. The CNPS9900DF comes with two fans, a 120mm intake fan that runs at a fixed speed of 1000RPM and a center, 135mm PWM fan than operates at 1400-900RPM. The center 135mm PWM fan operates at 1400RPM to 900RPM, thus even at its slowest speed airflow is pretty well balanced between the intake (front) fan and exhaust (center) fan. The open frames allow additional air mass to be drawn in from the sides as necessary.