Active clearance control system
The system provides fan discharge air for cooling the core compartment and the low-pressure turbine case. At low altitudes the core engine requires more cooling and the LPT case requires less cooling to prevent rub. At high altitude the core requires less and in the LPT core requires more to close clearances.
By means of a Y manifold and two shut-off valves, cooling air can be selectively directed to the core compartment or to the LPT case. The valves are not positively shut, but permit a required minimum flow at all altitudes and when activated added flow is directed. The valves are controlled by an altitude sensor which activates the core compartment valve below 19,000 feet +5000 feet and the LPT case valve above 19,000 feet +5000 feet.
Increased cooling airflow causes the cases to cool and shrink. This shrinkage closes blade tip to case clearances producing improved efficiency.