HP ML150 Optimizing facility operation in high density data center environment - Page 14

plenum beneath the raised floor. The computer room air conditioning (CRAC) unit draws in warm air from the top, cools the air, and discharges it into the supply plenum beneath the floor. Figure 7. Airflow pattern for raised floor configuration with hot aisles and cold aisles To achieve an optimum downdraft airflow pattern, warm exhaust air must be returned to the computer room air conditioning (CRAC) unit with minimal obstruction or redirection. Ideally, the warm exhaust air will rise to the ceiling and return to the CRAC unit intake. In reality, only the warm air closest to the intake may be captured; the rest may mix with the supply air. Mixing occurs if exhaust air goes into the cold aisles, if cold air goes into the hot aisles, or if there is insufficient ceiling height to allow for separation of the cold and warm air zones (Figure 8). When warm exhaust air mixes with supply air, two things can happen: • The temperature of the exhaust air decreases, thereby lowering the useable capacity of the CRAC unit. • The temperature of the supply increases, which causes warmer air to be re-circulated through computer equipment. Therefore, administrators need to minimize the mixing of hot and cold air by using the practices outlined in the following sections. 14