The rotary heat exchanger (also known as a Kyoto Wheel) is a form of air-side economizer that solves some of the problems associated with air-side economizers. Instead of introducing exterior air directly into the server room, the heat wheel transfers heat between separated data center ducts and outside air ducts. The heat wheel is bisected by two parallel ducts. Outside air flows across the top half of the wheel, while inside air flows across the bottom half. The wheel itself is a big honeycomb. A cold honeycomb cell enters the hot duct at 0 degrees of angle. The inside air warms the cell (and the wheel) as it rotates through the duct, until, at the 180-degree mark, the cell is hot at the front and cool at the back, effectively cooling the air passing over it. The now-warm cell rotates into the outside air duct, where it is cooled through a half-rotation. Then, the cool cell enters the warm side, and the cycle begins again.The Kyoto Wheel ranges between 12 and 20 feet in diameter. Rotation is slow—just a few revolutions per minute (rpm)—to optimize the heat transfer. In this design, there is minimal air transfer between the outside air and the computer room air (new designs reduce air transfer to 1% or less), eliminating the humidity and particulate contamination problems associated with direct-transfer, air-side economizers.