Here’s how the airflow works: The temperature in the data center is maintained at 80 degrees, somewhat warmer than in most data centers. That 80-degree air enters the server, inlet and passes across the components, becoming warmer as it removes the heat. Fans in the rear of the chassis guide the air into an enclosed hot aisle, which reaches 120 degrees as hot air enters from rows of racks on either side. As the hot air rises to the top of the chamber, it passes through the cooling coil and is cooled to room temperature, and then exhausted through the top of the enclosure. The flexible piping connects to the cooling coil at the top of the hot aisle and descends through an opening in the floor and runs under the raised floor.

via How Google Cools Its Armada of Servers » Data Center Knowledge.

Google’s custom servers also have a bare bones look and feel, with components exposed for easy access as they slide in and out of racks. This provides easy access for admins who need to replace components, but also avoids the cost of cosmetic trappings common to OEM servers.